Precipitation behavior and 2-step aging of 17-4PH stainless steel

The precipitation behavior of 17-4PH stainless steel, aged at 400-900 degrees C for 0.1-4 h, was studied to understand the strengthening effect of mainly Cu and Nb. MC, M6C, and Cu-rich phases were identified through the x-ray diffraction of the extracted residue, the measurement of electric resistivity, and TEM observation. Among these, the Cu-rich phase was most finely (around 20 nm) dispersed in the matrix, and was supposed to have a dominating effect to strengthen the steel. The precipitation of Cu-rich phase was considerably rapid to start, showing the nose of a c-curve with a shorter aging condition than 700 degrees CX0.1 h, and the high temperature limit of precipitation being about 700 degrees C. The Cu-rich phase was supposed to have the largest effect for strengthening at the earlier stage of precipitation, which was clarified using the tempering parameter separating the age hardening and the matrix softening by martensite recovery. A quantitative discussion, based on a morphological study of microstructure, was carried out through the mean free path calculation for dislocation. Also, several combinations of high-temperature and low-temperature aging (2-step aging) was tried for effective age hardening, and conditions for a better-balance of tensile strength, hardness and ductility than the H900 heat treatment in JIS was obtained. The difference between single and double-step aging was discussed.