| Although the alternating stress is lower than the yield stress when the material under static load, internal combustion engeine crankshaft,train axles and other parts may be fatigue fracture,it is a sudden brittle,and no apparent plastic deformation before fracture happen.This is because during their work to withstand cyclic loading,and have experience too many of stress cycles. So fatigue fracture often cause significant security incidents.Over the years people have been trying to improve the fatigue strength of parts,one of the simple and effective way is to have the surface rolled. Surface rolling is a non-cuting chip processing, which made the surface of parts smooth and produced residual compressive stress. Therefore, the formation of working-hardening layer will significantly improve the fatigue strength of parts. Warm surface-rolling is a rolling with the specimen under warm working temperature, compared to cold surface rolling(in the room tempeture), it has better strengthening effect. So research on Warm surface-rolling will promote the technology further development.In this paper the semicircular ring notched specimens was studied, which is 45 steel. Analyzed the rolling residual stress via numermical simulation, and predicted the optimum combination processing to get the maximize fatigue strength of the sample. Using the Warm surface-rolling equipment which designed by myself to roll 45 steel specimens under different temperature and feed value. In order to get the strengthening effect of specimens which mentioned above, rotating bending fatigue testing machine was adopted to test the surface-rolling specimen as well.Surface rolling numercal simulation started from the analysis of residul stress to research surface strengthening. All the results showed that: to get the maximum of residual compressive stress, the feed value is 0.06mm, 0.10mm, 0.15mm and 0.15mm respectively when tempeture is 20℃, 100℃, 200℃and 300℃. Summmaried and analyzed the results in total experimental tempeture, in the combination of 300℃and 0.15mm feed value, the specimen surface has maximum of the residual compressive stress. According to the micro-mechanism that the larger residual compressive stress, the higher the fatigue strength, the fatigue life was predicted for specimens under different process conditions. The conclusion of nunermical simulation consistent with the results of the specimen fatigue test, so it is feasible of using finite element method to analysis and get the optimization process of surface rolling.The rotating bending fatigue test results showed that: the specimen of surface rolling can significantly improve the fatigue strength of parts, and the warm surface rolling can get a further fatigue life than surface-rolling in room temperature. Surface rolling in room temperature, to obtain the greatest fatigue life, the optimum feed value of specimen is 0.06mm. Similarly, the optimum feed value of specimen is 0.10mm in 100℃. In the range of experiment feed value, both 200℃and 300℃, the optimum feed value are all 0.15mm. In the range of experiment temperature, in order to get the highest fatigue strength, the best surface-rolling process is to processing in 300℃with the feed value 0.15mm.Fatigue fracture analysis shows that: fatigue fracture belongs to brittle fracture. The long fatigue life specimen has a very clear but narrow bright band in section, while the short is not obvious but wider. Microstructure analysis shows that the warm surface-rolling produces non-uniform plastic deformation, the microstructure of the surface can be refined significantly, while the heart is still the original.
|
PDF Full Text Request