The microstructure evolution and dimensional stability of TiC steel-bonded cemented carbide during stabilizing heat treatments

Abstract

Stabilizing heat treatment is an ideal method to improve the performance of heat-strengthened steel-bonded cemented carbide for high-precision inertial instruments and some wear parts. In the present study, a TiC steel-bonded cemented carbide consisting of 35 wt% TiC hard phase and chromium‑molybdenum low alloy steel matrix was subjected to various stabilizing heat treatments, including annealing, quenching, tempering and thermal cooling cycling (TCC) treatments. X-ray diffraction analysis (XRD), scanning electron microscopy (SEM) and electronic probe micro-analyzer (EPMA) were applied to analyze the phase characteristics and microstructure of the material. In addition, the influence of microstructure evolution on the magnetic properties and dimensional stability of the cemented carbide was also investigated, and the underlying stabilizing mechanism was elucidated. The results showed that the microstructure of matrix was spheroidal pearlite after annealing and transformed to martensite after stabilizing heat treatments. The average TiC particle size of annealed cemented carbide was 2.22 μm, which decreased to 2.09 μm after quenching and increased to 2.31 μm and 2.29 μm after tempering and TCC treatments, respectively. In addition, the TiC particles in cemented carbide had obvious fractal characteristics. The ultimate strain of cemented carbide after tempering and TCC treatment was 3.01 × 10−4 and 8.38 × 10−5, and the dimensional stability of the tempered specimen was relatively poor. Therefore, the TCC treatment not only improved the strength of cemented carbide, but also significantly improved the dimensional stability of TiC steel-bonded cemented carbide. A variety of stabilizing mechanisms, including martensitic transformation, retained austenite-martensite transformation, martensite refinement, fine carbides precipitation, and residual stress release affected the microstructure and properties of cemented carbide during the stabilizing heat treatments, hence resulting in a significantly improvement of the dimensional stability of TiC steel-bonded cemented carbide.