Abstract: The article mainly introduces the characteristics and strengthening mechanism of laser transformation hardening, and the technology of laser surface hardening. The microstructure and properties of steel materials after laser transformation hardening were analyzed, and the research status of laser transformation hardening technology in Material Science in recent years was introduced. Since its advent in the 1960s, laser technology has gained important applications in various industries. In recent years, laser surface treatment technology has not only developed rapidly in research and development but also made great progress in industrial application, which has become a very active and emerging field of surface engineering. Laser surface treatment can not only change the microstructure of the surface material by laser transformation hardening (laser quenching) and surface melting, but also change the chemical composition and microstructure of the surface layer by laser cladding, gas phase precipitation and alloying. Laser transformation hardening is one of the most widely researched and applied methods in all kinds of laser surface treatment technologies. However, the application of laser surface transformation hardening technology is not as widely and mature as traditional heat treatment technology, but because of its unique advantages, it is being paid more and more attention, and has been applied and developed in the fields of mechanical manufacturing, transportation, oil, mine, textile, metallurgy, aeronautics and Astronautics, etc.
Laser phase change hardening layer depth (H) OCP/ (DxV) is known from the above formula that the depth of laser phase change hardening layer is directly proportional to P, and is inversely proportional to D and V. The three can compensate each other, and the similar hardening effect can be obtained by proper selection and adjustment. In addition, we should consider the selection range of each parameter value. D could not be too large and V could not be too small, so as to avoid the cooling rate be too low to realize martensitic transformation. Conversely, when the laser output power is too large, it will easily cause surface melting and affect the geometry of the surface. The smaller the ratio between the critical temperature of austenite transformation and the melting point of the material, the larger the temperature range that allows phase transition, the deeper the hardened layer will be.
If you have any problem with laser transformation, feel free to contact us at GLDMA.COM.
1. Mechanism and characteristics of laser phase change hardening
Laser transformation hardening is a local rapid thermal quench process. Because of the short heating time, the heat affected zone is small and the hardened layer is shallow, generally only 0.3 1 1.0mm. When the laser phase change hardening is heated, the surface heating rate is up to 104~106 /s, which makes the surface of the material reach the austenitizing temperature rapidly. The pearlite structure in the original material is transformed into austenite through non diffusion, and then the heat transfer of the material is quickly cooled at the cooling rate of 106 to 108℃/s, which can be both at the original grain boundary and the subgrain boundary. Nucleation can also be nucleated at the phase interface and other crystal defects, while the instant austenitizing under rapid heating makes the grain less mature, and will inevitably turn into fine martensite during martensite transformation; on the other hand, the laser is quickly heated to make the diffusion homogenization not to be carried out, and the carbon and alloying elements in the orb body are in the body. With the increase of the inhomogeneity of the concentration, the microstructure of the austenite with similar carbon content becomes smaller. Under the rapid cooling condition, the martensite formation temperature varies greatly in different microcosmic regions, which also leads to the formation of fine martensite. The microstructure of martensite after laser quenching is lath martensite and twin martensite. The dislocation density is very high, up to 10l2/cm2. The results show that grain refinement, high dislocation density of martensite and high solid solubility of carbon are the main reasons for obtaining super high hardness.2. Laser surface transformation hardening process
2.1Preprocessing of material surface
The absorption ability of the metal surface to the laser radiation energy mainly depends on the surface state. Generally, the surface roughness of metal materials is very small, and its reflectivity can reach to 80% - 90%, which will affect the efficiency of absorbing light energy on the surface of metal materials. In order to improve the absorption efficiency of the metal surface to the laser, surface pretreatment should be carried out before laser hardening. There are phosphating, surface roughness, oxidation, coating and coating, among which phosphating and spraying coating are the most commonly used. The coating on the original surface by absorbing laser material is the most effective one. In addition to greatly improving the absorptivity, these coatings must have the characteristics of cheap, non-toxic, non-pollution, solid bonding with the matrix, fast drying, no reverse jet in laser scanning, and convenient cleaning after laser treatment. Therefore, the coating which can be coated on the surface of the treated metal before laser quenching can greatly improve the absorptive capacity of the metal surface to the laser has been developed. It has become an important topic for the application of laser technology in the industrial field.2.2. The main parameters affecting the phase change hardening layer and their correlations
Laser transformation hardening process is a complex rapid heating and cooling quenching process. The size parameters of the laser hardened layer (hardened layer width, hardened layer depth, surface roughness) and performance parameters (microhardness, wear resistance, tissue change) depend on the laser power density (laser power, spot size), scanning speed, properties of materials (composition, original state) and surface pretreatment characteristics, etc. Meanwhile, it is also related to the geometry and size of the treated parts and the thermodynamic properties of the laser zone. The main technological parameters are laser output power (P), scanning speed (V) and the spot size (D) on the surface of the material (D). The comprehensive effect of the three factors directly reflects the temperature and time of the laser quenching process. The influence of three parameters on the effect of laser transformation hardening is as following:Laser phase change hardening layer depth (H) OCP/ (DxV) is known from the above formula that the depth of laser phase change hardening layer is directly proportional to P, and is inversely proportional to D and V. The three can compensate each other, and the similar hardening effect can be obtained by proper selection and adjustment. In addition, we should consider the selection range of each parameter value. D could not be too large and V could not be too small, so as to avoid the cooling rate be too low to realize martensitic transformation. Conversely, when the laser output power is too large, it will easily cause surface melting and affect the geometry of the surface. The smaller the ratio between the critical temperature of austenite transformation and the melting point of the material, the larger the temperature range that allows phase transition, the deeper the hardened layer will be.
2.3 Laser transformation hardening method
The scanning mode of the laser is narrow band scanning of circular or rectangular spot and broadband scanning of the linear spot. The width of the hardened strip is similar to the spot diameter, which is generally less than 5 ~. In order to require large area hardening, it is necessary to scan them one by one. Overlap is required between the scanning bands, and the tempering softening zone will be left in the overlap part. The width of the tempering softening zone is related to the characteristics of the light spot, and the tempered softening zone produced by the uniform rectangular spot is smaller. The width of the broadband scan can reach more than ten millimeters, which effectively reduces the adverse effect of the softening zone. Liu Wenjin and some others from Tsinghua University have used GaAs two element optical devices to focus on the line spot. The study of laser melting and alloying on the surface of 45 steel cam has improved the hardness and wear resistance of the cam surface.3.Conclusion
The technology of laser transformation hardening has been applied for more than 30 years with its application area constantly expanding. However, for the high technical content of this process, there are too many factors in the process and high cost of equipment. In addition to the simple shape, the basic molding process and the large batch, the production line can be set up specially, and the stable processing quality can be obtained. There are still a lot of problems in the application of the workpiece with more complex shapes. Basically, it is a laboratory technology with high cost, complex control, and special performance. However, because of the unique advantages of laser transformation hardening technology, it is still a high technology with wide application prospects. With the continuous progress in the research of numerical simulation and computer control technology, the development of real-time control system for laser phase change hardening technology, which can be applied to various conditions, will also succeed. Then it will pave the way for the laser transformation hardening technology to enter the automatic production line in an all-around way.If you have any problem with laser transformation, feel free to contact us at GLDMA.COM.
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