Effect of Post-weld Heat Treatment on Microstructure and Mechanical Properties of 309L/E5015 Fusion Zone of Bimetal Lined Pipe Weld

Abstract

The effect of different post-weld heat treatment (PWHT) on microstructure, low temperature impact toughness fracture morphology and hardness, of 309L/E5015 fusion zone of bimetal lined pipe weld was investigated by using optical microscopy (OM), electron probe microscope analysis (EPMA), scanning electron microscopy (SEM) and hardness test. The results showed that the microstructure of the fusion zone of the 309L/E5015 under non thermal treatment was martensite, and the lath was fine acicular. At 550 °C, the lath of martensite was bulky, and the feature was obvious; at 650 °C, the microstructure was tempered martensite, and the microstructure was obviously refined; at 750 °C, the microstructure was further refined and became multilateral; at 850 °C, the martensite microstructure was gradually analyzed, and the carbides were dispersed. With the increase of heat treatment temperature after welding, the element distribution gradient of fusion zone slowed down and the hardness first decreased and then increased. At 750 °C, the weld had the best toughness, because the lath of martensite was multilateral, and the hardness reached the qualified range, and the low temperature impact energy was maximum than other temperatures.