S. Y. Sato, M. Miyagi, H. Kokawa, Z. J. Wang, Y. S. Sato, Tohoku University, Sendai, Japan; M. Michiuchi, The University of Tokyo, Tokyo, Japan
The austenitic stainless steels have good mechanical properties and excellent corrosion resistance, so they are widely used in chemical and nuclear industries. However, when the austenitic stainless steels undergo a heat input such as welding process, they will become susceptible to intergranular corrosion. In recent years, the grain boundary engineering attracts attention as one of the new methods for improving corrosion resistance of materials. The present study examined development of 304 austenitic stainless steel having very high frequency of coincidence site lattice (CSL) boundaries by grain boundary engineering, evaluation of actual intergranular corrosion resistance of the grain boundary engineered 304 austenitic stainless steel and discontinuity of corrosion-prone random boundaries by percolation theory. The optimum parameters in the thermomechanical treatment during grain boundary engineering were revealed using orientation imaging microscope. The maximum frequency of CSL boundaries in the thermomechanical treated specimen was obtained at relatively smaller pre-strain and subsequent annealing for 72h. The orientation imaging microscopic study indicated that the grain boundary engineered 304 austenitic stainless steel is characterized by extremely high frequency of CSL boundaries and discontinuity of random boundary network, compared with the base material. The CSL boundaries in the grain boundary engineered 304 austenitic stainless steel were dominated by S3n boundaries. The corrosion rate of grain boundary engineered 304 austenitic stainless steel was lower than one-fourth of base material during the ferric sulfate-sulfuric acid corrosion test. The high intergranular corrosion resistance of grain boundary engineered 304 austenitic stainless steel was due to extremely higher frequency of coincidence site lattice boundaries and consequent discontinuity of random boundary network.
Summary: The present study examined development of 304 austenitic stainless steel having very high frequency of coincidence site lattice (CSL) boundaries by grain boundary engineering, evaluation of actual intergranular corrosion resistance of the grain boundary engineered 304 austenitic stainless steel and discontinuity of corrosion-prone random boundaries by percolation theory.
