STEEL FOUNDERS' SOCIETY OF AMERICA TECHNICAL SERVICE REPORT #l05

MICROSTRUCTURAL ANALYSIS OF HOT TEARS IN A WCB STEEL VALVE BODY

Published by the

STEEL FOUNDERS' SOCIETY OF AMERICA

Mr. Malcolm Blair Technical and Research Director

September, 1990

MICROSTRUCTURAL ANALYSIS OF HOT TEARS IN A WCB STEEL VALVE BODY

C.E. Bates1 and John Griffin2

ABSTRACT

Several hot tear defects were microstructurally examined. The cracks

occurred on a flanged section of a WCB steel valve body and appeared to initi-

ate on the inside surface and propagate through the wall and flange.

The steel had normal phosphorus and sulfur concentrations and no detect-

able Type II sulfides. It was concluded that the hot cracks were a result of

core restraint aggravated by the presence of microshrinkage caused by marginal

risering where the flange joined the main valve body. The thermal mass in

this area created a hot spot which caused the core to expand. Core expansion

caused plastic deformation in the flange, and the restrained thermal contrac-

tion led to the hot cracks.

Representative microstructures of hot cracks are presented.

1 C.E. Bates is the Head of the Metals Section for Southern Research

2 John A. Griffin is an Associate Engineer of the Metals Section for Institute.

Southern Research Institute.

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INTRODUCTION

Hot tear defects are jagged ruptures that occur at elevated temperatures

in steel castings. Hot tears usually initiate at the surface of a casting and

sometimes extend completely through the section. Fundamentally, hot tears are

caused by mold or core restraint preventing the steel from freely contracting

as it cools during and after solidification. If the mold material has suffi-

cient strength or density that it cannot accommodate the thermal contraction

of the steel, tensile stresses can develop sufficient to tear the steel.

Hot tears usually occur in hot spots in the casting associated with

section thickness changes, gates, or near risers where some portion of the

casting cannot freely contract. The lower strength, higher temperature metal

has a greater tendency to fracture under the thermal stress conditions,

Because of the high temperature and the oxidizing conditions that exist in the

mold at the time hot tear defects form, the surfaces of hot tears are usually

oxidized and decarburized.

Mechanism of Hot Tearing

The stress-strain mechanism proposed by Pellini, Bishop, and Akerlind is

the most widely accepted theory of hot tear formation in castings.(1-2)

mechanism suggest that in the final stages of solidification, thermal contrac-

tion of the steel begins and hot tears occur under conditions where the mold

or core restrains the contraction. Cores may actually be expanding as the

steel solidifies. The contraction of the casting is resisted by the core

which causes tensile stresses to develop in the solid metal.

The

The hotter areas of the casting may contain liquid indendritic or grain

boundary films. Separation of solid areas begins at low tensile stresses in

these films. When the strain in the solid material exceeds the ductility

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limit, separation occurs resulting in a tear in the casting visible at room

temperature.

Any condition that extends the metal solidification temperature range or

adds restraint to thermal contraction aggravates hot tearing. Hot tearing is

also aggravated by micro- or macro-shrinkage resulting from inadequate

risering. Hot tears initiating at the surface propagate through shrinkage

areas to form extensive internal crack networks.

Molding conditions that aggravate hot tearing include ramming to high

densities and the use of binders that have high strengths at elevated tempera-

tures. In general, using refractories with lower coefficients of thermal

expansion, such as zircon and chromite, can be used to reduce, but not elimi-

nate, hot tears. These refractories also have a higher thermal diffusivity

which causes more rapid solidification and after a given time, provides a

thicker solid shell of metal that may be capable of resisting the tearing

stresses.

Most hot tears have a dendritic appearance. A dendritic surface is

interpreted as indicating that the hot crack was formed by the separation of

solid metal regions through liquid or mushy grain boundary films. Conditions

that produce more segregation generally produce more hot tears. For example,

the presence of high sulfur and phosphorus concentrations causes liquid or

mushy films to form at the grain boundaries because steel enriched in these

elements has a lower solidification temperature compared to the bulk casting

composition.

Van Eeghem and Desy (3) proposed that a hot tear in cast steel proceeds

in the following way:

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1. Immediately after casting, a skin of metal is formed by cooling and

solidification of the metal in contact with the mold wall.

2. As the casting contracts during cooling, and as cores or certain

portions of molds expand on heating, tensile stresses are developed in

the solid metal.

If the yield strength is exceeded, the metal begins to flow and may

exceed the plasticity limit.

point in regions containing liquid films or in regions containing

micro- or macro-shrinkage.

The stress may exceed the yield strength of steel.

The solid shell tears at its weakest

3. As the solid skin becomes thicker, the tears may continue to prograte,

and their opening at the casting surface may become larger.

While hot tear theories suggest that liquid films aggravate hot tearing,

Tearing can also proceed liquid films are not necessary for cracks to occur.

after solidification is complete if the core restraint to metal contraction is

sufficiently great.

Mold or core density is a major factor in hot tearing because increasing

density usually produces higher compression strength.

stresses developed in a casting as it contracts during and after solidifica-

tion, the bulk density of the mold or core should be no higher than required

to maintain the dimensional stability of the casting.

density is one of the most practical methods of minimizing hot tear tenden-

cies. Since green sand molds are generally rammed to a lower density than

resin bonded molds, green sand molds are generally less prone to causing hot

tears.

In order to minimize

Reducing the mold

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BACKGROUND INFORMATION

Three sections from a valve body exhibiting hot tears in a flange were

submitted for examination.

shown in Figure 1.

A schematic illustration of the valve body is

Sample No. 1, from the barrel of the body, was submitted for micro-

structural and bend tests, and did not exhibit any tears. Specimen No. 2 was

cut from a torn region at the juncture of the flange with the body. Specimen

No. 3 was from the flange.

No details of the gating or risering on the casting were available.

The casting was poured from an 8800 lb ladle of WCB steel. The steel had

been dead melted (no oxygen blow), tapped into a pouring ladle, and deoxidized

during tapping with 4 lb aluminum, 10 lb titanium, and 17 lb Stelogen. The

casting was poured into a mold in which both the mold and core were prepared

from furan bonded silica sand.

The chemical composition of the heat is presented in Table 1.

RESULTS AND DISCUSSION

Sample No. 1 was cut from the barrel of the valve body several inches

from the hot tears and is illustrated in Figure 2.

metallographic examination was removed from the corner of the section as

marked in Figure 2.

A sample for

The microstructure of the section at magnifications of 100 and 200x is

illustrated in Figure 3. The microstructures are typical of unrefined steels.

Some of the oxide particles appear to be aligned along primary austenite grain

boundaries, as seen in Figure 3A, and there were some small patches of

microshrinkage along primary austenite grain boundaries as illustrated in

Figure 3B.

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A cracked region of the valve body taken from the junction of the flange

with the main body barrel is illustrated in Figure 4A. The hot crack after

complete fracture is illustrated in Figure 4B. Large parallel cracks are

visible on the interior of the specimen as well as on the exterior of the

specimen, as illustrated in Figure 4A. Typical hot cracks occurring in the

junction are illustrated in Figure 5 at a magnification of 10x.

At higher magnifications, some of the hot cracks can be seen propagating

Hot cracks extending through microshrinkage through areas of microshrinkage.

to the surface of the casting are shown in Figures 6A and 6B at a magnifica-

tion of 200x.

The microstructure of the steel near a hot crack is illustrated in Figure

7 at magnifications of 100 and 200x. Both oxides and sulfides are visible in

the microstructure, and there is some tendency for oxide and sulfide align-

ment.

aries,

These regions of alignment were probably primary austenite grain bound-

The visible oxides are deoxidation products formed with the addition of

complex deoxidizers to the metal during tapping and are relatively small and

angular in shape.

the deoxidation products.

residual aluminum content of 0.055% coupled with the presence of titanium and

vanadium.

The sulfides are angular Type III sulfides associated with

Angular Type III sulfides are expected based on the

The appearance of large fractures extending to the surface of the flange

in the hot tear sample No. 3 is illustrated in Figure 8A. Hot tears on the

interior of the specimen are illustrated in Figure 8B at a magnification of

25x.

Micrographs of hot tears in this specimen at magnifications of 25 and

200x are illustrated in Figures 9A and 9B, respectively. The hot tears appear

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to be propagating along primary austenite grain boundaries and through

microshrinkage associated with these grain boundaries. Some grain boundary

shrinkage is evident in Figure 9B.

Bend Tests

Three bend specimens were cut from the valve body for bend testing. The

specimens were 0.25 X 0.75 by 3 inches long. The specimens were loaded in

three point bending as illustrated in Figure 10.

One specimen fractured during bending and the fracture surfaces are

illustrated in Figure 11. The dark portion of the fracture was microshrinkage

which covered a substantial portion of the fracture surface. Typical

interdendritic microshrinkage areas from the fracture are illustrated in SEM

photographs in Figures 12A and 12B.

Bending was stopped when obvious cracks developed in two specimens as

illustrated in Figure 13. The bend angles were 103 and 115° on these speci-

mens. The cracks appeared to initiate at interdendritic shrinkage areas.

SUMMARY AND CONCLUSIONS

The hot tears observed in the sections of WCB valve body submitted for

examination resulted from core restraint to metal contraction after solidifi-

cat ion.

Oxides and Type III sulfides were found with microshrinkage along primary

austenite grain boundaries. No Type II sulfides, sometimes associated with

hot tearing, were found. Microshrinkage associated with marginal risering

aggravated hot tearing by producing an easy path for crack propagation along

grain boundaries.

The hot cracks developed because the casting had the highest thermal mass

at the junction of the flange barrel with the main body barrel. The high

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thermal mass kept this area hot, and when thermal restraint developed as the

casting cooled, the cracks formed in the hot, reduced strength material.

Microshrinkage was also present in other areas of the casting where

cracking did not occur such as in the barrel of the valve body.

areas, the body core restraint was not sufficiently high to cause hot cracks,

but the microshrinkage did contribute to cracking during three point bend

tests at room temperature.

In these

The best method for eliminating hot tearing would consist of reducing the

core density to provide some strain accommodation during cooling, adding a

combustible material to the core for the same reason, and providing a heavy

core wash to prevent metal penetration.

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REFERENCES

1. Pellini, W.S., "Strain Theory of Hot Tearing", Foundry, 80 (11), 124-133, 192, 194, 196, 199 (November, 1952).

2. Bishop, H.F., C.G. Ackerlind, and W.S. Pellini, "Metallurgy and Mechanics

3. Van Eeghem, J., and A. Desy, "A Contribution to Understanding the Mechanism of Hot Tearing of Cast Steel", Modern Castings, 48(1), 100-109, (July, 1965).

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Report No.: SRI-MMER-90-896-7048-002.F

To: Steel Founders' Society of America

This report and the information contained therein is the property of the dis-

(The Institute does not usually approve any type of endorsement

Southern Research Institute has used its professional experience and best

Cast Metals Federation Building 455 State Street Des Plaines, IL 60016

Date: September, 1990

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