Mechanisms of deformation-induced grain boundary chromium depletion (sensitization) development in type 316 stainless steels

Abstract

Deformation accelerates the development of grain boundary chromium depletion (GBCD), or sensitization, in type 316 austenitic stainless steels (SS). Quantitative assessment of the degree of sensitization (DOS) using the electrochemical potentiokinetic reactivation (EPR) test indicates that the acceleration in GBCD is a function of the amount of strain in the material and temperature of isothermal sensitization treatment. A systematic increase in strain from 0 to 20 pct yields a continuous increase in EPRDOS values below 700°C, while at higher temperatures, a threshold strain of 6 to 10 pct is required to cause accelerated GBCD development. Straining SS above 20 pct also produces higher amounts of chromium depletion, though the (intergranular) sensitization susceptibility of the material could not be quantitatively evaluated due to the presence of grain matrix or transgranular corrosion. Classical C-curve precipitation-sensitization behavior was also noted for strained and unstrained materials, though strain moved the C-curves to the left. Microstructural evaluation of sensitization revealed a systematic increase in grain boundary and twin boundary corrosion on EPR attack surfaces with strain, which corroborated the deformation-induced acceleration of EPRDOS. A time-temperature-strain dependence of transgranular corrosion was also identified on EPR-etched samples strained above 20 pct. These were also reflected in transmission electron microscope (TEM) observations of higher grain boundary carbide precipitation on strainedvs unstrained specimens and site-specific carbide precipitation on deformation sites in the material. Kinetic and thermodynamic modeling of deformation effects on carbide precipitation and depletion development in type 316 SS indicated that strain induces a reduction in the activation barrier to diffusion (Q) a and thermodynamic barrier to nucleation (ΔG *) during the precipitation-depletion process. The lowering ofQ a with strain caused chromium diffusivity and depletion development to be accelerated in strainedvs unstrained materials and appears to be due to increased dislocation pipe diffusion with strain. Reduction of ΔG * with strain was related to an increase in the free energy change of the grain boundary (ΔG) gb and accelerated carbide precipitate nucleation in deformed SS. The effect of strain on the kinetics and thermodynamics of the precipitation-depletion process decreases with increasing temperature.