Laser surface strengthening technology is widely used because martensite, austenite, carbide and other strengthening phases occur in the machining area. According to the research of zhuzuchang et al., in the laser processing process, the phase transformation process will preferentially occur in the high carbon region, such as near graphite, or ferrite / pearlite interface, or ferrite / cementite interface. On the contrary, in the carbon poor region, the corresponding phase transition cannot occur due to the lack of carbon atoms. Therefore, the solubility and distribution of carbon atoms will seriously affect the structure and properties of the processing area. For the structure of gray cast iron, graphite is dispersed in the matrix in the form of flakes, resulting in a very uneven distribution of carbon atoms, which will seriously affect the phase composition of the processing area.
Therefore, in order to make up for the deficiency of carbon atoms in different regions, artificially supplement carbon atoms to promote the occurrence of phase transition, this method is called ‘laser carburization’. In this chapter, using laser carburizing technology, we discussed the effects of different cladding carbon layers and different laser processing energy on the microstructure of the processing area and the fatigue wear resistance of the whole specimen.
By laser carburizing method, strip bionic units were prepared on the surface of gray cast iron to improve its fatigue wear resistance. Under the action of carbon coating, not only the absorption rate of laser energy on the surface of gray cast iron is greatly increased, but also the external carbon atoms are introduced into the molten pool under the action of thermal stress or the impact force of laser beam, which promotes the occurrence of phase transformation and makes the microstructure of carburized unit compact and evenly distributed.
Under different laser processing energy, the structure of the prepared unit is also different. The compactness decreases with the decrease of laser processing energy density, and the grain size increases with the decrease of laser processing energy density, showing poor mechanical properties. Therefore, the fatigue wear resistance of gray cast iron increases with the increase of energy density under different energy density treatment.
For different pre coating thickness, the microstructure of the prepared unit is also different. With the increase of pre coating thickness, the unit size increases and the density decreases, which seriously affects its mechanical properties. In addition, under the influence of thick coating, when the coating thickness is 0.3mm, the microstructure in the transformation zone is sorbite, while in the thin coating, the microstructure in the transformation zone is plate martensite. Therefore, the fatigue wear resistance of gray cast iron with different pre coating thickness decreases with the increase of coating thickness.
In the process of fatigue wear of gray cast iron, there will be a serious wear period in a short time, and then enter a long-term stable wear period. Because the surface of gray cast iron is treated by laser bionics, the time node when it enters the stable wear period is delayed. The average wear rate in the stable period is related to the unit structure and directly affects the fatigue wear resistance of the material. In addition, due to work hardening, the surface hardness of gray cast iron is significantly higher than the original hardness of gray cast iron in the stable wear period, and remains basically unchanged in the stable wear period.
For untreated samples, large areas of surface delamination and delamination appear on the surface after fatigue wear. In contrast, for the matrix part of treated samples, small areas of delamination and delamination appear, marking the improvement of fatigue wear resistance of gray cast iron materials. For the unit area, there is only slight surface pitting. Under the influence of the inconsistent deformation of the matrix and the element body, there are serious surface fatigue defects in the boundary area of the element body or at the junction of the matrix and the element body.
When the fatigue crack on the surface of gray cast iron propagates nonlinearly, a fan-shaped or ring-shaped closed surface fatigue defect is formed on the surface of gray cast iron, which is accompanied by the phenomenon of surface delamination; When the fatigue crack on the surface of gray cast iron grows linearly, there is continuous metal removal on one side of the crack. The bottom surface of metal removal connects the crack root with the surface contacting gray cast iron, and the inclination angle is about 20 °.