Research on Forming Temperature of Metastable Austenitic Stainless Steel Head Based on Strain-Induced Martensitic Transformation

Formation of strain-induced martensite (SIM) is found in metastable austenitic stainless steel (m-ASS) during cold forming. The presence of SIM may cause reductions in toughness, ductility and corrosion resistance of the m-ASS. Although these mechanical properties can be restored and improved after cold forming by using proper heat treatment, the manufacturing cost rises greatly. However, the cost of warm forming is cheaper and the SIM reduces with forming temperature increasing. Thus, the main purpose of this research is to investigate a suitable forming temperature, at which the strain-induced martensitic transformation (SIM-Tr) of m-ASS head in forming will reduce. Primarily, a series of static tensile tests were conducted based on several batches of S30408 plates. The tests were carried out at different temperatures varying from 20 °C to 180 °C, with the effect of deformation temperature on SIM evaluated. Moreover, according to the stacking fault energy (SFE) calculation method, the m-ASS’s chemical composition was taken into further consideration to investigate its effect on SIM. Eventually, a formula was established that related to SIM and chemical composition for optimizing the forming temperature. The results obtained by this formula were compared to the experimental results of 304 ASS head stamping tests, and satisfactory matching is found for the proposed forming temperatures and the predicted FN values (readings of the Ferritescope measurement, as a representation of the amount of martensite in this study). Additionally, an enhancement of the cryogenic impact properties and a fewer quantity of delta-ferrite in the microstructure are observed when stamping temperature is higher than 90 °C.