Abstract
With the aim of improving erosive wear resistance in hypoeutectic white cast irons with 18% Cr and 2% Mo, several samples of this grade were subjected to different heat treatments at 1000 °C to destabilize the austenite. The dwell times at this temperature varied from 4 to 24 h and the samples were cooled in air or oil. The existing phases were identified and quantified by applying the Rietveld structural refinement method. The results were correlated with the hardness of the material and with the microhardness of the matrix constituent. The greatest resistance to erosive wear was achieved in those samples that had a higher percentage of secondary carbides. The longer the dwell time at the destabilization temperature of austenite, the greater the amount of precipitated secondary carbides. However, the percentage of dissolved eutectic carbides is also higher. These eutectic carbides were formed as a result of non-equilibrium solidification. Low cooling rates (in still air) can offset this solution of eutectic carbides via the additional precipitation of secondary carbides in the 600–400 °C temperature range. A sharp decrease is observed in the percentage of retained austenite in those treatments with dwell times at 1000 °C equal to or greater than 12 h, reaching minimum values of around 2% volume. The percentage of retained austenite was always lower after oil quenching and the hardness of oil quenched samples was observed to be greater than those quenched in air. In these samples, the maximum hardness value obtained was 993 HV after a 12 h dwell, which result from the optimum balance between the percentages of retained austenite and of precipitated carbides.
Highlights
White cast irons highly alloyed with Cr are used in environments where resistance to abrasive wear is a priority requirement
The presence eutectic carbides and pearlite can be observed in a matrix with abundant retained austenite
The bars show the distance between the mean value and the destabilization of austenite. Anderror it seems less relevant for improved wear resistance the the resulting and minimum values
Summary
White cast irons highly alloyed with Cr are used in environments where resistance to abrasive wear is a priority requirement. Up to approximately a 16% Cr content, the eutectic constituent is ledeburitic, where the matrix phase of this constituent is mixed carbides with a 3:1 stoichiometry. When the weight content of Cr exceeds 16%, the eutectic constituent is no longer ledeburitic but austenite, being the matrix phase of this eutectic, while the dispersed carbides that accompany it are mixed carbides with a 7:3 stoichiometry [1], presenting a similar morphology to bars or plates [2]. As a result of the destabilization of austenite at temperatures around 1000 ◦ C, Cr-rich secondary carbides precipitate [3,4,5,6,7,8,9,10,11,12,13,14,15,16,17] which promotes enhanced abrasion resistance and an increase in the Ms temperature [18], thereby leading to a reduction in retained austenite [19].
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