| High strength structural steel has good mechanical properties and is widely used in construction,bridges,transportation and other fields.In the actual service environment,these structures are often subjected to fatigue loads,and fatigue damage seriously restricts their service life.Taking certain measures to improve the fatigue properties of materials has become an important element of reliability design in engineering structures.The improvement of fatigue performance of high-strength steels can be achieved by introducing residual compressive stresses to the surface of metallic materials to gradient the metal surface organization.Therefore,the paper proposes ultrasonic surface rolling(USR)treatment for ASTM A572 Gr65 steel to improve the fatigue performance of the material surface layer based on the generation of intense plastic deformation.This study has important practical significance for the application of highstrength structural steel in engineering practice.In the paper,the surface gradient treatment of ASTM A572 Gr65 steel was performed by ultrasonic surface tumbling technique.The gradient structure,microhardness and surface roughness of the gradient layer after ultrasonic surface tumbling treatment were analyzed,the fatigue properties before and after ultrasonic surface tumbling were investigated,the temperature evolution of the material under cyclic loading was investigated based on energy dissipation,and the fatigue limit was evaluated.The fatigue fracture behavior and fatigue life extension mechanism after surface tumbling were investigated.After the ultrasonic surface rolling treatment,the material shows a gradient distribution from the surface layer to the internal grains,which can be divided into three regions,namely,the surface fine-grained layer(FGL),the deformed grain layer(DGL)and the coarse grain layer(CGL).The gradient structure was characterized by electron back dissipative diffraction(EBSD),and the material after ultrasonic surface tumbling showed an increase in small-angle grain boundaries and an increase in the geometrically necessary dislocation density in the surface layer.the XRD patterns showed that no new phases were generated after ultrasonic surface tumbling,and the peak intensity increased and the half-height width broadened after tumbling.The surface roughness of the material was significantly improved after ultrasonic tumbling.The surface roughness of the material was Ra=1.573 μm after lathe finishing,and was significantly reduced to Ra =0.077 μm after ultrasonic surface tumbling treatment.The hardness in the gradient structure is distributed in a gradient,and the microhardness in the FGL zone is significantly higher than that in the CGL zone.The results of the tensile properties of ASTM A572 Gr65 steel before and after ultrasonic surface rolling showed that the yield strength of untreated and ultrasonic surface rolled specimens were 450 MPa and 545 MPa,respectively,which increased by 21%.The temperature evolution during the tensile process is divided into four stages.In the elastic stage,the thermoelastic effect leads to a decrease in the specimen surface temperature;in the plastic stage,the specimen surface temperature gradually increases,and when the specimen fractures,the accumulated energy release temperature increases abruptly,and the specimen temperature returns to room temperature after fracture.The fatigue test shows that when the peak cyclic stress does not exceed the fatigue limit,the ratchet strain remains unchanged after cyclic hardening,and when the peak cyclic stress exceeds the fatigue limit,the unstable cyclic softening of the ratchet strain after stabilization occurs under a certain number of cycles,resulting in a rapid increase of the ratchet strain and specimen fracture.In the cyclic loading process,accompanied by energy dissipation,the macroscopic manifestation is a rise in specimen surface temperature.Corresponding to the strain during cyclic deformation,the temperature evolution during fatigue shows the same change pattern as the strain accumulation of the material.The fatigue fracture of the specimens before and after ultrasonic surface rolling showed three regions,namely the fatigue crack sprouting region,the crack extension region and the instantaneous fracture region.The crack extension zone showed quasi-dissociative fracture characteristics,and the transient fracture zone showed plastic fracture characteristics.The fatigue crack source of the untreated specimen was located in the surface layer of the specimen,and the fatigue source of the specimen was transferred to the subsurface layer of the material after ultrasonic surface tumbling.The fatigue performance of the specimens before and after rolling was evaluated by S-N curve and infrared thermography.The results showed that the fatigue limits obtained by S-N curve method were 365.60 MPa and 454.15 MPa before and after ultrasonic surface rolling,and the fatigue limits obtained by infrared thermal imaging method were 337.55 MPa and464.91 MPa. |
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