A new post-weld heat treatment to improve toughness of 9Cr-1Mo steel weld metals

9Cr-lMo ferritic steels have been used in the conventional power generation plants due to their excellent creep resistance.However,one of the main obstacles in welding 9Cr-Mo steels is the formation of undesirable coarse columnar grains in weld metal which can severely compromise the toughness.A new post-weld heat treatment(PWHT) is developed in the present work.Unlike the conventional processes in which the post-weld heat treatment is carried out below Ac1,the use of temperatures above the Ac,of9Cr-lMo alloy is considered.The new PWHT at a temperature above Ac,improves the toughness of9Cr-Mo weld metals effectively.The improvement in toughness is mainly due to refinement and homogenization of microstructures.