Influence of Thermal Cycling Temperature on the Recrystallization of Cold Rolled Stainless Steel 316L

Abstract

Investment casting of an orthopedic implant plate based on stainless steel 316L was considered an economical process. Nevertheless, the mechanical properties of the investment casting product were found to be inferior as compared to the implant plate fabricated with other methods such as forging due to their differences in the microstructure. Investment casting mostly produced coarser grain as compared to those with forging or rolled process. In order to improve their mechanical properties, cold-rolling followed by a repetitive thermal cycling process is proposed. The goal is to generate finer grain size through recrystallization process leading to nucleation of new grain during the thermal cycling process thus increasing their strength. Stainless steel 316L was cold-rolled to 52% reduction in thickness and this process generate stored strain energy in the form of dislocation density in the material. The thermal cycling treatment performed within several cycles after cold rolling enabling gradual disperse of stored strain energy that facilitates the recrystallization process that initiates new grain formation. The short holding time within several cycles limits the grain growth that normally occurs during annealing. It was found that thermal cycling treatment at a temperature of 950 °C for 35 seconds within four cycles led to the formation of finer grain size of 22 µm on average as compared to the initial investment casting average grain size of 290 µm. The hardness also increases to 253 HV0.3 in this condition as compared to 155 HV0.3 of investment casting products. Lower thermal cycling temperature than 950 °C during the test did not result in grain refinement thus indicating that strain energy relieves were not enough to aid the recrystallization process.