Crystallographic Peculiarities of Shear α-γ Transformation in Austenitic Stainless Steel in the High Temperature Area

摘要

Structure-texture states in 18Cr-9Ni austenitic stainless steel after long-term operation of the tube at high temperatures and neutron irradiation have been investigated with orientation microscopy (EBSD). In the examined samples, cut out at the external surface, a significant concentration of α-phase with the lattice close to bcc has been detected. Phase transformation shows prominent crystallographic direction, caused by initial orientation of austenite grains and tensile stress effect, normally directed at a tangent to its external surface. High-angle boundary spectrum with the most prominent coincidence site lattice (CSL) boundaries, ∑3, ∑11, ∑25b, ∑33c ∑41c, is typical for α-phase. Thus, it can be claimed that austenite transformation was carried out by shear (bainite, taking into account high temperature) mechanism, according to orientation relationships (OR), intermediate between Kurdjumov-Sachs (K-S) and Nishiyama-Wassermann (N-W). Shear γ-a transformation began in austenite on twin boundaries (CSL Σ3), and was carried out in the range determined by initial orientation of γ-phase crystals and effective stress value. Based on high density of CSL boundaries Σ3 in α-phase it has been suggested that its nuclei are represented not by single crystallites, but crystallite couples in twin misorientation.