The influence of the cooling rate during induction hardening on residual stresses and fatigue strength
A vital part in the transmission for Scania lorries is the shaft in the rear axle. Since this shaft is divided into two identical parts, these are termed half shafts. Close control of the manufacturing process of these is required. Influence of flow rate, temperature and polymer concentration of the quenchant was investigated. An important controlling factor is the cooling rate used in the hardening process. This factor was selected for investigation in the present study. It was found that high flow rate, low temperature and low polymer concentration gives high cooling rates. Further investigation was performed how the residual stresses, surface hardness and the case depth of the half shafts were affected by the cooling rate. The results show that residual stresses are particularly affected by the cooling rate. Higher cooling rates results in higher residual stresses. Since residual stresses are important for fatigue lifer, a fatigue study was also undertaken.
Five half shafts were quenched with a polymer concentration of 5%, and another five with 15%. All ten half shafts were fatigue tested in torsion, using a Scania standardized method. The shafts were loaded in torsion torque with company confidential amplitude, mean equal to zero number of cycles to failure was recorded. In normal production the half shafts are quenched with 10% polymer concentration. No tests with a polymer concentration of 10% were carried out in this investigation since results from this concentration are available from earlier studies.
Residual stresses were measured using a relatively new method developed by Scania, called the core drilling method. A 20 cm long portion of the shaft was center drilled in steps, gradually increasing the drill diameter. After each step, the relaxation of surface strain was measured in the longitudinal and transversal directions. Having obtained these data, stresses can be calculated. Residual stresses were also measured by x-ray diffraction. Only surface stresses are obtained in this way, however. These measurements were made for each of the three polymer concentrations. An attempt was also made on trying to simulate the stress formation in the shafts during the heat treatment with FEM and also to calculate the residual stresses after the treatment. Results from the measurements were then compared with the FEM calculated results.
Since the number of tested shafts is small, results are not statistically relevant. One may conclude, though, that fatigue life increases with increasing cooling rate, i.e. with decreasing polymer concentration. The fatigue life requirement for all tested shafts was fulfilled. The compressive residual stresses for 5% and 10% polymer concentration are of same magnitude and higher than for those quenched with 15% polymer concentration. The result also shows that the cooling rate does not affect the surface hardness, nor the case depth.
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