Abstract
Hypoeutectic white cast irons containing 25% Cr are used in ore-processing industries due to their high resistance to erosive wear. Applying a Design of Experiments (DoE), the aim of this study is to analyse the influence of thermal processing factors on the microstructural variation of a white cast iron containing 25% Cr and 0.6% Mo. The carbides present in the as-cast state are of the M7C3, M2C, and M3C types. M2C carbides precipitate on the eutectic M7C3 carbides favoured by heterogeneous nucleation conditions. Two kinetics compete during the destabilisation of austenite. One dissolves those eutectic carbides precipitated as a result of non-equilibrium solidification (M7C3 and M2C), while the other enables the precipitation of secondary M7C3 and M23C6 carbides. The M7C3 carbides begin to precipitate first. Low destabilisation temperatures and short dwell times are insufficient to dissolve the precipitated eutectic carbides from non-equilibrium conditions, thus favouring the presence of M2C carbides, which are associated with Mo. The factor that has the greatest influence on hardness is the tempering temperature. The optimal tempering temperature is found to be 500 °C. Short tempering times maintain the distortion of the ferrite unit cell. The precipitation of Cr carbides during tempering requires a temperature of 500 °C and the prior dissolution of the carbon precipitated during the initial stages of said tempering. With short tempering times, the Cr atoms still remain dissolved in the ferrite, distorting its unit cell and increasing the hardness of the matrix constituent of the alloy.
Highlights
The microstructure of hypoeutectic white cast irons containing between 15% and 30% Cr is mainly composed of eutectic carbides in an austenite matrix, or of any of the phases or constituents resulting from their transformation [1,2]
Wiengmoon et al [5] conclude that the secondary carbides which precipitated during the destabilisation of austenite at 1000 ◦ C are M7 C3 -type carbides in cast irons containing 20% Cr
The aim of this study is to analyse the influence of thermal processing factors on the microstructural variation of a white cast iron containing 25% Cr and 0.6% Mo via the application of a Design of Experiments (DoE), analysing how the modifications of those factors that affect the destabilisation of austenite influence the percentage of the different carbides that are formed, the percentage of martensite, and the percentage of retained austenite
Summary
The microstructure of hypoeutectic white cast irons containing between 15% and 30% Cr is mainly composed of eutectic carbides in an austenite matrix, or of any of the phases or constituents resulting from their transformation [1,2]. This austenite can be destabilised by means of a high-temperature heat treatment, causing the precipitation of Cr-rich carbides. This precipitation results in an increase in the Ms temperature, reducing the risk of cracking during cooling [3,4]. Guitar et al [6] conclude
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