Microstructure, crystallography of phase transformations and multiple precipitations in PH 15-7Mo stainless steel

The microstructure and crystallographic features of a semi-austenitic precipitation hardening steel PH 15-7Mo during solution treatment, roddrawing and aging were investigated by means of optical microscope, X-ray diffraction analyzer and transmission electron microscope. It was found that the microstructure of the steel was consist of dominant austenite, small amount of martensite and 10-15 vol.% δ-ferrite after solution treatment at 1050 °C followed by cooling in water at room temperature. The austenite transformed into lath martensite during tensile roddrawing about 60% deforming companied with some coherent fine β-NiAl particles precipitated within martensite. With higher aging temperature and longer holding time, tiny carbide M23C6 particles precipitated from martensite, which kept the cubic-cubic orientation relationship (OR) with austenite and G-T OR with martensite which is different with all the reported orientations. The OR between tiny carbide M23C6 particles G-T OR with martensite was discussed in terms of crystallography of phase transformations.