Heat treatment of steel: optimization of heat treatment process of gearbox gear

Strength design and heat treatment of gear

The contact fatigue strength, bending fatigue strength, core toughness, surface hardness and wear resistance of the gear are the key indexes of the hot gear, which are directly related to the service life of the gear. The performance of raw materials and heat treatment process will significantly affect the bearing capacity of gear parts, so it is particularly important to select materials according to needs and reasonably prepare the process. Generally speaking, the bearing capacity of gear is mainly measured by the surface hardness, core hardness and effective hardened layer depth of gear after heat treatment. In GB/T3480.5-2008, the fatigue strength of gear is combined with the quality grade of material heat treatment, and the fatigue limit is divided into me, MQ and ml and illustrated. In the design of gears, the bearing capacity of gears shall be calculated based on the quality grade and the corresponding fatigue limit curve, taking into account both the use strength and the economy.

The key factors affecting the heat treatment deformation of gears

Pre heat treatment of gear blanks

The pre heat treatment of gear blanks usually includes quenching and tempering, normal normalizing, isothermal normalizing and forging residual heat isothermal normalizing. Ordinary normalizing treatment will result in great differences in the structure and hardness of different parts of different parts or the same part, which will reduce the processing performance and intensify the heat treatment deformation, thus affecting the accuracy level and service performance of gears. The final forging temperature of gear blank is generally about 900 ℃, and the blank is still in the austenite stage. The grain size of gear blank is significantly larger than that of reheating. However, the coarse grain has heredity and the transformation process of P + F lags behind, and bainite or pearlite are easy to appear, which makes the machinability worse. Isothermal normalizing is the process that the blank is completely heated to the proper temperature above the AC3 line to obtain uniform austenite, and then the blank is cooled to the “nose tip” temperature of the Austenite Isothermal Transformation Diagram by the way of rapid cooling, and the isothermal transformation is carried out in the low-temperature furnace, and then air-cooled to room temperature. Therefore, isothermal normalizing with forging waste heat or isothermal normalizing can be used for pretreatment. In production, three process parameters, namely cooling speed, isothermal temperature and isothermal time, should be reasonably selected and controlled according to the blank material and size factors to keep the blank in phase In order to obtain uniform microstructure and suitable hardness, the microstructure is transformed at constant temperature, i.e. the hardness is 160hb ~ 197hb, and the microstructure is uniform F + P. The characteristics of isothermal normalizing process are stable normalizing quality, small heat treatment deformation, suitable for mass production. For mass production of gearbox gear, the hardness and uniform structure of the appropriate blank can ensure the optimal cutting of the tool, which not only meets the high cutting efficiency, but also plays an important role in reducing the heat treatment deformation.

Hardenability and hardenability of gear steel

Hardenability refers to the ability of steel to obtain martensite or the depth of harden layer when it is quenched under certain conditions. It is an inherent attribute of steel and is mainly affected by alloy elements. There are many ways to calculate the hardenability value in work. The practical application of carburizing steel in gear industry
The following formula is recommended. Hardenability calculation formula of carburized steel with carbon content ≤ 0.25%

• In the formula, j6-40 – Hardness of each point within 6mm ~ 40mm from the water-cooled end; e – distance from the water-cooled end;

Hardenability also includes the relationship between cooling conditions and alloy elements, as well as the application in steel design and substitution, design material selection, heat treatment process parameter control, etc. In other words, hardenability is very important for the design and manufacturing process of gears. The experimental data show that the effective case depth of carburized layer with the same depth is very different due to different raw materials, modulus, overall dimension and cooling conditions. Even though the overall dimension, modulus and cooling conditions are very similar or the same, the effective case depth of workpieces can vary by as much as 0.3mm ~ 0.5mm. The reason is the difference in hardenability of carburized layer.
Hardenability is the highest hardness that can be obtained by normal quenching. It mainly depends on the carbon content of martensite. The higher the supersaturation of carbon, the higher the hardenability of steel. Generally, the quenching hardness increases with the increase of carbon content, but when the carbon content is ≥ 0.6% C, the quenching hardness of workpieces almost does not change. The hardness of common carburizing steel after carburizing and quenching is usually about 64hrc, and the surface hardness after low temperature tempering is often about 60HRC, which is the basic reason why the surface hardness of carburizing gear is generally specified as 58hrc ~ 63hrc.

Generally speaking, the deformation of gear heat treatment is caused by the comprehensive action and mutual influence of various factors

In addition to the pre heat treatment and hardenability, the shape, forging, machining and quenching specifications of the parts may cause deformation of the parts, thus affecting the accuracy and life of the gears. For the gear parts, the variable shape points are nothing but the tooth direction of the tooth shape, the accumulation of the circumferential pitch, the shrinkage of the internal spline, etc. the deformation rule can be judged by experience. The machining allowance or compensation amount shall be reserved in advance according to the process route, so that the finished parts are in the acceptable deformation range.

• (1) Gear geometry. The shape and structure of gear is one of the key factors to determine the deformation of heat treatment. The designer should fully consider the uniformity and symmetry of gear section structure to avoid the stress concentration caused by too large difference in thickness. Generally speaking, it is more difficult to grasp the deformation law of parts with complex structure and obvious stress concentration during heat treatment. Volvo company has studied the influence degree of three factors of gear design, raw material and heat treatment process on the deformation. The results show that the influence degree of design, material and process on the deformation of gear heat treatment is 50% ~ 60%, 20% ~ 30% and 5% ~ 15%, respectively.
• (2) Stress state before heat. After forging, normalizing, shot blasting and machining, residual stress, forging defects and poor organization will be accumulated more or less, and stress concentration has a significant effect on deformation. To eliminate or control the generation of residual stress is of great benefit to the subsequent heat treatment process to control deformation. In the forging process, the metal fiber streamline is controlled by means of management of upsetting direction to make it distribute symmetrically and evenly along the outer contour of gear blank; in the normalizing process, the formation trend of strip structure should be controlled to reduce the material anisotropy; in the machining process, attention should be paid to uniform cutting and tool life management to avoid excessive accumulation and uneven state of machining stress. Especially for the workpiece with complex shape, the residual stress produced by the former sequence has a great influence on the quenching deformation. The stress can be eliminated by stress relief tempering or homogenization.
• (3) Heat treatment process elements. The adjustment of process parameters, such as workpiece heating speed, carburizing temperature, quenching temperature, oil mixing speed, etc., as well as the difference of clamping mode, cooling medium and tempering process, will also affect the deformation and comprehensive mechanical properties of the gear.

Design and control of effective hardened layer depth of gear

In the design and production of automobile gearbox gear, effective hardening layer depth design is generally two methods. That is to say, according to the general range of gear module, apply the standard or calculate according to the empirical formula t = α * m (M module), α = 0.20-0.30, seldom analyze its applicability from the mechanical point of view. It is very important to design the best effective hardened layer depth for improving the tooth surface strength and saving energy.
The spalling failure of gears is not only related to the distribution of shear stress under the tooth surface, but also to the effective hardening depth, hardness gradient and other factors. The effective hardened layer depth of gear is often difficult to cover the transition area, and the peeling off of all kinds of hard tooth surface gear is often related to the transition area. The practice shows that the biggest characteristic of the effective hardened layer deep peeling off is that the fatigue crack occurs in the transition area between the hardened layer and the center, and the peeling pit formed is deep and large. In general, increasing the effective hardening depth is beneficial to improve the bearing capacity of gear and prevent fatigue peeling failure. However, too large hardening depth will make the process more difficult, process cycle increase, distortion increase and many other problems, resulting in the increase of gear production cost and energy consumption. The reasonable design of effective hardened layer depth is to ensure that the transition zone has enough strength to prevent deep peeling, and not over design.
The effective case depth of the case hardened gear is closely related to the strength, reliability and other properties of the gear, which is the key to ensure the full play of the bearing capacity of the gear. In the process of gear meshing, the local surface pressure stress is called contact stress, also called Hertz stress. The bearing capacity of tooth surface is related to Hertz contact stress. According to the formula, the magnitude of contact stress depends on the external load and the reciprocal of equivalent curvature radius of tooth surface. When the maximum contact stress is the same, the larger the equivalent radius of curvature is, the larger the effective depth of hardening layer is.

Other optimum design of gearbox gear

• (1) Gas nitriding process to strengthen the surface of gear. The load-carrying capacity of gear is usually three indexes: root strength, tooth surface strength and anti bite strength. As we all know, the anti seizing strength of nitriding gear is better than that of carburizing gear. Because of the application of pressurized gas nitriding technology and pressurized gas soft nitriding technology, the hardness of the material surface is improved and the hardness gradient of the carburizing layer is improved. Gear nitriding steel does not need hardenability control, but also can simplify the smelting management of the steel plant. The key point of gear nitriding steel smelting is to reduce the content of non-metallic inclusions and oxygen, which can further improve the fatigue strength of gear.
• (2) Strengthen the application of shot peening technology. When the gear is subjected to bending fatigue load, its Hertz contact stress reaches the maximum, so the fatigue core develops in the direction perpendicular to the maximum stress after forming on the tooth surface. When the micro crack develops into a macro crack, the hardened layer begins to fall off or even break the tooth. The results show that the bending fatigue resistance of carburized gear increases with the increase of its strength, and the bending fatigue resistance also increases with the increase of residual compressive stress on the gear surface. The so-called strengthening shot peening is a processing method that shot the steel shot at a high speed and form a certain depth of residual compressive stress on the tooth surface or the root of the tooth after continuous blow. It has the characteristics of wide adaptability, simple process, high production efficiency and remarkable strengthening effect. This kind of residual compressive stress can counteract the tensile stress of part of external load, restrain the micro crack from expanding again when the gear bears the contact stress, effectively eliminate the influence of stress concentration caused by the design and process, and also partially eliminate the intergranular oxide formation during the carburizing and quenching process The impact of success. Therefore, strengthening shot peening can effectively improve the contact fatigue strength and bending fatigue strength of gear teeth. The data show that after carburizing and quenching, the surface of gear is in the state of pressure stress distribution. Through strengthening shot peening, the pressure stress on the part surface will be further increased, that is to say, the contact fatigue strength of the part surface will be further increased.

Epilogue

In a word, the design and manufacture of gear is one of the key factors to improve the performance of gearbox. In the design, attention should be paid to the selection of gear material and process mode, the uniformity of structure, the design of effective hardened layer depth, etc.; technologists should pay attention to the elimination of adverse factors in the process of pre heat treatment, machining and heat treatment, and work together to improve product quality.
Source: China Pipe Fittings Manufacturer – Yaang Pipe Industry Co., Limited (www.yaang.com)