High Mn-TWIP/TRIP steels have been attracted much attention in the automotive field due to its excellent mechanical properties. However, the poor hydrogen embrittlement (HE) performance does limit its development. In this work, three kinds of high Mn-TWIP/TRIP steels with the almost same ultimate tensile strength (~1000MPa) were first prepared by different heat treatment processes. XRD test results show that the phase volume fraction of ε-martensite (PVFMs) is 0, 10% and 45%, respectively. Then the hydrogen-induced delayed cracking (HIDF) was studied by the slow strain rate test (SSRT) with hydrogen-charging. The results show that when the specimen contains a small amount of PVFMs (10%), hydrogen-promoted dislocation planar slip and dislocation transports the hydrogen atoms to the grain boundaries to reduce the bonding force of the grain boundaries at the slower strain rate (1×10-6 s-1), thereby promoting the initiation of hydrogen-induced intergranular cracks; The interaction between deformation twins and grain boundaries promotes hydrogen-induced intergranular crack initiation at the faster strain rate (1×10-5 s-1). When a large amount of PVFMs is contained (45%), the stress concentration caused by the inconsistency of ε-martensite during plastic deformation leads to the initiation of hydrogen-induced intergranular cracks. The Σ3 grain boundaries and the ε/γ interfaces with the orientation of the N-W orientation play a role in arresting the propagation of HICs. In addition, for TRIP steel with a large amount of PVFMs (45%), the ε-martensite with micro-orientation of ||RD at the cracks tip is more likely to cause the transgranular propagation of HICs and promote the formation of cleavage or quasi-cleavage fracture.
Posted by: Jinxu Li, Prof, University of Science and Technology Beijing, China (02-Dec-2019)