The effect of ageing upon the microstructure and mechanical properties of type 15-5 PH stainless steel

Microstructures developed in commercial 15-5 PH precipitation-hardened stainless steel after different heat treatments have been studied. In the as received condition, two types of carbides, NbC and M7C3, were present. Age hardening involves initial formation of fine precipitates rich in copper. Conventional transmission electron microscopy (TEM) and high resolution electron microscopy (HREM) studies have revealed the formation of a 9R structure copper precipitates (4 nm) in the samples aged at temperatures below 500 °C. However, at higher temperatures, in addition to the formation of these precipitates, the austenite phase was formed. After ageing at 500 °C for 128 h, M23C6 carbides were observed. The majority of the M23C6 carbides were in the interface of martensitic matrix and retained austenite. A second type of copper precipitate being, spherical in shape, were observed on ageing at 650 °C. TEM and X-ray microanalysis on thin foils and on carbon extraction replicas used for analysing the structure and metallic compounds of these precipitates. The mechanical properties following strain deformation has been carried out using scanning electron microscope (SEM), TEM and HREM. At peak aged, the 15-5 PH alloy exhibit brittle failure, the major fracture mode was cleavage and/or quasicleavage.