Dependence of Carbide Precipitation on Grain Boundary Structure in Sensitized Austenitic Stainless Steel

Abstract

Grain boundary carbide precipitation and intergranular corrosion in sensitized austenite stainless steel were examined by transmission electron microscopy (TEM) to clarify the effect of grain boundary structure on precipitation and corrosion. A type 304 steel, which had been solutionized at 1350 K was heat-treated at temperatures of 800-1300 K. Oxalic acid etch and Strauss tests showed that the frequency of grain boundaries with M23C6 carbide precipitation and corroded boundaries increased with holding time at sensitizing temperatures. The grain boundary carbide precipitation was observed during heat treatment at 1000 K by TEM. Grain boundaries were characterized on the basis of the Coincidence Site Lattice (CSL) theory using electron diffraction Kikuchi patterns. The observations revealed that the propensity to intergranular precipitation depends strongly on the grain boundary structure. Carbide precipitates tend to be detected at grain boundaries with higher Σ -values or larger deviation angles (Δθ) from low- Σ CSL misorientations. The border lines between precipitation and no precipitation can be drawn by a deviation parameter of Δθ/ΔθC, where Δθc is the maximum deviation angle by Brandon’s criterion. The border line of Δθ/Δθc decreased with the increase in the holding time at 1000 K. This means that the more ordered boundary needs the longer time for intergranular carbide precipitation and corrosion than less ordered or random boundaries.