Quenching and partitioning (Q&P) processing of a (C+N)-containing austenitic stainless steel

Abstract

This paper presents a systematic study of quenching and partitioning (Q&P) processing of a (C + N)-containing austenitic stainless steel with a special focus laid on the influence of partitioning conditions on the mechanical properties and its comparison with C-containing Q&P-treated stainless steels. Similar to C-containing stainless steels, martensite phase in the studied steel exhibited pre-precipitation phenomena such as interstitial segregation to defects below 250 °C and precipitation reactions such as the formation of paraequilibrium M3C carbides (M representing Fe and substitutional elements) above 300 °C. The interstitial partitioning between martensite and austenite and the dependence of mechanical properties on the partitioning conditions resembled those of C-containing stainless steels. With increasing partitioning temperature, interstitial enrichment in the austenite and ductility were enhanced while tensile strength decreased. Similar observations were made upon prolonged partitioning at 250 °C. In contrast, the ductility of steels partitioned at 350 °C and 450 °C barely changed with partitioning time. For partitioning at 450 °C, this was because most of the interstitial partitioning at 450 °C took place within the first 3 min of partitioning. For partitioning at 350 °C, the interstitial enrichment of fine austenite regions was completed rapidly as well and the later interstitial partitioning into coarse austenitic regions gave rise to little increase in austenite stability due to their large size. The optimal product of ultimate tensile strength and total elongation, 47 GPa%, was obtained after partitioning at 450 °C for 3 min. The sufficiency of a short partitioning to obtain superior mechanical properties is of great practical significance in the context of growing global concerns about energy demand and climate change.