Hardness-toughness relationship of a boron alloyed quench and tempering steel after different heat treatment cycles

The hardness-toughness relationship for the steel 42MnNiCrB4-3 was investigated by a dilatometer. This included standard heat treatment (850^oC 30min), short-cycle heat treatment (950^oC 16s), double short-cycle heat treatment (2 × 950^oC 16s). The tempering process lasted one hour for all samples and temperatures up to 500 ^oC were examined. Furthermore, isothermal transformation treatment with 320/360/400 ^oC 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 conditions. 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 can remove the retained austenite.