Role of ɛ martensite in tensile properties and hydrogen degradation of high-Mn steels

Effects of ɛ martensite on tensile properties and hydrogen degradation behaviors of a high Mn steel were investigated. For this purpose, a Fe–15Mn–2Cr–0.6C steel containing various amount of ɛ martensite was prepared and tensile tested at room temperature. Microstructures were examined by electron back scattered diffraction and transmission electron microscopy. Then, a series of electrochemical hydrogen pre-charging, slow strain rate tests, and thermal desorption spectrometry (TDS) analyses was conducted to examine the hydrogen degradation behaviors. Deformation of the steel without ɛ martensite (i.e. fully austenitic) was dominated by slip and mechanical twinning, but that of the steel containing ɛ martensite was mainly attributed to transformation induced plasticity in association with strain induced martensitic transformation during deformation, resulting in higher work hardening rate. However, tensile strength and elongation on the steel containing ɛ martensite were lower than those of the fully austenitic steel, since cracks were prone to be initiated and propagated at the region of ɛ martensite which is harder than austenite. Furthermore, it was found that ɛ martensite provided many diffusible hydrogen trapping sites. Consequently, the notch fracture stress of the steel containing ɛ martensite decreased significantly as the diffusible hydrogen content increased. The activation energy for hydrogen detrapping from its trapping sites was also calculated by means of the TDS analyses, ∼22 kJ/mol for the γ/ɛ interfaces, and ∼37 kJ/mol for dislocations/γ grain boundaries.