Critical Impact of Cooling Rate on Intergranular Carbide Formation and Service Performance in 4Cr5Mo2V Steel

This study investigates carbide precipitation along grain boundaries in 4Cr5Mo2V die‐casting die steel during quenching and its detrimental effects on high‐temperature service performance. Different cooling rates (0.05–1 °C s −1 ) from 1030 to 600 °C are applied to control intergranular carbide formation. Results show that slower cooling rates promote Cr‐rich carbide precipitation along prior austenite grain boundaries, forming distinct lamellar structures that significantly degrade steel properties. Specifically, thermal stability tests at 600 °C reveal that samples with extensive intergranular carbides exhibit accelerated hardness loss within the first 4 h. High‐temperature impact testing reveals severe degradation in toughness with increasing grain boundary carbide content. Fracture analysis confirms that increasing carbide precipitation induces a transition from quasi‐cleavage (RT) and ductile fracture (600 °C) to cleavage (RT) and intergranular fracture (600 °C), directly correlating grain boundary carbides with embrittlement. These findings highlight that intergranular carbide precipitation resulting from inadequate quenching rates in large die‐casting dies significantly compromises high‐temperature strength and toughness of steel. Avoiding such precipitation is critical for ensuring superior high‐temperature performance.