Graded Grain Structure to Improve Hydrogen-Embrittlement Resistance of TWIP Steel

The high strength of twinning-induced plasticity (TWIP) steels makes them vulnerable to the hydrogen embrittlement (HE) phenomenon, thereby limiting their potential applications. This study suggests inducing a graded grain structure (GGS) in a Fe-17Mn-0.8C TWIP steel through shot peening and subsequent heat treatment to solve the problem. The microstructures and fracture surfaces of GGS TWIP steel were compared with those of conventionally manufactured TWIP steel possessing a uniform grain structure (UGS). Compared with the conventional UGS TWIP steel, GGS steel showed similar tensile properties with a yield strength of 310 MPa, tensile strength of 1060 MPa, and elongation-to-failure of 135%. It also exhibited moderately enhanced low-cycle fatigue (LCF) resistance in terms of fatigue life (8196 cycles to failure) compared with the UGS steel (7201 cycles). Furthermore, GGS TWIP steel exhibited a marked improvement in HE resistance, both in the monotonic (by a slow-strain-rate test) and cyclic deformation modes (by the LCF test) in a hydrogen environment. A relatively fine-grained (d = 15.6 μm) surficial area enhanced the HE resistance by inhibiting hydrogen penetration and decreasing twin density, while the coarse-grained (d = 74.6 μm) interior promoted the LCF resistance by suppressing crack growth