Carbide Precipitation Behavior of High-Alloy Steel During Solidification and Cooling Process

The precipitation, growth, and transformation of carbides rich in Cr and Mo during solidification and cooling process of high-alloy steel for cold working dies are investigated. The Fe–C pseudobinary phase diagram of high-alloy steel is calculated by Thermo-Calc software to ascertain the critical phase transformation temperatures. The temperatures of phase transformation during the solidification process are determined by a differential scanning calorimeter. Then the microstructure of the samples at these characteristic temperatures is observed in situ by high-temperature confocal microscope. The results show that the characteristic temperatures for phase transformation and carbide precipitation are 1380, 1315, 1140, and 980 °C, respectively. At 1315 °C, thick rod-like or island-like M₇C₃ carbides enriched in Cr and Fe are formed. At 1140 °C, secondary M₇C₃, adhering to the primary eutectic M₇C₃, grows rapidly in a rod-like morphology and maintains a similar composition. The secondary M₆C carbides of segregated Mo elements precipitate in fan-shaped clusters near M₇C₃ through the dissolution of M₇C₃ and the conversion of M₂C. Thermodynamic calculation further indicates that the eutectic Cr₇C₃ precipitates at 1213 °C, and secondary Cr₇C₃ remains under conditions favorable for precipitation below the solidus, preceding the precipitation of secondary carbides Cr₂₃C₆ and Mo₂C.