Study of creep crack growth in a modified 9Cr-1Mo steel weld metal and heat affected zone

The creep crack growth (CCG) phenomena in the heat affected zone (HAZ) of Mod.9Cr-1Mo (P91) steel weldment is a lesser explored field than to that in the weld and the base material. In this study, constant load creep crack growth tests have been carried out at different applied loads at 898 K as per ASTM E1457 on the CT specimens prepared from 'V-type' and 'half-K' type of weld joints. The creep crack growth rate, as a function of C*, has been found to be higher in the HAZ as compared to the weld metal. Even the crack initiated in the weld has been found to deviate towards HAZ. These have been attributed to the combined effect of high constraint situation and high creep strain in the HAZ, leading to early crack initiation and its faster growth in this region. A detailed SEM based study of the crack initiation and growth mechanism in the HAZ has revealed that grain boundary cavitation, triple point cracking and subsequent sub-surface linking up of the cavities into microcracks have been responsible for extensive creep damage in the HAZ.