The hardness-toughness relationship for the steel 42MnNiCrB4-3 was investigated by a dilatometer. This included standard heat treatment (850°C, 30 min), short-cycle heat treatment (950°C, 16 s) and double short-cycle heat treatment (2 x 950°C, 16 s). The tempering process lasted one hour for all samples and temperatures up to 500°C were examined. Furthermore, isothermal transformation treatment (austempering) with 320 / 360 / 400°C were performed. To characterize these heat treatment conditions, three notched impact samples were used. After the dilatometer experiments, impact testing and Vickers micro-hardness testing was performed. Additional investigation on the tested samples comprised XRD, LOM as well as SEM. Regarding the hardness-toughness relationship, there are no significant differences between standard HT and short-cycle HT. The results showed a severe reduction in toughness properties due to the effect of 300°C - brittleness (blue brittleness). Tempering at 200°C provides a good combination of this relationship for each heat treatment condition. The best hardness-toughness relationship was achieved by isothermal treatment at 320°C and 360°C. There is no cleavage fracture at any conditions, but a significant change in fracture surface appearance could be found at the isothermal transformation treatment at 400°C. For all heat treatment conditions tempering at 250°C or higher removes the retained austenite.