MICROSTRUCTURAL ANALYSIS OF HOT TEARS IN A WCB STEEL VALVE BODY

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1 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

2 MICROSTRUCTURAL ANALYSIS OF HOT TEARS IN A WCB STEEL VALVE BODY C.E. Bates 1 and John Griffin 2 ABSTRACT Several hot tear defects were microstructurally examined. The cracks occurred on a flanged section of a WCB steel valve body and appeared to initiate on the inside surface and propagate through the wall and flange. The steel had normal phosphorus and sulfur concentrations and no detectable 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 contraction 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 Institute. 2 John A. Griffin is an Associate Engineer of the Metals Section for Southern Research Institute. 1

3 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 sufficient 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) The mechanism suggest that in the final stages of solidification, thermal contraction 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 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 2

4 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 temperatures. In general, using refractories with lower coefficients of thermal expansion, such as zircon and chromite, can be used to reduce, but not eliminate, 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: 3

5 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. The stress may exceed the yield strength of steel. If the yield strength is exceeded, the metal begins to flow and may exceed the plasticity limit. The solid shell tears at its weakest point in regions containing liquid films or in regions containing micro- or macro-shrinkage. 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, liquid films are not necessary for cracks to occur. Tearing can also proceed 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. In order to minimize 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. Reducing the mold density is one of the most practical methods of minimizing hot tear tendencies. 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. 4

6 BACKGROUND INFORMATION Three sections from a valve body exhibiting hot tears in a flange were submitted for examination. A schematic illustration of the valve body is shown in Figure 1. Sample No. 1, from the barrel of the body, was submitted for microstructural 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. A sample for metallographic examination was removed from the corner of the section as marked in Figure 2. 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. 5

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11 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 through areas of microshrinkage. Hot cracks extending through microshrinkage to the surface of the casting are shown in Figures 6A and 6B at a magnification 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. These regions of alignment were probably primary austenite grain bound- aries, 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 sulfides are angular Type III sulfides associated with the deoxidation products. Angular Type III sulfides are expected based on the residual aluminum content of 0.055% coupled with the presence of titanium and vanadium. 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 10

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18 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 specimens. 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 solidificat 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 17

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23 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. In these 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. 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. 22

24 REFERENCES 1. Pellini, W.S., "Strain Theory of Hot Tearing", Foundry, 80 (11), , 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), , (July, 1965). 23

25 USE OF THIS REPORT AND INFORMATION CONTAlNED THEREIN Publicity This report and the information contained therein is the property of the individual or organization named on the face hereof and may be freely distributed in its present form. We do ask, however, that no advertising or publicity matter, having or containing any reference to Southern Research Institute, shall be made use of by anyone, unless and until such matter shall have first been submitted to and received the approval in writing of the Institute. (The Institute does not usually approve any type of endorsement advertising.) Limitation of Liability Southern Research Institute has used its professional experience and best professional efforts in performing this work. However, the Institute does not represent, warrant or guarantee that its research results, or product produced therefrom, are merchantable or satisfactory for any particular purpose, and there are no warranties, express or implied, to such effect. Acceptance, reliance on, or use of such results shall be at the sole risk of Sponsor. In connection with this work, Southern shall in no event be responsible or liable in contract or in tort for any special, indirect, incidental or consequential damages, such as, but not limited to, loss of product, profits or revenues, damage or loss from operation or nonoperation of plant, or claims of customers of Sponsor. Report No.: SRI-MMER F To: Steel Founders' Society of America Cast Metals Federation Building 455 State Street Des Plaines, IL Date: September,

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STEEL FOUNDERS' SOCIETY OF AMERICA TECHNICAL SERVICE REPORT #108 CLEANLINESS OF CONVENTIONALLY DEOXIDIZED AND CALCIUM TREATED LOW CARBON STEEL Published by the STEEL FOUNDERS' SOCIETY OF AMERICA Malcolm