Hot workability of AISI 321 and AISI 304 austenitic stainless steels

Although the austenitic stainless steels 304 and 321 are often treated nominally as equivalents in their hot rolling characteristics, the question remains whether any subtle differences between the two allow further optimisation of their respective hot rolling schedules. The hot workability of these two types of austenitic stainless steels were compared through single-hit Gleeble simulated thermomechanical processing between 800 °C and 1200 °C while the strain rate was varied between 0.001 s−1 and 5 s−1. It was found that the constants for the hyperbolic sine equation for hot working of 321 steel are Q = 465 kJ/mol, A3 = 9.76 × 1017 MPa−1 s−1, α = 0.009 MPa−1 and n = 6.1 while for 304 steel the constants are Q = 446 kJ/mol, A3 = 2.14 × 1017 MPa−1 s−1, α = 0.008 MPa−1 and n = 6.1. It is shown that the occurrence of dynamic recrystallisation starts when the Zener Hollomon parameter Z ≈ 6.4 × 1017 s−1 for both steels but that the differences in the values of Q and A3 (the structure factor) between the two steels does lead to consistently lower steady state stresses for the steel 321 than is found in the steel 304 at the same Z values. This may, therefore, offer some scope for further optimisation of the hot rolling schedules and in particular in the mill loads of these two respective steels.