Effect of Salt Bath and Oil Quenching Heat Treatments on SKT4 Forging Dies for Sustainable Undercarriage Link Production

SKT4 steel, classified by the Japan Industrial Standard (JIS) as a hot-forming mold steel, is widely used for forging dies due to its favorable chemical composition and mechanical properties. During forging, dies are subjected to repeated impacts and high temperatures, which gradually reduce their performance and contribute to product defects. However, quantitative comparisons between salt bath and oil quenching treatments for SKT4 dies in high-cycle forging applications remain limited. This study evaluates the effects of salt bath and oil quenching, combined with tempering at 400°C and 450°C, on the microstructure, mechanical properties, and service performance of SKT4 dies used in undercarriage link forging. Forging trials were conducted for 3,000 and 6,000 production cycles. The results demonstrate that salt bath quenching significantly improves die hardness and wear resistance compared with oil quenching. The optimum condition was achieved with salt bath quenching followed by tempering at 400°C, producing the highest hardness of 46.1 HRC and the lowest specific wear rate of 6.98 × 10⁻⁵ mm³/kg·m. Dies treated under this condition showed no visible product defects after 6,000 forging cycles. Microstructural analysis revealed finer and more homogeneous tempered martensite laths in salt bath-quenched specimens. Unlike oil quenching, salt bath quenching eliminates vapor blanket formation, enabling direct convective cooling, more uniform temperature reduction, and minimal distortion. These findings indicate that salt bath quenching with 400°C tempering provides the optimal combination of hardness, wear resistance, and durability, supporting longer die life, reduced rework, and more sustainable forging operations.