On the material substitution technology of the hot

Material substitution technology of laser strengthened mold (Part 1)

mold manufacturing process according to the requirements of working conditions, the process system is composed of material selection, hot processing into billet, cold processing and forming, heat treatment strengthening, finishing and assembly. When the working environment has new requirements, or the manufacturer in order to reduce costs, the above process links can be reorganized to form a new process system

there are many intolerable actions in the mold manufacturing process system. In order to improve the hardenability of large molds, high hardenability materials must be used in order to harden the working layer of the mold. The high hardenability material is only high alloy steel. The metallurgy, hot processing and heat treatment processes of this material are very complex, and the cold processing is also very difficult, resulting in the increase of mold manufacturing cost. In order to reduce the influence of these factors, some molds that only need local strengthening adopt high alloy steel inlay modules, and the matrix adopts ordinary materials to make composite molds, which will reduce the accuracy of molds

I. metal materials science of laser phase change strengthening

laser surface strengthening technology is based on two processes: high energy density heating of laser beam and rapid self cooling of workpiece. In laser surface strengthening of metal materials, when the energy density of laser beam is at the low end, it can be used for surface phase change strengthening of metal materials. When the energy density of laser beam is at the high end, the spot on the surface of workpiece is equivalent to a moving crucible, It can complete a series of metallurgical processes, including surface remelting, surface carburization, surface alloying and surface cladding. The material substitution technology caused by these functions in practical application will bring huge economic benefits to the manufacturing industry

the so-called laser phase transformation strengthening is to quickly scan the workpiece with a laser beam. When the spot of the heating layer is removed, the heating layer whose temperature reaches above the critical point will undergo phase transformation due to the heat conduction of the workpiece matrix, and the process of phase transformation strengthening will be completed. The phase transformation strengthening process has the advantages of good surface quality, and the hardness and layer depth can be controlled according to different materials, the heat capacity of the workpiece, and the different laser processing parameters. For the depth of strengthening layer, the root experimenter can complete the basic control of the experimental machine. According to the test measurement, the steel is 0.7-lmm, and the cast iron is 0.35 o.65mm

in the traditional heat treatment process. The role of the technical factors in the laser phase transformation strengthening has changed greatly

1. Hardenability

hardenability is a crucial technical index for the traditional overall quenching process, which determines whether the material can obtain quenching structure and achieve the purpose of strengthening; For laser surface phase transformation enhancement, this index is irrelevant

take structural steel 45 and high-speed tool steel W18Cr4V, which are widely used, for example, the most unstable temperature of austenite in 45 steel is about 550 ℃, and the incubation time is about 1s. If it fails to exceed 550 ℃ within 1s, austenite begins to decompose and transform into pearlite; The most unstable temperature of austenite in W18Cr4V Steel is about 750 ℃. At this time, the inoculation time is about 8min. If it can exceed 750 ℃ within 8min, austenite can be transformed into martensite or bainite required by the impact tester for the environment

in laser quenching, when scanning with a spot of D = 5mm and a speed of 1200mm/min, the heating time of a fixed point is 0.2S

according to the field test, the temperature at 0.71mm from the surface of steel parts can rise above the AC3 critical temperature after heating for 0.2S; Conversely, when the heat capacity of the workpiece is large enough, the heated workpiece will be cooled from the AC3 critical point temperature to below the martensite transition temperature ms at a cooling rate of 104 100 ℃/s

so don't mention that the inoculation time of about 480s for W18Cr4V is about 1s for 45 steel, which is more than enough for austenite to cross the pearlite transformation region and complete martensitic transformation. Therefore, hardenability is irrelevant to laser surface phase transformation strengthening

2. Dispersion strengthening and distortion strengthening

laser transformation strengthening is to irradiate the workpiece surface with a high-energy density laser beam, so that the parts that need to be strengthened instantly absorb light energy, and the temperature rises sharply to form austenite. At this time, the matrix is in a cold state, and there is a very high temperature gradient between it and the heating zone. When the laser irradiation is stopped, the heating zone will be quenched due to quenching, so that the metal surface will undergo martensitic transformation

for austenite formed in this process environment, the austenitizing time of the surface layer is about 0.2S, and the innermost layer is instantaneous. There is no opportunity for austenite grains to grow in both the surface layer and the inner layer. Dispersed austenite grains form dispersed martensite phase, which makes martensite have lattice strengthening and dispersion strengthening effect at the same time

moreover, the martensitic lattice formed under the condition of quenching has a higher defect density than that of conventional quenching. At the same time, the residual austenite also obtains a very high dislocation density, which makes the metal material have distortion strengthening effect and greatly improve the strength

3. Non oxidation decarburization quenching

in the traditional heat treatment, if there is no protective measures during the heating process of the mold, oxidation and decarburization will occur, which will reduce the hardness, wear resistance, service performance and service life of the mold

the light absorbing coating used for laser phase transformation strengthening has the performance of protecting the surface of the workpiece from oxidation

4. Fatigue resistance mechanism of laser strengthening

one of the reasons that affect the fatigue resistance of metal materials is the initiation time of fatigue cracks. Wear and fatigue promote each other in the material damage process. Wear grooves can become the initiation point of fatigue cracks and accelerate the initiation of fatigue cracks. After fatigue cracks appear on the material surface, the surface roughness will deteriorate seriously and the wear will also intensify

the wear contrast test of Crl2MoV samples treated by laser and not treated by laser shows that the surface of the samples treated by laser is relatively smooth, the furrow is shallow, and the adhesion phenomenon is light; However, the wear surface of the sample without laser treatment is seriously damaged, with obvious furrows and surface adhesion marks. Therefore, the laser strengthened layer has strong resistance to plastic deformation and adhesion wear

the anti fatigue mechanisms of dispersion strengthening, distortion strengthening and laser strengthening make up for the carbide wear factor in high alloy materials

5. The hardness value of the equal strength working layer

conventional heat treatment has an obvious downward gradient from the outside to the inside. The hardness of the whole strengthening layer of the workpiece strengthened by laser is almost the same

the hydraulic universal testing machine mainly tests the physical properties of raw materials, finished products and semi-finished products

the cooling direction of conventional heat treatment is from the outside to the inside, the cooling speed of the surface is the fastest, and the cooling speed gradually decreases from the outside to the inside, so the gradient distribution of the decrease in hardness value from the outside to the inside is obtained

the heating direction of laser phase transformation strengthening is from the outside to the inside. Not only the surface temperature is higher, but also the heating time is longer, up to 0 25s, while the austenitizing of the inner layer is completed instantaneously, resulting in higher C concentration in the surface austenite. The cooling direction of laser quenching is opposite to that of conventional heat treatment, from the inside to the outside. Although the temperature of the inner layer is low, the cooling speed is the fastest. Although the surface temperature is high, the cooling speed is the slowest. Although the concentration of C in the inner layer is slightly low, the distortion strengthening and dispersion strengthening are stronger

in this way, almost constant hardness value distribution is formed in the hardened layer. Strong working layers such as laser strengthened parts avoid the phenomenon that once the surface of conventional heat-treated parts is worn, the wear rate will accelerate

6. Hardenability of cast iron mold

cast iron is widely used as mold material because it has excellent casting and wear resistance by pressing the "input 1/5" key. However, because the material properties of cast iron molds are not fully understood, the use effects of cast iron molds in the market are very different

the hardening of cast iron mold is achieved through the phase transformation strengthening of the matrix, and the hardenability of cast iron mold also depends on the structure of the matrix. If the ferrite content of the matrix structure is low, the hardenability of the cast iron mold becomes higher; If the ferrite content of the matrix structure is high, the hardenability of the cast iron mold is poor

the carbon content in cast iron has sufficient carbon source for the matrix to obtain 100% pearlite. However, if the graphitization force in the casting process is too strong, the carbon in the matrix will be completely graphitized, resulting in ferrite in this region of the matrix, and even more than 50% ferrite in the whole matrix. This kind of cast iron mold is not wear-resistant, and the potential to improve wear-resistance through phase transformation strengthening is relatively small

in addition, there are a large number of cast iron molds produced under simple production conditions in the market because the cast iron production equipment does not require much investment. The cast iron mold produced under simple production conditions has three characteristics:

(1) there are many casting defects. (2) The structure is thick, especially the graphite. (3) The graphitization degree cannot be controlled, and the ferrite content in the matrix is high