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1. International Journal of Engineering Research and Development
e-ISSN: 2278-067X, p-ISSN: 2278-800X, www.ijerd.com
Volume 9, Issue 9 (January 2014), PP. 07-10
Evaluation Of Blowholes And Shrinkage Defects In Investment
Casting Of Low Alloy Steels
Smt R.Sri Rama Devi
Associate Professor, Department of Metallurgical Engineering, JNTU-H, Kukatpally, Hyderabad-500085
Abstract:- Low alloy steels produced through investment casting technique suffer from Gas holes / Blowholes,
Shrinkage cavities, Slag inclusions, cracks and hot tears, cold shuts etc. Blowholes are entrapped bubbles of gas
with smooth walls. The causes for blowholes are Inadequate metallostatic pressure, use of rusty scrap, moist
conditions presence of liquid slag, improper degassing of liquid metal, improper gating system, turbulence while
filling, too low head pressure, gas entraption due to inadequate flow-offs, risers & back pressure, interrupted/
slow pouring, cold pouring, low pouring pressure etc. The volumetric contraction accompanying solidification
of molten metal manifests in defects like shrinkage cavity, porosity, centerline shrinkage, corner shrinkage and
sink. Shrinkage defects can be minimized by adopting carefully designed gating system which promotes
directional solidification i.e. from thin to thick sections in the casting, leading to feeders. Parameters that
control the feeding system are location of the feeder, size and shape of the feeder, material from which the
feeder is manufactured. In this project work it is decisive to know the effect of these parameters on level of these
defects.
I. INTRODUCTION
Investment casting technique is the best process for producing a product to the near net shape and size
and that can be fitted into the assembly without any machining. This process is best suitable for mass production
of small size casting of size few grams to 300 Kg. Better surface finishes and dimensional accuracy, the normal
tolerance being ±0.005mm, Intricate machining of the casting is avoided. Thin sections of the order of 0.75 mm
can be cast by this method, so that saving a great extent of machine and man hours and finally the cost of the
end product will be cheaper. But the casting technique lacks the quality of the product due to defects those are
inherent of the product. Castings having the shorter life span than the products from other routes due to these
defects. Producing castings without any defect is a challenging job to the foundry men. Since, so many
parameters affecting the quality of the casting that includes raw materials, various stages of operations/
processes to the end product. Out of those some vital parameters needs high attention of the producer and the
quality man. Variation in shape and size of product and material and workman ship always gives scope to do
some new work.
II.
THE PURPOSE OF THE PROJECT
Ordnance Factory Medak is one of the largest castings producer having facilities for production of
various types and sizes of castings used in defense production. Castings such as Housing to Drg No GCF-TM199(M) and Clamp/Web to Drg No GCF-TM-175 are produced through Investment casting with radiography
acceptance standards of these castings are given in the table No. 3.1
Defects
Gas porosity
Table I: Acceptance Standards
Acceptance level
up to Level –V
Acceptable Standard
As per ASTM E-192.
Slag inclusions /foreign materials
up to Level –V
Shrinkage Cavity/Porosity
up to Level –III
Cracks, Hot tears and Cold shuts
Not Acceptable
The casting are still to be established with the above Radiographic acceptance standards. With
presently following procedures and gating system, defects such as shrinkage and gas porosity levels obtained are
above the acceptance standard.
Above components are required for 30mm Automatic gun with high firing rate, which are fitted on
ICV-II Vehicle. Achievement of acceptance standards is very much essential for firing the gun at higher rate
7

2. Evaluation Of Blowholes And Shrinkage Defects In Investment Casting Of Low Alloy Steels
with more safety of crew. Development a procedure and gating system which improves the quality of the
components improves effective function of the components and also the safety of the crew. Hence, the main aim
and objective of this project is to study the various parameters affecting the Shrinkage and Blow holes in low
alloy steel castings produced through Investment casting technique.
III.
EFFECT OF BLOW HOLES AND SHRINKAGE CAVITIES ON CASTINGS
It is a fact of life that any material or product produced after undergoing various manufacturing
processes with utmost care does contain a flaw and material with a flaw is an acceptable norm in all the
industries. Material is never free from flaws. Flaws affect structural integrity and have a considerable influence
on the engineering application of the material [10].
The largest cavities are most often isolated; the smallest (pinholes) appear in groups of varying
dimensions. In specific cases, the casting section can be strewn with blowholes of pinholes. The interior walls of
blowholes and pinholes can be shiny, more or less oxidized. The defect can appear in all regions of the casting.
Porosity may be the most persistent and common complaint of casting users. Porosity in castings contributes
directly to customer concerns about reliability and quality.
At porosity levels above this critical value there is no longer interconnected solid throughout the
volume of material. At this point the material loses all loads carrying capability, and the elastic modulus is zero
[11]. Experimental work on the effect of porosity on the fatigue strength of cast steel reveals that reductions in
fatigue strength of 35% and 50% are observed for “sizes” (areas of cavities) of less than and greater than 3
mm2, respectively, for cast 13 Cr stainless steel. For low alloy steel, fatigue strength reductions from 8 to 30%
were found when shrinkage porosity cavities covered 3 to 7% of the fracture surface [13].
The presence of shrinkage discontinuities reduces the level of fibrous fracture energy in bending impact
specimens. This reduction is greater for normalized and tempered than for quenched and tempered cast steels.
The amount of reduction increases with the severity of shrinkage. The presence of shrinkage discontinuities has
a greater effect on the behavior of cast steel sections tested in tension impact than in bending impact[13].
The bending fatigue properties of cast steel sections were reduced because of the presence of shrinkage
discontinuities. The fatigue properties were considerably reduced, however, when both Class 2 and Class 6
shrinkage extended to the surface of the test specimen [13].
The presence of Class 2 and Class 6 internal shrinkage reduces to an appreciable extent the torsion
fatigue strength of cast steel sections. The decrease in the ratio of the alternating shear stress to tensile strength
at the endurance limit is about 17 percent for Class 2 shrinkage and 32 percent for Class 6 shrinkage for the high
strength, quenched and tempered cast steel. The decrease was not so great for the lower strength annealed cast
steel ; namely, 15 percent for Class 2 shrinkage and 20 percent for Class 6 shrinkage[13].
IV.
OBJECTIVE AND SCOPE OF THE PROJECT
This Project, “Evaluation of Blowholes and Shrinkage defects in Investment Casting of Low Alloy
Steels”, has ambitious objectives; to validate the analysis methods for casting soundness through radiographic
testing, and to ultimately propose new design and improved product quality standards. Main objectives of the
project are:
The main objective of this project work is to find out the effect of various parameters on level of
Blowholes and Shrinkage defects in low alloy steels made through investment castings. The areas selected for
this work are as under:
1.
2.
3.
4.
Literature Review
(a)
Gating design
(b)
Risering system
(c)
Melting and Pouring
Experimental
(a)
Chemical composition
(b)
Pouring temperature
(c)
Pouring time
(d)
Gating and feeding system
Analysis of Data & Result and discussion
Conclusion
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3. Evaluation Of Blowholes And Shrinkage Defects In Investment Casting Of Low Alloy Steels
V.
METHODOLOGY
The methodology of this project is to study the ordnance factory investment casting plant and their
procedures, collection of data pertaining to low alloy steel castings and study and analysis. Root cause analysis
of defects related to items Housing and Clamp and experimental work changing gating and feeding system of
the two castings. Results and discussion of the out come and conclusion.
VI.
EXPERIMENTATION
In casting processes, there are so many parameters which affect the quality of the product. Any minor
change to some critical parameters leads to defective casting/product. For each type of defect, many causes have
been listed under different categories such as design, molding and pouring/melting related parameters, raw
material etc.
The methodology to achieve optimized process parameters are as given below:
(a) Any defect is selected which is needed to be analyzed. For example, many internal defects (Gas
holes/blow holes, Shrinkage cavities, etc.) largely depends on the melting, feeding system.
(b) The target of process is to achieve “lower casting defects” by adjusting the process parameters.
(c) Select the most significant parameters that cause the defects in casting. These parameters can be
identified by the cause effect diagram.
(d) Plan the experiments as per either design of experiments. Based on the experimental conditions, collect
the data.
(e) Analyze the data. Using statistical tools prepare graphs to determine the preferred levels for each
parameter of the process.
(f) Decide optimum settings of the control parameters. Verify the optimum settings result in the predicted
reduction in the casting defects.
The following parameters have been identified as key Parameter which affecting the radiographic
acceptance standard of the castings:
(a)
(b)
VII.
Pouring temp & Time
Risering & Gating
IMPLEMENTATION OF THE IDENTIFICATION OF KEY PARAMETERS
The data collection and implementation of identified key parameters have been done and results
obtained as per expectations. Brief about implementation of key parameters is as follows:
A. Pouring temp & Time
Table II
Specification
C%
Si%
Mn% Ni%
Cr%
Mo% Cu%
S%
P%
Casting
BS- 3146: Pt-I:
0.35 - 0.20 - 0.40 - 0.40
0.30
0.10
0.30
0.035 0.035
Bracket,
1974 Type
0.45
0.60
1.00
max
max
max
max
max
max
Drg. No.
CLA1C
GCF-TM153 (J)
BS- 3146: Pt-I:
0.450.300.500.40
0.80 - 0.10
0.30
0.035 0.035
Body, Drg.
1974 Type
0.55
0.80
1.00
max
1.20
max
max
max
max
No. GCFTM-174 (J) CLA12A
IS: 10343 Gr
0.350.200.401.30 - 1.00 - 0.200.30
0.045 0.040
Depressor,
0.45
0.60
0.70
1.80
1.40
0.35
max
max
max
Drg No 64C „4D‟
1024 250 F3
VIII.
RESULTS AND DISCUSSION
This data with different pouring temp and pouring time of castings is analyzed in a graphical
representation based on acceptance or rejection of various castings having defects like Gas Porosity and
Shrinkage Porosity/Cavity etc according to acceptance standard of castings.
A. Pouring temperature
The data on pouring of casting „Bracket‟ at various temp and affect on soundness and rejection% is
analyzed and a graph is plotted which is shown at fig 1.
9

4. Evaluation Of Blowholes And Shrinkage Defects In Investment Casting Of Low Alloy Steels
10
9
8
% Rejection
7
6
5
4
Rejection%
3
2
1
0
1540
1550
1560
1570
Pouring temperature deg C
Fig. 1: Pouring temperature v/s % Rejection of casting ‘Bracket’
IX. CONCLUSION
Pouring temperature and pouring time had a vital role in obtaining desired quality of casting Pouring
temp should be kept lower side for castings with thick sections and the alloys freezing with mushiezone at
liquidus and solidus zone. So that casting will be free from shrinkage porosity defects. Pouring temp should be
kept higher side for castings with thinner sections to increase the fluidity of the melt so as to allow the liquid
melt to reach extreme thinner section and far end sections of the casting. So that casting will be free from cold
shut and cold lap defects. Holding time of liquid metal in the furnace or in the ladle should be kept as little as
possible to avoid pickup of harmful gases i.e. Hydrogen, Nitrogen and Oxygen etc. Pouring should be done in
controlled manner smoothly and without any agitation otherwise it will leads to turbulence in the liquid metal
and resulting gas porosity. Sprue/Riser height should be maintained as high as possible in order to maintain
hydrostatic pressure of liquid metal in the Sprue/Riser all over the casting to feed it during solidification and
avoid formation of shrinkage defects. For thin & thick section i.e. complex castings gating system should be
designed in such a way that thinner sections solidify first and then proceed towards thicker section, so that
thicker section acts as hot spot and feeds thinner section and riser should be provided at thicker section so that
desired radiography standards can be obtained with out any difficulty.
REFERENCE
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[4].
[5].
[6].
[7].
[8].
[9].
[10].
[11].
[12].
[13].
[14].
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Ph.D TechForm. Advanced Casting Technology. L.L.C.
Metal Casting Principles & Practice by Prof. TV Ramana Rao.
Feeding system design and evaluation using Temperature Gradient (Feed path) Maps by M. Sutaria, D.
Joshi, M. Jagadeshwar & B. Ravi.
Foseco Foundryman‟s Handbook, by John R. Brown.
Progress in Investment Castings, by Ram Prasad.
The Effect of Pour Time and Head Height on Air Entrainment, by Malcolm Blair, Raymond W.
Monroe, Christoph Beckermann.
Science and Technology of Casting Processes (Progress in Investment Castings) by Ram Prasad.
Riser design, copy right© 2009 ASM International.
Foundry Technology, by OP Khanna.
Significance Of Flaws In Performance Of Engineering Components, by Mr . Vijay V Vesvikar.
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Hardin1and C. Beckermann.
Effect of Shrinkage on Service Performance of Steel Castings, by Richard Hardin and Christoph
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