Kinetics of precipitation in 17–4 PH stainless steel

Abstract

The sequence of precipitation and its kinetics in 17–4 PH (precipitation hardening) stainless steel were studied by observing the electrical resistivity and microstructure of the alloy during isothermal aging at various temperatures in the range 320–600°C. By the absence of an incubation period for the onset of precipitation it is shown that there is no free energy barrier to nucleation. The electrical resistivity of the specimen decreased on prolonged aging approaching a steady value asymptotically with time. The alloys aged above 550°C were found to have higher final resistivity values than those aged at lower temperatures. By transmission electron microscopy, local reversion of the martensite to austenite, attributed to enhanced diffusion and concentration of copper atoms at the lath boundaries, was revealed in the specimens aged at temperatures above 550°C. The kinetics of precipitation in the system obeyed the Johnson–Mehl equation. The activation energy Q of the precipitation process was estimated to be 112·2 ± 3·6 kJ mol^−l from the resistivity measurements. This may be understood in terms of an enhanced diffusion of copper atoms in the supersaturated matrix caused by a higher dislocation density and a higher concentration of quenched-in vacancies.