| Cemented carbide is an important structural material used widely in metallurgy, mechanical, petrochemical, mining, electronic and medical industry. Since 1950s, Cemented carbides became hot topics in the world, one of them was to evaluate and predict reasonably the service performance, and improve service efficiency of cemented carbide products. Some physical metallurgy issues such as property change, phase structure, microstructure evolution, deformation, fracture and their mechanisms under some simulated real working conditions were systematically studied by OM, SEM, TEM, XRD, hardness tester, thermal simulation instrument, finite element analysis method. These simulating real working conditions included high temperature, thermal shock, thermal force combined fatigue, impact fatigue, oxidation and so on. Some study content and conclusion are as follows:(1) The effect of thermal interaction on structure and property of cemented carbide was studied, and crack growth rate of cemented carbide with different grain size under thermal shock condition. The results indicated that, the strength of cemented carbide was same to that at room temperature below 300℃, and the strength decreased remarkably between 400℃～500℃, with the temperature higher than 500℃the strength decreased slowly. Under thermal shock cycling condition, the content ofα-Co increased. The solution of the elements W and C in binder phase could strengthen the alloys. Thermal stress and oxidation resulted in the damage of alloy. The interaction of many factors improved the transverse rupture strength and fracture toughness a little after one cycle, while transverse rupture strength and fracture toughness decreased when thermal shock cycle time increased. Under thermal shock cycling condition, the crack initiation of cemented carbide was explosive, and the crack might grow in step style. With the decrease of precut notch, the incubation period of the second propagation of crack became short, and the effect of notch depth on crack propagation rate was small. With the temperature difference of thermal shock and the thermal conductivity of cooling medium increased, crack propagation became faster. The critical thermal shock temperature difference△Tmin of the crack propagation of 3540 alloy was between 330～360℃under water quenching.(2) The hardness variation and microstructure evolution of WC-12Co cemented carbides were investigated. The fracture mechanisms of alloys were discussed under thermal and force fatigue conditions. The results indicated that, the hardness decreased with the testing temperature, load and stress amplitude. As temperature and stress increased, some WC/WC grain boundaries became separated, meanwhile, bonding phase layers became thicker and wider, and then formed bonding phase strips, the WC skeletal structure was destroyed. As temperature and stress getting higher, the grain boundary slipping was more obvious, and the binding phase strips increased and became thicker. A large number of micropores formed, and the grain of hard phases became spherification and crushed. The deformation mechanism of WC-12Co alloy under thermal and force was that, plastic deformation of alloy under low temperature and stress was achieved by the dislocation moving of WC phases and fcc-hcp transformation of binding phase. As temperature and stress increasing, the dislocation moving of WC phase intensified and martensitic transformation of binding phases took place in all directions. However, with the increase of deformation degree stacking fault appeared in the alloy. The process of crack iniation and propagation was as followings, stress concentration occurred at defects, As fatigue going on, binding phases began to crack, hard phases were also prone to deform or crack in these places, and defects were able to grow along length section and generated micropores. Afterwards, micropores grew up and captured neighbor micropores which lead to the short crack and connection of micropores. Finally, the short crack continued to extend, and resulted in the fracture of the sample.(3) The mechanical property, life time and microstructure evolution of WC-6Co cemented carbides under impact fatigue condition were investigated. The results indicated that, the impact fatigue life decreased in exponential order with increasing of impact energy and experiment temperature. In room temperature, impact fatigue limit is between 0.3 and 0.4 Ak. The fatigue life not effected by impact frequency when the impact energy and temperature is low, and slightly decreased with the impact frequency increased at high temperature and impact energy level Flexural strength and vicker hardness of WC-6Co declined with the increasing of testing temperature, impact fatigue energy and impact fatigue cycles. The decreasing extent of vicker hardness is much lower than that of flexural strength. The cracking of WC/WC interface and binding phase in the main reason for the impact fatigue fracture of WC-6Co alloy. Cleavage fracture occurred on minute quantity of WC grains with big size which also confirmed by the observation of fatigue striations. with the testing temperature increased, the cleavage fracture of WC disappeared. With the increasing of impact fatigue cycles, the density of dislocation in binding phase increased and more precipitates separated with bigger size. Dislocation tangles formed in rigid phases and the grain boundaries of WC/WC slipped with the smoothing of hard phase grains. The continued skeleton construction destroyed. Small holes and micro-cracks were also observed in WC-6Co alloy.(4) Oxidation weight gain was measured on 3555 and 471 cemented carbides. The microstructures and phase component after oxidation were examined by SEM and XRD, respectively. The oxidation thermodynamics and dynamics were studied. SEM analysis showed that, oxidation did not occur below 300℃, slight oxidation at fracture location at 400℃, became aggravating between 500～600℃, above 700℃, hardness phase oxidized and formed porous oxides. After oxidation above 700℃, the grains at out layer changed from regular polygon to columnar. The results of thermodynamics showed that the oxidation reaction Gibbs free energy of bonding phase soluted with W and C atoms was lower than that of WC phase. Oxidation weight gain curve of 471 alloy was a straight line indicating low oxidation resistance. The activation energy of 471 alloy was 254.7 kJ·mol-1. Oxidation weight gain curve of 3555 alloy was a parabola and the activation energy was 254.7 kJ-mol-1.(5) A modeling of stress distribution, deformation and failure of cemented carbide during three point bending is constructed by RFPA method. The modeling results were coincident with experimental results well. During the loading process, the interface of bonding phase and hard phase fails firstly, and then the bonding phase failed, finally penetrated to form a crack. When a crack propagated to a hard phase, it would inflex when the angle between crack and hard phase was small. If the angle was big, the crack would stop. When the stress accumulated to some degree, the crack would propagate through the hard phase. The final failure was not caused by only one crack, the main crack depresses others. Analysis results about stress distribution showed that, stress concentration occured at bottom face of samples, then cracks initiated. With the propagation of cracks, tensile stress concentrated to the crack tip to form a type of 'T' crack. Compress stress appeared at loading point and catch points. With the increasing volume ratio, the wider variation range of particle size, or the larger average particle size of the hard-phase, this alloy often exhibits higher strength.
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