HEAT TREATMENT PROCESS OF GEARBOX GEAR

Gearbox gear is an important part of engine such as automobile and tractor, which is used to change the speed ratio of engine crankshaft and driving shaft. Therefore, the tooth surface works under high load (impact load, alternating load, etc.), so the wear is fast. In the process of working, the dynamic force is transferred through the contact of the tooth surface. In the process of relative motion of the two tooth surfaces, there are both rolling and sliding, and there is great pressure stress and friction. The tooth end is impacted by frequent shifting. The gearbox gear is required to have high bending strength, contact fatigue strength and good abrasion resistance.

(1) MATERIAL SELECTION

According to the working characteristics of the gear and its application requirements, low-alloy carburized steel should be selected. Firstly, there are many alloy elements, which ensures the quenching permeability after carburizing and reduces the deformation of the gear. Secondly, the strength and toughness of the gear matrix are guaranteed, which can meet the working requirements of the gear. Generally use 20CrMnTi, 20CrMnMo, 18Cr2Ni4WA and other carburized steel, they can make large gear.

According to the service condition of gearbox gear, it is suitable to use 20CrMnTi low alloy carburized steel, which has high hardenability and core strength. Low carbon content can make the core of gear have good toughness; The presence of chromium and manganese improved the hardenability, and low carbon martensite structure was obtained at the core, which enhanced the strength of steel. Moreover, chromium can promote carburization and increase carburization speed. Manganese has the effect of reducing excessive carbon content on carburizing surface. Titanium prevents the grain from growing and increases the strength and toughness of steel. The shape and size of the gearbox gear is shown in figure.

Car gearbox gears

(2) HEAT TREATMENT OF GEARS

Machining process: blanking, blank forming, preheat treatment (normalizing), cutting processing, carburizing, final heat treatment, shot blasting (sand), finishing.

The thermal treatment technology requirements of gear are: tooth surface hardness 58 ~ 62(HRC); Heart hardness 33 ~ 48(HRC); The deformation amount shall not be greater than 0.25mm; The surface of the metallographic structure is tempered martensite + uniformly distributed fine granular carbide + a small amount of austenite, and the center of the metallographic structure is low-carbon martensite + a small amount of ferrite.

The pre-heat treatment of 20CrMnTi steel gearbox gear in the finished blank requires high finish and good cutting machinability, which makes the shavings of inserted teeth show a curly ribbon and reduces the machine load and extends the life of the cutter. Therefore, after the normalizing treatment, the hardness should be between 179 ~ 217(HB), and the uniform distribution of sheet pearlite + iron cable body should be obtained, which ensures the above requirements. The normalizing process temperature is 950 ~ 970 ℃, through air cooling after burn or blow, spray cooling, if hardness is low but after the first water-cooled air cooling measures.

(2) the gear carburized gear in the well type carburizing furnace carburizing, considering the carburizing velocity and the deformation in the process of carburizing, carburizing temperature generally choose between 920 ~ 940 ℃, carburizing heat preservation time depends on the requirements of the depth of carburizing layer, the actual depth of requirement of carburized layer depth plus twice the amount of unilateral grinding gear, so in the actual operation in the process of carburizing time should be reasonable. The carburizing time can be executed according to the formula:

T = X squared / 0.63 squared

Where, X is the depth of carburized layer, mm; T is the carburizing time, h.

The gear above requires a carburized layer with a depth of 0.8 ~ 1.3mm, and a carburized layer with a carbon content of 0.8% ~ 1.05%. The tissues after carburized are: hypereutectoid + eutectoid + subeutectoid.

After the final heat treatment of the belt-driven gearbox gear, the gear gets a hard, strong surface layer and a well-hardened heart.

According to the characteristics of the performance requirements and materials for transmission gears, the gear can stay in pit type furnace after carburizing furnace temperature dropped to 850 ~ 860 ℃ from direct quenching.