Abstract: There were a large number of low-hardness P91 pipes in the main steam and hot section system of in-service thermal power units.The high temperature endurance strength of P91 steel decreased with the decrease of hardness.The yield strength and tensile strength of 160 HB-hardness P91 at 540 ℃ decreased by 57% and 33%, respectively.The extrapolated endurance strength after 1×105 h was reduced by half compared with the recommended value.Low-hardness P91 pipe had great operational risk, which required life assessment or replacement treatment.When welding and heat treatment were carried out for P91 steel pipe that was already at the lower limit of hardness, the key was to control the decrease of hardness.In this paper, the whole process of hardness testing and verification, residual life calculation and replacement treatment of low-hardness P91 main pipe in a power plant was studied.The results indicated that when Richter hardness of P91 steel was lower than 160 HB, it was recommended to use portable chain Brinell hardness tester for verification, which was more accurate.The hardness of the new purchased P91 pipeline was recommended between 200～230 HB, leaving a quantity for welding and post-weld heat treatment.By calibrating the temperature control instrument of heat treatment equipment, optimizing and adjusting process parameters of post-weld heat treatment, hardness decrease of the base metal near P91 weld would be controlled within 10 HB, which provided technical reference for the comprehensive treatment of low-hardness P91 steel pipe in thermal power units.Highlights:(1)Reasonable hardness range of the new P91 pipeline was proposed.(2)Precise control measures of hardness decrease in base metal near weld zone were put forward during post-weld heat treatment of P91 steel.(3)Comprehensive treatment measures of low-hardness P91 steel were raised.