1 Traditional gear heat treatment process and its quality characteristics 1) Surface quenching Surface quenching The most commonly used are high frequency quenching and flame quenching. Generally, the machine tool gears are selected by medium-carbon steel after high-frequency quenching, and the hardness, strength and toughness can meet the requirements, and have the advantages of not easy oxidation, decarburization and deformation, and high productivity. For large modulus gears (m>4), the gear-by-tooth heating quenching method is often used, and the small modulus gears are generally subjected to integral heating and quenching. Flame quenching is used under the simple conditions of the production site, and there are certain effects.
The biggest weakness of this process is that the root is easy to stress concentration, and it is prone to breakage during use, and the high-frequency coil is impossible to achieve complete tooth-tooth matching, resulting in uneven distribution of tooth surface hardness.
2) Chemical heat treatment of large load high-speed gears such as automobiles and tractors. In order to make the base body have strong toughness and wear resistance, the surface of the low carbon steel is hardened and tempered after carburizing, or the surface of the medium carbon steel is tempered. Chemical heat treatment such as nitrogen. Although such a process can meet the performance requirements at present, the process flow is long, the energy consumption is high, the cost is high, and environmental pollution is caused. In addition, the gear of the carburizing treatment is greatly deformed, and the finishing work amount is increased in the later stage; the gear of the nitriding treatment has a peeling phenomenon during use.
3) Overall quenching and tempering treatment Gear shaft and large load Medium and low speed large gears are quenched and tempered to prevent brittle fracture as an important indicator. In order to meet various performance requirements with the same process, high quenching materials are used for quenching and tempering. The treatment ensures that the shaft and the base do not break under heavy load. This compromise process sacrifices the hardness of the tooth surface and reduces the wear resistance.
2 Laser phase change enhancement characteristics Laser phase transformation enhancement is to use laser beam to scan the workpiece, so that the surface layer of the workpiece rapidly rises above the critical point of AC3. When the thermal layer moves away, the temperature instantaneously enters the martensite due to the heat conduction of the workpiece substrate. In the zone or bainite zone, martensite transformation or bainite transformation occurs, and the phase transformation strengthening process is completed.
1) Hardenability Hardenability refers to the rate of decomposition of austenite when it is in the pearlite transformation zone during the cooling process after heating to austenitization. The hardenability of decomposition is poor, and the hardenability of decomposition is slow. There is a gestation time before the austenite begins to decompose. The material can obtain a completely quenched structure (martensite or bainite) as long as it passes this temperature interval during the incubation time; otherwise, only partially quenched tissue or The quenched structure is completely unavailable.
In laser quenching, when scanning with D=5mm spot and V=1200mm/min speed, the heating time of a fixed point is 012s. According to the field test, the steel piece can be heated by 012s to make the surface 017~1mm away. The temperature rises above the critical temperature of AC3. Conversely, when the heat capacity of the workpiece is sufficiently large, the heated workpiece will be cooled from the AC3 critical point temperature to the martensite start transition temperature MS at a cooling rate of 104 to 106 ° C / s, for the austenite to cross the pearlite transformation region, It is sufficient to complete the martensitic transformation conditions. Therefore, hardenability is not critical for laser surface phase transformation strengthening.
2) Dispersion strengthening and distortion enhancement Laser phase transformation enhancement is to irradiate the surface of the workpiece with a high energy density laser beam, so that the part to be strengthened absorbs light energy instantaneously, the temperature rises sharply, austenite is formed, and the austenitizing time of the surface layer is formed. At about 012 s, the austenite grains have no chance to grow up, and the dispersed austenite grains form a diffuse martensite phase, which makes the martensite have lattice strengthening and a dispersion strengthening effect. Moreover, the martensite lattice formed under chilling conditions has a higher defect density than conventional quenching. At the same time, the retained austenite also obtains a very high dislocation density, so that the metal material has a distortion strengthening effect and the strength is greatly improved.
3) Non-oxidative decarburization The light-absorbing coating used in the phase transformation strengthening has the property of protecting the surface of the workpiece from oxidative decarburization.
4) Strengthening the anti-fatigue performance The laser-treated and non-laser-treated Cr12MoV specimens were subjected to wear comparison tests. It was found that the surface of the laser-treated sample was smooth, the furrow was shallow, and the sticking phenomenon was light; The surface of the worn surface is seriously damaged, and there are obvious furrows and surface sticking marks. Therefore, the laser strengthening layer has strong resistance to plastic deformation and anti-adhesive wear, and enhances the fatigue resistance of the material.
5) Uniform hardness of the strengthening layer The conventional heat treatment has a significant decreasing gradient from the surface and the hardness of the workpiece. The hardness of the entire strengthening layer is almost the same.
311 gear material laser gear does not need to be quenched as a whole, medium carbon steel should be used. If low carbon steel is used, the gear base will have no strength guarantee and reduce bending fatigue strength.
The optimal original state of the 312 gear original state laser quenching gear is the quenching and tempering state. The specific operation can be combined with the stress relief heat treatment after the gear blank forging. In the traditional process, the original post-forging stress relief heat treatment can be annealed or normalized. As a blank of the laser hardened gear, it can be determined to be normalized.
313 laser scanning mode 31311 circumferential continuous scanning gear continuously rotates, the spot moves axially, forming a spiral spacer hardening zone on the tooth surface. The main problems of this method are as follows: 1) A narrow tempering zone between the hardened zones is formed on the tooth surface, the hardness of the tempering zone is about 10HRC lower than the hardness of the hardened zone; 2) The spot moves from the root to the top of the tooth During the process, the power density is gradually increased, and if the laser parameters and scanning motion parameters are improperly selected, the tip portion is easily melted. The larger the modulus of the gear, the more significant this situation is. Therefore, the circumferential continuous scanning of the gear is generally only applicable to medium and small modulus (m ≤ 315 mm) gears; 3) the optimum strengthening point using circumferential continuous scanning is difficult to control at the position where reinforcement is most needed.
The 31312 axial split tooth scanning spot reciprocates axially along the gear, and the gear rotates.
After the scanning operation of the tooth flanks of the same side of the gear teeth is completed, the spot is moved to the other side, and the above motion is repeated to complete the scanning work of the other gear surface of the gear teeth.
The axial split tooth scanning has the following characteristics: 1) The axial split tooth scanning should adopt a wide-band mirror, and the appropriate bandwidth can be selected according to the modulus of the tooth teeth. The full tooth surface can be scanned at one time to avoid overlapping on the tooth surface, which can be ideal. Strengthen the deep distribution of layers; 2) The scanning method is especially suitable for medium and large modulus gears with m≥4; 3) The axial split tooth scanning has back tempering. Since a single beam of light scanning is used, for one gear, there must be a problem of sequential scanning on both sides of the gear teeth, so that back tempering may occur, so that the two sides of the gear teeth have obvious hardness differences. The smaller the modulus of the gear, the greater the difference in hardness.
314 Solution to back tempering The important reason for back tempering is that the temperature rise of the gear base exceeds the tempering temperature, and the temperature rise of the substrate is directly related to the laser parameters, although proper heat dissipation measures are taken to reduce the tooth base. The temperature can effectively avoid back tempering, but complicates the equipment and creates environmental pollution.
Through a large number of experiments and theoretical analysis, it is known that the appropriate laser parameters (power, scanning speed) can be selected according to the gear modulus to avoid back tempering. For a given gear modulus, back tempering does not occur as long as the selected laser power P and scanning speed V are located to the lower right of the modulus curve.
Gear laser quenching parameter selection Figure 315 Deformation problem Because the laser quenching heating speed is extremely fast, up to 103 ~ 104 ° C / s, and the phase transition temperature dwell time is very short, only 10-2 ~ 10-1s, so the heat affected zone is very small . In addition, the matrix has a great inhibitory effect on the deformation of the tiny local heat-acting zone, so that the thermal stress deformation and the phase-change stress deformation can be controlled within a small limit. However, due to the large range of modulus variation of various gears, the shape and size of the gears, and the thickness of the spokes, when the gear is laser quenched, if the adopted method is improper, especially the deformation of the thin spoke gear can not be ignored. Appropriate pretreatment methods and temperature rise control techniques in the process can significantly reduce the amount of deformation.
Before and after laser quenching, the variation values ​​of each measurement item are about several micrometers, and the deformation amount is extremely small. Since the amount of deformation is extremely small, the original accuracy level is not lowered, so no grinding is required.
4 laser quenching gear performance 411 fatigue performance, the gear material is 45 steel two sets of gears for fatigue performance test, one of which uses laser surface strengthening, a set of quenching and tempering treatment. Under the action of maximum rated load (SH=1), the fatigue life exceeds 514×107413. The performance is based on the tracking investigation of the use of the steel. The same use environment, the service life of the non-heat treated gear is quenched and tempered (hardness) 50 gears of 35HRC); the gears used in the blast furnace are 35CrMo. The previous manufacturers provided the quenching and tempering state, the service life is 3 months, and the service life after laser surface strengthening has exceeded 6 months. In use).
According to a follow-up survey by a steel company in Tianjin, the wire rod mill is made of 34CrNiMo gear shaft. The service life of the gear after quenching and tempering is 115 months, the service life of high frequency quenching is 3 months, and the service life of laser surface strengthening has exceeded 6 months (still in use).
In summary, the service life of gears after quenching and tempering is double that of heat-treated gears; the service life of high-frequency quenching (including flame quenching) is double that of quenched and tempered gears; the service life of gears strengthened by laser surface It is double the high frequency hardened gear. The service life of the laser surface strengthened gear is slightly longer or substantially equal to that of the chemical heat treatment gear.
5 Conclusions Due to laser phase transformation strengthening, it has dispersion strengthening, distortion strengthening, non-oxidative decarburization, unique anti-fatigue and uniform hardness of the strengthening layer, which makes the material have both high hardness and high strength, greatly improving the fatigue performance of the gear and Wear resistance, and there are no hidden troubles such as coarse crystal, delamination, and root stress concentration.
The axial split tooth scanning is used to select the appropriate laser parameters according to the gear modulus, especially for large and medium modulus gears with m≥4. In order to avoid back tempering, the small-modulus gears have a lower power density and a thinner reinforcement layer.
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