EFFECT OF CRYOFORMING OF AUSTENITIC CR NI-STEELS AT 77 K ON MARTENSITIC-TRANSFORMATION AND WORK-HARDENING CHARACTERISTICS

The effect of cryoforming at 77 K on the flow and work-hardening characteristics was investigated considering the martensitic transformation behaviour in austenitic Cr Ni steels with different nickel contents. The test steels can be divided into two groups relating to the flow and work-hardening characteristics and martensitic transformation behaviour at 77 K. The first group comprises steels with less than 16 % nickel, the second group those with more than 16 % nickel. The flow curves of the first-group steels show two inflection points on the basis of gamma --> alpha'-transformation. alpha gamma'-martensite is observed and epsilon- and alpha(epsilon)'-martensite too. The flow curves of the second-group steels do not show any inflection points. The gamma --> alpha'-martensitic transformation is not induced. epsilon- and alpha epsilon'-martensite are provable by light and scanning microscopy. The stress-strain intervals were determined for the individual martensite transformations at 77 K in the test steels. They are dependent from the nickel content. The stress which specifies the first inflection point on the flow curve and the minimum of the work-hardening rate, respectively, characterizes the stress for initiating the deformation-induced alpha gamma'-martensite formation. Transformation of the austenite to alpha' martensite will end in achieving a stress of 1200 to 1400 MPa, i.e. in achieving the second inflection point of the flow curve and the maximum of the work-hardening rate, respectively. The stress interval is not dependent from the nickel content.