Gear shaft spalling failure is prevented and treated by heat treatment with high-frequency induction annealing machine

A certain gear shaft is an important part of the gearbox mechanism, and the workpiece material is 20CrMnMo steel. Workpiece size: The diameter of the tooth tip circle is 175mm, the gear shaft length is 400mm, the module is 9mm, and the number of teeth is 16. The workpiece processing flow is: forging - normalizing at 920°C - machining - carburizing at 920°C - machining - salt bath heating at 860°C and quenching oil - tempering at 170°C and machining. After heat treatment of the workpiece, the tooth surface hardness is 59-60HRC, the core hardness is 32--3HRC, and the carburized layer depth is 1. 50 - 1. 60mm. During production, it was found that after 8 months of use, the gear shaft suffered early failure due to severe tooth surface peeling, network cracks and plastic deformation indentation defects. Macroscopic inspection of the gear shaft revealed that 7 adjacent teeth had large groove-shaped peeling, network cracks and plastic deformation marks, and the network cracks showed signs of falling off, and multiple grinding cracks appeared on the tooth surface; other teeth appeared on 1/ 5. The same defects mentioned above appear on the tooth surface. There are network cracks on both sides of the grinding tooth cracks. The peeling is mainly caused by the grinding tooth cracks. There are many cracks on the grinding teeth at the surface peeling place, and there are fewer cracks on other tooth surfaces. The peeling tooth cross-section epidermis The distribution of cracks can be seen below, and the cracks are basically parallel to the tooth surface and extend to the tooth surface. Metallographic examination revealed that multiple intergranular cracks appeared on the peeled tooth surface from the surface to a depth of 0.5 mm. The structure there was martensite + carbide + retained austenite with a volume fraction of about 15%. Sometimes lower bainite was found. Structure; toward the inside, a large amount of retained austenite appears, with a volume fraction of about 40%. It was observed that the martensitic needles in the tooth surface area where spalling occurred were thick, and the amount of retained austenite was significantly greater than that of the normal tissue. This shows that the heating temperature at the gear peeling point is too high, and the workpiece is locally overheated during heating. The overheated part is exactly the failure part of the gear peeling.

Analysis shows that overheating occurs during the quenching and heating of the gear shaft. After quenching, coarse martensite needles and a large amount of retained austenite are produced. The thermal stress generated during gear grinding promotes the transformation of the retained austenite into martensite. During the transformation The phase transformation stress causes grinding cracks on the tooth surface or intergranular crack defects in the carburized layer. When the gear shaft is running, the cracks expand and extend under the action of alternating contact stress, and new cracks appear on both sides of the grinding tooth cracks, thus forming network cracks and developing to cause the hardened layer to peel off. In addition, the cracks in the carburized layer are subject to internal and external stresses Under the action, the cracks expand and increase in number, promoting peeling damage of the hardened layer.

Analyze the characteristics and causes of workpiece defects and failures from the heat treatment and heating methods of gear shafts. One end of the gear shaft is heated downward, and the furnace temperature near the bottom of the furnace is too high, causing the heating temperature of the lower gear shaft to be too high. This is due to the large amount of residue at the bottom of the furnace. After analysis, a high-frequency annealing furnace is selected for quenching. The gear shaft is being heated. Due to the large size of the workpiece, it will not cause overheating due to long processing time of the gear.

To prevent gear shaft defects and failure, the preventive measures are as follows:

(1) The workpiece is strictly operated according to the heat treatment process specifications of the high-frequency induction annealing machine to prevent overheating and overburning defects in the workpiece.

(2) During salt bath heating, prevent the workpiece from being too close to the electrode position to ensure uniform heating temperature of the workpiece.

(3) During salt furnace heating production, slag should be removed regularly and the furnace bottom should be cleaned to prevent the furnace bottom temperature from being too high due to slag accumulation or excessive slag, and the corrosive and damaging effects of slag on workpieces.