IN-SITU ANALYSIS OF REVERTED AUSTENITE IN SUPERMARTENSITIC STAINLESS STEEL WELD DEPOSITS

Abstract

To increase the toughness, ductility and resistance to localized corrosion in supermartensitic stainless steel weld deposits, suitable post weld heat treatments are required, since these properties are controlled by the microstructure. In this sense, the austenite transformation or stability is a key issue. The aim of this work is to analyze “in-situ” the evolution of austenite during the heating, maintenance and cooling in post weld inter-critical heat treatment in a supermartensitic stainless steel weld deposit, by means of a thermomechanical simulator integrated on a synchrotron X-ray diffraction line. All weld metal coupon was welded with a semi-automatic process. On samples extracted from the welded coupon, dilatometry measurements were performed at different heating rates (1, 10 and 100 °C/s) in the thermomechanical simulator, in order to determine the critical transformation temperatures. To study phase transformation during the apply post weld heat treatment cycles (heating to 665 °C at 1 °C/s, maintenance during 15 min and cooling to room temperature at 2 °C/s), in-situ synchrotron X-ray diffraction measurements were carried in the facilities of National Laboratory Synchrotron Light (Campinas, Brasil). The microstructural evolution was discussed in terms of volumetric phase fractions, micro-deformation and crystallite size. The techniques used make it possible to detect the critical transformation temperatures, phase transformations and their kinetics, monitoring particularly the austenite evolution during the post weld heat treatments cycle. At the inter-critical temperature 71% of austenite was formed, after 15 min of permanence. Finally, at room temperature 29% of stable reverted austenite was obtained.