Influence of Warm Forming on Microstructures and Cryogenic Mechanical Properties of Metastable Austenitic Stainless Steels

For metastable austenitic stainless steels (M-ASSs), warm forming is an effective processing technique to control the martensite content during and after forming and enhance the plasticity and toughness of materials for pressure components. However, considering that the working temperature of cryogenic pressure equipment is in a low-temperature environment (below −150 °C), the mechanical properties of materials after warm forming under cryogenic conditions deserve greater attention. Further, comprehensively researching on the influence and mechanism of warm forming on the mechanical properties of materials is of great significance for improving the warm forming technique and establishing lightweight design and manufacturing methods for cryogenic pressure equipment. In this work, the deformation and facture evolution scheme of cryogenic mechanical properties of warm-formed (WF), cold-formed (CF), and as-received (ASR) S30408 were obtained by tensile, Charpy impact, and low cycle fatigue tests at 77 K and 293 K, as well as microstructure analysis. For tensile fractures, the morphology of WF and CF S30408 at cryogenic temperatures resembles that of ASR, respectively exhibiting ductile and shear fracture characteristics. For the impact fracture, the area of the fibrous zone in the ASR material is significantly larger than that in the WF and CF materials, while the cryogenic impact toughness of the WF material is intermediate between the ASR material and CF material. For fatigue properties, when the fatigue life higher than 103, the cryogenic cyclic stress of WF material is lower than that of CF material, but it is still significantly higher than that of ASR materials.