Transmission electron microscopy investigation of precipitation reactions in coarse-grained heat effected zone in two 13%Cr supermartensitic stainless steels

Precipitation reactions in the coarse-grained heat affected zone (HAZ) during welding and post weld heat treatment (PWHT) of two 13%Cr supermartensitic steels have been studied. The major difference between the two steels was the titanium content. One of the steels contained 0,1% Ti and the other one was not alloyed with titanium. Light microscopy revealed regions in HAZ with high density of grain-boundary particles at prior austenite grain boundaries, and carbon replicas were prepared from these regions, for the non-titanium steel in as-welded condition and for the titanium steel after PWHT at 650 0C for 5 min. Diffraction studies together with X-ray energy-dispersive spectroscopy were applied to identify the grain-boundary particles, which appeared to be MC titanium carbides in the titanium steel and M23C6 chromium carbides in the non-titanium steel. Transmission electron microscopy of thin foils revealed the presence of zones depleted in chromium near prior austenite grain boundaries in as-welded condition. After PWHT the depletion was eliminated by chromium diffusion from the grain interior. The precipitation mechanisms in the steels and the possible influence of carbides on stress corrosion resistance of the non-titanium steel are also discussed in the paper.