1987 Volume 73 Issue 1 Pages 167-174
It has been shown in previous paper that good toughness of 1/2Mo steel could be obtained when the rate of cooling from the normalizing temperature was controlled to obtain a bainite content of 40% or more in the microstructure and tempering was conducted at a temperature about 650°C1). In this paper, metallurgical investigations were carried out relating to the deterioration in toughness caused by hydrogen attack and the effect of the above-stated toughness improving conditions on this resistance.
Hydrogen attack is observed in 1/2Mo steels utilized in heat exchangers, pipes and their fittings which were operated in a hydrogen environment below the safety limit shown in the API Nelson chart revised in 1983. In recent years, therefore, the reexamination of the susceptibility to hydrogen attack of 1/2Mo steels has been a matter of concern.
The effects of Mo content, microstructure and tempering temperature on hydrogen attack susceptibility of steel for pressure vessel were investigated by the specimens exposed to hydrogen at 450 and 500°C under 150kgf/cm2 G.
From the test results, it was confirmed that the resistance to hydrogen attack was improved with increasing Mo contents. In terms of the relationship between the resistance to hydrogen attack and the toughness behavior of 1/2 Mo steel, it was also confirmed that the hydrogen attack deterirated the toughness, but the toughness-improving conditions of 1/2 Mo steel mentioned above reduced hydrogen attack susceptibility.