The residual stress on the surface of cemented carbide rods is an important factor affecting its fatigue life and cutting stability. This stress state originates from multiple links such as material preparation, processing and subsequent processing. The difference in its distribution and size will have a complex impact on the performance of the rod. Residual stress is essentially an unbalanced internal stress in the material, which can be divided into compressive stress and tensile stress. The different distribution forms of the two on the surface of the rod will directly change the stress concentration degree and microstructure evolution path of the material under stress.
In terms of fatigue life, the residual tensile stress on the surface of cemented carbide rods often becomes the cause of fatigue crack initiation. When the rod is subjected to cyclic load, the surface tensile stress will be superimposed on the stress generated by the external load, so that the local stress exceeds the fatigue limit of the material, and then cracks will be induced at the interface between the hard phase particles and the bonding phase or at the location of small defects. These cracks will gradually expand with the increase in the number of cycles, and eventually lead to fatigue fracture of the rod. On the contrary, if there is residual compressive stress on the surface, it can offset part of the external tensile stress, reduce the stress concentration effect, delay the generation and expansion of cracks, and thus significantly improve the fatigue life of the rod. This compressive stress state is like forming a "protective barrier" on the surface of the material, hindering the process of fatigue damage.
The relationship between cutting stability and residual stress is reflected in the dynamic response of the rod during the cutting process. When cemented carbide rods are used as cutting tools, the surface residual stress will affect its deformation behavior under the action of cutting force and cutting heat. If there is uneven residual stress on the surface, the stress release during the cutting process may cause micro-deformation of the rod, destroy the geometric accuracy of the tool, and then cause cutting vibration. This vibration will not only affect the quality of the machined surface, but also aggravate tool wear and even cause chipping. For example, in high-speed cutting or high-feed processing, the release rate of residual stress interacts with the generation of cutting heat, which may cause a sudden change in the stress state in the local area of the rod surface, further deteriorating the cutting stability.
The effect of residual stress on the performance of cemented carbide rods is also closely related to the depth distribution of stress. The residual stress in the shallow layer of the surface mainly affects the fatigue resistance of the material, while the stress state in the deeper layer will interfere with the overall rigidity and deformation resistance of the rod. During the cutting process, the stress field inside the tool will form a coupling effect with the external load. If the deep residual stress distribution is unreasonable, the rod may be bent or twisted during cutting. This deformation will not only affect the processing accuracy, but also make the cutting force unevenly distributed and aggravate the wear of the tool. Therefore, controlling the gradient distribution of residual stress on the cross section of the rod is a key link to improve its cutting stability.
The sintering, cooling rate, and subsequent grinding, coating and other processes in the preparation process of cemented carbide rods will have a significant impact on the surface residual stress. For example, rapid cooling after sintering will cause a temperature gradient between the surface and the inside of the rod, thereby forming residual stress; the pressure and grinding heat of the grinding wheel during grinding will also cause changes in the surface stress state. Unreasonable process parameters may lead to the accumulation of residual tensile stress, while by optimizing the cooling rate, adjusting the grinding parameters or using surface shot peening and other processes, beneficial residual compressive stress can be introduced on the surface of the rod. The core of these process control methods is to change the microstructure state of the material surface through physical or chemical effects, thereby adjusting the stress distribution.
From the perspective of microscopic mechanism, the influence of residual stress on cemented carbide rods also involves the plastic deformation of the bonding phase and the stress transfer of the hard phase particles. There is a difference in the thermal expansion coefficients of the WC particles and the Co bonding phase in cemented carbide, and this difference will cause residual stress at the interface during the cooling process. When the rod is subjected to external load, the residual stress will change the supporting effect of the bonding phase on the hard phase and affect the efficiency of stress transfer at the interface between the two phases. If there is too high residual tensile stress at the interface, it may cause the bonding phase to peel off from the surface of the hard particles, weaken the overall strength of the material, and accelerate fatigue damage. Reasonable residual compressive stress can enhance the bonding force of the two-phase interface, improve the fatigue resistance of the material and the structural stability during cutting.
In practical applications, in order to reduce the negative impact of residual stress on the fatigue life and cutting stability of cemented carbide rods, it is necessary to control it from multiple levels such as material design, process optimization and usage specifications. The residual stress state can be effectively improved by accurately controlling the sintering cooling curve, using low-temperature annealing to eliminate stress concentration, or performing stress relaxation after surface processing. At the same time, it is also an important measure to ensure cutting stability to reasonably select cutting parameters during cutting to avoid abnormal release of residual stress due to overload or overheating. Only by fully understanding the mechanism of residual stress and implementing precise regulation can the performance advantages of cemented carbide rods be maximized to meet the needs of high-end cutting.
