Microstructural evolution in a 17-4 PH stainless steel after aging at 400°C

The microstructure of 17-4 PH stainless steel at various stages of heat treatment, i.e., after solution heat treatment, tempering at 580 degrees C, and long-term aging at 400 degrees C, have been studied by atom probe field ion microscopy (APFIM) and transmission electron microscopy (TEM). The solution-treated specimen consists largely of martensite with a small fraction of delta-ferrite. No precipitates are present in the martensite phase, while spherical fcc-Cu particles are present in the delta-ferrite. After tempering for 4 hours at 580 degrees C, coherent Cu particles precipitate in the martensite phase. At this stage, the Cr concentration in the martensite phase is still uniform. After 5000 hours aging at 400 degrees C, the martensite spinodaly decomposes into Fe-rich alpha and Cr-enriched alpha'. In addition, fine particles of the G-phase (structure type D8(alpha), space group Fm (3) over bar m) enriched in Si, Ni, and Mn have been found in intimate contact with the Cu precipitates. Following spinodal decomposition of the martensite phase, G-phase precipitation occurs after long-term aging.