Evaluation of post weld heat treatment quality of modified 9Cr–1Mo (P91) steel weld by magnetic coercive force measurements

• Magnetic coercive force of modified 9Cr–1Mo steel weld is studied after PWHT.
• Reduction in MCF of weld is proportional to reduction in its hardness after PWHT.
• Reduction in MCF after PWHT is dependent on degree of precipitation in weld.
• Effect of PWHT is reflected in decrease in MCF of weld.
• MCF of modified 9Cr–1Mo weld not exceeding 10.4 A/cm indicates satisfactory PWHT.

This paper presents a novel technique for evaluation of quality of post weld heat treatment (PWHT) of modified 9Cr–1Mo (P91) steel weld by correlation with its magnetic coercive force (MCF). The MCF of modified 9Cr–1Mo steel was studied in respect of different PWHT cycles involving ordered variations of soaking temperature unto 770 °C and soaking time unto 140 min. It was observed that MCF of the weld metal reduced with increase in both the soaking temperature and soaking time of PWHT. The drop in MCF of welds after PWHT was found to be dependent on the drop in the hardness of the weld and on the degree of carbide precipitations seen in the weld microstructure. Based on the study, a criterion was evolved for evaluation of heat treatment quality of the weld.