Influence of Thermal Processing Factors, Linked to the Destabilisation of Austenite, on the Microstructural Variation of a White Cast Iron Containing 25% Cr and 0.6% Mo

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.

Keywords

• white cast irons containing 25% Cr
• hardness
• destabilisation of austenite
• distortion of the ferrite cell
• secondary carbides
• M₂C carbide