Study on Non-uniform Softening and Formation Mechanism of P91 Steel for High Temperature Steam Pipeline during Service

Author of the article:WANG Jinhai 1 , WANG Yansong 2 , LIU Fuguang 2 , LIU Liying 2 , LI Gang 1 , HAN Tianpeng 2

Author's Workplace：1. Huaneng International Power Co., Ltd., Dezhou Power Plant, Shandong 253024, China; 2. Xi'an Thermal Power Research Institute Co., Ltd., Xi'an 710054, China

Key Words：P91 steel; mechanical properties; microstructure; hardness

Abstract：The chemical composition, microstructure and mechanical properties of P91 original steel and long-service steel
were analyzed by means of carbon and sulfur analyzer, oxygen, nitrogen and hydrogen tester, optical microscope, scanning
electron microscope (SEM), transmission electron microscope (TEM), hardness tester and universal tester. The results show
that all elements of P91 steel after service are within the standard and meet the requirements of the standard, among which
the mechanical properties of the low hardness area do not meet the service conditions. The microstructure of P91 steel is
changed, part of martensite is transformed into massive ferrite, there are a lot of precipitates (mainly M 23 C 6 phase) in grain
and grain boundary, and part of precipitates gather at grain boundary and form short chains. The coarsening of
microstructure and the accumulation and growth of carbide are the main reasons for the decrease of hardness and strength
of P91 steel after long-term service.