Metallurgical investigations on dual heat cycled boron alloyed P91 ferritic martensitic steel

Welding of ferritic/martensitic steels degrades the metallurgical properties at heat affected zone (HAZ) near the fusion line in comparison to virgin steels. It has been reported that adding boron upto 100 ppm to modified P91 steel increases creep resistance. The aim of present work is to assess the effect of boron addition on microstructural variations and hardness changes by subsequent post weld heat treatment (PWHT) of simulated HAZ subzones representing single-pass welding of P91B steel. Due to gradual decline of peak temperature from fusion line towards virgin metal during welding produces HAZ which is further composed of subzones like GCHAZ (grain coarsened heat affected zone), GRHAZ (grain refined heat affected zone) and ICHAZ (inter-critical heat affected zone). To carry out preliminary studies on actual sub-HAZs cut from weldment is always difficult, since they are usually produced by both single-pass and multi-pass welding resulting in narrow HAl width; which reduces the volume of material of each subzone for metallurgical investigations. Therefore, thermal simulation is employed to generate individual subzone of HAZs by giving an equivalent thermal cycle. Gleeble3800, thermo-mechanical simulator was used to separately reproduce each subzone of HAZ for P91B steel having larger volume than actual welding. This thermal cycle corresponds to first heat cycle of welding and then PWHT for 760 °C/3h is done on these simulated HAZs which constitute second heat cycle. The selected PWHT condition gave an improvement in microstructural features, stability of fine precipitates and significant drop in hardness gradient among simulated subzones/specimens of P91B-HAZ. These results are found better than P91 for similar PWHT as stated in literature. Significant hardness drop was observed after PWHT of simulated HAZs which lies in the range of hardness recommended for virgin P91B steel. These findings signify the 'boron memory effect' which result in more stable microstructures for creep resistance in subzones of P91B - HAZ after PWHT of 760 °C/3h and also validates it as an optimum tempering condition. (author)