EVALUATION OF STRESS-RUPTURE BEHAVIOUR OF SIMULATED WELD JOINT (HAZ) MICROSTRUCTURES OF P91 STEEL

摘要

P91 steel is used in subcritical and supercritical boilers in welded condition. Welding results in different microstructural features in the heat affected zone (HAZ), and the performance of the weld Joint during service depends on the above microstructural conditions. Failure often takes place in the intercritical region of the HAZ. The study involved simulation of various microstructural conditions encountered during welding through beat-treatment at different temperatures and correlating them with stressrupture properties. P91 steel samples were heattreated in the temperature range of 800 to 1300℃, and were subjected to stress-rupture testing at 500, 550, 575, 600 and 625℃ at stress range varying from 6 to 30 kg/mm{sup}2. Creep-rupture properties are lowest for P91 steel beat-treated at 9000C which represents intercritical zone, and are very high for the material bear-treated at 1200 or 1300℃ Creeprupture properties, room and high temperature tensile strengths are found to be increasing with increasing grain size when austenitised above 900℃, whereas the impact properties exhibited an opposite trend and decreased with increasing grain size. The role of carbides was also studied using Transmission Electron Microscope. It is inferred that the austenitising temperature of P91 steel could be beneficially increased up to 1100℃ if higher creep-rupture properties are desired, at the cost of marginal reduction in impact strength.