Microstructural evolution in the intercritical heat affected zone of a boron containing modified 9Cr-1Mo steel

Type IV cracking observed in high Cr ferritic steels is attributed to poor creep properties of the intercritical heat affected zone (ICHAZ) of the weld joint which in turn associated with the partial transformation that take place in this zone during the weld thermal cycle and resulting refinement of the microstructure. Recent studies on steels with controlled addition of boron has shown that creep strength of ICHAZ in these steels are comparable to that of the base metal and the microstructure of this zone is significantly different from that of the steels with out boron. Hence in this paper microstructural evolution in the simulated ICHAZ of two different P91 steels, one without boron and another with boron and reduced nitrogen content has been studied. Results indicate that during heating part of the weld thermal cycle, austenite nucleate and grow into two different morphologies, globular and acicular which transform to martensite during cooling. The former is more prevalent along the prior austenite grain boundaries (PAGB) of the base metal while the latter along the lath boundaries of the tempered martensite within the grains. Results also show the transformation to austenite is delayed in the boron containing steels and austenite with both the morphologies are formed. However, growth of the globular austenite, formed along the PAGB is sluggish and hence PAGBs are discernable even after the transformation. This explains the difference in the microstructure of the of ICHAZ of steels with boron and without boron and this could also be the reason for the improved creep resistance of the weld joint of high Cr ferritic steels with controlled addition of boron