| In this paper, by adjusting the size(50 microns and 100 microns) and content(25wt%, 35wt% and 45wt%) of WC particulates, four kinds of WC particulates reinforced steel matrix composites, as well as the 5CrNiMo steel were prepared by the composite electroslag melting and casting technology. Then the material was processed with annealing and forging after casting. A total of six kinds of process including 950℃, 1000, and 1050℃ as quenching temperature, 180 ℃, 220 ℃ and 300 ℃ as tempering temperature were choosed to the composites for heat treatment.Following conclusions can be drawn from tests and analysis in this study.The casting microstructure of WC particulates reinforced steel matrix composites on the original state is mainly composed of martensite, residual austenite, eutectic ledeburite, and all kinds of carbides. By annealing treatment, long strip carbides dissolve or partly dissolve in the steel substrate and big blocky carbides break down. And then through forging processing, dendritic, bone shape and fishbone eutectic organization with high thermal stability are broken. When tempering at low temperature, the organization transformation of the composites was mainly that of the steel matrix, including the segregation of carbon within the matrix, the decomposition of martensite, the transformation of residual austenite, and the precipitation, segregation and spheroidizing of carbides. The microstructure is mainly composed of aphanitic tempering martensite, carbides and retained austenite. The type of the carbides were mainly raw WC particles, larger crumby particles of Fe3W3 C, dendritic carbides of Fe3W3 C or M7C3, secondary carbides of Fe3W3 C or M23C6 as distributing dispersively.The dissolution of WC grains will make the edges and corners passivate, and great white WC particles are wrapped with a circle of black stripe for Fe3W3 C. Analysis made by EBSD and EDS is that after high temperature quenching and low temperature tempering, the highangle boundary of the composites increases significantly, the grain size gets smaller markedly, and the grains distribute uniformly, resulting in the effect.of fine-grain strengthening. Cr elements are mainly in the larger grain of steel matrix, and Ni elements are on the contrary.Rockwell hardness of the composites will reach HRC 60 to 66 when quenching at 950℃ to 1050℃, with the fluctuation of first rising and then falling. The nanometer hardness and elastic modulus of steel matrix for the composites all increase after heat treatment, with smaller change of that for WC particles. The bending strength of the composites will reach 1600 to 1650 MPa when quenching at 950℃ to 1050℃, with the fluctuation of first rising and then falling. Under the condition of forging and annealing, the bending fracture is compound fracture of quasi-cleavage and dimples. Under the condition of quenching and tempering, the composites showed the fracture mechanism of cleavage fracture and dimples within partial matrix.Fractal research based on the Sierpinski’s measurement of fractal dimension and calculation method, showing that fractal dimension of WC with the change of the heat treatment process presents different varation. When quenching at high temperature and then tempering, WC exists two groups fractal structure of different particle size and quantity. The higher the quenching or tempering temperature is, the larger the difference of fractal dimension is, and the greater the morphology of the WC change.Thermal fatigue cracks with shorter incubation period, whose initiation was from the crack in the root of the v-shaped notch, extends discontinuously and intermittently mainly in the form of a main crack. The extended modes of the main cracks are mainly extending along the interface of the carbides and steel substrate, passing through large WC particles and crumby carbides, extending along reticular carbide chain, passing through accumulation area of small WC particles, the fishbone carbide, and the steel matrix. The main morphology of the cracks extending on the surface of the sample are linear, bent or trapezoid, circular arc and bifurcation, including the "bypass" type crack.With the increasing of WC content or the particle size in a certain range, the friction coefficient of the composites is increased. The wear mechanism of two-bady abrasion is abrasive wear and oxidation wear. For three-body abrasive wear, the friction coefficient in the run-in period presents a larger leaps with longer run-in period than two-body abrasion. Serving under the environment of two-body abrasion will play the best wear resistance of the composites. The main wear mechanism of three-body abrasion is fatigue fracture mechanism of the deformation layer caused by multiple plastic deformation or micro pressure.To sum up, in a certain range, the larger the WC particles size is, the higher the content is, the higher hardness of the composites will be, the lower bending strength of the composites will be, the worse impact resistant ability of the composites will be, while the better the two-body abrasion resistance and three-body abrasive wear resistance will be. When the composites with 45 wt % coarse WC particulars are heated at near 1000℃ of quenching temperature, and near 220℃ of tempering temperature, the better organization structure, strength and toughness can be guaranteed, while obtain comprehensive performance with the high hardness, high wear resistance, and high temperature resistant at the same time. |
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