Determination of the hardness-toughness-relationship for different short-cycle heat treatment conditions on the steels 51CrV4, 74NiCr2 and 100Cr6

The paper covers the effect of different heat-treatment conditions, with a focus on short-cycle heat-treatment, on the mechanical properties hardness and toughness of the steels 51CrV4, ~74NiCr2 and 100Cr6. To determine this property-relationship, micro-impact samples were heat treated under various conditions in a dilatometer. The investigated parameters cover standard quenching and tempering cycles as well as short heat-treatment cycles, including overheating, with tempering at low and medium tempering temperatures. Afterwards the samples were tested regarding their impact toughness and Vickers hardness. Furthermore, additional investigations of the microstructure (optical), the retained austenite content (XRD) and the fracture behaviour (SEM) were performed. Selected dilatometric measurements on the transformation behaviour completed the investigation program. Tempering at temperatures in the range of 200°C always provided a favourable property-relationship. While short-cycle heat-treatment conditions only lead to a slightly reduced toughness at similar hardness, overheating resulted in a more significant loss in toughness with different severity for the three steel grades. Microstructural investigations provide substantial information to explain this behaviour.