Cable Engineering for Local Area Networks (Barry J. Elliott) (Z-Library).pdf

Cable engineering for
local area networks
BARRY J ELLIOTT
W O O D H E A D
P U B L I S H I N G L I M I T E D
Cambridge England

1. Introduction to Austenitic Stainless Steels
Geogy George' and Hasan Shaikh'
Abstract The Family of austenitic stainless steels has a wide variety of grades precisely tailored for
specific applications such as household and community equipment, transport, food industry, industrial
equipment, chemical and power engineering, cryogenics, and building industry. The optimum choice of
the gradcs would depend on service needs and this would require a clear understanding of the metallurgical
parameters, which control the microstructure and thus the mechanical properties, formability and corrosion
resistance. This chapter. in brief, deals with the physical metallurgy, welding metallurgy, and physical and
mechanical properties of austenitic stainless steels. In the physical metallurgy of stainless steels the
tendency of alloying elements to form different phases, the transformation of austenite to martensite
during cooling or straining, hardening processes and formation of intermetallic phases, have been discussed.
The influence of chemical composition and temperature on the various physical properties of austenitic
stainless steel such as coefficient of expansion, thermal conductivity and magnetic permeability is highlighted.
Variation in mechanical properties, such as tensile, fatigue and creep strengths of austenitic stainless steels
with temperature, composition and microstructure has been discussed. The mechanisms to strengthen thc
austenitic stainless steels by appropriate thermo-mechanical treatments, grain refinement etc. have also
been addressed. Austenitic stainless steels lend themselves remarkably to deep drawing and cold rolling,
where their work-hardening characteristics enable high strength levels to be attained. Weldability is
excellent. and welds, which do not transform to martensite during air-cooling, have mechanical properties
similar to base metal.
Key Words Austenitic stainless steels, high nitrogen steels, physical metallurgy. physical properties.
mechanical properties, welding, martensite.
HISTORICAL BACKGROUND TO STAINLESS STEELS
As a class of matefials, stainless steels stand apart and are considered the backbone of modern
industry since they find wide applications in chemical, petrochemical, off-shore, power generation
and allied industries. In 1889, Riley of Glasgow discovered that additions of nickel significantly
enhanced the tensile strength of mild steel, and in 1905,Portevin observed that steels containing more
than 9% chromium were resistant to acid attack. The transition from the laboratory to the first attempts
to confirm the practical applications of stainless steels took place principally from 1910 to 1915. To
cite the pioneers of this work would go beyond the scope of this introduction; nevertheless, a few
'Scientific Officers, Corrosion Science and Technology Division. Indira Gandhi Centre for Atomic Research.
Kalpakkam-603 102, India.

2 GEDKGE
AND SHAIKH
~~
important names must be mentioned: the Englishman Brearley for martensitic steels, the Americans
Dansitzen and Becket for ferritic steels, the Germans Maurer and Strauss for austenitic steels [11.The
term "stainless" (inoxydable in French or rostfrei in German) is now popularly used for iron alloys
containing greater than 12 wt. % Cr. In a relatively short span of time since the discovery, the
applications of stainless steel have grown rapidly with its image changing from that of an expensive,
high-technology wonder alloy to that of a cost-effective, everyday material of construction.
The design of stainless steel alloys has been motivated primarily by chemical, mechanical and
thermal stability considerations. The base for the various stainless steels is the binary Fe-Cr system [2]
(Fig.l), the properties of which are modified by
the addition of several major alloying elements
such as Ni, Mo, Mn etc. as well as minor ones
such as C and N. Fe-Cr-Ni alloys are the most
predominantly used austenitic stainless steels.
Important phase relationships in Fe-Cr-Ni stainless
steels can be considered to stem from the properties
of the binary Fe-Cr and Fe-Ni phase diagrams. A
convenient way of understanding the phase
relationship in the Fe-Cr-Ni ternary system is by
the use of cross-sections through the ternary
diagram, such that the proportion of one element
is constant. Fig. 2 shows a section of the Fe-Cr-Ni
diagram at a constant Fe content of 70% [3]. It is
clear from the diagram that austenite is the stable
phase in the Ni-rich side of the diagram while
delta-ferrite is the equilibrium phase in the Cr-
rich side.
The important factors, which must be considered
in the design of the various types of stainless steel,
are:
(i) Corrosion and oxidation resistance in the
operating environment
(ii) Mechanical and physical properties
1800
1700
1600
1500
1400
a
'1300
c!
% 900
e
c-" 800
700
600
500
400
300
Fe 20 40 60 80 Cr
Cr, wt %
Fig. 1. Fee-Crbinary phase diagram [2].
(iii) Fabrication characteristics from the point of view of both hot and cold working
(iv) Welding; many of the stainless steels are required to be readily weldable, and welding must not
There are many different stainless steels, and the main types, are grouped according to their
impair the corrosion resistance, creep resistance or general mechanical properties.
metallurgical structure as follows 141.
MartensiticStainless Steels
They are Fe-Cr-C alloys with or without addition of other alloying elements. Chromium content is
12-18 wt. 9
6 and carbon is 0.1-1.2 wt. 9
6 [5].Several other additions are also made such as Mo. V, Nb,
Ti and Cu to get certain desirable properties. These alloys are austenitic up to 1050"C,but transform
to martensite on cooling. The additions of alloying elements lower the martensite start (M,) and finish

Introduction to Austenitic Stainless Steels 3
1600
1400
P
v
k
5
3 1200
1000
800
I ,
L
-
Ni % 30 20 10 0
Cr 7% 0 10 20 30
Fig. 2. Vertical sectionof Fe-Cr-Niphase diagramshowingthevariationof solidification
mode with compositionfor a constant Fe content of 70% [3].
(Mf)temperatures. Therefore, controlled addition of alloying elements is necessary to maintain the M,
temperature at a reasonably high value above room temperature.
These stainless steels are characterised by very high strength and low toughness. Temperingis done
to increase the toughness. Typical applications include turbine blades, springs, aircraft fittings, surgical
instruments,knives, cutlery,razor blades and other wear-resistingparts.Typical examples of martensitic
stainless steels are AISI Types 403, 410, 420 and 431 and their compositions are shown in Table 1.
Table 1. Chemical compositionof martensitic stainless steel grades [7]
Grade C Si Mn P S Ni Cr
AISI Max MaX MaX MaX M
a
X
403 0.15 0.50 1.o 0.040 0.030 - 11.5-13.0
410 0.15 I .o 1.o 0.040 0.030 - 11.5-1 3.5
420 0.15 min 1.o 1.o 0.040 0.030 - 12.0-14.0
431 0.20 1.o 1.o 0.040 0.030 1.25-2.50 15.0-17.0
The fabricability of martensitic stainless steels is poor because of a hard microstructure. Therefore,
a special class of these steels has been developed that can be termed as controlled transformation
steels. The carbon content is limited to a maximum value of 0.1 wt. % and chromium is in the range
of 16-19 wt. %. Substantial alloying additions (Ni, Co, Mn, Mo and Cu) are required to depress the
M, temperature to well below 0°C. The alloys remain austenitic at room temperature and are amenable
to various forming operations. The martensite is formed on freezing the alloy below M, temperature.
These alloys are heat treated to achieve precipitation strengthening. Typical examples of this class

4 GEORGE AND SHAIKH
of stainless steels are 17-7PH, PH 15-7Mo, PH 14-8Mo and their compositions are listed in
Table 2.
Table 2. Typical chemical composition of some precipitation-hardenedstainless steels [7]
Grade C Mn Si Cr Ni Mo A1 N
17-7PH 0.07 0.50 0.30 17.0 7.1 - 1.2 0.04
PH 15-7Mo 0.07 0.50 0.30 15.2 7.1 2.2 1.2 0.04
PH 14-8Mo 0.04 0.02 0.02 15.1 8.2 2.2 1.2 0.005
AM-350 0.10 0.75 0.35 16.5 4.25 2.75 - 0.10
AM-355 0.13 0.85 0.35 15.5 4.25 2.75 - 0.12
Precipitation Hardenable Stainless Steels
They are chromium-nickel grades that can be hardened by an aging treatment at a moderately elevated
temperature [6].
These grades may have austenitic, semi austenitic, or martensitic crystal structures.
Semi austenitic structures are transformed from a readily formable austenite to martensite by high
temperature austenite-conditioning treatment. Some grades use cold work to facilitate transformation.
The strengthening effect is achieved by adding such elements as copper and aluminium, which form
intermetallicprecipitates duringaging.In this solution-annealed condition,these gradeshave properties
similar to those of the austenite grades and therefore readily formable. The precipitation-hardened
grades must not be subjected to further exposure to elevated temperature by welding or during service,
because overaging of the precipitates can result in loss of strengthening.
Ferritic Stainless Steels
The ferritic stainless steels are Fe-Cr alloys with 15-30 wt. o/o Cr, low C, no Ni and often Mo, GI,Nb
or ‘Ti. The formability and weldability of these steels are poor but they possess moderate to good
corrosion resistance. However, these alloys are prone to high temperature embrittlement. Modern
meltingand refiningtechniqueslikeVacuum-Oxygen-DecarburisationandArgon-Oxygen-Decarburisation
have achievedconsiderablereduction in C and N contentsin these alloys.The steels with low interstitial
content have improved formability,weldability and toughness. Some typical steels in this category are
AISI Types 405,409,410S,
430,434and 446 whose compositions are shown in Table 3.
Table 3. Chemical composition of important ferritic stainless steel grades [7]
Grade C Si Mn P S Ni Cr Olhers
AISI max max max I I l U max
405 0.08 I.o I.O 0.040 0.030 - 11.5-14.5 A1 0.1O-O.30
409 0.08 1.o 1.o 0.045 0.045 - 10.5-11.75 T
i 6xCmin but
410s 0.08 I.o 1.O 0.040 0.030 0.60max 11.5-13.5 -
430 0.12 1.O 1.o 0.040 0.030 - 16.0-18.0 -
434 0.12 1.O 1.o 0.040 0.030 - 16.0-18.0 MO0.75-1.25
446 0.20 I .o 1.5 0.040 0.030 - 23.0-27.0 N 0.25max
0.75rnax
Austenitic Stainless Steels
These stainless steels contain 18-25 wt. % Cr and 8-20 wt. %
I Ni and low C. These steels may also

have additions of Mo, Nb or Ti and are predominantlyausteniticat all temperatures,althoughdepending
on composition and thermomechanical history some delta-ferrite may be present. The austenitic alloys
constitute the largest group of stainless steels in use, making up 65 to 70 % of the total. They occupy
their dominant position not only because of their excellent corrosion resistance, but also because of an
extensive inventory of ancillary properties, which include strength at elevated temperatures, stability
at cryogenic temperatures and ease of fabricability including weldability. Some representative Fe-Cr-
Ni stainless steels, arranged in order of Ni and Cr concentrations, are shown in (Fig. 3). Table 4lists
the chemical compositions of important austenitic stainless steels and the specifications used for
austenitic stainless steel by different countries.
AISI Type No.
310
309 ezz?zzz
305 emzzl
316 c?zzzzm
304 -
302 m
301
0
U
6 8 10 12 14 16 18 20 22 24 26
Weight % N i ( m ) Or Cr (
0
)
Fig. 3. Some representativeFe-Cr-Ni stainless steels arranged in order of Ni and Cr contents [71.
High Nitrogen Stainless Steels High-nitrogen stainless steels are becoming an increasingly important
new class of engineering materialswith theirbetter
propertycombinationssuch as strength,toughness,
creep resistance, non-ferromagnetic behaviour,
corrosion resistance and stress corrosion cracking
resistance [8]. These steels are considered ‘high
nitrogen’ if they contain more than 0.08 wt % N
with a ferritic matrix or 0.4 wt % N with an
austenitic matrix [8]. The solubility of nitrogen in
a Fe-Cr-Ni alloy is much lower than Fe-Cr-Mn
alloys with comparable chromium content as
illustrated in Fig. 4[9].
The high nitrogen austenitic
stainless steel having the composition 18 % Cr,
18 % Mn, 0.5-0.6 9
iN with very low carbon has
the highest product of strength and toughness
[Klc . 00.2 = 3 x lo5 MN2. m-7’2] (Fig. 5 ) [8].
Yield strengths of 2400 MPa can be achieved
Fe-2OCr-15Mn
3 -
Fe-20Cr-15Ni
0 1 2 3 4
N2 Pressure (MPa)’.’
Fig. 4. Nitrogen solubility in liquid Fe-based alloys
at 1873K asa functionofnitrogen-gaspressure
VJI.

6 .GEORGE
AND SHAIKH
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Introduction to Austenitic Stainless Steels I

8 GEORGE
AND SHAIKH
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10 GEORGE
AND SHAIKH
through cold work due to the high work hardening
coefficient in these steels with added benefit of
absence of deformation induced martensite
(Fig. 6) [lo]. Some of the Ni-free high nitrogen
stainless steels are given in Table 5 [ll].
Duplex Stainless Steels
These steels contain austenite and ferrite in equal
proportions and are characterised by superior
toughness, as compared to fully ferritic stainless
steels, and excellent corrosion resistance. The
balance between ferrite and austenite is achieved
by adjusting the amounts of Cr (18-26-wt. %),
Ni (5-6-wt. %), Mo (1.5-4 wt. %) and nitrogen.
Table6 showstypical compositionsof someduplex
stainless steels.
This chapter deals with physical metallurgical
aspectsand the mechanical and physical properties
of austenitic stainless steels. The corrosion
properties of austeniticstainlesssteel arediscussed
in the remaining chapters in the book.
2.0 AUSTENITIC STAINLESS
STEELS
600
500
-
c"
ci
'E
3 400
u
x
5 300
u
2
g 200
Y
3
0
Ct:
100
0
I I I
I Strength ' toughness
./ Steel x5 CrMnN18 18
1000 2000 3000 4000
Yield strength, 00.2, [MN/rn2]
Fig. 5. Chronological development of commercial
.steels with progressively better values of the
product of strength and toughness 181.
The wrought austenitic steels are either single ( y )
or duplex(y+ a or 6) phase structures, where yrefers to the face-centred-cubic (fcc) austenite and a
or 6refers to the body-centred-cubic(bcc)ferriterespectively,at the usual solutiontreatmenttemperatures
of above 1000"C. This is evident from (Fig. 7 )which shows part of the isothermal section of the iron-
chromium-nickel equilibrium diagram at 1100 "C [12]. Type 310, 20Cr- 25Ni -Nb and 12R72HV
steels fall within the single-phase (y) field whilst the other steels listed in the figure lie just within the
two-phase ( y + a)field. The effect of various austenitising and ferritising elements on austenite and
ferrite can be expressed in terms of Ni equivalent and Cr equivalent respectively [131.The following
nickel and chromium equivalents can be used to locate typical compositions of the commercial steels
in the isothermal section of the Fe-Cr-Ni diagram [14].
Ni equivalent (wt. %) = % Ni + % Co + 0.5% Mn + 30% C + 0.3% Cu + 25% N (1)
Cr equivalent (wt. 5%) = % Cr + 2% Si + 1.5% Mo + 5% V + 5.5% A1 + 1.75% Nb
+ 1.5% Ti + 0.75% W (2)
Apart from removing the y-stabilizing elements, nitrogen and carbon, from solid solution, alloying
elements like titanium and niobium act as a-and 6-stabilizingelementsin their own right. Consequently,
their a- and o-forming effects can be significantly enhanced 115, 161.

Introduction to Austenitic Stainless Steels 1I
500
Yield strength of
stable austenitic steels
-
2000 -
h
k
z
$ 1500 - 1I500
80C
24C
0
1912 1920 1940 I960 1980 2000
Year or introduction
Fig. 6. Effect of cold work (without formation of deformation induced martensite)
on yield strength in stable austenitic stainless steels [8].
Table 5. Chemical compositions of some Ni-free high nitrogen austenitic stainless steels [I11
SrcT?l Composition (wt %)
Cr M O Mn N c Si
NML-82 20.90 - 13.80 0.69 0.12 0.70
NML-B4 22.07 - 16.63 1.01 0.04 0.40
NML-BS 17.90 - 16.93 0.69 0.05 0.28
VSG Steel 18.00 2.00 18.00 0.90 0.10 -
Swiss A 17.10 3.20 11.40 0.92 0.008 1.20
Swiss D 16.40 4.20 11.80 0.98 0.009 1.60
Swiss G 1I .80 7.80 11.10 0.80 0.016 1.40
Ring Steel
Retaining 18.00 - 18.00 0.5-0.6 Very low -
Constitution of Austenitic Steels
The constitution of the austenitic steels at ambient temperature, following rapid cooling from the
solution treatment temperature, can be predicted using the Schaefler diagram (Fig. 8) which shows the
phase fields in terms of the nickel and chromium equivalents [13]. The high temperature phases in
majority of the austenitic steels can be made stable at ambient temperature either by adjusting the
chemical composition or by rapid cooling. However, partial transformation of the y+ a
' martensite

12 GEORGE
AND SHAM
1
2
3
4
Table 6. Qpical chemical composition of some duplex stainless steels [7]
UNS No. Cr Ni Mo N Others
AlSl 301 5 AlSl 316 9 M316
AlSI 302 6 AISl 321 10 FV548
AlSl 304 7 AlSl 347 1 1 12R72HV
AlSl 310 8 20Cr-25Ni-Nb
S32304 23
S3I803 22
S32760 25
s32550 25
S32750 25
4
5
7
6.5
7
0.2
2.7
3.6
3.8
3.8
0.1 -
0.I4 -
0.24 0.7Cu. 0.7W
0.26 1x u
0.27 -
10 20 30 40 50 60
Ni. wt-%
0 6 12 18 24 30 36
Chromium equivalent (Cr + Mo + 1.5 Si + 0.5 Nb)
Fig. 8. Schaefflerdiagram 1131.

phase occurs in AISI type 301 stainless steel during cooling due to the relatively high M, (d)
temperature of this steel [17].
The addition of nickel to 18% Cr steels enlarges the gamma loop considerably [12, 19, 20, 211.
Increasing nickel has two main effects on the constitution and microstructure, which are as follows:
(i) It increases the amount of austenite present at the solution-treatment temperature. However, at
low nickel contents this austenite may transform wholly or partially to martensite on rapid
cooling to room temperature [4].
(ii) It decreases the M, temperature such that, with about 8% Ni, the M, temperature is just below
room temperature and stable austenite is retained after cooling from the solution-treatment
temperature to room temperature [21, 22, 231.
An 18Cr-8Ni carbon-free steel is a borderline with respect to a fully austenitic structure and may
contain a little delta ferrite [24]. About 12% Ni is required to produce a fully austenitic structure at
solution-treatment temperatures of about 1050"C.
An 18Cr-8 Ni-O.1C alloy is fully austenitic above
about 900 "C because carbon is a powerful
austenite-forming element. However, the M,
temperature is onlyjust below room temperature,
leading to partial transformation of austenite to
martensite either during a refrigeration treatment
or duringcold working [4].The interactionbetween
chromium and nickel in promoting the formation
of stable austenite in 0.1 % C steels, after cooling
from 1050 to 1100 "C, is therefore of utmost
importance (Fig. 9) [24]. Some of the main effects
are: 5
(i) At low chromium contents, chromium acts
as an austenite stabilizer by expanding the
gamma phase up to the minimum in the 5 10 15 20 25
gamma loop. Chromium, %
(ii) At 18% Cr, a minh"Jm nickel content is Fig. 9. Effect of nickel and chromium equilibrium
required to promote a fully austenitic struc- on constitution of 0.1 % C.
ture, which is stable at room temperature.
(iii) With more than 18%Cr, the ferrite-forming tendency of chromium predominates and increasing
nickel is required to eliminate delta ferrite, although the austenite becomes increasingly stable
with respect to martensite formation.
The ferrite-stabilizing character of molybdenum is illustrated in Fig. 10 which shows the room
temperature structure of an 18%Cr-8% Ni-2% Mo stainless steel to be dual phased, consisting of both
austenite (A) and ferrite (F). In order to maintain a fully austenite structure the nickel content of an
18% Cr- 2% Mo steel must be greater than 10%. Molybdenum also promotes the formation of
intermetallic phases, particularly sigma, which causes room temperature embrittlement [25]. It extends
the range of stability of this phase and shifts the A/A+F boundary to lower chromium contents. Like
carbon, nitrogen is also a strong y stabilizer, whose influence is shown in Fig. 11.
25
20
t
i
? ' 5
~
3
2 10
Q)
0

14 GEORGE
AND SHAIKH
%J Ni
Fig. 10.
22 -
,
8
-
% Cr
16 18 20 22 24
Effect of molybdenumon the structureof
Fe-Cr-Ni aloys air cooled from 1100 to
1150 O C [25].
18
16
14
12
10
* 8
6
4
2
0
18 19 20 21 22 23 24 25
% Cr
Influence of nitrogen on the austenite (A)
and ferrite (F) phase boundaries,and on the
structure oP Fe-Cr-Ni stainless steels [25].
Fig. 11.
The effect of other alloying elements is also important in that, depending upon whether they are
austenite or ferrite-forming elements, they will decrease or increase the tendency for delta-ferrite
formation at the solution-treatment temperature. Many workers [141 have considered the effect of
alloying elements on the phase stabilities of austenitic stainless steels by using chromium and nickel
equivalent compositions and superimposing them on the Schaeffler diagram [131.
Transformationof Austenite to Martensite
Austenite in the lower range of highly alloyed stainless steels may be transformed to martensite. This
can occur either in the solution-treated condition when the M, temperature is above room temperature
or it may occur during refrigeration in more stable alloys in which the M, temperature is below room
temperature. Martensite may also be formed by deformation, above room temperature in the case of
unstable steels and below room temperature in the case of stable steels, depending on Mdtemperature.
Apart form cobalt, almost all alloying elements depress the M, temperature [21, 22, 231. Recently,
linear equations relating the M, temperature to the composition have been developed for austenitic
stainless steels [26]. This type of relationship is important, particularly if used to establish the Md
temperature, in assessing the cold formability of austenitic stainless steels [27]. Equations have also
been established for austenitic steels relating the Md30temperature, at which 50% of martensite is
produced under the action of a true strain of 0.30, to the composition of the steel [28]. Md is always
higher than Ms,
M, ("C) = 1302 - 42(%Cr) - 61(%Ni) - 33(%Mn) - 28(%Si) - 1667(%C+ %N) (3)
Md(OC) = 413 - 462 (%C + %N ) - 0.2 (%Si) - 8.1 (%Mn) - 13.7 (%Cr)
- 0.5 (%Ni) - 18.5 (%Mo) (4)

Carbide and Nitride Precipitation
Unstabilizedgrades Austenitic stainless steels can contain up to 0.15% of carbon. The solubility limit
for carbon in a type 18-8 alloy is indicated in Fig. 12by the line separating the single-phase yregion
from the y+carbide field. The carbides correspond to the type M2& where M is principally Cr, but
it can be partially replaced by Fe, Mo and Ni, whence the general designation (Cr, Fe, Mo, Ni)23C6.
h
Y
4 4
5
fi
G
Fig. 12.
Austenite + MZ3C6
. .
-.
.
-.
0 0.2 0.4 0.6 0.8
% C
Solubilityof carbon with respect to MZ3C6
carbides (M = Cr, Fe, Mo, Ni)
in an 18% Cr-8%Ni stainless steel.
After austenitizing at around 1100OC, the carbon is retained in solution only by rapid cooling. If
the annealed alloy is held in the temperature range between 450 and 850"C, either during service or
h
Y
0.08%C
900-
700-
600
-
Austenite
Austenite +
.02%c
M23C6
500' I I I I
0.1 1.o 10 too
Time (h)
Fig. 13. Influence of C content on the kinetics of M23C6precipitationa type
18-10austenitic stainless steel [25].

16 GEORGE
AND SHAIKH
cooled slowly after a welding operation, the excess carbon precipitates at grain boundaries in the form
of chromium-rich M23CGcarbides (Fig.13). The M23CGcarbides precipitate initially at grain and
incoherent twin boundaries, before forming with the austenite grains.
Molybdenum decreases the solubility of carbon in austenite and accelerates the M& precipitation
[5].An increase in nickel content has a similar effect [30], while nitrogen retards the precipitation and
coalescence of M23C6 (Fig.14) [29].
1000
900
800
h
p!
1
Y
E
700
'& 600
8
500
0
*
._
Y
Y
.-
I I I I
0.01 0.1 1 10 100 1000
Time (h)
Fig. 14. Influence of nitrogen on the kinetics of MZJC6
precipitation in a
17 Cr-13 Ni-5 Mo-0.05 C steel [29].
Stabilized grades The addition of titanium or niobium retards the precipitation of chromium-rich
M & jcarbides, thus increasing the resistance to intergranularcorrosion [5].The austenite is depleted
in carbon due to the selectiveformationof Ti(C,N) and Nb(C,N) carbonitridesand Ti4C2S2
carbosulfides
The following relations give the solubilityof titanium and niobium carbides in 18Cr-12Nisteel [5]:
PI.
log [Ti] [C] = 2.97 - 6780/T
log [Nb][C] = 4.55 - 9350/T
Corresponding to the general equation:
log [M][X] = A - H/RT
where A is a constant, H is the heat of dissolution, R is the perfect gas constant and T is the absolute
temperature [25].
The M(C, N) particles precipitate essentially within the grains. However, intergranular precipitation
occurs under certain conditions, particularly at high austenitising temperatures. This phenomenon is
observed in weld zones of 18 Cr-10 Ni-titanium stabilised alloys.
lntermetallic phases Alloys containing transition elements A, such as Fe, Ni, Mn, Co, etc., together
with transition elements B, of the type Cr, Ti, V, etc., can form intermetallic phases with formula
ranging from A4B to AB4,On high temperature exposure, austenitic stainless steels are known to result
in precipitation of a host of secondary phases [31]. Someof these phases commonly occur and are well
understood with respect to their impact on mechanical and corrosion properties. This section discusses

some of these phases such as sigma, chi, carbides and R-phase, Lave’s, G-phase, mu phase, Z-phase
etc., also precipitate in austenitic stainless steels [32, 331.
Sigma phase Sigma phase has a body centered tetragonal structure. The values of the Cr and Ni
equivalents can be used to evaluate the possibility of sigma phase formation in a Fe-Cr-Nialloy at high
temperature (Fig. 15).The propensity of sigmaphase precipitation in austenitic stainlesssteels depends
on the chemical composition of the residual austenite after precipitation of carbides and nitrides,
which always form first [31].
Cr
- -
10 20 30 40 SO 60 70 80 90 N
% Ni
Fig. 15. A section of the Fe-Cr-Ni ternary equilibrium diagram at 65OoC[25].
The tendency to sigma phase formation of an austenitic stainless steel can be known from the
formula proposed by Hull [34]. The formula for Equivalent Chromium Content (ECC) is,
ECC = % Cr +0.31 % Mn + 1.76 % Mo + 0.97 % W + 2.02 % V + 1.58 % Si + 2.44 % Ti
+ 1.7 % Nb + 1.22 % Ta - 0.266 % Ni - 0.177 % Co.
If the equivalent Cr content (ECC) is greater than 17-18 wt %, the steel is susceptible to sigma
formation. This equation was modifiedby Gill et al. to account for the strong influenceof carbon [35].
As per these authors, normalised equivalent chromium content, NECC =ECC/% C. This suggeststhat
sigma phase precipitation becomes easier as the carbon content of the matrix reduces [36]. Fig. 16
showsthe influenceof various alloyingelementson the kineticsof sigmaphase precipitation.Chromium,
molybdenum, titanium and niobium all promote sigma formation, while the precipitation rate is also
accelerated by the addition of 2 to 3% of silicon [37]. Incorporation of nitrogen in the weld deposit
avoids/delaysnucleation of sigma and chi phases [38].The presence of delta-ferrite and low interstitial
content affect the growth kinetics of sigma and other intermetallic phases but not the total content of
these phases [39].

18 GEORGE
AND SHAIKH
2 -
1 -
0
% sigma phase
17Cr-12Ni-2.3Mo
19Cr-9N
18Cr-lONi-0.87Nb
18Cr-lONi-0.35Ti
P
/ , H 18Cr-lONi-O.13Nb-0.06Ti
Cold work decreases the incubation period for sigma phase formation [25]. On the contrary, an
increase in grain size, due to annealing at a very high temperature, retards sigma phase precipitation
[37]. The presence of delta ferrite, particularly in welds, can reduce incubation period for sigma
formation in an austenitic stainless steel [5].Stress accelerates sigma phase precipitation and extends
its range to lower temperature [40].
The precipitation of sigma phase is controlled both by the rate of diffusion of chromium and other
sigma-forming elements and by the mode of nucleation [40]. The chemical composition of sigma
phase, determined for different types of austenitic steels (17 Cr-12 Ni-2.5 Mo-Ti, 25 Cr-20 Ni-0.03 &
0.13 C- 0.6 & 2 Si) exposed for times between 10and 5000 hours at temperatures from 650 to 900OC,
was found to vary with time and temperature [41]. The compositions of delta-ferrite and sigma phase
are close to each other [42]. Hence, delta-ferrite, in an austenitic stainless steel weld metal, easily
transforms to sigma phase by a crystallographic re-orientation [42]. Heat input during welding has a
significant say in the precipitation kinetics of sigma and other intermetallic phases. Higher heat input
to the weld metal retards the decomposition kinetics of delta-ferrite and thus the precipitation kinetics
of sigma phase [43].Kokawa et al. reported faster precipitation kinetics of sigma phase in vermicular
ferrite than in lacy ferrite [44]. Sigma phase is known to affect the tensile and creep ductilities of the
stainless steel [45, 461.
Chiphase The importance of chi phase in austenitic stainless steel has been lucidly brought out by
Weiss et al. [47]. Chi phase has a body centred cubic structure and is a stable intermetallic compound
containing Fe, Cr and Mo [47]. Chi is a carbon-dissolving compound of the type MlsC [48]. The
composition of chi can vary appreciably with a high tolerance for metal atom interchange [48]. Upon
addition of carbon, the metal atom proportion within the chi phase is shifted towards Mo at the cost
of Fe and Cr i.e. towards the strongest carbide former [49]. The precipitation diagrams for Mo-
containing and Ti and Nb stabilised stainless steels are shown in Figs. 17 and 18, respectively [25].
Weigand and Doruk reported that chi and lave's phases form simultaneously with carbides [50].
Presence of delta-ferrite in the steel favoured the precipitation of sigma and chi phases [50]. Solomon

and Devine showed that chi phase precipitates at lower temperatures of aging as compared to sigma
phase [51].
1100 I I I I I I I I
1000
- 900
E
E *0°
700
600
6
500
400
10-2 10-1 100 10' 1o2 103 104 10)
Time (h)
Fig 17. TTT diagram for precipitation in an 18 Cr-12 Ni-2 Mo austenitic stainless steel [5].
Weiss and Stickler showed that at liquid nitrogen temperatures, presence of M & , led to a sharp
decrease in impact strength while the presence of chi phase did not lead to a further significant
drop [47]. Shankar et al. showed that copious precipitation of chi phase beyond 100hours of aging,
at 850 "C,in nitrogen containing AISI type 316L stainless steel led to a sharp decrease in the tensile
ductility [52]. However, good resistance to brittle microcracking in presence of chi phase and its
interfaces has been observed during creep crack growth [48, 531.
R-phase It is a Fe-Cr-Mo intermetallic phase with a hexagonal structure having unusually large lattice
spacings [54]. Dyson and Keown considered the atomic movements in ferrite to accommodate the R-
phase structure [55].They showed that only small atomic movements and lattice strains were required
for R-phase to form from ferrite. Tavassoli et al. reported its presence on aging Mo-bearing alloys
[54]. R-phase is reported to precipitate inside the delta-ferrite of the weld metal, with lath morphology
[53,54]. Formation of R-phase has been reported during stress relief of austenitic weldmentscontaining
higher amounts of ferritisers in the weld metal [53].
Carbides MZ3C6
carbides are face centered cubic structured precipitate [48]. Carbide precipitation
usually precedes the formation of intermetallic phases [47, 561. As the formation of intermetallic
phases increases, the carbides redissolve due to thermodynamic considerations, to replenish the matrix
in Cr, Mo, C & N. During this period, the precipitation rate of intermetallic phases decreases after
which an increase is again observed as shown in Fig. 19 [56, 571. The precipitation kinetics of MZ3C6
carbide phase in an AISI type 316Lweld metal [57] is shown in Fig. 20. Precipitation of these carbides
at grain boundaries is known to impair impact property more than any other mechanical property [47].
Their precipitation is extremely deleterious to the localised corrosion behaviour of austenitic stainless
steels, as discussed extensively in this book.
SolidsolutionhardeningThe interstitialalloyingelementsN, C and B produceconsiderablestrengthening

20 GEORGE
AND SHAIKH
450
950 I
- -
TiC+TiN+Ti4C2S2
t
2 750
650
Ei
550
---
0.02 0.2 2.0 20 200 200h
Time (h)
(a)
800
h
750
E
700
B
650
NbC
I I I
1o2 I
d 104 1o5
Time (h)
(b)
Fig. 18. TTT diagram for precipitation in (a) an 18 Cr-10Ni-Ti austeniticstainless
steel and (b) an 18 Cr-10 Ni-0.9 Nb austeniticstainlesssteel
(Fig. 21) [25]. Increase in yield strength caused by substitutional solid solution elements, particularly
by austenite stabilizers, is moderate. Hardening is due to the inhibition of dislocation movement by
the lattice distortion associated with soluteatom [5].The most effective method of increasing the yield
strength of austenitic stainless steels is by introduction of nitrogen, an addition of 0.1% leading to a
gain of about 50 MPa [51.
Hardening by grain refinement For austenitic stainless steel, strengthening can be obtained by grain
refinement, according to the Hall-Petch relation:
0 = 0, + Kd-It2
where d is the mean grain diameter, a the yield stress, and a
, and K are temperature dependent
constants for the material considered. The hardening due to grain refinement is due to the difficulty

-40
14
12
10
i 8
.-
rA
s$
r 6
4
2
0
-
I I I I I I I I
I 1 I I I 1
0.5 2 20 200 SO00
Ageing time (h)
Pig. 19. Growth kineticsof sigma at 873 K
and 973 K [57].
0.8
0.7
0.6
< 0.5
0.4
0.3
0.2
0.1
0
2
z
773 K
0 973 K
A a73 K
T
0.5 2 20 200 SO00
Ageing time (h)
Fig. 20. Amount of M& formed during
aging [57].

22 GEORGE
AND SHAIKH
ductility is lowered. The greater the amount of plastic strain, the higher is the stress required to deform
the material further. This phenomenon is known as strain (or work) hardening. The cause for strain
hardening is the increaseddifficultyof dislocationmovement,as theirdensity increaseswith deformation,
due to their interaction with each other or with vacancies and other crystal defects.
Certain elements increase the already high work-hardening rate of the austenitic steels. Low nickel
grades are less stable and will tend to gradually transform to martensite during cold working, leading
to pronounced hardening (Fig. 22) [30].
I350
I200
10.50
900
$ 750
600
450
300
I 50
0
h
Y
!
A
G5
I I I I I
/ 17Cr-7Ni 1
1
I I I I I
8 16 24 32 40 48 xlO-’
True strain (In ( [ I f o )
Fig. 22. Effect of nickel content on the true stress-strain curves for 17% Cr austenitic stainless steel [30].
Low carbon austenites(0.02%) work harden faster than those with larger amounts of this element
(> 0.06%).Copper reduces strain hardening, whereas nitrogen and silicon increase it [5].In unstable
steels, apart from alloy chemistry, which determines the Md temperature, the quantity of martensite
formed depends on the amount of strain and the deformation temperature. An increase in strain rate
also leads to more rapid hardening (Fig. 23) [25].
Precipitation hardening Intragranular precipitation of particles based on elements such as C, N, B, V,
Nb, or Ti is an important strengthening mechanism in austenitic stainless steels. Fine precipitates
uniformly distributed in the matrix act as efficient obstacles to dislocation movement [29]. After
solution treatment, the precipitation of Tic and TiN occurs within the grains, and can be used to
increase the creep strength. Austenitic grades with large boron content on cold working can develop
a uniform dispersion of fine and stable M23(C,B)6 particle [25].
Welding Metallurgy
Austenitic stainless steels can generally be readily welded, since no hard structures are formed in the
heat-affected zone. However, a number of detrimental effects can occur. They are :
(i) A fully austenitic weld metal can produce hot cracking because of the stresses set up during

800I
2OOt ;,i;x'o-; I I I
0
15 30 45 60 75
Elongation (%)
Stress-strain curves for an 18 Cr-12 Ni-2 Mo austenitic stainless
steel deformed at different strain rates 1251
Fig. 23.
contraction that accompanies solidification of the weld. This can be overcome by ensuring that
the weld metal contains a little delta ferrite [4].
(ii) Various forms of liquation cracking can occur in both the weld metal and heat-affected zone
close to the weld metal, if low melting point phases, e.g. borides, are present. The problems in
fully austenitic welds can be minimized by reducing the joint restraint and heat input during
welding, by decreasing the concentrations of the detrimental elements and trace impurities in
the steels, and by using consumable electrodes with balanced compositions such that 5-10%
&ferrite is produced in the weld deposit [58].
(iii) In stabilized steels, especially those containing niobium, the high temperatures in the heat-
affected zone of a weld can dissolve some NbC. Subsequent strain induced precipitation of the
NbC can occur during a post-weld stress-relieving treatment, and this can lead to a form of
low-ductility creep-rupture cracking [59, 601. This can be overcome by stress relieving at
higher temperatures at which the NbC overages. Alternately, a full solution treatment may be
used.
(iv) During welding, parts of the heat-affected zone are heated in the range in .which Cr2&
precipitates at the austenite grain boundaries. This locally lowers the chromium content, so that
preferential corrosive attack occurs in the chromium-depleted zone. This is known as weld
decay. Remedial measures could be full solution treatment at 105OOC to dissolve any grain-
boundary carbides precipitated in the heat-affected zone or annealing at about 9OOOC to allow
chromium to diffuse from bulk into the impoverished zone ('healing' treatment) [
4
]
.
Austenitic
stainlesssteelsstabilizedwith Ti or Nb aresusceptibleto knife lineattack [
4
]
.
Becausestabilization
is entirely effective, there has been a trend to produce austenitic stainless steels with very low
carbon contents of 0.03% maximum [61]. In the absence of efficient stabilization, low heat
input during welding may be used to minimize the time for which the heat-affected zone is in
the sensitization temperature range, and to decrease the width of the sensitized region [4].

24 GEORGE
AND SHAIKH
The possibilityof formationof delta-ferriteduringcoolingof weld metal and the primary solidification
mode can be known from the 70%iron isopleth (Fig. 2). The amount of delta-ferrite that is retained
in the weld metal can be known from the constitution diagrams, if the chemical composition is known.
Schaefler diagram is the most popular of these constitution diagrams. Fig. 24 locates important
stainless steels on this diagram with respect to its propensity to form delta-ferrite [62]. However,
Schaefler diagram does not account for the influence of Nitrogen, a very potent austenitiser. Effect of
nitrogen has been accounted for in the diagram proposed by W.T.Delong (Fig. 25) [63]. The latest
constitution diagram that has been produced is by the Welding Research Council in 1992 (Fig. 26)
~ 4 1 .
4
40
36
38 i
Austenite
0 2 4 6 8 10 12 14 16 18 20 22 24 26 28 30 32 34 36 38 40 42
Cr Eq.= %Cr+ %Mo + 1.5 x%Si + 0.5x%Nb
Hg. 24. Schaefflerdiagram-Position of some common grade [5]
The amount of delta-ferrite to be retained in the weld metal is governed by the needs of service.
Generally, delta-ferrite embrittles a weld metal [65] and deteriorates corrosion property [66], details
of which arediscussedelsewherein the book.However,Shaikhet al. recently reported that embrittlement
of austenitic stainless steel weld metal was not due to presence of delta-ferrite but because of cold
work present in the weld metal [67].
Physical Properties
The physical properties of austenitic stainless steels that are considered are in regards to the functional
properties of stainless steels and related alloys [68]. The major physical properties considered are
melting range, density, coefficient of expansion, modulus of elasticity, electrical resistivity, thermal
conductivity, specific heat and magnetic permeability.
1. M
e
l
t
i
n
g range: Table 7 shows the solidus and the liquidus temperatures for various grades of
austenitic stainless steels.These values weredeterminedby differential thermal analysis (DTA).

18 20 22 24 26
1u-
16
Chromium equivalent (Cr + Mo + 1.5 Si + 0.5 Nb)
Fig. 25. W.T.DeLong diagrams for welds in stainless steels. If the nitrogen content of the metal has not been
determined by analysis, the followingvalues will be taken as a function of different welding processes:
covered electrode welding, under shielding gas nonconsumable electrode (G.T.A.W.),
plasma, under
solid flux: N = 0.06%; under shielding gas with consumable electrode (G.M.A.W.): N = 0.08%; with
self-shielded flux-cored wire: N = 0.12% 1631.
18 20 22 24 26 28 30
18
0
+
10
Fig. 26. WRC-1992 diagram [64].
The magnitude of the interval between the solidus and liquidus temperatures varies considerably
for the different grades. This interval is greater for the highly alloyed grades and elements like
niobium and molybdenum tend to increase it [5].

26 GEORGE
AND SHAIKH
Table 7. Melting range for a number of austeniticstainless steels [S]
AISI Solidus ("C) Liquidus ("C) Solidus-Liquidus ("C)
202 1398 1454 56
302 1400 I447 47
304 1405 1448 43
304L 1394 1440 46
305 1400 1435 35
310 I350 1395 45
314 1322 1388 66
316 1392 1444 52
316L 1405 1445 40
316 Ti 1378 1432 54
316 Nb 1370 1431 61
32I 1398 1448 50
347 1394 I446 52
2. Density: The density of a number of alloys at room temperature is given in Table 8.The density
varies little in the 18-10Cr-Ni and 13-17% Cr range of stainless steels. For these steels, it is
of the order of 7.7 to 7.9 g/cm3. [5].
3. Coefficient of expansion: Table 9 gives Density of the principal stainless steels
the mean coefficients of linear expansion (various sources) [S]
Table 8.
for a series of grades at high temperatures
and Table 10at low temperature [69].The
Designation A N Density (g. ~ r n - ~ )
coefficient of expansion of the 18-10 type 201 7.7
austenitic steels is significantly high, of 202 7.7
the order 17 x 104/"C, which increases 301 7.7
of products. The coefficient of thermal 304L 7.9
expansion increases with the rise in 305 7.9
temperature. For weld deposits of stainless 308 7.9
alloys, Elmer, Olson and Matlock have 310 7.9
302 7.9
303 7.9
304 7.9
the problems related to changes in
dimension during the heating and cooling
316 7.9
321 7.9
347 7.8
studied the influence of composition and
structure on the coefficient of expansion
(Fig. 27) 1701.
4. Elastic modulus: Table 11 gives the values
of both Young's modulus and the shear or torsional elastic modulus for a series of grades.
Nunes and Martin [71] have shown that strain hardening causes an increase in the elastic
modulus for unstable austenitegrades.On the otherhand, for stable steels,the modulusdecreases
to a strain hardening level of 80 to 85% and then increases if strain hardening is continued [5].
Rise in temperature will result in the decrease in elastic modulus of austenitic stainless steels
as shown in Table 12.
5. Electrical resistivity: Table 13 [69] gives the resistivity values of austenitic grades at room
temperature.Table 14 [69jshows its variationsat high and low temperature. Resistivity changes
little in the standard range of 13 to 18 %ICr grades. Resistivity increases with temperature for
austenitic stainless steels as shown in Table 14.

Mean coefficient of expansion in 104."C-' of some stainless steels-variations as a function of
temperature (various sources) [5]
Table 9.
Grade
AISI 20-200 "C 20-400 O C 20-600 "C 20-800 O C 20-1000 O C
Mean coejjicienr of expansion (I@. "
e
l
)
304 17
316 16.5
314 15
18
17.5
16
19
18.5
17
19.5
19.0
18
20.0
19.5
19
Table 10. Mean coeff'icient of expansion in lod * "C-' of some stainless steels at low temperatures [5]
Grade Temperature ("C)
AISI -1841021 "C -129 to 21 "C -73 lo 21 O
C -18 10 21 "C
301 13.7 14.1 14.8 15.7
304 13.3 13.9 14.8 15.7
316 12.8 13.3 14.1 14.8
347 13.5 14.6 15.3 1.5.7
310 12.6 13.5 14.1 14.4
8 10
0 8 16 24 32
Chromium equivalent
Fig. 27. Mean coefficient of expansion of alloys deposited by welding (inlO4/'C
between 0 and 400 "C) superimposed on a Schaeffler diagram [70].
6. Thermal conductivity and Specific heat: Table 13and 15 [69] give the specific heat values for
a range of alloys at room and high temperatures. The thermal conductivity and specific heat
increase with temperature [5].

28 GEORGE
AND SHAIKH
~ ~ ~ ~~
~ ~~
Table 11. Modulus of elasticity of some stainless steels [5]
Grade Young's modulus Shear modulus
AISI (KN/mm2) (KN/mm2)
302
304
310
316
193
193
193
196
79
79
73
78
Table 12. Changes in elastic modulus of some stainless steels as a function of temperature; static values [5]
~ ~~ ~~
~ ~~
Grade AlSI Temperature("C)
-196 20 100 200 400 600 800
302 200 193 191 183.5 168.5 153.5 139
304 208 193 191 183 168 148 128
310 - 193 192 184 173 155 134
316 - 193 192 185 168.5 151 132
321 - 193 192 182 166 151 132
347 208 193 184 168 152 152 I34
Young's modulus E (KN/mm2)
Grade AISI Temperature("C)
20 100 200 400 600 800
304 79
310 73
316 78
321 76
75 72 64 54 50
72 70 66 59 50
76 73 65 59 52
74 72 64 58 52
Shear modulus G (KN/mm2)
7. Magnetic permeability: Table 16 gives the permeability values for different austenitic steels.
They are non-magnetic when their structure is fully austenitic and have Curie points much
lower than room temperature.When austenite transforms to martensite by strain hardening, the
metal becomes ferro-magnetic, as seen in AISI 301. Similarly,AISI 308 type ingot can contain
up to 15% of ferrite content and will thus be ferro-magnetic. The transformation of ferrite to
0- phase by heating in the 873-1 173.Ktemperature range also causes the material's magnetism
to disappear. The 18-10 type austenitic stainless steels retain their non-magnetism at much
lower temperature. But the less stable austenitic grades like the 18-8 type may transform
partially to martensiteat low temperatureand thus become ferro-magnetic.Nickel-rich austenitic
stainless alloys remain austenitic at low temperature and have Curie points, the level of which
depends on their composition [5].

Table 13. Resistivity, thermal conductivityand specific heat of some stainless steels at
room temperature [5]
Grade AISl Electrical resistivity Thermal conductivity Specific heat
(DuBcm) (Wm-'r') (J.kg-'K-')
202
201I
301
304
305
3
310
14I
316TI
316Cb
347
69
72
90
74
72
14.6
14.6
14.6
14.6
14.6
500
500
500
SO0
SO0
Table 14. Variation in resistivity(pQ -cm) of some stainless steels as a function of temperature [S]
Grade AISl Temperature ("C)
-196 -78 20 200 400 600 800 I000
301 - - 12 83 94 105 114 -
302 - - 72 84 96 106 115 119
304 55 65 72 85 98 1 1 1 I20 -
31
0 - - 90 100 1
1
0 120 I25 130
316 60 68 74 85 98 108 - -
321 - - 72 90 103 115 123 -
347 52 60 12 88 97 110 1 I9 -
Table 15. Influence of temperature on the specific heat of some stainless steels (J kg-'K-') [5]
Grade AISI Temperature ( "C)
-I 96 -78 20 200 400 600 800 1000
~ ~~
301 285 394 456 527 571 595 628 695
316 284 393 452 523 561 582 628 722
347 285 393 452 520 561 595 636 741
314 - - 502 544 586 627 710 795
Mechanical Properties
- - - - -
304 136 . 408 444
Tensile strength and Toughness characteristics Pickering [4] and Irvine et al. [72] derived empirical
relationshipsbetween compositional and microstructuralparameters,and tensile propertiesof austenitic
stainless steels, as shown by the following equations:

30 GEORGE
AND SHAIKH
Table 16. Magnetic permeability of various austeniticstainless steels [5]
Grades Permeability
(H1OOA000 Oe)
Z8CN 18.12
Annealed
90% cold work
Z 12CN 17.07
Annealed
Heavily cold worked
Z 6CND 17.11
Z 6NCTD V 25.15
NC 15 Fe
Annealed or cold worked
KC20N 16 FeD
1.001to 1.005.
1.1
I to 1.1
10 to 20
1.1
1.005
1.005
4 .0 5
0.2% Proof stress (MN m-2) = 15.4 (4.4 + 23(C) + 1.3(Si) + 0.24(Cr) + 0.94(Mo) + 1.2(V)
+ 0.29(W) + 2.6(Nb) + 1.7(Ti) + 0.82(Al) + 32(N)
+0.16(S-ferrite) + 0.46~f-”~] (5)
Tensile strength (MNm-2) = 15.4 (29 + 35(C) + 55(N) + 2.4(Si) + O.ll(Ni) + 1.2(Mo)
+ 5.O(Nb) + 3.0 (Ti) + 1.2(A1)+ 0.14(S-ferrite) + 0.82 t-’/’](6)
where &ferrite is the percentage of S-ferrite, d the mean linear intercept of the grain diameter (mm),
t the twin spacing (mm) and the brackets indicate the alloying addition in weight percent.
The relationships in equation (5) and (6) indicate that high proof and tensile strengths are correlated
with the high carbon specification of stainless steel. Type 316 steel and stabilised steels (types 321 &
347) containing high titanium and niobium have high tensile strengths. Chromium has a positive
effect on property [5].Molybdenum and silicon increase the strength either by solid solution hardening
or by their effect on the stacking fault energy [5]. The twin spacing does not affect the proof stress
because the stacking fault energy, which controls the work-hardening rate, has little or no effect at the
low strains at which the proof stress is measured [4]. The twin spacing is much more important than
the grain size in controlling the tensile strength because the effect of stacking fault energy on the
work-hardening rate, and hence on the tensile strength, is quite significant [5]. However, in high
stacking fault energy austenites, in which there are relatively few twins, the tensile strength will
depend on the grain size, following a Hall-Petch type of relationship. In this case, increasing the grain
size decreases the proof stress value. S-ferrite increases the proof stress and tensile strength values by
a dispersion-strengthening effect [5].Table 17 gives the mechanical properties of the main austenitic
stainless steels at room temperature in the annealed state.
The austenitic stainless steels retain a high ductility and good impact strength at low temperature,
which makes them particularly useful for cryogenic applications [77]. The tensile strength greatly
increases at low temperatures. At high temperatures, the yield strength and the U.T.S.
decrease for
austenitic steels as shown in Table 18.
Fatigue The fatigue limit for a material is the maximum alternating stress that may be applied

~~ ~
Table 17. Mechanical properties at room temperature of austenitic stainless steels [5]
Crude Mechanical properties Solution
AISI treatnienc
0.2% Y. s.
( MPa)Min.
UTS (MPa) El 70 (water quench)
("C)
~~~ ~
302 215 490-690 45 1050
304 195 490-690 45 1050
304L 185 470-670 45 I050
321 205 500-700, 40 1075
347 205 500-700 40 1075
316 205 500-700 45 I075
316L 195 480-680 45 1075
316 Ti 215 510-710 40 1075
316 Cb 215 510-710 40 1075
304N 250 550 45 '1025
316N 280 600 45 1050
309 240 540 30 1120
314 240 540 30 1120
Table 18. Changes in tensile properties of four austenitic stainless steels as a function of temperature [5]
Crude Properties Temperature "C
AISI 20 100 200 300 400 SO0 600 700
0.28Y.S.(MPa) 247 243 I69 148 136 133 125 I09
304 UTS (MPa) 599 496 456 449 443 416 367 268
El (%) 62.6 56.1 46.4 41.6 43.1 41.7 41.1 47.7
0.2%Y.S. (MPa) 254 200 172 161 157 144 141 125
316 UTS (MPa) 588 493 483 479 472 457 421 327
El (%) 60.1 52.1 46.0 41.9 41.9 41.7 42.6 49.6
0.2%Y.S. (MPa) 234 206 194 163 161 152 145 138
321 UTS (MPa) 588 506 452 435 436 39I 376 269
El (%) 53.9 47.5 42.0 42.0 36.4 34.8 36.0 48.4
0.28Y.S. (MPa) 250 213 195 179 168 157 155 144
347 UTS (MPa) 609 540 475 451 448 422 387 292
El (8) 49.2 46.8 40.7 36.8 35.3 34.1 35.3 49.4
indefinitely without causing fracture. Fig. 28 [73] schematically shows the principal parameters and
phenomena taken into consideration as regards to fatigue. The fatigue limit for austenitic steels is 0.4
times the tensile strength. Thus, this is of the order of the yield strength. In case of low cycle plastic
fatigue, austenitic stainless steels undergo co,nsiderablestrain hardening in cyclic imposed deformation.
The fatigue limit also increases with strain hardening, in proportion to the tensile strength. This
proportionality is maintained up to strengths of the order of 1100 MPa [74].
For nuclear engineering applications, complete absence of crack initiation is required for stainless
steels for which the calculation codes provide curves relating the amplitude of the alternating strain

32 GEORGE
AND SHAIKH
- - - Initiation
-Rupture
Number of cycles N
(a)
1o-2
2 10-3
:
:
lo4
10-5
2
-u
2 10-6
10-7
10-8
GyGx-$
* rupture
' p
I B
I m
I
:gion C
Threshold AK
(b)
Fig. 28. (a) "Stress-number of cycles" curves giving the number of cycles to initiation and fracture. (b) The
crack propagation rate dddN as a function of stress intensity factor AK. Region A: low crack rate
(threshold), Region B:intermediate range (Paris relation), Region C: high crack rate (K,).
to the number of admissible cycles as shown in Fig. 29 [5].Fatigue strength increases with decreasing
temperatures.
%
Fig. 2
9
. Fatigue limit curves for imposed strain testing for the AISI 304 and AISI 316austenitic steels
(transcribed from the Boiler and Pressure Vessel Code-ASME-CodeCase 1592).
Creep The rate-controlling creep deformation processes in austenitic steels, at a given temperature,
are dependent on the applied stress. The deformation at high stresses occurs primarily by a dislocation
climb or glide process whereas other mechanisms such as grain boundary sliding, solute drag and
vacancy diffusional processes determine the creep rate at lower stress [17]. The dependence of the

creep deformation rates on temperature and applied stress and the activation energies for the different
processes have been tabulated elsewhere [75].
The austenitic stainless steels, by virtue of their microstructure, are the ones that best resist high
temperature creep. The alloying elements have a strong influence on the creep resistance of austenitic
steels. Titanium strongly increases the creep resistance of austenitic stainless steels. The titanium
content giving the best creep resistance lies between 0.25 and 0.5% which corresponds to a Ti/C ratio
higher than the stoichiometryof the carbideTic.The titanium action depends on the creep temperature.
At low temperatures, as per the mechanisms suggestedby Williamsand Harris [25,76]the deformation
occurs within the grains and the small carbide precipitates nucleated on the dislocation will prevent
further deformation or else the titanium atom in the solution will restrict the motion of the dislocations
to which they are strongly bound [5]. At high temperature, the intergranular carbides ensure the
resistance of grain boundaries. Swindeman and Binkman [77] have shown that increasing the niobium
content from 20 to 100ppm in a AISI type 304 alloy distinctly increases creep life while decreasing
ductility (Fig. 30) [77].
10
8
.-
8 8
a 4
c)
& 6
8
2
0
0 4 8 12 16 20 24 28 30 (x103)
Time (hours)
Fig. 30. Comparisonof creepbehaviorat 866K andunder 117MPa stressasa function
of Nb content of a AISI type 304 steel. a = 20 ppm, b = 30 ppm, c = 50 ppm,
d = 80 ppm, e = 100 ppm. After Swindeman and Brinkman [77].
Vanadium, like niobium and titanium, increase creep life but to the prejudice of the ductility [5].
Molybdenum ifnproves the creep properties of stainless steels, as it is a substitutional element as well
as a carbide former [5]. Nitrogen increases creep life but reduces the secondary creep rate and fracture
ductiliity.Boron has a beneficial effect on the creep strength of stainless steels containingMolybdenum
Low strainhardeningenhancesresistancetocreepparticularlyat low temperature.Foreachtemperature,
there is an optimum amount of strain; the strain value decreases as the temperature is raised. The
influence of grain size (G) depends on the temperature [5]. Large grain size is preferable at high
temperature for better creep strength (G > 3 for AISI 316 steel).
151.
CONCLUSION
Austenitic stainless steels are most commonly used in major activity sectors such as house hold and

34 GEORGE
AND SHAIKH
communityequipments,transport,food industry,industrialequipments,chemicaland power engineering,
cryogenics, and building industry.
The family of austenitic stainless steels has a wide variety of grades precisely tailored for a specific
application. The optimum choice of the grades would depend on service needs and this would require
a clear understanding of the metallurgical parameters, which control the microstructure and thus the
mechanical properties, formability and corrosion resistance.
This chapter, in brief, has dealt with the physical metallurgy, welding metallurgy, physical and
mechanical properties of austenitic stainless steels. In the physical metallurgy of stainless steels the
effect of alloying elements in forming different phases, the transformation of austenite to martensite
during cooling or straining, hardening processes and formation of intermetallic phases, have been
discussed. The influence of chemical composition and temperature on the various physical properties
of austenitic stainless steel has been emphasised. Austenitic steels are distinguished by a higher
coefficient of expansion, a lower thermal conductivity and, when the structure is entirely austenitic, a
non-magn,etismthat is retained at low temperatures.
The mechanical properties, like tensile strength, fatigue and creep strengths of austenitic stainless
steelsvary with temperature,compositionand microstructure.Austenitic stainlesssteelshaveparticularly
low yield strengths, and several processes are used to improve them: appropriate thermomechanical
treatments, hardening with nitrogen, precipitation hardening. Their creep strength is excellent up to
973 K, and can be further improved by alloying with N, Mo, Nb, Ti, W, V or B. In austenitic steels,
at temperatures as low as -73 K, toughness is relatively unaffected, whereas their strength increases.
They lend themselves remarkably to deep drawing and cold rolling, where their work-hardening
characteristics enable high strength levels to be attained.Weldability is excellent, and welds, which do
not transform to martensite during air-cooling, have mechanical properties similar to base metal.
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32. Robinson P.W. and Jack D.H., Proceedings of the Conferenceon New Developments in Stainless Steel Technology,
Ed: Lula, R.A., American Society for Metals, 1985, p. 71.
33. Powell D.J.,. Pilkington R., Miller D.A., Proceedings of the Conference on Stainless Steels 84, Gothenburg,
1984, p. 382.
34. Hull, EC., Welding Journal, 52, 1973, p. 104-s.
35. Gill, T.P.S., M. Vijayalakshmi, J.B. Gnanamoorthy and K.A. Padmanabhan, Welding Journal, 65, 1986,
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38. Leitnaker, J.M., Welding Journal, 61, 1982, p. 9-s.
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40. Barick J., Metal Science, 14A. 1983, p. 635-641.
41. Barcik J., Brzycka B., Metal Science 17, 1983, p. 256-260.
42. Barcik J., Journal Applied Crystallography, 16, 1983, p. 590.
43. Verma, D.D.N., M.S. Thesis, Indian Institute of Technology, Madras, 1983.
44. Kokawa. H., Kuwana, T. and Yamamoto, A., Welding Journal. 68, 1989, p. 92-s.
45. Shaikh, H., Pujar, M.G., Sivaibharasi, N., Sivaprasad, P.V. and Khatak, H.S., Materials Science and Technology,
10, 1994, p. 1096.
46. Mathew. M.D., Sasikala, G., Gill, T.P.S., Mannan, S.L. and Rodriguez, P., Materials Science and Technology,
10, 1994, p. 1104.
47. Weiss, P. and Stickler, R., Metallurgical Transactions A, 31A. 1972, p. 85 I .
48. Lai, J.K. and Haigh, J.R., Welding Journal, 58, 1979, p. I-s.
49. Goldschmidt, H.J., Interstitial Alloys, Butterworths, London, 1967.
50. Weigand, H. and Doruk, M., Arch. Eissenhuttenw, 8, 1962, p. 559.
51. Solomon, H.D. and Devine, T.M., ASTM STP, 672, 1979, p. 430.
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Venugopal, S.,Sundararaman, D. and Khatak, H.S., Journal of Nuclear
Materials, 264, 1999, p. 29.
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Nuclear Applications o
f Stainless Steels at Elevated Temperatures, Varese, Italy, Metals Society, London.
54. Tavasolli, A.A., Bisson. A and Soulat, P., Metals Science, 18, 1984, p. 345.
55. Dyson. D.J. and Kcown, S.R., Acta Metallurgica, 17, 1969, p. 1095.
56. Shaikh, H.. Khatak, H.S., Seshadri, S.K., Gnanamoorthy, J.B. and Rodriguez, P., Metallurgical and Materials
Transactions A. 26A, 1995, p. 1859.
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1989, p. 11 15.
p. 122-s.
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36 GEORGE
AND SHA~KH
58. Gooch, T.G., and Honeycombe, J., British Welding Journal, 2,1970, p. 375.
59. K.J. Irvine et al.: Journal of Iron & Steel Institute, 196, 1960, p. 166.
60. N.E. Moore and J.A. Griffiths: Journal of Iron and Steel Institute, 197, 1961, p. 29.
61. D.G. Benvick: Metallurgia, May 1966, p. 218.
62. Schaeffler A.L., Metal Progress, 56, 1949, p. 680.
63. Delong W.T., Metal Progress, 77, 1960, p. 98.
64. Kotecki, D.J. and Siewerd, T.A., Welding Journal, 71, 1992, p. 171-s.
65. Ward, A.L., Nuclear Technology, 24, 1974, p. 201.
66. Gill, T.P.S., Gnanamoorthy, J.B. and Padamnabhan, K.A., Corrosion, 43, 1987, p. 203.
67. Shaikh, H., Vinoy, T.V. and Khatak, H.S., Materials Science and Technology, 14, 1998, p. 129
68. Hochmann J.. Aciers in oxy dables et Refractaries ( Techniques de I’Ingenieur) 1974-1981.
69. Documentation International Nickel, B 24-25-26-31.
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71. Nunes, J., Martin, A., Journal of Material Science, 1975, p. 641.
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73. Bathia C., Bailon J.P., La Fatigue des materiaux et des structures, Maloine Publisher.
74. Colombier L., Hochmann J., Aciers inoxydables et Refractuires, Dunod, 1965.
75. J.M. Silcock and G.Willoughby :Proceedings of the Conferenceon CreepstrengthinSteel and High Temperature
Alloys, The Metals Society, London, 1974, p. 1.
76. Williams, T.M., Harries, D.R.,
Meeting on Creep Strength in Steels and High TemperatureAlloys, Sheffield
1972.
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78. Albert. S. Melilli Editor, Handbook o
f Comparative World Steel Standards.

Basic applied mathematics
There are many excellent books on physics, maths and applied
maths, suitable for all levels, and it is not the intention to reproduce
large tracts of maths and physics here.There are certain fundamen-
tals however that are necessary to adequately understand the
process of selecting, designing, procuring, installing and testing a
cable system. Someof these basic elements,such as decibels,occur
over and over again in this particular subject of cable engineering,
and it is topics such as these that will be explored in this chapter to
give the reader a sufficient foundation of knowledge to make best
use of the remaining chapters.
2.1 Working with indices
To handle very large or very small numbers,we use a convention of
representingthese numbers in the following notation, for example:
109 = I ooooooooo
10-3 = 0.001
The small number in the superscript, or the index, if it is positive, rep-
resents how many zeroes there are, or more precisely, how many
factors of ten are involved. So 10’ means one thousand million, or a
billion.

Basic applied mathematics 5
If the index is negative, then the number is less than one, and the
index number reveals how many places after the decimal point there
should be, or how many factors of divisions of ten there are. So
means one-thousandth. An expression of 6.3 x lo6 means
6300000.
If two numbers in this notation are multiplied together then simply
add the index numbers together. For example:
10" X1o7=loii
I O - ~
x lo7=lo4
(2 x lo4)x (8 x 106)=1.6 x loi1
To divide two numbers, subtract one index from the other. For
example:
109/106
= lo3
1ol*/l o - ~
= 1oi5
8 x 108/2x lo4= 4 x lo4
Simple addition or subtraction of two numbers in this form can only
take place if the indices are the same. For example:
(8.5 x 106)-(3.1 x 106)=5.4x lo6
8.8 PreAxes to denote size
There are accepted prefixes we can use to denote the size of
a number more simply than always writing it out or pronouncing it
in its entirety, e.g. kilometres means one thousand metres; the kilo
part representing one thousand. Table 2.1 gives the full list. For
example:
1pF is one picofarad, or 1 x lo-'* farads.

6 Cable engineering for local area networks
Table2.1 Pdxnotation
Decimalnumber Index Prefix Symbol
1o0Ooooo0Oo0Oo0O
1o0Ooooo0Oo0O
1ooooooo0O
1OOOo0O
loo0
100
10
0.1
0.01
0.001
0.o0OoO1
o.OOoOOo001
0.o0OOOoo0O001
0.o0OOOoo0Oo0O001
o.o0OOOoo0Oo0Oo0O001
1015
1012
1OQ
l
O
a
109
102
10
1
10-1
10-2
lP
10-8
10-0
lC’2
lo-’*
lo-15
peta
tera
s
i
g
a
mega
kilo
heCt0
deca
deci
centi
milli
micro
neno
pic0
femto
att0
P
T
0
M
k
h
da
d
m
CI
n
P
f
a
C
2
.
3 Logarithms
Logarithms of numbers also make the large and the small easier to
manipulate.A logarithmof a number is that numberthat you haveto
raise another number to the power of to get back to the first number.
For example:
The logarithm of 100 is 2, because you have to raise 10 to the
power of 2 (lo2)
to get 100.
This is working to base ten, but logarithms can be expressed in
any base. To be precise we should write loglo, but it is always
assumedthat if no basenumberisspecifiedthen we aretalkingabout
calculations made to the base ten. These are sometimes referred to
as common logarithms. Sometimes base 2 is used in communica-
tions theory because digital transmission only has two states, that is
‘ones’ and ‘zeroes’. The log, of 16 is 4, becausewe have to raise 2
to the power of 4 (24)to get 16.
Antilogarithms are simply working the other way round. For
example:

antiloglo2 = 100
antilog103 = 1000
antiloglo-3 = 0.001
2.4 Decibels (dB)
Decibels are ten times the logarithm of a ratio. They are used in
all branches of engineering, and can be used to represent differ-
ences in electrical power, light or even sound. Using decibels makes
calculations much easier to comprehend and even do in your head.
For example, if the attenuationof one piece of cable is 4dB, and you
add onto it another length of cable with 3dB of attenuation then the
resultingattenuationof the whole channel is 7dB. Decibelsare a con-
venient shorthand which show how energy is absorbed or produced
regardless of what levels of energies are actually involved:
gain (attenuationif it is negative)= 1010glo(Pl/f2), [2.11
where P1is one power measurementand P2is another power mea-
surement that we wish to compare with the first.
For the remainder of this book we will adopt the conventionthat all
logarithmsareto the base 10unlessotherwisedenoted. Forexample:
if power level 1 (output) equals 1mW and power 2 (input)equals
0.002mW then
gain =I OIog1/0.002
= 1Olog500
= 10 x 2.7 = 27dB
for attenuation we have
10log0.002 (10logP&)
= -27dB
Note the absolute value does not change, only the sign. Many
writers leave out the negative sign altogether if it is clear they are
talking about attenuation, so as to avoid the uncertainty of a double
negative.

8 Cable engineeringfor local area networks
Sometimes it is presumed that the comparison is being made to
1mW of power, so PI or P2 in the equation will always be one. To
denote this the resulting answer has the units dBm.
Measuring the power in a device or a cable is not nearly as easy
as measuring the voltage across it relative to ground or any other
potential. We can take account of this by knowing that:
power = volts x amps, or P = V x I L2.21
But from Ohms law we also know that V = /R and hence / = V/R and
therefore power also equals V x V/R or V2/R.
R is the resistance of the cable and is presumably the same
from one readingto another, so we can cancel it out in the following
equation:
gain =1Olog -
G;;)
=1olog(v,/v2)2 ~2.31
= 2Ol0g(V,/V2)
So we can obtain decibel readings by simple voltage measure-
ments and incorporating a factor of 2 in the standard decibel
equation.
It should be rememberedthat power only equalsvoltage multiplied
by the current for the special case of direct current. For alternating
current the correct formula is:
power (w) = voltage x current x the cosine of the phase
difference between the two.
The cosine o
f the phase difference is known as the power factor.
If the current and voltage are in phase then the angle is zero and
the cosine of zero is one, so in that special case power does indeed
equal voltage times current. Any reactive load, i.e. capacitance
or inductance, will cause the current to lead or lag the voltage in
phase.

2.8 Sine waves and phase
A sine wave or sinusoidal wave is the most natural representation of
how many things in nature change state. A sinewave shows how the
amplitude of a variable changes with time. The variable could
be audible sound for example. A single pure note is a sine wave,
although it would sound a very plainand flat note indeedwith noneof
the harmonicswe normallyhear in nature.A straightforwardoscillating
or alternatingcurrent or voltage within a wire can also be represented
by a sine wave. The number of times the sine wave goes through a
completecycleinthespaceof 1secondiscalledthefrequency.Indeed
the unit used to be cycles per second, but now the unit of measure-
ment is hertz (Hz).A frequency of IOOOHz, or 1kHz, means that the
sinewave goesthrough 1000complete cycles in 1s. Ifwe are consid-
ering audible soundwaves then the humanear hasa frequency range
of about 20Hz-20 kHz. The electrical mains frequency in Europe is
50Hzand 60Hz inAmerica. Figure2.1 shows a sinewave.
1
0
" 45" 90" 135" 180" 270" 3
6
0
'
Phase
Fig. 2.1 A sine wave.
The sine of any angle can vary from -1 to +I.
For examplethe sine
of 0" is 0 and the sine of 90" is 1. The sine of 270" is -1 and when
we get to 360" we are back to zero again. A cosine is 90" out of
phase with a sine wave as we can see in Fig. 2.2.

1
0
" 45" 90" 135" 180" 270" 360"
Phase
Fig. 2
.
2 A sine and cosine wave.
The cosine of 0" is 1 and the cosine of 90" is 0. So we say that a
cosine is 90" out of phase with a sine wave. Any number of sine
waves can exist at any one time and have any manner of angular
phase differencesto each other. Whenever a phase angle is men-
tioned it is always relative to something else. Digital 'ones' and
'zeroes' can be encoded as two signals of identical amplitude and
frequency but with different phases to each other or some other ref-
erence marker. This would be called phase modulation.
When we apply an alternating voltage across a resistor, a current
flows through the resistor. If we looked at the voltage and current
waveforms on an oscilloscope we would see two sine waves that
superimpose each other,when differencesof amplitudearetaken into
account. The two signals are in-phase with each other. If we add a
capacitor in series with the resistor we would see the current and
voltage signals diverge so they were out of phase with each other.
When an electrical current flows in a circuit we are observing the
effect of the flow of the fundamental particlescalledelectronsflowing
through the wire from a negative to a positive terminal. We can
imagine a capacitor is like a big bucket for electrons. When the
voltage is applied to the circuit, the electrons flood into the bucket.
But as the rising voltage reaches its peak, the bucket is nearly full,
andthe flow of electrons. or current tails off. The flow of currentthere-

fore seems to lead the voltage and is out of phase with the voltage.
An inductive load works the other way round. The rising voltage is
needed to draft the electrons into the inductor where they fight
against the magnetic field they have created. The current therefore
lags the voltage. A circuit with a capacitive and/or inductive load is
called reactive.The actual phase of the current relativeto the voltage
will depend on the values of resistance, capacitance and inductance
in the circuit and may be represented as a complex number.
2.6 Complex numbers
Complex numbers take the form a +jb, where a is the real number
andjb is the imaginary number.j is supposed to be the square root
of minus one, that's why it's called imaginary (just try getting an
answer for the square root of minus one on your calculator and you'll
270" phase shift
Positive'real' value
0"phase
ay' value, j
90" phase shift
Negative 'real' value
180"phase shift
Fig. 2
.
3 Complex vector.

see why). This strange format is useful to express a value which has
a phase and amplitude component. The real component can repre-
sent the in-phasecomponent, such as the current flowing through a
resistor,and the imaginarycomponent representsthe current flowing
out-of-phase due to the reactive load. As we can see from Fig. 2.3
the resulting value is a vector which has an amplitude and a phase
angle caused by the complex combination of two or more out-
of-phase components. Note that most books on mathematics use
lower-casei to represent the imaginary component. Howeverin elec-
trical engineering,i more often means a current, so the letterj is used
instead.

Basic physics -electrical
3.1 SI system and fundamental units
As long as people have been measuring things they have needed
units of measurement to make any kind of meaningful recording of
the event. Noah would not have understood God’s instructions to
make the Ark unless both understood the concept of the cubit. As
empires have risen and fallen they have introducedtheir own units of
measurement to make sense of their own mathematics, civil engi-
neering and commerce. Today the world has evolved towards the
metric system and away from a hotchpotch of measurement units
vaguely grouped together under the heading of ’imperial’. Only the
United States, the United Kingdom and Ireland use imperial mea-
surements, although frequently mixed with metric units. The rest of
the world is solidly metric! The number of imperial units in common
circulation has declined and the era of British schoolchildren, pre-
1970, having to struggle with rods, chains, pecks and bushels is for-
tunately over. Imperial units still hold some stings however; the Irish
acre is not the same as the English acre, the US gallon and British
gallon have little in common and the meaning of the nautical mile is
open to interpretation!
Getting units wrong can be expensive. The Mars Climate Orbiter
spacecraft crashed into the surface of Mars in 1999 because one
programmer in NASA had been calculating in metric units whilst
another had been using imperial units.

Whilst metric units are universally accepted for engineering and
science they are more accurately described under the SI system,
or Systeme Internationale d’llnites, which originated in 1948 and is
now enshrinedas an IS0 standard. SI contains seven base units and
some derived and supplementary units. The seven base units are:
1 The metre, m, the unit of length.
2 The kilogram, kg, the unit of mass.
3 The second, s, the unit of time.
4 The ampere, A, the unit of electric current.
5 The kelvin, K, the unit of temperature.
6 The candela, cd, the unit of luminous intensity.
7 The mole, mol, the standard amount of a substance.
All other units can be derived from these base units, e.g. the unit of
force is the newton, N, but it can also be expressed as mass times
length over time-squared. The SI unit of pressure is the pascal, Pa,
this is force per unit area and so can be expressed as mass over
length times one over time-squared. The SI unit of frequency is the
hertz, Hz, which is the reciprocalof time.
Supplementary units include the radian and the steradian. Be-
fore the SI system some countries used the cgs system, mean-
ing the basic units were centimetres, grams and seconds rather
than the SI units of metres, kilograms and seconds. Other units
still in popular use, but not recognised SI units, are the micron
(104m), the metric tonne (1000kg) and minutes, hours, days and
years.
In electrical engineering the base unit is the ampere. One amp is
defined as that constant current which, if maintained in each of two
infinitelylong straight parallel wires of negligible cross-sectionplaced
1m apart, in a vacuum, will produce between the wires a force of
2 x 10-7N/m length.
From this we have the potential difference, whereby 1V is the dif-
ference in electric potential between two points of a wire carrying a
constant current of 1A when the power dissipation between these
two points is 1W.
One ohm of resistance is defined as the electrical resistance be-
tween two points of a conductor when a constant potential differ-

Basic physics -electrical 15
Table 3.1 SI electrostatic and electromagnetic units
Quantity Symbol SI unit Abbreviation
Mass
Length
Time
current
Charge
Potenti difference
POWW
Resistance
Conductance
Inductance
Capacitance
Magnetic flux
Magnetic induction
m
I
t
I
Q
v
P
R
G
L
C
#
B
kilogram
metre
second
ampere
coulomb
volt
watt
ohm
siemens
henry
farad
weber
tesla
kg
m
S
A
C
v
W
n
S
H
F
Wb
T
I EI&ricfieldstrength E volt metre-’ Vm-’
I
ence of 1V applied between these two points produces in the con-
ductor a current of 1A.
Thefull list of SI unitsrelevantto electricalengineeringis inTable 3.1.
Atoms, elements and compounds
The atom is the basic building block of matter in which the matter
still retains the unique identity of an element. Copper is an element
and it is made up of copper atoms. Atoms themselves are made up
of building blocks such as electrons and protons, but if the copper
atom is reducedto its constituents then it is no longer a copper atom
in the same way as if a wool coat was unravelledinto a ball of wool,
cotton and buttons, the identity of the original coat would be lost,
and those constituents could go to make something else.
Elements are made up of atoms of the same type. Iron, copper,
aluminium are all elements; steel is not, it is a mixture of iron and
carbon and a few other things.
Molecules are groups of atoms. Oxygen atoms, for example, go
around in pairs, so the atmosphereis full of oxygen molecules,which

have the symbol 02.
Compounds are two or more elements that are
chemically combined, such as water, H20,which is two atoms of
hydrogenand one of oxygen.Salt is sodium chloride, NaCI, one atom
of sodium (Na)and one of chlorine, (CI).
Allotropes are ways in which the same element can exist in differ-
ent forms. For example, both diamond and graphite are allotropesof
carbon, as is fullerene.
There are 92 elementsfound on earth and a further 14 have been
produced by scientists.
Atoms are made up of three basic subatomic particles, called pro-
tons, neutrons and electrons. In the last 30 years ‘atom-smashers’,
or high velocity particlecolliders, have brokenatoms into a large and
bizarre list of sub-subatomic particles. However we merely need to
graspthe fundamentalsof the proton, neutron and electron.An atom
is made up of a nucleus containing protons and neutrons. Around
this in orbit are electrons that group together in various shells. Figure
3.1 gives a representation of an atom.
A proton has one positive charge and an electron has one nega-
tive charge. The proton is many times more massive than an elec-
tron however. The neutron has no charge but the same mass as the
proton.The atom has a neutral overall electric charge because there
are as many electrons as there are protons. If an atom loses or gains
electronsthen it would become charged and would be known as an
ion. The process of losing electrons is called ionisation.
The mass of an atom is made up of the combination of neutrons
and protons, the electrons add very little to the mass. The number
of protons and neutrons together is the mass number. The number
of protons is called the atomic number. Most of the volume of an
atom is just empty space. When some stars collapse at the end of
their lifeto form neutron stars the atoms havetheir electronscrushed
down towards the nucleus by immense gravity and pressure.This is
why neutron star material has unimaginableweight such as around
a million tonnes per spoonful!
The electrons orbit in shells. One or two in the first shell, up to 8
in the second shell, up to 18 in the third shell (an inner group of 10
and an outer group of 8) and up to 32 in the outer shell (includingan
outer group of 8). Electronsfill up the lower shells first and each shell

Fig. 3.1 The atom.
is associated with a particular energy level. Phosphorescent ma-
terials allow photons of light to boost some of the electrons to higher
levels. When the electron falls back a level it releases a photon to
account for the energy, and that is what we would see as light.
Hydrogenis the simplest element. It has one proton and one elec-
tron and zero, one or two neutrons depending upon which isotope
of hydrogen it is. Next is helium with two of everything. The more
subatomic materials in an atom the heavier and more dense it will
be. Uranium has a mass number of 235. The number of electrons in
the outer orbits determines the chemical properties of an element.
All the members of the group of elements known as the halogens
(fluorine,chlorine, bromine, iodineand astatine) have seven electrons
in their outer shell so they all react very similarly. A group of gasses
known as the noble gasses have all of their outer electron shells filled

up, so they are extremely unreactive. This group includes helium,
neon, and argon.
The elements are usually listed in something called the periodic
table. Elements, which have the same number of electrons in the
outermost shell, fall into vertical columns, of which there are eight.
The horizontal listings are called periods, of which there are seven
plus two ‘rare earth’ periods called the lanthanum series and the
actinium series. The latter two periods contain the mostly artificially
producedand very unstableelementssuch as curium, plutoniumand
californium etc.
In between the very reactive metals such as lithium and sodium
which lie on the left side of the periodic table, and the non-metals
such as chlorine and fluorine, on the right of the table, lie the transi-
tion metals, and here we see the more familiar metals such as iron
(Fe),copper (Cu)and gold (Au).At the border of the metals and non-
metals we find the metalloids which have some metallic and some
non-metallic properties. Here, for example, we find silicon and
germanium.
To be an atom of the same element it must havethe same number
of protonsas every other atom of the same element. So, for example,
every single atom of magnesium must contain 12 protons, or else it
wouldn’t be a magnesium atom. The number of neutrons however
may differ. Forms of an element which differ in the number of neu-
trons in the atom are called isotopes. Isotopes are expressed in the
form AzX,wherexis the chemicalsymbol,A isthe massnumber(neu-
trons plus protons) and Z is the atomic number, i.e. the number of
protons. For example, two isotopes of chlorine are 35,7CI
and 3717CI.
Carbon, for example, has three isotopes, known as carbon-I2,
carbon-I3 and carbon-I4. The latter is radioactive,with a half-lifeof
5700years, and is usedto ‘carbondate’ materialthat was once alive,
typically wood.
3.3 Conductors, semi-conductorsand insulators
An electrical current flows in a conductor when a potentialdifference,
i.e. a voltage, is applied across it. The flow of current is the flow of
electrons within the conductor moving under the influence of the

applied electric field. Metals are conductors and copper is a very
good conductor. The atoms within a metal are held together by what
is known as the metallic bond. The outer electrons of the atoms
are able to break free and roam about between the metal ions. The
motion of the electrons is random except when under the influence
of an electric field.
Not all metals can conduct electricity to the same extent. Copper
is much better than aluminium for example, with copper having only
two-thirds the resistivity of aluminium.
An insulatorisa materialwithinwhichtheelectronsaresecurelyfixed
in chemical bonds and are not free to move about under the influ-
enceof an electricfield. Eventuallythough, when the voltage was high
enough, a current would flow through the material and that is called
the breakdown voltage. Insulators are also sometimes known as
dielectrics.An insulator,or dielectric,hasthecapacityto storeacharge
to some extent as the atoms are pulled apart slightly by the electric
field. It isa storeof energy ina similarway that a stretchedelastic band
is a store of potential energy. This capacity is known as the material’s
permittivity.A vacuum could not hold any charge because it contains
no matter. A material is thus measured by its relative permittivity, i.e.
how much better is it at storing a charge than a vacuum.
A capacitor is two conductors separated by a dielectric. If the
capacitance of the two plates is measured in dry air, then the rela-
tive ability to store a charge is measuredwhen different materials are
inserted between the two plates, and we can then determine the
relative permittivity of that material. The relative permittivity of air is
1.000536, because air is not that far removed from a vacuum, but
the relative permittivity of mica is 6.7, so a piece of mica between
two plates of metal would make a far better capacitor than two par-
allel metal plates on their own. Capacitance is also proportional to
the surface area of the two conductors and inversely proportional to
their distance apart.
Capacitance is an unwelcomephenomenonin cables.The capac-
itancetakes time and energy to charge and discharge as high-speed
electronic signals pass down the wire. It also provides a mechanism
whereby noise can be coupled into the cable.
Conductors have the property of inductance, which has the sym-
bol, L, and the unit of the henry, H. Inductance is the ability of a

conductor to store a magnetic field. Like capacitance, inductance
consumes time and energy from the signal by having to charge and
discharge the magnetic field.
Inductance is a property of the conductor, whereas capacitance
has to be between two conductors or more. We can thus have
capacitance between conductors in the same cable, between the
conductors and a screen or shield, and capacitance between
the conductors and the ground plane. The propertiesof a cable can
thus be changed by the cable’s proximity to an earthed/grounded
sudace.
Finally we have semiconductors, such as silicon and germanium.
Semiconductors have properties between metals and non-metals
and have proved invaluable in the development of semiconductor
microcircuits based primarily on silicon.
3.4 Electricity and circuits
We have seen that electrical current is the flow of electrons through
a conductor under the influence of an electric field. If the voltage
stays at the same polarity then the flow of current will always be
in one direction. This is called direct current, or dc. If the polarity of
the voltage changes then so will the current. This is called al-
ternating current, or ac. The domestic voltage system we use in our
houses and in the national grid is ac, primarily because it makes
it easy to change from one voltage to another by the use of
transformers.
The main parameters used to quantify an electrical circuit are:
voltage
current
resistance
capacitance
inductance
impedance
We also have conductance, G, which has the unit of siemens, con-
ductance is the reciprocal of impedance or resistance and may be

considered as the leakage between conductors due to imperfect
insulation.
For a dc circuit we can relate the resistance, voltage and current
by Ohm’s law, given in equation 3.1.
V = l x R
V = voltage, in volts
I = current, in amperes
R = resistance, in ohms.
i3.11
For an ac circuit the resistanceto the current isthe combinedeffect
of the dc resistance, and the reactance (X) of the capacitance and
the inductance. The effect is called the impedance, and it has the
symbol, Z. The units are still in ohms. Equation 3.2 gives the
relationshipof R and X to create the ac impedance:
Z = R + j X
Z = impedance, in ohms
R = resistance, in ohms
X = reactance of the circuit, in ohms.
Reactance is the complex (in the mathematical sense, hence the
operator ‘j‘in front of it to denote a complex number. Pure mathe-
maticianswould use the symbol ‘i’ as the complex number operator,
but to engineers this more often means current and so engineers
usually use the symbol 7’) value of impedance inherent in a ca-
pacitor and inductor. It is frequency dependent and the complex
operator denotes that it has a phase value. Equation 3.3 gives the
reactance of a capacitor:
x, = I/iOC P.31
X, = reactance, in ohms
C = capacitance, in farads
o = angular frequency, i.e. 2nf,where f = the frequency in hertz.
Equation 3.4 gives the reactance of an inductor:
X, =jwL [3-4
1

X, = reactance, in ohms
L = inductance, in henries
o = angular frequency, i.e. 2nf,where f = the frequency in hertz.
Resistances and impedances, when in series, can simply be added
together to give the circuit total, i.e. Rt = R1+R2+R3+. . . .When in
parallel, the total value becomes l/Rt = l/R1 + I/& + 1/R3+. . . .
Figure 3.2 shows this.
Resistorsin series
7
Resistors in
parallel
& &
Fig. 3
.
2 Resistors in series and parallel.
Of more use in communications is the characteristic impedance
of a circuit, rather than the absolute impedance. The characteristic
impedance, Z,, is the impedance of an infinite length of line. Even
though real-life cables are never infinitely long the characteristic
impedance is important for matching components in a circuit
together. Energy can only betransferred from the sourceto the cable
and onto the load efficiently, if the characteristic impedances of
each component are the same. Otherwise energy will be reflected
back at every point where there is a discontinuity of characteristic
impedances. Equation 3.5 gives the equation for characteristic
impedance in ohms:
We may assume that the conductance, G, goes to zero for a cable
with a good insulator. Also at high frequencies R will become very
small compared to the OL term and so the equation simplifies to:

And then to:
z
o =g
[3.61
P.71
as thejo terms cancel out.
The amount of energy reflected back can be derived from:
[3.81
z
s -zo
z
s +zo
R, =
where R, = the reflectioncoefficient (alsogiven the symbol p in some
textbooks)
Z,= the impedance of the source
Z
,= the characteristic impedance of the cable.
From equation 3.8 we can see that the reflection coefficient will be
zero when Z
,= Zo
[3I 9
1
A twisted pair telephone cable can be represented as two resistors
in series with a capacitor in parallel with one of them. The capacitor
appears as an open circuit at dc and tends towards a short circuit
as the frequency gets higher thus shunting out the effect of the
second resistor; see Fig. 3.3.
We can deduce from the diagram that the impedance of the line
is around 600ohms at low frequencies and tends towards 1OOR at
return loss = 20 loglo R,
Fig. 3
.
3 Equivalent impedance model for a twisted pair, 0.5mm copper.

1600 1
1400
1200
1000
aoo
c:
600
400
200
I I I I I I I I I I I I
0 25 50 75 100 125 150 175 200 225 250 300 325
kHz
Fig. 3.4 Impedanceagainst frequency for a 0.5mm copper pair.
higher frequencies. A standard test on a telephone line is to look for
600sZ impedance at 1200Hz. Figure 3.4 shows the impedanceof a
0.5mm copper pair against frequency. At audio frequencies, i.e.
30Hz-3.3kHz the impedance is of the order of 1400-600sZ, but at
higher frequencies, and remember that Local Area Networks will
generallybeoperatinginthe 10-1 00MHzband,the impedancetends
towards 100R.
Electrical power is measuredin watts. Energy is power times time.
Hence we pay our electricitybills in units of kilowatt-hours, being the
amount of power we are consuming times the period that con-
sumption goes on for. If we run a 2kW electric fire for 3 hours then
we will haveconsumed6kW-hoursof energy. Although kW-hours (or
simply ‘units’ as they may be referred to on your electricity bill) is a
convenient unit of measurementfor a power company,the correct SI
unit of energy is the joule, J, or kilojoule, kJ.
We can derive the power consumption in watts of a dc electrical
circuit from the following equations:
power = V x /
power = l2 x R
power = V2/R
[3.10]
[3.1I]
[3.12]

For an alternating current circuit we can replace the resistanceby
a value for the impedance, but equation 3.10 then requires modifi-
cation. In a purely resistive circuit the voltage has the same phase
as the current. If we envisage a sinusoidal voltage then the resulting
current is exactly the same shape and the peaks and troughs of the
two waveforms are exactly coincident, i.e. they are in phase. If the
load is reactive, i.e. it has capacitance and/or inductance then
the current will not be in phase with the voltage. If, for example, we
apply a voltage across a capacitor, the current will rapidly flow into
the capacitor as electronsseek to 'fill-up' the bucket they see before
them. The voltage will rise slowly as the current flows but as the
bucket of electronsfills up the current will slow down and the voltage
will rise to its maximum potential. If the two waveforms, current and
voltage, were observed on an oscilloscope we would see the current
apparently leadingthe voltage wave by 90". The oppositewill happen
with an inductor. If we tried to arrive at the power generatedby multi-
plying the current by the voltage we would not get a correct answer
as when the voltage was at a maximum, the current flow would be
zero. Power dissipation in an ac circuit is given in watts by equation
3.13.
[3.13]
power = V x I x cose
cose = the cosine of the phase angle between the voltage and
current.
Cose is also known as the power factor. The cosine of 90" is 0, so
that in the worst case, with 90" phase lag, no power would be
dissipated. To avoid confusion or ambiguity many machines and
generators quote their output or consumption in terms of kVA,
or kilovolt-amperes, demonstrating that absolute power dissipated
depends upon the reactance of the load applied.

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