Early failure analysis of spline shaft deformation and wear of sprocket and new heat treatment process using medium frequency induction heating machine

The sprocket spline shaft is an important part of the ML-722 coal mine conveyor. Its working environment is quite harsh, so the requirements for the sprocket spline shaft are also extremely strict. It generally requires high hardness, wear resistance and service life. We used En24 steel quenching and tempering treatment before. During production, it was found that the life of the sprocket spline shaft was low, and most of the spline shafts failed and scrapped after only 2-3 months of service. Generally, only 150,000 tons of coal were mined, and the economic loss was huge.

The number of teeth of the spline shaft is 19, the pressure angle is 30°, and the pitch circle diameter is 120 and 65mm. The hardness value of the failed spline shaft 20mm from the tooth top to the axis center is 248-302HBW (24.5-32.5HRC). The hardness difference is large, resulting in a large difference in the strength of the workpiece. Insufficient hardness and strength of the workpiece are the main reasons for the failure of the workpiece. There are two main failure modes, one is failure caused by severe wear and thinning, and the other is failure caused by extrusion, deformation and twisting of the tooth shape into a spiral shape. Based on the above analysis, in order to prevent plastic deformation and excessive wear of the workpiece, the bending strength and surface strength of the spline shaft must be improved, that is, the hardness of the workpiece after quenching and tempering is increased from 225-270HBW to 349HBW. In order to achieve the above requirements, appropriate materials can be selected and heat treatment process improvements can be made. After analysis, the selected materials should meet the following conditions:

(1) Good hardenability. At the 15mm part of the moment tooth item, the hardness is required to be above 38HRC.

(2) The cutting performance is good, which is conducive to the processing of workpieces.

(3) Good welding performance.

(4) Under appropriate heat treatment process, the deformation is small.

(5) No obvious temper brittleness.

After comprehensive comparison, the choice of 42CrMo steel can achieve a workpiece with a diameter of 120mm, a distance of 15mm from the tooth top after quenching, a high hardness of 349HBW, and excellent weldability and cutting performance. The improved process flow is formulated as follows: forging-annealing-rough turning-quenching and tempering-finish turning-drilling-tapping-gear hobbing-heat treatment-assembly welding-grinding-finished product. The main process and improvement measures are as follows:

(1) Forging; the initial forging temperature is 1130-1180°C, the final forging temperature is 850-900°C, slow cooling.

(2) Annealing. The annealing of the spline shaft can be carried out using a medium frequency heating machine, the temperature is 850°C, the heat preservation is 150-180min, and the furnace is cooled with air cooling below 600°C. The hardness is 208-225HBW.

(3) Conditioning. Adding quenching and tempering process after rough turning can reduce the final heat treatment deformation, improve segregation defects, and improve the strength and toughness of the workpiece. The process is: heating at 890-900°C with a medium frequency heating machine, heat preservation for 25min and 45min, high temperature tempering (680°C±10°C) x 210min, air cooling, and the hardness is 216-235HBW.

In this process improvement, the original process of quenching and tempering then hobbing the spline shaft is improved to hobbing first and then quenching and tempering, so as to reduce the deformation of the workpiece.

After the improvement of the above process, the performance of the 42CrMo steel spline shaft meets the technical requirements and indicators, and the quality of the workpiece is excellent. Production and application have shown that after process improvement, the working life of the spline shaft is five times longer than that of the original product, and exceeds that of similar foreign products. After the process improvement, the appearance of the spline shaft was good after 7 months of operation.