Evaluation Of Blowholes And Shrinkage Defects In Investment Casting Of Low Alloy Steels

8/10/2019 Evaluation Of Blowholes And Shrinkage Defects In Investment Casting Of Low Alloy Steels

1/4

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

7

Evaluation Of Blowholes And Shrinkage Defects In Investment

Casting Of Low Alloy Steels

Smt R.Sri Rama DeviAssociate 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 solidificationof 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 thatcontrol 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. INTRODUCTIONInvestment 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 theend 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 PROJECTOrdnance 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-TM-

199(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

Table I: Acceptance StandardsDefects Acceptance level Acceptable Standard

Gas porosity up to LevelV

As per ASTM E-192.Slag inclusions /foreign materials up to LevelV

Shrinkage Cavity/Porosity up to LevelIII

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


2/4

Evaluation Of Blowholes And Shrinkage Defects In Investment Casting Of Low Alloy Steels

8

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 CASTINGSIt is a fact of life that any material or product produced after undergoing various manufacturingprocesses 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 influenceon 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 3mm2, 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 PROJECTThis 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 ofBlowholes and Shrinkage defects in low alloy steels made through investment castings. The areas selected for

this work are as under:

1. Literature Review

(a) Gating design

(b) Risering system

(c) Melting and Pouring

2. Experimental

(a) Chemical composition

(b) Pouring temperature

(c)

Pouring time

(d)

Gating and feeding system

3. Analysis of Data & Result and discussion4. Conclusion


3/4


9

V. METHODOLOGYThe 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 analysisof 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. EXPERIMENTATIONIn 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, collectthe 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) Pouring temp & Time

(b) Risering & Gating

VII.

IMPLEMENTATION OF THE IDENTIFICATION OF KEY PARAMETERSThe 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

Casting Specification C% Si% Mn% Ni% Cr% Mo% Cu% S% P%

Bracket,

Drg. No.

GCF-TM-

153 (J)

BS- 3146: Pt-I:

1974 Type

CLA1C

0.35 -

0.45

0.20 -

0.60

0.40 -

1.00

0.40

max

0.30

max

0.10

max

0.30

max

0.035

max

0.035

max

Body, Drg.

No. GCF-

TM-174 (J)

BS- 3146: Pt-I:

1974 Type

CLA12A

0.45-

0.55

0.30-

0.80

0.50-

1.00

0.40

max

0.80 -

1.20

0.10

max

0.30

max

0.035

max

0.035

max

Depressor,

Drg No 64C

1024 250 F3

IS: 10343 Gr

4D

0.35-

0.45

0.20-

0.60

0.40-

0.70

1.30 -

1.80

1.00 -

1.40

0.20-

0.35

0.30

max

0.045

max

0.040

max

VIII. RESULTS AND DISCUSSIONThis 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.


4/4


10

0

1

2

3

4

5

6

7

8

9

10

1540 1550 1560 1570

Pouring temperature deg C

%R

ejection

Rejection%

Fig. 1: Pouring temperature v/s % Rejection of casting Bracket

IX.CONCLUSIONPouring 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 bekept higher side for castings with thinner sections to increase the fluidity of the melt so as to allow the liquidmelt 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 andavoid 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[1].

Characterization and correction of casting defects. By Timothy L. Donohue and Dr. Helmut F. Frye,

Ph.D TechForm. Advanced Casting Technology. L.L.C.

[2]. Metal Casting Principles & Practice by Prof. TV Ramana Rao.

[3]. Feeding system design and evaluation using Temperature Gradient (Feed path) Maps by M. Sutaria, D.

Joshi, M. Jagadeshwar & B. Ravi.

[4]. Foseco Foundrymans Handbook, by John R. Brown.

[5]. Progress in Investment Castings, by Ram Prasad.

[6]. The Effect of Pour Time and Head Height on Air Entrainment, by Malcolm Blair, Raymond W.Monroe, Christoph Beckermann.

[7]. Science and Technology of Casting Processes (Progress in Investment Castings) by Ram Prasad.

[8]. Riser design, copy right 2009 ASM International.

[9].

Foundry Technology, by OP Khanna.[10]. Significance Of Flaws In Performance Of Engineering Components, by Mr . Vijay V Vesvikar.

[11]. Simulation of the Mechanical Performance of Cast Steel with Porosity: Static Properties, by R.A.Hardin1and C. Beckermann.

[12].

Effect of Shrinkage on Service Performance of Steel Castings, by Richard Hardin and Christoph

Beckermann.

[13]. The effect of internal shrinkage discontinuities on the fatigue and impact properties of cast steel

sections, by Charles w. Briggs.

[14]. Principles of Metal Casting, by Richard W Heine, Carl R Loper and Philip C Rosenthal.

Documents

Evaluation Of Blowholes And Shrinkage Defects In Investment Casting Of Low Alloy Steels