Effects of W on microstructure of as-cast 28Â wt.%Cr-2.6Â 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. MxC 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.