17-4 Precipitation-Hardening Stainless Steel: Soft-Tough Heat Treatment Mechanism and Thermal Fatigue Characteristics

This study selected 17-4PH (Type 630) precipitation-hardening stainless steel as the raw material. After subjecting the material to precipitation hardening and softening heat treatments, the effects of these treatments on the microstructural characteristics and mechanical properties were investigated, along with a thermal fatigue test. The results showed that after the precipitation hardening heat treatment, the ultimate tensile strength and hardness of 17-4PH stainless steel increased. However, the ductility was around 2%, indicating a severe brittleness effect, which is unfavorable for ordnance applications. A double aging treatment for 12 h resulted in a combination of softness and ductility, with an ultimate tensile strength of 900 MPa, ductility of 26%, and hardness HRC 28. These material properties are suitable for cold forging in ordnance and other applications. In addition, a thermal fatigue test was conducted on the softened 17-4PH material from 750 °C to room temperature. After 3000 thermal cycles, there was no significant change in material properties, ensuring it can withstand 3000 rounds of gunfire in a barrel with excellent durability. Furthermore, XRD and fracture surface analysis confirmed that the softening mechanism of 17-4PH stainless steel and its high-temperature phase stability could provide a reference for cold forging and high-temperature applications.