For this study, Fe-13Mn alloy was quenched from 1000℃ to room temperature to have those crystal structures of α-ferrite (BCC), α'-martensite (BCT), ε-martensite (HCP) and γ-austenite (FCC).
To study how much the charged hydrogen content affects to the hydrogen embrittlement, the hydrogen was charged electrolitically in the electrolyte of 5% $H_2SO_4$ with 10 mg of $As_2O_3$ per one liter of the solution.
The cathode, the specimen, was placed between two anodes, the platinum plates. The current density was applied to be 15 mA/㎠, constant.
Measurements of the hydrogen content were made by the method of the hot extraction, using the LECO hydrogen analyzer.
The specimens were pulled by the Instron testing machine at a crosshead speed of 0.01cm/min.
Increasing the hydrogen charging time, the surface layer of the specimen may be oversaturated, and this makes γ-austenite transformed to ε-martensite or to α'-martensite, and ε-martensite to α'-martensite.
Almost all hydrogens diffuse in through α'-martensite lattices. During the tension test, the hydrogen oversaturation is increased due to the strain-induced phase transformation from ε-martensite to α'-martensite because of the solubility difference between the two phases. The more the charging time, the hydrogen is saturated. By charging hydrogen less than 6 ppm, the solid solution hardening is observed, that is the fracture strength and the ultimate tensile strength increase. As the hydrogen content increases more than 4 ppm, the elongation decreases until the hydrogen content reaches a critical value of 10 ppm the elongation does not decrease any further. The fracture strength and the ultimate tensile strength decrease as the elongation.
The hydrogen-induced phase transformation, occurred possibly near the subsurface, does not affect to the mechanical properties.
Summarizing all the experimental results, it may be suggested that the hydrogen embrittlement in the Fe-13Mn alloy is due to the interaction between the hydrogen gas and the phase transformation.
