Effects of Deformation (Strain) and Heat Treatment on Grain Boundary Sensitization and Precipitation in Austenitic Stainless Steels

Sensitization, particularly the degree of sensitization (DOS) in type 316 stainless steel pipe is critically dependent upon the solution anneal of the mill-annealed or commercial material, and is particularly sensitive to low-temperature aging when the starting material is solution annealed between about 1,000°C and 1,100°C. It is observed that when the DOS is above about 10 C/cm² (quantitative electrochemical potentiokinetic reactivation units in Coulombs/cm²), noticeable carbide precipitation occurs in the grain boundaries and increases with increasing DOS. Transmission electron microscopy (TEM) examination of precipitation occurring in type 316 stainless steel pipe grain boundaries has shown them to exhibit many microstructural features that seem to be coincident with grain boundary microstructures, particularly ledges. Uniaxial, tensile strain prior to aging of solution annealed pipe was observed to have a dramatic effect on DOS of type 316 stainless steel pipe; DOS increases with increasing strain. This effect appears to be correlated with the propensity of dislocations to be generated near grain boundaries, and for grain boundary microstructural changes to be related with strain, as observed in type 304 stainless steel. The effects of strain on DOS, combined with instabilities connected with annealing of type 316 stainless steel, indicate that in the practical utilization of pipe, it is imperative to reduce deformation prior to service and during service applications, to avoid sensitization and, thus, minimize the probability of grain boundary cracking.