Isothermal and Cyclic Aging of 310S Austenitic Stainless Steel
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I.
INTRODUCTION
AFTER standard processing routes, stainless steels of the 310 series show a metastable austenitic (c) matrix with some residual high temperature d ferrite and M23C6 carbides. These carbides form at the solidification end and are generally called primary carbides. During service at intermediate temperature, formation of r phase and secondary M23C6 carbides is observed. More precisely, precipitation of r phase has been reported after aging in the temperature range from 773 K to 1173 K (500 C to 900 C).[1–4] 1138 K (865 C) has been identified as the most critical temperature for r phase formation[5] and certainly corresponds to the nose of the Time-Temperature-Transformation curve. Secondary M23C6 carbides start precipitating below 1323 K (1050 C).[6,7] The r phase is one of the most frequently observed intermetallic compounds in stainless steels and is promoted by alloying elements such as chromium.[1] r phase formation thus induces a ductility loss and a decrease in corrosion resistance. This intermetallic phase belongs to Topologically Close-Packed (TCP) phases[8] characterized by close-packed layers of atoms that are separated from one another by large interatomic distances. This particular structure gives r phase a low ductility and high hardness at 940 HV[9,10] which lead to numerous industrial issues by drastically decreasing the lifetime of austenitic stainless steels.[11,12]
CORALIE PARRENS is with the CIRIMAT, Universite´ de Toulouse, INP-ENSIACET, 4 alle´e Emile Monso - BP44362, 31030 Toulouse, France, and also with Safran Landing Systems, Site de Bidos, 9 Rue Guynemer, 64400 Bidos, France. Contact e-mail: [email protected] JACQUES LACAZE, BENOIT MALARD, and DOMINIQUE POQUILLON are with the CIRIMAT, Universite´ de Toulouse, INP-ENSIACET. JEAN-LUC DUPAIN is with Safran Landing Systems, Site de Bidos. Manuscript submitted August 12, 2016. METALLURGICAL AND MATERIALS TRANSACTIONS A
In wrought 310S stainless steel and for short aging time, long bands of r phase are firstly formed from residual d-ferrite.[13] This d fi r transformation seems to be nucleation controlled.[14] As aging time is increased, precipitation of r phase then proceeds directly from austenite, mainly at grain boundaries and especially on triple grain boundaries,[15] but also at twin boundaries and nearby carbides.[16] However, r phase formation from austenite requires much longer heat treatments than does the formation from ferrite[17] though its precipitation kinetics increases gradually with hot rolling ratio.[18] Precipitation of r phase from austenite in austenitic stainless steels has also been reported to be nucleation controlled,[14] further growth being controlled by diffusion of chromium and other r-forming elements.[14] Other alloying elements such as silicon have also been reported to favor r phase precipitation.[19] Secondary M23C6 carbides precipitate below 1323 K (1050 C)[6,7] and usually show an increased number density with aging time. With increasing exposure time, M23C6 carbides first precipitate
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