The Strengthening and Substructures in Cr-Ni Steel by Ausforming

Abstract

The effect of lattice defects in the deformed austenite of Cr-Ni steel on the strengthening by ausforming was investigated by means of the micro-hardness test, and optical and transmission electron microscopy.
(1) The micro-hardness test showed that the austenite strain-hardened remarkably by up to about 20%-ausforming, beyond this, it reached a constant hardness level. The ausformed martensite did not harden so much by up to 20%-ausforming, beyond this, it hardened rapidly. (2) Numerous fine parallel and crossed striations in the 10%-ausformed austenite were observed by optical microscopy, and they were confirmed as ε-phase and deformation twins by transmission electron microscopy. It was observed that the martensite plate originated from these lattice defects and formed along or across them. The orientation relationships among austenite, ε-phase and martensite were (111)_γ\varparallel(0001)_ε\varparallel(011)_α′ and [10\bar1]_γ\varparallel[11\bar20]_ε\varparallel[1\bar11]_α′. (3) In the case of the martensitic transformation from the austenite which contained the ε-phase, both austenite and ε-phase could be transformed to a martensite, and the ε-phase would act as a transitional phase in the martensitic transformation without being inherited into the martensite as a kind of lattice defects of martensite. (4) The deformation twins in the austenite have a possibility of leaving the {110}_α interface in martensite. (5) In the case of 30%-ausforming, the austenite did not contain the ε phase and deformation twins but dislocations, and the martensite contained only dislocations. Both martensite and austenit were high in dislocation density. (6) The non-ausformed martensite had dislocations with only one kind of Burgers vector, and the ausformed martensite had dislocations with two or more kinds of Burgers vector. The dislocation density was very high in the ausformed martensite. (7) From the above results, it can be considered that lattice defects contributing to the increasing of dislocation density and to the strengthening of ausformed martensite are dislocations in the ausformed austenite.