The 17-4 PH (precipitation hardening) stainless steel is a martensite stainless steel containing approximately 3wt% Cu and is strengthened by the precipitation of highly dispersed copper particles in the martensite matrix . This alloy is more common than any other type of precipitation hardened stainless steels due to its excellent mechanical properties and excellent corrosion resistance. The chemical composition of 15-5PH stainless steel is listed: 0.04 mass% C, 0.3 mass% Mn, 0.6 mass% Si, 0.023 mass% P, 0.013 mass% S, 16.39 mass% Cr, 4.32 mass% Ni, 0.36 mass% Nb, 3.4 mass% Cu, balance mass% Fe.
The study of detailed microstructure in 17-4PH stainless steel after solution treatment and aging has assumed great significance due to the treatment influence on the mechanical properties. The aim of the present work is to investigate the phase transformation and the aging hardening behavior of this stainless steel at various stages of aging and to try to give an insight of the relationship of hardening behavior with microstructure of the alloy
(1) When 17-4PH stainless steel aging at 595°C for about 20 min, the bulk hardness of the sample attains its peak value (HRC 42.5), and then decreases at all time.
(2) After aged at 595°C, there precipitates the fine spheroid-shape copper, with the fcc crystal structure and has the K-S relationship with martensite. Synchronously, the fiber-shape secondary carbide M23C6 precipitate from the martensite matrix.
(3) The precipitation copper and M23C6 are the reasons for strengthening of the alloy at this temperature.