| Contact fatigue is a common form of fatigue failure of metal parts,which is common in bearings,wheels and rails with contact motion.Contact fatigue damage is often related to fatigue cracks.With the growth of fatigue cracks,the degree of fatigue damage gradually increases,which aggravates the potential safety hazards.Therefore,it is very important to improve the anti-contact fatigue ability of parts in practical production and application.Contact fatigue belongs to fretting fatigue.Research shows that the high residual compressive stress on the surface of material can effectively improve the fretting fatigue performance of parts.Laser Shock Peening(LSP)is a new surface treatment technology which can improve the surface strength and mechanical properties of materials.It can improve the anti-fatigue ability of material surface and inhibit the initiation of fatigue cracks by generating high residual compressive stress on material surface.At present,there are few studies on the effect of laser shock strengthening on the contact fatigue properties of materials at home and abroad.In this paper,316 stainless steel is taken as the research object,and the effects of laser shock strengthening on the rolling contact fatigue properties are studied.The main research contents are as follows.:(1)Laser shock hardening of 316 stainless steel with energy of 3J,4J,5J and 6J was carried out,and then the surface mechanical properties of the samples were tested.The results show that laser shock hardening causes plastic deformation on the surface of 316 stainless steel,and the roughness and hardness are improved.The greater the laser energy,the greater the surface roughness and hardness.With the increase of depth,the increment of microhardness decreases,and the gradient decreases.At the same time,the depth of the influence layer increases with the increase of laser energy.When the impact energy is 6J,the depth of the affected layer reaches 0.9 mm,the hardness increases by 20%,and the residual compressive stress reaches the maximum value of 280 MPa.The experimental results show that the improvement of surface hardness and residual stress of laser shock 316 stainless steel is an important factor to improve its contact fatigue performance.(2)Rolling contact fatigue tests were carried out on 316 stainless steel specimens without impact,laser shock hardening and laser shock polishing respectively.The results show that surface polishing has little effect on the contact fatigue life of 316 stainless steel specimens.After fatigue test,the stress-life(S-N)curves of the specimens without impact and laser shock were drawn and compared.It was found that the rolling contact fatigue life of the specimens after laser shock increased significantly.When the contact stress is 848 MPa and 708 MPa,the fatigue life of non-impact specimens is 47 164 times and 107 800 times,respectively.After laser shock,the fatigue life of specimens increases by 2.1% and 15% respectively under the same stress conditions.This shows that the increasing effect of laser shock on fatigue life of 316 stainless steel decreases gradually with the increase of fatigue stress.(3)The surface micro-morphology of fatigue failure specimens is analyzed.The results show that the fatigue cracks occur and propagate on the surface of the specimens after contact fatigue failure.At this time,the materials in this area will spall,and when other cracks extend to this spalling area,the "secondary spalling" will occur.When contact fatigue failure occurs,different fatigue failure zones influence and penetrate each other,which can cause new fatigue failure,thus causing more serious surface defects.After laser shock,the fatigue damage degree of 316 stainless steel contact surface is reduced,the irregular spalling of surface layer is reduced,the number of pitting pits is reduced,the length of fatigue crack is reduced,and the greater the impact energy is,the better the fatigue damage resistance effect is.(4)The distribution of residual compressive stress field produced by laser shock and its effect on rolling contact motion stress field were simulated by combining finite element analysis with experimental study.The results show that laser shock strengthening can produce a residual compressive stress layer with a depth of 1 mm on the surface of the material.When the laser energy is 6 J,the residual compressive stress in the model reaches 500 MPa.The simulation of rolling contact on model surface shows that the Mises stress produced by rolling contact of non-impact model is 690 MPa,and the Mises stress value of laser shock model is reduced by 29.6% compared with that of non-impact model.According to von Mises criterion,laser shock delays the occurrence time of model surface fatigue and improves the anti-contact fatigue performance of materials.In summary,laser shock can produce residual compressive stress on 316 stainless steel surface,improve its mechanical properties,reduce the maximum Mises stress generated in contact motion,delay the initiation and propagation of fatigue cracks,and effectively improve the contact fatigue resistance of materials. |
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