Microstructural characterization and the kinetics of ageing of 15 PH stainless steel is studied in the fusion zone and in solution-treated and quenched (as-quenched) samples. Fusion zone had a finer structure than the as-quenched samples due to melting and subsequent solidification. This had a major effect on the amount of the hardness received in the fusion zone. The ageing structure of both samples was essentially the same, except for the smaller solidification cell size of the fusion zone. Strengthening was achieved by the formation of coherent precipitation of copper in the bcc martensite. Copper precipitates were found to be related to the parent martensite with a Kurdjumov-Sachs orientation relationship in both samples. Precipitates were spherical in shape and they nucleate and grow both on the dislocations and in the matrix. No incubation period was observed in the hardening curves. Kinetics of precipitation was studied from an Arrhenius type equation for both samples. It was found that, at high ageing temperatures, the activation energy for precipitation hardening was approximately the same as that of the activation energy for substitutional diffusion in bcc ferrite. At lower ageing temperatures, the calculated activation energy was consistent with the activation energy for short circuit diffusion of substitutionals in bcc structure. Microstructural characterization and the activation energy calculations showed that precipitation of copper in both samples was controlled by the diffusion of copper in bcc martensite. At high ageing temperatures, mass transport of copper was through the lattice. At low temperatures, the contribution of high dislocation density to the apparent diffusivity was large.