Abstract
This work presents the investigation of the solidification and cooling behaviour of a newly developed Fe92.65Cr4.2V2.1B0.05C1 wt% (FeCrVBC) tool steel with special emphasis to phase formations and transformations. JMatPro simulations were compiled to predict the characteristic temperatures of phase crystallizations and transformations under equilibrium conditions. Differential thermal analysis (DTA) was used to cool down samples from 1500 °C to room temperature with various cooling rates (5, 10, 20, 40, and 50 K/min), whereby differences in the number and position of the resulting peaks occurred. To correlate the peaks with specific phase reactions, the microstructure of the variously cooled samples was analysed by means of optical (OM) and scanning electron microscopy (SEM), energy-dispersive X-ray spectroscopy (EDS), wavelength-dispersive X-ray spectroscopy (WDS), electron backscatter diffraction (EBSD) and microhardness tests. The systematic analyses show that the microstructure evolution of the FeCrVBC alloy strongly depends on the cooling rate. For low cooling rates up to 10 K/min, the austenite-ferrite transformation and a marked formation of M7C3 and M23(C,B)6 secondary carbides were observed. At rates of 20 K/min and above, the austenite-martensite transformation takes place, accompanied with a strong increase of the hardness in dendritic areas. By cooling with rates of 40 K/min and higher, the precipitation of M7C3 or M23(C,B)6 secondary carbides cannot be observed anymore, whereas primary M3(C,B) besides MC carbides were detected. Furthermore it was determined, that the very small content of boron (0.05 wt%) strongly influences the (trans-)formation of special carbides, shown by the comparison to its base alloy without boron, the formerly investigated Fe92.7Cr4.2V2.1C1 wt% (FeCrVC). Consequently, strongly depending on the formed carbides the materials hardness and wear resistance can be significantly affected by boron addition. Concluding, the obtained knowledge about the thermal behaviour of the FeCrVBC alloy and the strong impact of a small addition of boron offers the possibility for tailored adjustment of phases and respective properties for a load-adapted tool design.
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