Enhancing combined cryogenic mechanical properties of metastable austenitic stainless steel by warm forming

Abstract

Abstract Forming temperature is an essential factor for metastable austenitic stainless steel (M-ASS) products to accommodate microstructures and cryogenic mechanical performance. Warm forming has been proposed to suppress the strain-induced martensitic transformation (SIMT) of M-ASS during plastic deformation with effective costs recently. However, rational ranges of forming temperature and cryogenic properties of warm-formed M-ASS have not been studied thoroughly yet, which is more essential to evaluate the mechanical performance of M-ASS cryogenic equipment under service environment. Thus, in this research, the effect of warm-forming on cryogenic tensile and impact properties of S30408 (equivalent to grade of 304 SS) was investigated at liquefied nitrogen temperature (-196 °C) on specimens after uniaxial pre-strained 35 % at 20 ∼ 180 °C. The transformed microstructure after warm-forming was also studied by electron backscatter diffraction, X-ray diffraction, and Ferritescope. Results show that after warm-forming, not only the content of SIM but also the dislocation density are decreased. Besides, the combined mechanical properties of cryogenic ductility and impact toughness are enhanced though the yielding strength degrades as compared to cold-formed materials that were pre-strained at 20 °C. Moreover, the optimal warm-forming temperature 200 °C was determined as an initial forming temperature for M-ASS based on comprehensive consideration of experimental results about the sensitivity of forming temperature on strength difference and static toughness. Furthermore, the S30408 head EHA 1000 × 14 was warm-formed at temperatures proposed above for verification, the cryogenic ductility and impact toughness of materials from the knuckle section and skirt section of the head are increased, while the strength is decreased as compared to cold pre-strained 35 % materials. It indicated that via warm-forming, the cryogenic combined mechanical properties of ductility and impact toughness could be enhanced significantly as compared to cold-forming, also strength increased than that of as-received materials due to the SIM. Finally, semi-empirical correlations of ultimate tensile strength and Charpy impact energy versus martensite contents were proposed for predicting mechanical properties of M-ASS.