Effect of Postweld Heat Treatment on Critical Temperature in Hydrogen Attack of 1/2 Mo Steel Welds

Abstract

Relationship between a critical temperature in hydrogen attack and postweld heat treatment conditions was shown by a metallurgical investigation that was conducted to make clear a preventive measure of failures occurring in 1/2 Mo steel welds of heat exchangers and pipe lines which had been used in oil refineries and petrochemical plants.
The authors examined degradation of tensile properties, susceptibility to intergranular cracking and bubble formation behavior in both actual welds and a simulated-heat-affected zone material (simulated HAZ) after a hydrogen exposure in the temperature range from 320 to 500°C under a pressure of 100 or 150 kgf cm² (9.81 or 14.71 MPa).
Postweld heat treatment at a temperature range from 550 to 650°C produced a significant improvement in hydrogen attack resistivity of the actual weld. In addition, an examination result of the simulated HAZ exhibited to produce a maximum critical temperature in hydrogen attack by excuting a heat treatment at around a temperature of 650°C. It was concluded that the highest resistivity to hydrogen attack was able to maintain provided a postweld heat treatment would be performed in the temperature range between 625 and 650°C.