Microstructural effect and wear performance of high chromium white cast iron modified with high boron contents

Abstract

The present study discuss the effects systematic additions of boron (up to 1.197 wt%) on the microstructure, bulk hardness and abrasive wear of a 17Cr– 3C–1Ni–1Mo white cast iron. The alloys were melted in an open induction furnace and cast into 25.4 mm × 12.5 mm cross-section bars. Characterization was carried out by optical and electronic microscopy, energy dispersive spectroscopy, and X-ray diffraction in the as-cast conditions and after a destabilization heat treatment. Hardness was also measured in both conditions. The results show that, boron addition produced a considerable increase of the carbide volume fraction from 27.1 to 53.84% (percentage by volume) and promotes the transition from the Cr rich M7C3 eutectic carbides to the lower C and Cr content M23(C,B)6carbide. However, the hardness values of the experimental alloys were not affected by this transition, and thus, the hardness values increased with the boron content due to a higher carbide volume fraction and the strengthening of the iron matrix reaching a maximum value of 767 HV for the higher B content. Likewise, a secondary hardening effect was produced by the destabilization heat treatment, increasing the hardness values of the different alloys. However, as the boron content increased, this hardening effect becomes less significant. Abrasive wear test were consistent with the hardness values in the as-cast condition by decreasing the wear losses with the increase in B for high and low loads. Similarly, for the destabilized irons, which exhibited higher harness values, a decrease in the wear volume losses were observed for B contents up to 0.598 wt% and the wear losses increased with the increase in applied load.In the case of the heat treated 1.197 wt% B added iron, the wear losses were significantly affected by the applied load during the wear tests. For this alloy, a volume lost of 0.639 mm3 was observed for the test with 54 N a load, which corresponds to the lower value obtained for any B content and experimental condition, while in the case of the test performed with 130 N a relatively high volume lost of 1.319 mm3 was measured which is considerably higher compared with the lower boron alloys in the same condition. This behavior was attributed to the massive carbide cracking observed after heat treatment promoting the carbide detachment during the test with 130 N of load.From the author's review of the literature, the effect of such a high boron addition on the microstructure and wear behavior have never been performed or published for high chromium white cast irons. And the based in the experimental results of the present study, the addition up to 1.197%wt boron in the as cast condition represents an effective way to increase the hardness and wear resistance of these kind of alloys in the as-cast condition suppressing the destabilization heat treatment.