Studies on the Microstructure and Toughness of Simulated Heat Affected Zone in a Modified 9Cr-1Mo Steel

The impact toughness of various regions of heat-affected zone (HAZ) of modified 9Cr-1Mo steel has been carried out on physically simulated specimens. For this, modified 9Cr-1Mo steel specimens were subjected to simulated thermal cycles in a Gleeble thermo-mechanical simulator at four different peak temperatures, viz. 860°C and 900°C for the inter-critical HAZ (ICHAZ), at 975°C for the fine grain HAZ (FGHAZ), and at 1200°C for the coarse grain HAZ (CGHAZ). HAZ simulated specimens were subjected to simulated post weld heat treatment (PWHT) at 760°C for 4h. Charpy V-notch impact toughness, hardness measurement and microstructure studies were carried out in the as-simulated and after simulated PWHT. Impact toughness after PWHT for all the four different peak temperatures is comparable to base material toughness. Microhardness measured across the simulated specimen showed substantial reduction in hardness after PWHT and hence recovery of ductility and toughness. This paper presents and discusses the results of this experiment investigation on the microstructure, hardness distribution and impact toughness of different regions of HAZ in modified 9Cr-1Mo steel.