Aging behavior of high nitrogen austenitic stainless steel

Time-temperature-precipitation (TTP) characteristics of high nitrogen austenitic Fe-18Cr-18Mn-2Mo-0.9N stainless steel (all in weight percent) were investigated in terms of precipitation behavior, and crystallographic features of the second phases. The precipitation reactions can be categorized into three stages, i.e. (i) high temperature region (above 950 degrees C) : mainly coarse grain boundary (interganular) Cr2N; (ii) nose temperature region : intergranular Cr2N -> cellular Cr2N -> intragranular Cr2N + sigma (sigma); (iii) low temperature region (below 750 degrees C) : intergranular Cr2N -> cellular Cr2N -> intragranular Cr2N + sigma + chi (chi) + M7C3 carbide, respectively. Based on the analyses of selected area diffraction (SAD) patterns, the crystallographic features of the second phases were determined : (i) Cr2N (trigonal, P (3) over bar 1m) - [001](gamma) [1 (1) over bar 00](Cr2N), (1 (1) over bar1)(gamma) (0001)(Cr2N);(ii)chi(b.c.c., 1 (4) over bar 3m) - K-S orientation relationship; (iii) M7C3 (orthorhombic, Pnma) - [001](gamma) [101](M7C3)(111)(gamma) ((1) over bar 31)(M7C3).