Part VIII - Papers - Cyclic Martensitic Transformation and the Structure of a Commercial 18 Cr-8 Ni Stainless Steel

One complete cycle, allstenite to martensite to aus-tenite, of Martensitic transformation was induced in a commercial 18 Cr-8 Ni stainless steel. Transmission electron microscopy showed that the reverse transfarmation, martensite to austenite, perpetuated the dense defect substructure of the martensite produced by the direct transformation, austenite to 'martensite, and reproduced the retained austenite matrix orientation. A general precipitation of lathlike M,,C, particles was noted to be present in, and is suggested as cause for, regions of martensite that Izad formed after a complete cycle of transformation. The habit plane of the precipitate particles was found to be within 5 a plane previously established as the habit of nitride precipitates of similar morphology in bcc iron. The direct transformation of austenite (y) to martensite (a) in metastable 300 series stainless steels has received considerable attention in the past,'-8 and the martensite product given special emphasis in recent years,9"*u but little information has been published about the reverse martensitic transformation during which bcc a martensite is converted to fcc y austenite."-" The reversal mechanism and the resulting crystallographic relationship between parent and product austenite are of fundamental concern. The reversal product, fully austenitic, and hereafter referred to as reversed austenite, exhibits a defect structure far different from its virgin counterpart. Some studies have been reported in which the structure of reversed austenite in iron nickel alloys is compared to annealed austenite with respect to austenite strengtheningM and Stabilization.' The martensite in these Fe-Ni alloys, however, shows a basic difference from that produced in stainless steels. Structurally, the Fe-Ni martensite is profusely twinned," while the (low-carbon) Fe-Cr-Ni martensite contains a high density of tangled dislocations and is apparently twin-free.I0 reediis" has recently shown in an Fe-16 Cr-12 Ni stainless alloy that this typical martensitic defect structure and the reverse transformation directly influence the structure of the reversal product. He found that much of the reversed austenite had returned to the orientation of the virgin austenite and that fine twins were formed in the austenite after the reverse transformation. This investigation was undertaken in order to describe as completely as possible the structural changes produced by a cycle of transformation in a typical 18-8 stainless steel. EXPERIMENTAL PROCEDURE The 301 stainless steel under investigation (17.44 Cr, 7.75 Ni, 0.80 Mn, 0.60 Si, 0.40 Mo, 0.40 Cu, 0.08 C, 0.03 N, bal Fe)was received as 0.060-in.-thick