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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.

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