AUSTENITE REVERSION DURING TEMPERING OF MARTENSITIC-FERRITIC STAINLESS STEEL

摘要

Martensitic-ferritic stainless steels (MFSS) have been developed by Vallourec for oil and gas application. Seamless tubes in this material are produced by quenching and tempering in order to assure yield strength higher than 110 ksi and better SSC resistance than supermartensitic stainless steels. During tempering, some amount of austenite can nucleate and grow inside the martensite. In the subsequent cooling, part of this austenite may transform into untempered martensite, while the other part remains as "reversed austenite". In order to improve the material and process design is essential to understand the microstructure evolution during the tempering process. In the present work, the austenite reversion was studied through in-situ high temperature XRD and the specimens were further characterized using SEM and TEM. Partitioning of elements during cooperative formation of austenite and Cr-rich phases (M_(23)C_6 carbides and Chi-phase) were evidenced through EDX maps. The factors that control this partitioning process are discussed.