Part 1: Hydrogen Attack Limit of 2 ¼ Cr-1Mo Steel

Chrome-molybdenum steels are in common use in pressure vessels for high temperature and high pressure hydrogen applications. The proper alloy is usually determined from the Nelson Curves. The 2 ¼ Cr-1 Mo steel has found particular favor in services such as hydrodesulfurization. For 2 ¼ Cr-1 Mo, the mode of damage expected is surface decarburization-not the more far-reaching hydrogen attack. A more detailed confirmation of all the effects of temperature and hydrogen pressure was thought useful.

To carry out such studies, Task Group I of the Japan Pressure Vessel Research Council Subcommittee on Hydrogen Embrittlement conducted a joint research program on 2 ¼ Cr-1 Mo Steel assessing the effects of exposure time, temperature, and hydrogen pressure on deterioration of properties. This report presents the results as determined by changes in mechanical properties and microstructure of base material and simulated heat affected zones. The report was first presented at the First International Conference on Current Solutions to Hydrogen Problems in Steel, ASM, Nov. 1982.

Part 2: Embrittlement of Pressure Vessel Steels in High Temperature, High Pressure Hydrogen Environment

The cooperative effects of temper embrittlement and hydrogen embrittlement had been investigated using high strength steels, and it was pointed out that these effects should be considered even with lower strength pressure vessel steels. Based on this, TG II focused its attention on the relationship between both embrittlement phenomena in Cr-Mo steels, which absorb high amounts of hydrogen during operation in high temperature, high pressure hydrogen services and may be temper embrittled through long time operation in the temperature range, 350-500°C. (662 ~ 932°F).

Part 3: Hydrogen Embrittlement of Bond Structure Between Stainelss Steel Overlay and Base Metal

Many of the heavy wall reactors for hot H₂/H₂S service are made of 2 ¼ Cr-1 Mo steel weld-overlaid with austenitic stainless steel to combat against aggressive corrosion by hot H₂S. In such reactors, hydrogen dissolves in the reactor wall during operation and the hydrogen retained during shutdown may sometimes cause unforeseeable damage to the materials, such as hydrogen-assisted crack growth of the Cr-Mo steel and hydrogen embrittlement cracking of the austenitic stainless steel weld metal. Therefore, to avoid such hydrogen damage care must be taken when designing, fabricating, and operating such reactors.

Recently, attention has been paid to the hydrogen-induced disbonding of stainless steel weld overlay, as a new type of the material degradation due to retained hydrogen, and research has been carried out on the problem. Disbondings in a desulfurizing reactor and a hydrocracking reactor are also reported in actual plants.