Study On Ultrasonic Nanocrystal Surface Modification And Supersonic Fine Particle Bombardment Strengthening Of DZ2 Steel

As a transmission and load-bearing component,axle has a vital impact on operation safety of high-speed trains.DZ2 axle steel,an axle material independently developed and designed by China,has better performance than those imported EA4 T steel.However,due to the low hardness and wear resistance of DZ2 axle steel,its service performance still needs to be improved.In this thesis,ultrasonic nanocrystal surface modification(UNSM)and supersonic fine particle bombardment(SFPB)technology were used to modify the surface of DZ2 axle steel,which improved the service performance of DZ2 axle steel.To get the changes of material properties after modification,the microstructures,mechanical properties,and wear resistance of DZ2 steel were all studied.Micro-structures were observed by an optical microscopy,a NPFLEX 3D surface metrology system,an X-ray diffractometer,an X-ray residual stress analyzer,and a scanning electron microscopy.Mechanical properties were tested by an HV-1000 microhard-ness tester.Wear resistance of DZ2 axle steel was evaluated by an UMT-2 ballon-disk friction wear tester.This thesis draws the following conclusions:(1)Both the UNSM treatment and the SFPB treatment could cause a severe plastic deformation of DZ2 axle steel,refine the surface grains,and form a surface deformation layer with gradient changes in grain size.After four times of UNSM treatment,the surface grain size of the sample was refined to 16 nm,and the thickness of the surface modification layer was up to 310 ?m.The grain size of the sample surface was refined to 29 nm and the thickness of the surface modification layer was about 56 ?m when the gas pressure SFPB treatment under 2 MPa.The XRD analysis showed that no phase transition occurred during the two modification processes.(2)After the UNSM treatment,the machining marks on the sample surface were severely flattened,and the roughness was significantly improved.Grain refinement and work hardening increased the surface microhardness of the material from 220 HV0.2 to283 HV0.2,an increase of 28.6%.After the SFPB treatment,many pits were formed on the sample surface,and the roughness increased significantly.The SFPB treatment also hardened the sample surface,and the surface hardness increased from 220 HV0.05 to246 HV0.05,an increase of 11.8%.After treated by the two methods,the surface hardness of the samples changed in a gradient,and gradually tended to the matrix hardness with increase of the depth.(3)Both the UNSM treatment and the SFPB treatment introduced compressive residual stress on the sample surface and formed a certain depth of compressive residual stress layer.The maximum compressive residual stress introduced by the UNSM treatment on the sample surface was-703 MPa,and the effective depth was 1500 ?m.The maximum compressive residual stress introduced by the SFPB treatment was-480 MPa,and the effective depth was 200 ?m.(4)Since the UNSM treatment improved the surface roughness of sample,and increased the surface microhardness,the friction coefficient was reduced and the wear resistance was improved.However,the SFPB treatment increased the surface roughness,so the actual contact area with the counter-grinding pair during the friction and wear processes was small,leading to many abrasive particles.The main forms of wear were abrasive wear and adhesive wear.The wear resistance of samples treated by the SFPB has not been improved,but worsened.Through the above research,we compared two surface modification methods,and provided necessary data support for the surface strengthening of DZ2 axle steel.It has an important application for improving the fatigue life of key components such as axles.