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This study investigated the effects of hardness, stress relief annealing, surface decarburization, rolling/phosphating sequence, under-round fillet roughness, and under-round fillet transition on the fatigue properties of high-strength bolts. Effect of hardness change: A comparative fatigue test of two sets of samples was carried out by taking a 12.9 MlXI.25 bolt. In addition to the first group of higher hardness (lower tempering temperature) is 42 soil 0.SHRC, the second group is lower hardness (higher tempering temperature) is 40.5 soil 0.SHRC, the rest of the conditions are completely the same. The average fatigue strength of the specimen with higher hardness is 8.0kN, the standard deviation is 0.88kN; the average fatigue strength of the specimen with lower hardness is 7.9kN, and the standard deviation is 1.4kN. Comparing, the hardness difference is not big, fatigue The difference in intensity is also small. Effect of tempering: The sample of the same batch of the first batch will be tempered again at 470 ° C (corresponding to the original tempering temperature). After the tempering treatment, the average fatigue strength of the sample was 6.2 kN, and the standard deviation was 0.75 kN. After the tempering, the fatigue strength of the bolt decreased by 22.5%. Effect of stress relief annealing: The stress relief annealing is to be subjected to stress relief annealing at 20 ° C in the same batch of the first batch. After stress relief annealing, the fatigue strength of the sample was 5.8 kN and the standard deviation was 0.96 kN. The fatigue strength of the bolt after stress relief annealing decreased by 27.5%. Effect of surface decarburization: Compare the fatigue strength of bolts without surface decarburization and surface decarburization, and investigate the effect of surface decarburization on bolt fatigue performance. The test bolt is an engine connecting rod bolt with a thread size of M12xl.25 and a strength class of 12.9. The bolt is rolled after heat treatment, and decarburization occurs during the heat treatment. The average fatigue strength of bolts without surface decarburization is 9.6kN, and the standard deviation is 0.6kN. The average fatigue strength of bolts with surface decarburization is 7.7kN, and the standard deviation is 11kN. The fatigue of bolts in the case of surface decarburization The intensity decreased by 19.8%. Effect of rolling/phosphating sequence In general, the surface of the bolt is phosphated to prevent rust and to stabilize the friction during assembly. When the transition of the crest of the rolling wheel thread is not smooth, or the radius is too small, a local excessive pressing force will be generated during the rolling process, and a bad stress distribution will be formed at the bottom of the thread groove of the bolt after the rolling process is completed. In severe cases, local microcracks may be formed due to excessive pressure, which will adversely affect the fatigue performance of the bolt and the service life of the spinning wheel. The design of the bolts with high fatigue performance and the corresponding thread shape of the rolling wheel shall be in accordance with the requirements of the MJ thread.
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