Microstructure Stabilities of Newly Developed Hot‐Working Die Steel and H13 Steel at Elevated Temperature

Abstract

Hot-working die steels have an important role in the development of the industry and society, and they are widely used in industrial manufacturing due to their good mechanical properties, wear resistance and thermal resistance. [1][2][3][4] With the rapid increase in industrial and manufacturing requirements, dies need to operate under severe conditions of high temperature and pressures. The peak temperature at the surface of dies can reach even 700 °C, and exceed the tempering temperature. This inevitably leads to an evolution of the microstructure and degradation of mechanical properties, such as the hardness, strength, and thermal fatigue properties, strongly related to the service life of dies. [4,5] Therefore, the microstructure stability of hot-working die steels is of significance.The martensite lath structure is in a metastable state with a low thermal stability. The strength and hardness of the steel rapidly decrease with the recovery and disappearance of the lath structures in a martensitic steel. [6][7][8] Alloying with high levels of alloying elements is usually regarded as an effective approach to improve the thermal stability of a metallic material, but it is not a universally applicable method and is not conducive to some important properties of steels. For example, Cr is an important alloying element in a heat-resistant steel, and 9-12% Cr is usually added in the heat-resistant steel to form M 23 C 6 carbides always distributed along lath boundaries and grain boundaries to hinder their migration and recrystallization during creep. [9,10] However, the M 23 C 6 carbide is not conducive to the thermal fatigue performance and thermal stability of hot-working die steels owing to its fast coarsening. [5,11] The growth rate of M 7 C 3 in low-Cr alloys is slower than that of M 23 C 6 , which can be used as a type of precipitation pinning the microstructure interface. [12,13] Owing to the excellent thermal stability, the MC carbide can effectively hinder the recovery and recrystallization of the microstructures at temperatures lower than 600 °C. [14] Moreover, at a temperature below 0.5 T m , the deformation mechanism of steel materials is still the slip of dislocations. The stable and fine MC carbide can provide an excellent strengthening effect for the matrix under high-temperature conditions. [15,16] Therefore, the typical microstructures of hot-working die steels contain carbide forming elements such as Cr, Mo, and V to form the fine dispersed MC or M 2 C nano-carbides. [17] However, the microstructures of the present commercial hot-working die steels still becomes unstable when the temperature is increased to 600-650 °C. [6,11] In a previous study, a Cr-Mo-W-V HWD700 steel was developed, which exhibits a high strength as well as excellent fatigue performance at temperatures up to 700 °C strengthened by MC (M = V, Mo, W) nano-carbides. [18][19][20] However, the microstructure stability of the HWD700 steel at an elevated temperature has not been extensively investigated. In this study, the