Microstructure stability and softening resistance of a novel Cr-Mo-V hot work die steel

Abstract

To enhance the high-temperature stability of hot-work die steel, a novel hot-work die steel was designed on the basis of 55NiCrMoV7 steel by increasing the content of Mo and V. The hardness variation trend of the two steels during high-temperature service was investigated through a thermal stability test. Test methods such as X-ray diffraction, scanning electron microscope, high-resolution transmission electron microscope, and energy spectrum analysis were employed to analyze the microstructure and carbide evolution. Combined with the Johnson–Mehl–Avrami equation, the influence of Mo and V on the high-temperature aging kinetic process was analyzed. The results indicated that the evolution of carbide and the matrix recovery process exhibit a coupling appearance. The thermal stability and tempering resistance of material experiment steel (MES) 2 steel at the test temperature were optimized, and its tempering activation energy is 34.9 kJ·mol−1 higher than that of MES 1 steel. Under the same tempering conditions, the matrix recovery of MES 2 steel is slow, and carbide coarsening is effectively inhibited. This is mainly because Mo and V are strong carbide-forming elements that preferentially combine with C to form MC over Cr, Mn, and other elements. As a result, more Cr and Mn dissolve in the matrix. This not only plays the role of solid solution strengthening but also reduces the diffusion rate of C in the matrix at high temperatures and effectively inhibits the precipitation and growth of carbides.

Keywords

• 55nicrmov7
• high-content mo and v hot-work die steel
• thermal stability
• jma equation
• tempering activation energy
• carbide