Embrittlement of Mn-Si and Mn-Mo High Strength Steels Caused by Heating in High Pressure Hydrogen at High Temperatures

Three kinds of high strength steel were heated for differing periods of time in 200 kg/cm2 hydrogen at high temperatures of 300∼500°C, and the changes in their mechanical properties and structures were studied at room temperature. The results obtained are as follows: (1) The steels which were heated in high pressure hydrogen at high temperatures became brittle and showed a remarkable decrease in the mechanical strength and ductility. The concurrent decrease in both hardness and carbon content occurred with many inner cracks at grain boundaries. The fissures at grain boundaries gave rise to deterioration in the quality of materials. (2) At a constant temperature and pressure, there was an incubation time (t) prior to embrittlement and a plot of the ln t vs. reciprocal of absolute temperature holds a linear relationship. (3) The activation energy of embrittlement on the kinetics of hydrogen attack was calculated, but the mechanism of embrittlement by hydrogen attack could not be clearly explained. (4) The addition of molybdenum, a carbide forming element and such heat treatments as normalizing and tempering increase the resistance to hydrogen attack and raise embrittlement limits.