Sources of variability and improvement in impact toughness of a low Cr–Mo steel weld metal. (November 2020)

• Record Type: Journal Article
• Authors: Yan, Yingjie; Liu, Gang; Cao, Rui; Li, Xinyu; Jiang, Yong; Huang, Yifang; Dou, Guishan; Wu, Kefan; Chen, Jianhong
• Is Part Of: International journal of pressure vessels and piping. Volume 187(2020)
• Publication Date: 2020-11
• Subjects: Heat resistance steel weld metal -- Post-weld heat treatment -- Impact toughness -- PWDT -- PWNT -- Microstructure; Pressure vessels -- Periodicals; Pipe -- Periodicals; Récipients sous pression -- Périodiques; Tuyaux -- Périodiques; Pipe; Pressure vessels; Periodicals; 681.76041
• DOI: 10.1016/j.ijpvp.2020.104175
• Languages: English
• ISSNs: 0308-0161

Abstract

Abstract: For the heat resistance steel weld metal, special post-weld heat treatments need to be performed to maintain the creep resistance, improve the impact toughness, and relieve the welding residual stress. In this paper, firstly, the three factors affecting variability and lower values of toughness were revealed. Secondly, two different post-weld heat treatment (PWHT) methods have been used to improve the impact toughness of a heat resistance steel weld metal. Two PWHT methods include post-weld direct tempering (PWDT) process at 690 °C/26 h/air cooling and post-weld normalized tempering (PWNT) process with re-austenitization at 960 °C/1.5 h/air cooling + tempering at 690 °C/26 h/air cooling. For three type specimens with as-welded (AW), PWDT, PWNT processes, the impact toughness and the hardness were measured, and the fracture surface, microstructures were observed and analyzed by scanning electron microscope (SEM). Results indicated that PWDT heat treatment can improve the impact toughness of the weld metals from the original 79 J - 169 J to 82 J–290 J because the microstructure is changed from granular bainite to block ferrite and granular bainite. PWNT heat treatment can remarkably improve the impact toughness of the weld metals from the original 79 J- 169 J to 148 J–325 J. It is attributed to that PWNT heat treatment process can remove the boundaries of the columnar grain zone and reheated zone, make the difference of the microstructure between the columnar grain zone and reheated zone decrease, and improve the homogeneity of the microstructure. Highlights: The three factors jointly contribute to give rise to variability and lower values of toughness. PWNT process with re-austenitization at 960 °C/1.5 h/air cooling + tempering at 690 °C/26 h/air cooling was used PWDT process can improve the impact toughness of the weld metals because the microstructure is changed from GB to BF and GB. PWNT process removes the boundaries of the columnar grain and reheated zone, improves the homogeneity of microstructure.