Steel hardening is a very old manufacturing process to improve the mechanical properties of components. Distortions and residual stresses after temperature balance are influenced by the hardening process. The subsequent rework and the life cycle depend on distortion and on residual stresses. During the heat treatment process the development of stress and deformation cannot be measured in situ. Only numerical methods allow the investigation of temperature, stress, deformation and microstructure development in the whole component during the quenching process. For this purpose heat conduction, phase transformations, and mechanical behaviour of the steel as well as the couplings and the interactions between strain and phase transformation under consideration of transformation plasticity and different volume changes have to be modelled in the used FE-Program. Residual stress and distortion measurements should be carried out to verify the numerically determined results. The choice of the quenching media has a large influence on the development of stress and deformation. For vaporizing liquids like oil the heat transfer depends on the temperature and the location of the specimen. Gas quenching is a modern, efficient process. Compared with oil, gas quenching leads to a more homogeneous and reproducible cooling process and generally to lower residual stresses. In this work numerically determined and experimental results for cylinders quenched with gas in a nozzle system which was developed at our institute are compared and discussed. Different gas pressures and gas media were investigated. Finally selected results of cylinders quenched with gas and oil will be presented and discussed.