Fracture and the formation of sigma phase, M 23 C 6 , and austenite from delta-ferrite in an AlSl 304L stainless steel
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INTRODUCTION
MANY austenitic stainless steels solidify by the formation of delta-ferrite. Figure 1 shows that in AISI 304 austenitic stainless steels, delta-ferrite should be completely transformed to austenite at temperatures below 1200 ~ However, in castings, weld metal, and wrought heavy sections of austenitic stainless steels, delta-ferrite persists as a component of the microstructure, because of rapid cooling or insufficient hot working. Small amounts of primary delta-ferrite are desirable in the fusion zone of welded austenitic stainless steels, because the ferrite getters phosphorus and sulfur and thereby prevents hot tearing or cracking.J2] On the other hand, deltaferrite is prone to decomposition to sigma phase if exposed to temperatures in the range of 600 ~ to 900 ~ The formation of sigma phase adversely affects ductility, toughness, and corrosion resistance of the austenitic stainless steels.J3-5[ For example, cracking during forging of a 21Cr-6Ni-9Mn austenitic stainless steel has been attributed to sigma phase formation from deltaferrite.t6~ Sigma phase has a complex, body-centered tetragonal structure with 30 atoms per unit cell, and in Fe-Cr alloys it forms around the equiatomic Fe-Cr composition. I7~Thus, sigma phase forms readily in austenitic stainless steels, such as AISI 310, which contain 25 mass pct chromium. tSl In these steels, chromium partitions to ferrite in amounts well above the nominal alloy content, and this concentration of chromium enhances sigma-phase formation in the ferritic areas, tS[ C.C. TSENG, Graduate Research Assistant, S.W. THOMPSON, Associate Professor, and G. KRAUSS, John Henry Moore Professor, are with the Department of Metallurgical and Materials Engineering, Colorado School of Mines, Golden, CO 80401. Y. SHEN, formerly with the Department of Metallurgical and Materials Engineering, Colorado School of Mines, is deceased. M.C. MATAYA, Associate Scientist, is with EG&G Rocky Flats, Golden, CO 80401. Manuscript submitted July 7, 1993. METALLURGICALAND MATERIALS TRANSACTIONS A
Depending on composition, there have been a number of mechanisms proposed to explain the formation of sigma phase in austenitic stainless steels. Barcikt91 showed that sigma phase could form directly from austenite in austenitic steels with high chromium contents, provided that the carbon concentration was below a critical level. However, several investigations showed that sigma phase formed only at M23Ct/austenite or delta-ferrite/austenite interfaces, c3,4,~~ For example, Beckittt41 showed that a lamellar/cellular structure of M23C 6 and austenite initiated at delta-ferrite/austenite interfaces. Subsequently, sigma phase formed in association with the austenite present in these lamellar aggregates. Gray et al. ,[31 in an AISI 304 stainless steel, showed evidence for the transformation of delta-ferrite to a mixture of sigma phase and austenite, but reported no M23C6 formation. The AISI 304 stainless steel plate examined by Gray et al. contained about 5 vol pct of delta-ferrite stringers and
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