Study of tempering brittleness of tool steel 55NiCrMoV7

Horáková Klára - PRIKNER - tepelné zpracování kovů s.r.o. (Czech Republic)

Tempering brittleness is a negative phenomenon, which occurs after tempering of some kinds of steel (primarily according to the chemical composition) in certain temperature intervals. The phenomena is characterized by reduced impact toughness and fatigue properties. In practice it means that the steel should not be tempered in this interval. In some cases this fact complicates the fulfilment of prescribed requirements to mechanical properties of the tempering part, because it is impossible to reach the requirement by different combinations of tempering temperature and time.

The study examines the effect of various tempering regimes of tool steel 55NiCrMoV7 to the appearance of tempering brittleness. The steel 55NiCrMoV7 by two different producers was used for the experiment. It is known that the tempering brittleness occurs in this material in the range of tempering temperatures 350-450 °C after cooling in the still air. But it is not described how different kinds of cooling affect the phenomena. There were chosen seven tempering temperatures ranging from 200 to 600 °C for the experiment and three types of cooling methods: On still air, in nitrogen with pressure 0,5 bar and in nitrogen with pressure 3 bars. Charpy impact test, hardness, metallographic and fractographic analysis are measured. To determine the effect of the tempering regime to the appearance of tempering brittleness, the results obtained by measurement for different regimes were compared and also results for the same tempering regime, but different producer of the steel were compared.

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