Predicting microstructure and strength for AISI 304L stainless steel forgings

Abstract

Specially designed billets of AISI 304L stainless steel were forged at two temperatures, 843 °C and 941 °C, and with varying transfer times. The machined 304L billets were thick near the center (51 mm) and thin near the periphery (10 mm) such that forging with a flat platen resulted in regions of high strain (center) and low strain (periphery). Multiple metallographic samples were extracted from one experimental forging for each temperature (water quenched forging with nominal transfer times). Microstructural analysis revealed that recrystallization initiated at about 1.4 plastic strain for forgings made at 843 °C and 1.1 plastic strain for 941 °C. Mechanical tests—performed using tensile bars from multiple locations from each forging—revealed that the resulting yield strength increased with strain up ~10–20% recrystallization, then decreased. Coupled thermo-mechanical simulations estimated a temperature increase of up to 162 °C in the center of the forging due to rapid deformation in the High Energy Rate Forging (HERF) press. Simulation using updated BCJ model parameters for 304L stainless steel predicted the strain distributions using the scalar value, equivalent plastic strain. The predicted trends for percent recrystallization and yield strength from this simulation (using the internal state variable BCJ model) agreed qualitatively with experimental tensile test results, but the quantitative comparison leaves room for improvement. Both the simulation and the experimental results clarify that the microstructure and strength of 304L forgings are difficult to control in a forging with excessive strain variation. It is recommended that strain variation be minimized and overall strain be kept under ~0.5 to best control the strength and grain structure of 304L stainless steel. Excessive transfer time appeared to have little effect on the forging yield strength, but air cooling and die cooling after forging indicated softening for the forgings made at 941 °C.