The Thermodynamic Behavior, Microstructures And Properties Of CrMo Wear Resistant Cast Steel Under The Impact Of Niobium

Niobium is a frequently employed element in steel due to its effects of dispersion strengthening and grain refining strengthening. In this paper, the thermodynamics calculation and the heat treatment were performed to investigate the equilibrium phase diagram, phase transformation temperature, microstructure and properties of CrMo wear resistant cast steel ZG85Cr2 MnMo influenced by the addition of niobium. The present work includes:(1) The software Thermal-Calc was employed to calculate the effects of niobium on equilibrium phase diagram of the steel with niobium element covering the temperature of liquidus, solidus and niobium carbide precipitation. The calculation results showed that the addition of niobium element can decrease the liquidus temperature, raise niobium carbide precipitation temperature. The solidus temperature will experience a promotion and then decrease trend when the niobium content is gradually increased.(2) The thermal expansion measurement showed that the Ac1 and Accm temperature raised 2.9℃ and 3.4℃ when niobium element increased from 0.019 wt.% to 0.1wt.%. A structure transition from pearlite to martensit was observed when the cooling rate is over 0.5℃/s.(3) The influence of niobium on carbides precipitation during the tempering treatment was also investigated via using the software JMatPro. The results showed that the several types carbides would form during the tempering treatment, including M3 C, M23C6, M7C3, M(C,N), M2(C,N), M6 C carbides. The addition of niobium element could decrease the volume fraction of formed M3 C, M23C6, M2(C,N), M6 C carbides, raise the volume fraction of M7C3 carbide. The volume fractio of M(C,N) carbide would grow firstly and then reduce with the increase of niobium content.(4) The austenite grain size in the steels quenched at 880℃ and 910℃ was concerned under the influence of niobium element. The results showed that austenite grain size was significantly reduced by the niobium element. The finest grains can be observed in the steel with the 0.052 wt.% niobium.(5) The influence of niobium on microstructures and hardness was investigated in the steels tempered at 450℃ and 500℃, 550℃ and 600℃, respectively. The results presented that that addition of niobium element did not generate the obvious influences on the microstructure and hardness of steels tempered at 450℃ and 500℃. However, the refined microstructures and higher hardness can be achieved by the niobium element when the steels was tempered at 550℃ and 600℃.