The goal of the present work was to investigate the influence of hot working on the microstructure of the precipitation hardening martensitic steel PH 13-8 Mo. In addition to understand the influence of hot working parameters on the grain size of the recrystallized microstructure, a possible formation of deformation-induced delta ferrite was also investigated. Initially, single-hit upsetting tests were performed and recorded at different hot forming parameters, delivering a set of flow curves. After correction of the flow curves under consideration of friction, they were smoothed as well as fitted and the relevant strains as well as flow stresses for dynamic recrystallization were determined. Double-hit upsetting tests were used to investigate meta dynamic recrystallization and the recrystallized fraction of the microstructure was determined after different holding times between the forming steps. Furthermore, the individual specimens were prepared and the hardness and on optical microscopy images, the grain size, were determined for comparison with the forming parameters. EBSD measurements were also carried out to visualize the martensite structure as well as the prior austenite grains. Finally, the samples were analyzed for the presence of delta ferrite and the results were documented. The dependence of grain size and hardness on the forming parameters demonstrated in this work follows the technical findings available to date, but nevertheless, minor deviations were found. It could also be shown that the material recrystallizes within a very short time and that the martensite blocks and the former austenite grains do not exhibit a preferred orientation or texture. However, a deformation-induced formation of delta ferrite could not be clearly verified.