A comparison of sensitization kinetics in 304 and 316 stainless steels

The effects of tensile and cold rolling strain (up to 40%) over a range of grain sizes ranging from 300 μm to 10 μm on sensitization (and desensitization) were observed and compared for 304 and 316 stainless steel having a constant carbon content of 0.05%; at 670°C. Rapid sensitization-desensitization was observed for both materials at the smallest grain size, and plots of degree of sensitization (DOS) data with time, temperature, and tensile strain coupled with chromium diffusivity data for 304 stainless steel allowed activation energies to be calculated from corresponding Arrhenius plots utilizing supplemental data from Beltran, et al. [1] at 625°C and 775°C. Values of 1.9 and 2 kcal/mol were found for unstrained and 20% strained samples for 11 μm grain size while corresponding values at 175 μm grain size were 55 and 32 kcal/mol respectively. Activation energies for unstrained and 10% strained 316 stainless steel for 135 μm grain size were found to be 76 and 64 kcal/mol, respectively. Sensitization was more rapid for cold-rolling versus tensile straining in both stainless steels, and there was no detectable sensitization for the largest grain size regime of the 316 stainless steel up to 10 h aging time at 670°C.