Influence of lowered austenitisation temperature during hardening on tempering resistance of modified H13 tool steel (Uddeholm Dievar)

摘要

The surface of large tools will be exposed to the hardening temperature for longer times than the core. This might in occasions, result in grain growth. In order to prevent this, it has become practice to lower the hardening temperature. This paper presents the effect of this practice on the precipitation of tempering carbides and the tempering resistance of Uddeholm Dievar. Composition of equilibrium austenite and the undissolved carbides at two different hardening temperatures were estimated by Thermo Calc simulations and the calculations predict that the balance between the amounts of molybdenum and vanadium in the austenite is shifted towards more molybdenum at the lower austenitising temperature. Since molybdenum stabilises M_2C precipitates, it was predicted also that the tempering carbides would be almost only M_2C in the sample with the lower austenitising temperature, whereas for the higher austenitising temperature, the subsequent tempering would yield a mixture of the much more stable MC together with M_2C. Samples were hardened at the simulated temperatures and tempered. The existing carbides were investigated with help of SEM and TEM. The result shows that a lowered austenitisation temperature decreases the tempering resistance. However, the transmission electron microscopy reveals that both samples have the same mixture of tempering carbides, as the samples do not reach thermodynamical equilibrium during the holding time at the hardening temperature. The lower austenitising temperature gives less tempering carbides as less alloying elements are dissolved.