Through the study of fatigue wear failure forms and influencing factors, it is understood that fatigue defects mainly appear on the surface of gray cast iron. Therefore, strengthening the contact surface can effectively improve the contact fatigue performance of gray cast iron. In modern industry, the original gray cast iron is generally strengthened by special treatment, such as carburizing, heat treatment, laser remelting, or the composite layer of matrix + strengthening layer is prepared, so as to improve its service life. In this regard, the common surface treatment methods are as follows:
Using the principle of electrochemistry, metals and alloys are deposited on the surface of the workpiece to form a dense, uniform and well bonded deposition layer, so as to achieve the way of surface strengthening. This method can make the surface of gray cast iron obtain excellent properties, such as high wear resistance, high corrosion resistance and so on. However, compared with other processes, the electroplating process is more complex and the processing cost is relatively higher.
(2) Thermal spray
Thermal spraying is a coating technology that uses a certain heat source to heat the coated gray cast iron to the molten or semi molten state, atomizes it with the help of flame flow or high-speed gas, and promotes these atomized particles to spray onto the substrate surface to deposit into a coating with a certain function. This process can prepare the reinforced coating according to the different service environment of the device, and the temperature change of the matrix gray cast iron is small in the spraying process, so it is not easy to produce stress or deformation due to heating.
(3) Vapor deposition
Vapor deposition is to deposit vaporized molecules or ions on the surface of gray cast iron to form a surface strengthening layer by physical or chemical means, that is, physical vapor deposition (PVD) and chemical vapor deposition (CVD). This treatment method also has diversity in the selection of chemical composition of the strengthening layer, and the thickness and purity of the coating can be controlled, the deposition speed is fast, and the quality of the film is also very stable. However, the film prepared by this treatment method has small thickness and harsh processing conditions (vacuum environment), so it is not easy to be applied to heavy load and large devices.
(4) Shot peening
The inertia force of the high-speed rotating impeller, i.e. centrifugal force or compressed air, is used to accelerate the pellets to 60-90 M / s, so as to impact the surface of metal parts and produce a certain residual compressive stress on the surface, so as to improve its fatigue life [67-69]. The utility model has the advantages of simple equipment, low cost, no limitation of workpiece shape and position, and convenient operation. However, the processing environment is poor and it is easy to introduce impurities or second phase particles, which seriously affects the fatigue life of gray cast iron. Moreover, the bombardment effect on the surface of gray cast iron is random, which is easy to cause many uncertain factors.
(5) Plasma sputtering
Plasma sputtering belongs to the treatment process of metal surface modification by glow discharge and plasma in a specific atmosphere lower than 0.1MPa, using the glow discharge generated between the cathode and anode of the workpiece. In addition, there are plasma carburizing, nitriding and sulfurizing processes. Plasma treatment can significantly improve the wear performance of treated gray cast iron, but the coating obtained is too thin to be applied in high load machinery, and the processing environment is relatively harsh, so it is difficult to apply to large components.
(6) Laser treatment
The surface of gray cast iron to be treated is irradiated by high-energy laser to form an instantaneous high temperature in the irradiation area. Under the action of thermal diffusion, it is cooled at a very high cooling rate to form a strengthening layer with a certain thickness. This technology can effectively improve the mechanical and physical properties of the surface of gray cast iron, and then improve the wear and fatigue resistance of gray cast iron, so as to meet the application requirements of practical engineering. It is a non-contact processing method. At the same time, the energy, moving speed and processing path of the laser beam can be adjusted. In addition, the requirements for the processing environment are not very high and the controllability is strong, which determines that the laser surface treatment method can be widely used in practical production.