Microstructural modification of a static and dynamically solidified high chromium white cast iron alloyed with vanadium
Alexeis Sánchez-Cruz, A. Bedolla-Jacuinde, F. V. Guerra, I. Mejı́a
Abstract
The objective of this study is to analyze the effect of dynamic solidification and vanadium additions in a range from zero to 6.42 wt% to a high chromium white iron with the purpose of refining the microstructure and to improve hardness. The alloys were characterized by optical and electron microscopy, EDS and X-ray diffraction and complemented with phase quantification undertaken by image analysis and XRD data. Bulk hardness was measured for different casting thicknesses in both as-cast and after a destabilization heat treatment at 900 °C for 45 min. The results show that V addition produces a refining effect on the M7C3 carbide as well as the VC carbide formation which can nucleate as primary carbide for V contents higher than 4.75 wt%. Dynamic solidification of the experimental irons produced a notable decrease in the size of the carbide phase and increased its volume fraction. This solidification technique also produced a transformation of the iron matrix of the base alloy from predominantly austenitic to a fully pearlitic matrix, while for the 6.42 wt% V alloyed iron a notably smaller, slightly elongated and isolated VC carbides embedded in a highly martensitic matrix were observed. This microstructural modification resulted in the maximum harness values obtained in the as-cast conditions. After the destabilization heat treatment, a softening of the microstructure of the V alloyed irons was observed due to the C depletion of the matrix inhibiting the martensitic transformation; conversely, the base alloy experienced a considerably increase in hardness as a result of the high amount of precipitated carbides embedded in a highly martensitic matrix. The results of the present study show that, V alloying and the dynamic solidification technique can be used as effective ways to refine the carbide phase obtaining high hardness levels in the as-cast condition, avoiding this way heat treatments.