Influence of Rolling Amount on the Surface Integrity of 300M Steel Bolt Backing Groove
Because of their high strength,precision,and hardness,300M steel bolts are widely used in the assembly of various aircraft structural parts.Owing to long-term service in extreme environments,such as severe vibrations and large temperature differences,fatigue damage is prone to occur at the bolt-backing groove,resulting in major safety accidents.Rolling processing,which has dual functions of surface finishing and surface strengthening,is the primary processing technology for bolt surface strengthening.The selection of rolling parameters has significant influence on surface integrity and fatigue life.The rolling amount is an important parameter in the rolling process.To effectively improve the surface quality and performance of the bolt,a rolling strengthening test of the bolt-back groove under different rolling amounts was conducted on a self-developed special rolling tooling for bolts.The effect of the rolling amount on the surface integrity of the 300 M steel bolts was studied.Based on the previous test,the rolling amounts were selected as 0.05 mm,0.08 mm,and 0.11 mm,respectively.The surface roughness,surface morphology,microhardness,residual stress,and surface microstructure of a 300 M steel bolt after back-groove rolling were tested using a Zygo surface roughness tester,super depth-of-field microscope,microhardness tester,X-ray stress diffractometer,and scanning electron microscope.The results showed that with an increase in the rolling amount,the surface roughness initially decreases and then increases.When the rolling amount is 0.08 mm,the surface roughness Sa reaches the minimum value of 0.058 μm,which is 75％lower than that before rolling.The surface morphology of the bolt reaches the optimal state when the rolling amount is 0.08 mm,the turning marks basically disappear,and the surface is the most polished.As the rolling amount increases,the surface quality is destroyed,and a large number of pits and ploughs appear.The rolling process causes plastic deformation of the bolt surface structure,and the martensite grains are elongated and refined.Based on the results of the local orientation difference,the KAM value of the bolt after rolling increases from 0.619° to 0.875°,proving that the rolling process introduces high-density dislocations on the surface of the bolt.As the rolling amount increases,the content of the small-angle grain boundary increases,and the degree of plastic deformation of the bolt increases gradually.When the rolling amount is 0.11 mm,the depth of plastic deformation layer reaches 166 μm.In the surface layer of the bolt,the grain boundaries become blurred owing to severe plastic deformation.Cold work hardening and plastic deformation significantly increase the microhardness of the bolt surface,with a maximum value of 660 HV,which is 16％higher than that of the matrix.Within 0-400 μm,the microhardness gradually decreases as the distance from the rolling surface increases.The rolling process introduces a large residual compressive stress on the bolt surface.When the rolling amount is 0.11 mm,the dislocation density between the microstructures reaches the maximum value,and the residual compressive stress reaches 848.4 MPa,which is 662％higher than that without rolling.In this study,the surface integrity of a 300 M high-strength steel bolt cutter groove after rolling is systematically studied,and the influence of the rolling amount on the surface quality is analyzed,which can provide a reference for process personnel to optimize bolt rolling process parameters.Because the fatigue life of bolts is closely related to the surface roughness and residual stress,the research results can also be applied to the analysis of bolt fracture causes and prediction of fatigue life,providing technical support for the anti-fatigue manufacturing of high-strength bolts.
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