Phase Analysis and Measurement of Local Carbon Contents in Hypoeutectic Alloys in the System Fe-C-B-Cr-W

Abstract

Cold work tool steels of the Fe‑0.4C-1B-Cr alloy system use the element boron (B) for hard phase formation. B should facilitate a reduction in the contents of carbide-forming elements and carbon (C) to the extent that C is used exclusively for martensitic hardening, while allowing the practical utilization of hard borides as hard phases. The formation of carboborides stands in the way of this concept. To further develop the alloying system, an interaction of the alloying elements tungsten (W) and C during solid-state microstructure formation during heat treatment is investigated. The main objective is to investigate the influence of hardening as well as solution annealing on the solution state of C in the martensitic matrix in different W‑alloyed high B‑content cold work tool steels. For this purpose, two laboratory melts of Fe‑0.4C-1B‑2.5Cr‑W are cast with a W content of 2 and 9 mass%, swaged, austenitized at 1100 °C for varying time steps, and quenched. A microstructure and phase analysis is carried out by means of scanning electron microscopy (SEM), electron backscatter diffraction (EBSD), and X-ray diffraction (XRD). Furthermore, the local C contents of the martensitic matrix and hard phases are measured by wavelength dispersive X‑ray spectroscopy (WDS). The results are discussed considering thermodynamic equilibrium calculations. It is shown that the addition of W leads to the eutectic formation of W‑rich carboborides, which are thermodynamically stable at austenitization temperature. However, high B contents in the carboborides lead to a significant reduction in the boride content of these alloys, which results in significant differences to the thermodynamic calculations.