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

Microstructures of as-cast 28 wt.%Cr-2.6 wt.%C irons containing (0-10)wt.%W with the Cr/C ratio about 10 were studied and related to their 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 irons with 1 to 10 wt.%W addition was hypereutectic containing large primary M₇C₃, whereas the reference iron without W addition was hypoeutectic. The matrix in all irons was austenite, partly transformed to martensite during cooling. The volume fractions of primary M₇C₃ and the total carbides increased, but that of eutectic carbides decreased with increasing the W content of the irons. W addition promoted the formation of W-rich M₇C₃, M₆C and M₂₃C₆. At about 4 wt.%W, two eutectic carbides including M₇C₃ and M₆C were observed together with primary M₇C₃. At 10 wt.%W, multiple carbides including primary M₇C₃, fish-bone M₂₃C₆, and M₆C were observed. M_xC where x = 3 or less has not been found due possibly to the high M/C ratio in the studied irons. W distribution to all carbides has been determined increasing from ca. 0.3 to 0.8 in mass fraction as the W content in the irons was increased. W addition led to an increase in Vickers macro-hardness of the irons up to 671 kgf/(mm)² (HV30/15) obtained from the iron with 10 wt.%W. The formation of primary M₇C₃ and aggregates of M₆C and M₂₃C₆ were the main reasons for hardness increase, indicating potentially improved wear performance of the as-cast irons with W addition.