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

Effects of epsilon 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 epsilon 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 e martensite (i.e. fully austenitic) was dominated by slip and mechanical twinning, but that of the steel containing epsilon 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 epsilon martensite were lower than those of the fully austenitic steel, since cracks were prone to be initiated and propagated at the region of epsilon martensite which is harder than austenite. Furthermore, it was found that epsilon martensite provided many diffusible hydrogen trapping sites. Consequently, the notch fracture stress of the steel containing epsilon 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, similar to 22 kJ/mol for the gamma/epsilon interfaces, and similar to 37 kJ/mol for dislocations/gamma grain boundaries. (C) 2011 Elsevier B.V. All rights reserved.