Effects of Cold Deformation Prior to Sensitization on Intergranular Stress Corrosion Cracking of Stainless Steel

Abstract

The effects of deformation, prior to sensitization, on intergranular stress corrosion cracking (IGSCC) were studied on the AISI 304 (UNS S30400) stainless steel (SS). The degree of sensitization (DOS) was quantified by the double loop electrochemical potentiokinetic reactivation (DL-EPR) method. The susceptibility to IGSCC was investigated by the slow strain rate test (SSRT) carried out in polythionic acid (PTA) solutions. The results were complemented by scanning electron microscopy (SEM) fractographs. Deformation was found to accelerate sensitization, and a peak in sensitization vs. deformation was always observed. This peak was found to shift toward lower deformations with an increase in sensitization temperature. At 700°C, prior deformation is able to desensitize or heal the SS after 24 h. IGSCC was observed in AISI 304 SS after some treatments. No one-to-one correspondence was observed between IGSCC and DOS; this could be explained by the fact that the DOS measured by the DL-EPR indicates the depleted regions below ~15% Cr, whereas IGSCC depends on the availability of continuous grain boundary paths that are chromium-depleted, along with strain rate and environment (pH, temperature, etc.). Deformation prior to sensitization causes carbide formation and chromium depletion to occur near dislocations within the grain interiors, in addition to along grain boundaries. The DOS does not differentiate between these interior regions and the grain boundary regions, and shows Sensitization is a common phenomenon in stainless steels (SS) when they are exposed to temperatures ranging from about 400°C to 800°C.1-20 Classical sensitization results from the nucleation and growth of chromium carbide along grain boundaries (in solution-annealed SS and nickel alloys) with simultaneous depletion of chromium in adjacent grain boundary regions. The extent of chromium depletion in near grain boundary regions is limited by the equilibrium concentration of chromium at the carbide-matrix interface. The equilibrium chromium level depends on the temperature, the chromium activity coefficient, the carbon activity, and the equilibrium constant for carbide formation. Hall and Briant showed the equilibrium chromium concentrations to be 6.6, 8.4, and 10.8 wt% in AISI 316LN(1) (UNS S31603)(2) sensitized at 600, 650, and 700°C, respectively.21 Sensitization occurs in the temperature range where carbide is thermodynamically stable (<900°C) and chromium has sufficient diffusivity (>500°C).