Quenching, cooling, annealing and other heat treatment processes for automobile half shafts

The half shaft is an important structural part of the rear axle of the car that receives torque and a certain impact force. It is one of the main components that transmits power. Improving the service life of half shafts has become a common concern. Induction hardening is a surface strengthening technology that improves the service life of parts. The emergence of the overall surface quenching technology of the half shaft has improved the production efficiency of the half shaft and saved energy. Today, Zhengzhou Gou's will briefly introduce the heat treatment process of automobile half shafts and the influence of water temperature on the quenching quality of automobile half shafts.

(1) Selection and alignment of cooling methods

The quenching cracks and early fatigue fractures that occur during the heat treatment of the half shaft are mostly related to the presence of a large amount of network (block) ferrite at the connection between the disc and the rod, where the hardness is low. It is generally caused by poor quenching process and improper operation. The disk enters the water too early or is too close to the water surface. The stress after quenching is too large, resulting in quenching cracks. During quenching, the disk is too far away from the water surface and the connection between the disk and the rod is damaged. Producing a ferrite structure with low strength and hardness will cause early fatigue fracture of the half shaft. Therefore, in order to prevent quenching cracking and improve its fatigue life, when the axle shaft is heated as a whole, the disc is first oil-cooled and then the oil is self-tempered, and finally the whole body is water-cooled. For example, intermediate frequency quenching is performed again. Fatigue strength will be increased exponentially. It is very important to inspect the surface quenching defects of induction hardened parts. Timely detection can avoid the waste of subsequent processing. Magnetic particle inspection is usually used to detect parts with quenching cracks and material cracks, which takes advantage of the ferromagnetic properties of steel parts themselves.

(2) The influence of water temperature on quenching quality

During intermediate frequency quenching, water is used as the quenching medium and spray cooling is used. The quenching cooling rate is very fast. Therefore, in the actual heat treatment of the axle shaft, practical cooling methods must be adopted to avoid cracking of the axle shaft caused by too fast cooling rate. The influence of quenching water temperature on spline quenching cracks is very obvious. In addition, the maximum cooling rate of 50°C cooling water in the temperature range of 200 to 300°C is lower than that of static water immersion cooling, so the occurrence of quenching cracks can be eliminated. This shows that the quenching cooling performance is poor and the cooling method is poor (such as cooling speed and uniformity). property) will also cause quenching cracks. For splines with thin hardened layers (2~3mm) and some parts with simple shapes, there is no risk of quenching if tap water is used as the quenching medium.

The alignment of the axle shaft should be done cold as much as possible, because after heating with an oxygen-acetylene hot spot, once the temperature is higher than the phase transformation temperature, coarse martensite will be formed after rapid cooling, causing great stress, and the axle shaft will be subjected to repeated loads. , gradually expands and fatigue fracture occurs.

(3) Process improvement of half shaft

Most of the early manufactured automobile half shafts made of 40Cr material used the process of forging, annealing, machining, normalizing, surface medium frequency quenching, low temperature tempering, etc. In fact, the early brittle fracture of the half shaft that occurred during the working process is related to this. The process is related, that is, it is related to the insufficient fatigue strength of the half shaft after heat treatment. Therefore, in order to improve the fatigue strength, we can start by improving the surface roughness and surface strength, improve the structure and size, and improve the heat treatment process. Considering that the easily damaged parts are the fracture of the arc connection between the rod and the flange and the connection between the spline and the rod at both ends, the other is the wear of the spline end face. The carbon content of 40Cr material is 0.37% ~ 0.44%, which is lower than 45 steel. Therefore, after normalizing (870 ~ 890℃ heating and air cooling), a fine pearlite structure is obtained. Its hardness is around 200HB. After medium frequency quenching Although the wear resistance of the half shaft meets the process requirements, the hardness of the core is low, resulting in deviations in strength and impact toughness, and cannot meet the requirements of the service conditions of the half shaft.

In order to overcome the inconvenience of mechanization and automation in the induction heating treatment of the half shaft, the spline area near the half shaft bar will produce a soft band with low strength, which will break at the tail of the spline during use. The use of overall one-time induction heating increases the heating effect of the rounded corners. The heating temperature here is uniform and the heating time is short. Therefore, the depth of the hardened layer and sufficient hardened area are ensured, which not only improves the fatigue strength, but also increases the hardness of the spline teeth. has been improved.