The Effect of Boron on a Cr-Mo-V Hot-Work Tool Steel

In this thesis, the effect of boron on phase transformations and microstructural development in a Cr-Mo-V hot-work tool steel was investigated. Dilatometry, high-temperature laser scanning confocal microscopy (HT-LSCM), optical microscopy, electron backscatter diffraction (EBSD), high-energy X-ray diffraction (HE-XRD) and hardness testing were used to analyse a standard alloy and a boron-containing alloy. The as-delivered material exhibited a heterogeneous microstructure with alternating coarse, carbide-poor and fine, carbide-rich areas. The latter contained Cr-rich and Mo-rich carbides whereas the carbide-poor regions only contained Cr-rich carbides indicating macro-segregation during ingot casting as the cause of local carbon fluctuations, thus influencing the transformation behaviour. HT-LSCM observations showed that austenitisation at 1020 °C resulted in partial dissolution of fine carbides and moderate grain growth, while at 1100 °C extensive carbide dissolution led to by excessive grain coarsening. Dilatometry confirmed that the martensite start temperature (MS) decreased with increasing austenitisation temperature in the standard alloy but remained almost constant between 1000 and 1020 °C in the boron-modified steel. Continuous cooling experiments indicated that boron had little effect on martensitic transformation but affected diffusion-controlled transformations. At low cooling rates, the boron-containing steel showed a slight reduction in pearlite formation, while at higher rates, transformation behaviour was similar in both alloys. Boron raised the bainite start temperature by up to 30 °C, although this effect diminished with faster cooling. At 20 K/s, both alloys formed fully martensitic microstructures with minimal retained austenite. Hardness increased from about 400 HV10 at slow cooling rates to 670 HV10 at high rates, with the boron-containing alloy showing marginally higher values under intermediate conditions. Overall, boron mainly affected bainitic transformation, with minimal influence on pearlite and no impact on martensite transformation, while only minor changes in hardness were noted. Its limited effect on martensite is likely due to partial incorporation into stable carbides. Future work should focus on homogenising the starting material and investigate the effects of boron at higher cooling rates and during tempering to optimise heat treatment for enhanced tool steel performance.