ESSC & DUPLEX 2019

The 17-4 PH is a precipitation hardening stainless steel with good mechanical and corrosion properties. Mechanical properties strongly depend on the heat treatment of the steel, namely, the isothermal ageing process. The formation of Cu-rich precipitates dictate the steel hardness, as they start to form the hardness rises, they coarsening however causes a substantial drop in hardness. However formation of reversed austenite ca also be observed during ageing. The evolution of the microstructure during ageing was thoroughly investigated in order to explain the processes that have effect on formation of the reversed autenite. The reaustenitization was analyzed with a dilatometer, while the coarsening of Cu-rich precipitates was observed by transmission electron microscope. The amount of austenite was measured with X-ray diffraction and the impact of austenite on the fracture appearance transition temperature was observed. It was found that the amount of reverse austenite does not only depend on the amount of transformed austenite during ageing but also on its chemistry, as it dictates its ability to transform into martensite during cooling.