The Press Forming Behaviour of Austertic and Metastable Stainless Steels

Abstract

The present state of knowledge of austenite stability in 18/8 type nickel-chromium steels, transformation of austenite to a' martensite, and the deformation and press formability of austenitic stainless steels has been reviewed. Experimental work is described for a range of chromium=nickel stainless steels of different austenite stability used in press forming. The steels were processed to give a wide range of properties at the final gauge and the effects of prior deformation and of prior transformation on mechanical properties and press formability were assessed with respect to austenite stability. An analysis has also been made of the complex true stress~true strain relationships obtained with steels of this type for the three stress systems, un.taxial tension, biaxial tension, and plane strain compression, and the results related to press formability. The results showed that transformation to α’ martensite during deformation is beneficial to stretch formability and to those mechanical properties governed by strength, but only beneficial to deep drawing when its presence does not impair ductility. The presence of α' martensite was shown to resist localised necking. Unlike mild steel, the drawing capacity of stainless steel is not influenced markedly by the anisotropy ratio R. The stress-strain curve for stainless steels does not obey the Ludwik relationship and an alternative function has been derived. α' martensite formation markedly affects the shape of this curve and this is reflected in the constants governing the derived function. However, the strain level at which a change in the slope of the stress-strain curve occurs, does not correspond to the strain level at which α' martensite is initiated and an alternative theory is offered. Using the derived function, a theoretical prediction of stress-strain data, and various mechanical and press forming properties can be obtained from knowledge of the austenite stability of the steel.