Effect of Post Weld Heat Treatment on the Hydrogen Trapping Behavior of Pressure Vessel Steel

As the demand of low quality crude oil containing a large amount of H₂S gas has been increased for economic reason, the sour resistant pressure vessel steel used widely in the field of petrochemical industry has been developed. The post weld heat treatment (PWHT) is inevitably carried out to relieve the residual stress after welding process. However, the effect of PWHT on hydrogen trapping behavior has not been fully clarified. Changes in microstructure such as the distribution of cementite and center segregation show different hydrogen induced cracking (HIC) property, expressed numerically in terms of crack length ratio (CLR). It means that the hydrogen diffusion behavior in the steel is different before and after PWHT and it is closely related with change in the hydrogen trapping sites in steel. Therefore, it is necessary to investigate relationship between the changes in microstructure caused by PWHT and hydrogen trapping parameters. In this study, an electrochemical hydrogen permeation technique (HPT) is utilized to quantitatively obtain the trapping parameters in the pressure vessel steel (A516-70). Experimental conditions of the HPT are determined using ISO17081 standard and the NACE-A solution is used as the hydrogen charging solution. In this presentation, the effect of PWHT on various hydrogen parameters of the amount of reversibly and irreversibly trapped hydrogen, diffusivity and permeability will be discussed.