Effect of Postweld Heat Treatment on Microstructure and Properties of Thick S11306 Ferritic Stainless Welded Joints

Abstract

This study investigated the evolution of microstructure and mechanical properties of 25 mm thick S11306 ferritic stainless steel welded joints during Post weld heat treatment (PWHT) by a series of tests, including the optical microscope observation, hardness test, tensile test, bending test, and scanning electron microscope tests. The experimental results show that at as-welded and 870 °C, the toughness of welded joints was poor; and when the heat treatment temperature is between 770 and 820 °C, the content of the composite phase of lower bainite and martensite (LB/M) in the weld zone increased to 43.9~47.6%, and the mean values of tensile strength and yield strength were 476 MPa and 309 MPa, the elongation was close to 40%, and the welded joints show good toughness. The bending test of the welded joints under several groups of PWHT was qualified. Heat preservation of welded joints was conducted at 820 °C for 30 s~1 h and with the increase of holding times, the content of the LB/M in weld zone showed an upward trend, and the recrystallization of base metal zone was basically completed at 15 min. In addition, a recrystallization kinetic model of the base metal zone was also established by measuring the hardness of the base metal zone at 820 °C for different holding times, which provides reference opinions for obtaining the favorable microstructure of steel grades during PWHT in the engineering.