The Thermo-Mechanical Response to a General Loading Path of a Martensitically Transforming Steel

Abstract

An extensive experimental program on a 9% Ni, 12% Cr, 2% Mo steel is introduced. This material transforms from the austenitic (γ) phase into the martensitic (α′) phase at a low temperature level around 150°C upon cooling on air only, thus making it especially suitable for testing purposes since it exhibits practically no creep effects during and after transformation (Fischer et al., 1996). Dilatometric tests are carried out for two types of specimens (longitudinal specimens (LSs) with a rolling texture and radial specimens (RSs)). Interestingly, the dilatation loops do not close after cooling down to room temperature. For an increasing annealing temperature the gap becomes smaller and closes for RSs. It turns out that the dilatometric loops close for preloaded specimens, pointing to an initial backstress in the material in the same order of magnitude as the load stress so it must not be neglected. Monitoring the martensite start (M_s ) and finish temperature (M_f ) for different loading conditions and stress levels reveals a strong influence of the type of loading (tension, shear, compression) on M_s . The overall yield stress as a function of temperature is noticeably disturbed immediately after transformation starts. As the temperature approaches M_f the composite effect in the α′-γ region prevails. Experiments including mixed, nonproportional loading paths for cooling as well as at constant temperature have been performed. The orientations of the martensitic variants account for the different deformation behavior for different loading types but equal global equivalent stresses (see also Fischer et al., 2000a). A comprehensive micromechanical modeling concept is presented based on the numerical implementation of a transformation condition (Fischer and Reisner, 1998) governing the variant selection into a finite element algorithm (see Reisner et al., 1998). Finally, the issue of an improved constitutive law for the TRIP strain rate has been tackled. A novel approach for an evolution law including the backstress has first been presented by Videau et al. (1996) and has later been refined by Fischer et al. (2000b).