Effects of prestrainning on the impact response and twinning structure of 304L stainless steel

An investigation has been made into the effects of prestraining on the impact response and twinning structure of 304L stainless steel by means of a split Hopkinson bar and TEM metallography. Annealed 304L stainless steel vas pre-deformed at prestrains of 0.15 and 0.3 using a Saginomiya 100 metal forming machine, providing specimens with different microstructural parameters and mechanical properties. Cylindrical specimens were machined from the pre-deformed material and tested at room temperature at strain rates of 10^-3, 8 × 10², 2.3 × 10³ and 4.8 × 10³ s^-1, with the true strains varying from 0.1 to 0.3. The results indicate that the overall stress-strain response was strongly dependent on applied strain rate and prestrain level. Flow stress increased with strain and strain rate for each prestrain condition. The higher prestrain specimen was generally stronger because of the different initial microstructure and degree of work hardening. Strain-rate sensitivity and activation volume varied with prestrain and work hardening stress (σ - σ_y). Increased prestrain and work hardening stress resulted in increased strain-rate sensitivity, but the inverse was observed for activation volume. TEM thin foil observation revealed deformation twinning was operative in 304L stainless steel over a broad strain and strain rate range, and can combine with prestrain effects for different work hardening characteristics and impact strength enhancement. Twin density was observed to increase with prestrain and work hardening stress.