Effects of Mo on microstructure of as-cast 28 wt.% Cr–2.6 wt.% C–(0–10) wt.% Mo irons

Microstructures of as-cast 28 wt.% Cr–2.6 wt.% C irons containing (0–10) wt.% Mo with the Cr/C ratio of about 10 were studied and related to hardness. The experimental irons were cast into dry sand molds. Microstructural investigation was performed by light microscopy, X-ray diffractometry, scanning electron microscopy, transmission electron microscopy and energy-dispersive X-ray spectrometry. It was found that the iron with about 10 wt.% Mo was eutectic/peritectic, whereas the others with less Mo content were hypoeutectic. The matrix in all irons was austenite, partly transformed to martensite during cooling. Mo addition promoted the formation of M23C6 and M6C. At 1 wt.% Mo, multiple eutectic carbides including M7C3, M23C6 and M6C were observed. M23C6 existed as a transition zone between eutectic M7C3 and M6C, indicating a carbide transition as M7C3(M2.3C) → M23C6(M3.8C) → M6C. At 6 wt.% Mo, multiple eutectic carbides including M7C3 and M23C6 were observed together with fine cellular/lamellar M6C aggregates. In the iron with 10 wt.% Mo, only eutectic/peritectic M23C6 and M6C were found without M7C3. Mo distribution to all carbides has been determined to be increased from ca. 0.4 to 0.7 in mass fraction as the Mo content in the irons was increased. On the other hand, Cr distribution to all carbides is quite constant as ca. 0.6 in mass fraction. Mo addition increased Vickers macro-hardness of the irons from 495 up to 674 HV30. High Mo content as solid-solution in the matrix and the formation of M6C or M23C6 aggregates were the main reasons for hardness increase, indicating potentially improved wear performance of the irons with Mo addition.