Orientation relationships associated with austenite formation from ferrite in a coarse-grained duplex stainless steel

Studies on the morphology and crystallography of austenite precipitated from ferrite were performed in an Fe-22.5 pct Cr-4.7 pct Ni-3 pct Mo-duplex stainless steel. Samples were solution treated at 1325°C (yielding a grain size of approximately 2 mm), water quenched, and then aged at temperatures ranging from 700 °C to 1100 °C for times ranging from 5000 to 30,000 seconds. The morphology of grain boundary precipitates depends on the grain boundary segment at which the precipitates are formed, and may be adequately described by the Dubé classification system. Orientation relationships (ORs) between austenite and the ferritic matrix were determined with electron backscattered diffraction (EBSD) analysis, employing graphic and algebraic methods. Grain boundary precipitates exhibited Kurdjumov-Sachs (K-S) or Nishyiama-Wassermann (N-W) ORs with at least one of the adjacent grains, and in some cases relationships intermediate between K-S and N-W appeared. In approximately 60 pct of the cases examined, grain boundary precipitates show K-S or N-W ORs with both grains forming a boundary, though with small deviations (up to 5 deg) from the exact relationships. This article is based on a presentation made in the “Hume-Rothery Symposium on Structure and Diffusional Growth Mechanisms of Irrational Interphase Boundaries,” which occurred during the TMS Winter meeting, March 15–17, 2004, in Charlotte, NC, under the auspices of the TMS Alloy Phases Committee and the co-sponsorship of the TMS-ASM Phase Transformations Committee.