A mathematical model was used to calculate the internal stresses and strain produced when plates of a high hardenability steel (835M30) were quenched in water, polymer and oil. A prerequisite for the calculations was the experimental determination of relevant physical and mechanical property data not available from other sources? this included the surface heat transfer coefficient during quenching and the mechanical properties of 835M30 steel in the metastable austenitic condition. The results of the thermal stress calculations showed that the production of residual strain was markedly dependent on both the surface heat transfer coefficient and the flow strength of the steel during the early stages of the quench. It was predicted that the low stability of the vapour blanket, found during the early stages of a water quench, gave large variations in distortion. The more consistent values of the surface heat transfer coefficient, obtained during oil quenching, were associated with more reproducible levels of distortion. The predicted residual stress distributions were less sensitive to variations in either the surface heat transfer coefficient or the flow strength of the steel during the early stages of the quench. However, the residual stress distributions were strongly dependent on the flow strength of the material during the martensite transformation. A correlation of the results of the calculations with experimental behaviour showed that agreement was good in the cases of both the residual stresses and strains obtained after a water quench. However, in the case of oil quenched plates, a similar correlation was only obtained with residual strains. A possible explanation for the lack of good correlation in the case of oil quenched plates may be found by consideration of the effect of strain rate on the mechanical properties of the material, particularly during the martensitic transformation.