Effect of PWHT on properties of 485MPa tensile strength vessel steel

Effect of Post-Welding Heat Treatment (PWHT) on properties of 485MPa tensile strength vessel steel was studied by changing carbon contents and parameters of heat treatment. The conclusions are: The strengths of vessel steel decrease after PWHT. And the amounts of strength reduction increase with the carbon contents rising. To get the tensile strength≥485MPa vessel plates, of given Mn content 1.45%, the carbon content is designed over 0.17% in case of 900°C normalization, over 0.20% in case of 620°C holding temperature in 2 hours, over 0.23% in case of 620° holding temperature in 2 hours by 3 times, and over 0.20% in case of 620° holding temperature in 6 hours. The impact toughness are decreased by PWHT at all carbon contents. The SEM images of morphology of microstructure are studied to research on mechanism of strength reduction by PWHT. Then it's found that the pearlites are broken and cementite phases precipitate along grain boundary along with the process of PWHT, which brings about a reduction of strength