Microstructures and Mechanical Properties of TaC Added to Vanadis 4 Tool Steel through Vacuum Sintering and Heat Treatments

Abstract

In this study, different amounts of TaC powders (10, 15 and 20 mass%) are mixed and added to Vanadis 4 tool steel powder. The composite powders are sintered at 1250, 1300 and 1325°C for 1 h, respectively. After that, a series of heat treatments is performed to determine the optimal parameters of the Vanadis 4 composites. The experimental results show the optimal vacuum sintering temperature for the Vanadis 4 composites to be 1300°C. The Vanadis 4 specimens with a 10% TaC added possess the highest transverse rupture strength (TRS) value of 1974.2 MPa and a suitable hardness (82.6 HRA). When the optimally sintered specimens of the Vanadis 4 composites then undergo a heat treatment, the TRS shows a significant increase to 2069.4 MPa, while the hardness declines slightly to 81.5 HRA. In addition, the microstructural evaluation reveals that the plate-shaped carbides (M₇C₃) located on the grain boundaries disappear after the TaC powders are added and that clustered TaC carbides appear in the grain boundaries instead. Meanwhile, with TaC as the nucleation sites of the VC carbides, the result is the generating of VC carbides in the grain boundaries. Moreover, the VC carbides decompose and re-precipitate the refined carbides (Fe₃C), which are uniformly distributed within the grains. The results also show that heat treatment effectively improves the microstructure and strengthens the Vanadis 4 composites.