Scale-up fabrication of gradient AGS in austenitic stainless steels achieves a simultaneous increase in strength and toughness

Abstract

Simultaneous strengthening and toughening of austenitic stainless steel are of great importance in safety-critical applications. Gradient structured steels have thus attracted significant interest as they can achieve unprecedented strength and toughness synergy. However, the traditional fabrication process of gradient metals is relatively complex and limited to small depths of gradient. In this contribution, we have developed a novel processing route consisting of inhomogeneous rolling and annealing to fabricate a large gradient profile (∼5 mm) in the austenite grain size. The fabricated bulk gradient austenitic steel exhibits higher tensile strength (∼690 MPa) and Charpy impact energy (11.8 J) compared with the uniform-grain-size structural steel (∼650 MPa and 10.9 J, and ∼580 MPa and 11.3 J for fine grained (FG) and coarse grained (CG) steels). The deformation mechanisms of the bulk gradient austenitic steel were further explored. The combined contribution of the hetero-deformation induced (HDI) strengthening and transformation-induced plasticity (TRIP) effect results in the ideal work-hardening capability and tensile strength of the gradient austenitic steel. Thanks to the deep gradient, the fraction of deformation-induced martensite decreases along the fabricated gradient. The pronounced TRIP effect near the coarse-grained V-notch improves the crack initiation energy (Ei), while the less martensite formed in the fined-grained interior raises the crack propagation energy (Ep). This study provides new insights into the circumvention of the strength-toughness trade-off and the fabrication of gradient structured steels with a large gradient profile.