Precipitation reactions in the coarse grained heat affected zone of high grade (0 . 1%Ti) and medium grade (Ti free) 13%Cr supermartensitic stainless steels have been studied. During multipass welding, a high density of (Ti,Cr)C and (Cr,Fe,Mo,Si) 23 C 6 particles formed at prior austenite grain boundaries in high and medium grade steels respectively. Chromium depleted zones along prior austenite grain boundaries were found in the medium grade steel, and these zones are assumed to be the reason for the well known sensitivity to intergranular corrosion of these steels. After post-weld heat treatment the depletion was eliminated, explaining the improved corrosion resistance generally observed after such a treatment. The replacement of chromium carbides with titanium carbides in the high grade steel explains the enhanced resistance to grain boundary corrosion generally observed in this type of steel. A possible precipitation mechanism based on repeated heating during multipass welding is advanced for both steels.

Two pass weld simulation has been used to investigate the precipitation mechanism of carbides in the coarse grained heat affected zones (HAZ) of two supermartensitic stainless steels, one high grade steel containing titanium and the other medium grade alloy without titanium. Recently, a model for such carbide precipitation has been proposed. 1 According to this model, reheating of the HAZ in the martensitic stage is needed to provoke strong carbide formation. In the present work, the peak temperatures of the first and the second heating passes, the holding time during the second (reheating) pass and the holding temperature between the two passes have been varied in order to find the most favourable conditions for carbide precipitation in the two steels. In addition, measurements of the chromium content across prior austenite grain boundaries have been made for the titanium free steel.