2. What are stainless steels ?
“ stain less”
Steels containing 10.5 - 30%
Chromium
The chromium oxide forms a thin
passive layer on the surface when
exposed to atmosphere.
This prevents corrosive attack and
gives the steel its property.
Minimum 10.5% Cr needed to protect
against atmospheric corrosion.
Elements like Ni, Mo, Cu, Nb, Ti etc
added to improve mechanical
properties and corrosion resistance
11. Effect of Alloying Elements and their Purpose
Chromium (Cr)
Oxidation & Corrosion Resistance
Formation of ferrite
Nickel (Ni)
Increases resistance to mineral acids
Produces tightly adhering high temperature oxides
Increases Toughness properties
12. Effect of Alloying Elements and their Purpose
Molybdenum (Mo)
Increases resistance to chlorides.
Resistance to pitting corrosion
Copper (Cu)
Provides resistance to sulfuric acid.
Manganese (Mn)
Increases the solubility of nitrogen
13. Effect of Alloying Elements and their Purpose
Sulfur (S)
Improves resistance to chlorides.
Improves weldability (Penetration) of certain
austenitic SS.
Improves the machinability of certain austenitic SS.
Titanium (Ti)
Stabilizes carbides to prevent formation of Cr carbide.
Precipitation hardener
14. Effect of Alloying Elements and their Purpose
Niobium (Nb) & Columbium (Cb)
Carbide stabilizer.
Precipitation hardener
Aluminum (Al)
Deoxidizer – Precipitation hardener
Carbon (C)
Carbide former and Strengthener
15. Welding of stainless steels
All standard welding processes is SMAW, GMAW,
FCAW, GTAW, PAW and SAW maybe used depending
on the application.
Generally corresponding grades of filler metal
composition are used to match corrosion and / or
heat resistance properties along with strength.
Weldability problems are different for different
types of stainless steels eg martensitic, Ferritic,
austenitic, duplex and precipitation hardening.
Weldability considerations are similar for wrought
and cast alloys
16. Martensitic stainless steels
ASME: P.No-6
Martensitic SS – 4xx
Martensitic SS
High Carbon
Low Chromium
Cr – 11.5-17.0 %
Ni – 1.25-2.50 %
Mo – 0.50-2.0%
C – 0.05-0.20%
17. Martensitic stainless steels
AISI 403, 410, 416, 420, 431 & 440
A/B/C grades
Can be hardened by heat treatment
Martensitic structure - higher carbon
grades used in tempered condition.
Used for cutlery, surgical instruments,
steam, gas & hydel turbine blades, ball
bearings and races.
18. Welding of Martensitic Steels
Higher carbon grades used in the quenched and
tempered condition
Problem of Hydrogen induced cracking in HAZ.
Pre-heat and post-weld heat treatment required if
welded with matching composition martensitic SS
electrodes.
Austenitic SS electrodes generally used which
avoids cracking problems without pre and post
heating
20. Ferritic stainless steels
ASME: P.No-7
Ferritic SS
Low Carbon
High Chromium
Ferritic SS – 4xx
Cr – 11.5-30.0 %
Ni – 0.50-2.50 %
Mo – 0.75-4.2%
C – 0.07-0.12%
21. AISI 405, 409, 430, 446 grades
Ferritic structure - higher ductility and resistance
to SCC & pitting corrosion.
Used as thin sheet for corrosion, oxidation & heat
resisting applications and decorative purposes eg.
Automobile exhausts, Solar water heater, catalytic
converters and automobile trim.
Ferritic stainless steels
22. Welding of Ferritic steels
Softer and more ductile than martensite steels
Cannot be hardened by heat treatment
Ferrite phase does not transform to martensite but
susceptible to 475 deg embrittlement and sigma phase
formation in higher chromium grades.
Problem of grain growth during welding leading to
brittle structure in HAZ.
Generally welded with austenitic SS electrodes
25. Duplex stainless steels
AISI 2205, 2304, 2507
Almost twice the strength of austenitic steels
Excellent pitting + SCC resistance
Used for plant and piping in oil and gas production,
corrosive applications to resist chloride ion media. Higher
strength structurals
26. Weldability of Duplex steels
Duplex stainless steels have fairly good weldability.
All standard welding processes can be used.
Not quite as easily welded as the austenitic grades but low
thermal expansion in duplex grades reduces distortion and
residual stresses after welding.
All grades
Solidify as ferrite, austenite formation during cooling
Austenite/Ferrite ratio dependent on 2 primary variables
Alloying effects – Cr & Ni equivalents
Heat input/cooling rate
27. Welding metallurgy of Duplex steels
Faster cooling rate produces higher Ferrite which leads to
reduced low temperature impact strength and corrosion
resistance
Slow cooling through 1050 – 550 C produces carbides,
nitrides, sigma etc which affect corrosion resistance and cause
embrittlement
Fast cooling Correct cooling Slow cooling
Too high Ferrite Between 30 -40 %
ferrite
Nitrides, Carbides
& intermetallics
28. Welding of Duplex Stainless Steels
–Overmatch nickel in filler metal
–Control heat input and cooling rates carefully
–Use N2 in shielding gas
–Ensure low impurities in base and filler
30. Precipitation Hardening stainless
steels
Cr 12 –18%
C 0.05 – 0.15%
Ni 3.0 – 27.0% +
Mo, Cu, Al, V, Cb, Ti
Can be hardened by heat treatment
This sub-group provides a combination
of austenitic and martensitic properties.
Hardening is achieved by adding one or
more elements such as aluminium,
molybdenum, niobium, titanium, and
copper.
It is capable of developing high tensile
strength through heat treatment
Available as forgings, castings, bar and
plate and used for compressor blades,
pumps, gears for metering chemicals etc
34. AISI 304, 310, 316, 321 & 347 grades
Austenitic structure gives good weldability with
excellent ductility and toughness down to
cryogenic temperatures.
Nickel improves general corrosion resistance in
high temperature
Widely used for Boiler components, chemical,
petrochemical, fertilizer plant and food
processing. Also used for nuclear and cryogenic
plant
Austenitic stainless steels
35. •Has 50% higher coefficient of linear expansion, than carbon
steels
•Has poor thermal conductivity, 30% less than carbon steels
•Results in much higher distortion after welding
•Steps to prevent distortion
- closer tacking
- greater use of jigs and fixtures
- use of balanced and skip welding
techniques
- limit heat input by use of low currents and
stringer beads
Physical properties of austenitic stainless
steels
36. Welding of Austenitic steels
Generally good weldability as there is no martensitic
transformation but following problems encountered:
Sensitization leading to inter-granular corrosion –IGC
Hot cracking
Stress corrosion cracking –SCC
Sigma phase formation leading to embrittlement
Higher distortion during welding
37. Sensitization and inter-granular
corrosion
Sensitization refers to the
precipitation of carbides at grain
boundaries in a stainless steel or
alloy, causing the alloy to be
susceptible to intergranular
corrosion.
austenitic stainless steel may become
sensitised if they are heat treated or
used at temperature in range 450-
900c. the heat affected zones of welds
may also be sensitised in some
circumstance
39. Steels to prevent IGC
Steels with elements having higher affinity for carbon
eg. Ti , Nb/Cb – stabilised steels. Form carbides in
preference to Cr.
321 grade – Ti stabilised
347 grade - Nb stabilised
Steels with low carbon 304L & 316L grades( 0.03% C
max)
40. Hot cracking in austenitic welds
Hot cracking or solidification cracking is
caused due to low melting eutectics (S, P, Nb,
Ti, N) formed at the grain boundary.
Due to presence of 5 – 10 % ferrite phase in
weld deposit as the weld solidifies, in
combination with shrinkage stresses, leads to
cracks in fully austenitic welds
Promoted by S, P, Nb, Ti, N etc.
Prevented by adjusting weld metal
composition to give 5 – 10 % ferrite phase in
the deposit.
Also prevented by reducing heat input and
controlling design stress.
41. THE BASICS OF COLD WORKING AND ANNEALING
Cold working makes alloys stronger and
harder while reducing the material’s ductility.
annealing reduces strength and hardness
while restoring ductility.
r – nominal outside radius of pipe or tube
R – nominal bending radius to centerline of
pipe or tube
42.
43.
44. Before welding of stainless steel
Preserve the materials in clean (carbon-free) environment
Provide wood support / insulation material support
Avoid contact with carbon steel, Low alloy steel (to avoid formation of
Iron oxide)
Removing the oils, Dust, contaminant
Edge preparation - Cobalt based tool
Grinding/ polishing – Aluminum oxide wheels
Cleaning – acetone
use fixture
46. During welding of stainless steel
Argon purging (root & hot) - water soluble dam – to avoid oxidation (pitting)
in root pass
Ensure shielding gas flow
Avoid arc strike
Inter pass temperature – 150 deg.
Inter pass cleaning – SS wire brush
Avoid Sudden stoppage of arcing
47.
48. After welding of stainless steel
Cleaning – Removal of heat tint
preserve the weldment in clean environment
49. Types of weld Cleaning
Mechanical Weld Cleaning
Chemical Weld Cleaning
Electrochemical Weld Cleaning
50. Mechanical Weld Cleaning
Mechanical weld cleaning is a common and low cost method used for cleaning
stainless steel
involves grinding machines and abrasives to clean the top layer of metal surfaces
where rust and other slag particles can form
51. Chemical Weld Cleaning
chemical pickling paste for cleaning after a welding job. The paste is
applied to the affected areas using a brush or spray and left on the surface
for some time to interact with the metal
chemical pickling paste contains a variety of toxic acids, including
hydrofluoric, nitric, and sulfur acids. These chemicals are quite dangerous
for the human body and they can cause serious, long-term damage to the
skin and internal organs if they are consumed or breathed in.
52. Electrochemical Weld Cleaning
Electrochemical weld cleaning, also known as electropolishing, is
considered the most effective method of cleaning stainless steel
It is faster, safer and preferred by welders, compared to the other two
methods. It doesn’t pose any major health risks for the welder.
