Improvement in Creep Resistance of Modified 9Cr-1Mo Steel Weldment by Boron Addition

The microstructural evolution in the heat-affected zones (HAZs) of boron-added modified 9Cr-1Mo steel (P91B) weldment is distinctly different from that of steel without boron addition (P91). This is accompanied by less reduction in hardness in the inter-critical heat-affected zone (ICHAZ) of the weldment of the boron-containing steel than what is observed in boron-free steel.

Though, tensile properties of both the boron-containing steel weldments and boron free-steel weldments do not differ much at room and at high temperatures, there is a significant improvement in the creep strength of the weldment of the boron-containing steel over that of the weldments of boron-free steel.

In fact, for creep tests so far completed for weldments of P91B, fracture occurred in the base metal, not in typical Type IV mode (fracture at the ICHAZ or fine grained heat- affected zone) as occurred for the P91 weldment for identical conditions of testing. This improvement is explained based on the role of boron in modifying the HAZ microstructure of the steel.