| Ultrasonic impact technology as a new type of surface deformation enhancement technology has been widely recognized for its high efficiency,controllability,cleanliness,and flexibility.So far,ultrasonic impact technology is mainly used to improve the fatigue performance of the weld zone in industrial production,and is widely used in repair and extension projects of offshore platforms,steel bridges,and large aircraft.After the ultrasonic impact treatment,the surface structure of the metal material is refined,and a layer of residual compressive stress is generated at a certain depth,which significantly improves the material's strong hardness and fatigue properties.However,there is a lack of systematic research on the relationship between ultrasonic impact parameters and impact strengthening effects.How long can the ultrasonic impact surface hardening effect be maintained and how the residual compressive stress will change under long-term service conditions,Whether or not there will be stress relaxation and the extent and speed of stress relaxation have not been studied so far.However,these problems are urgently needed to be understood in practical projects.The experiments were performed on the surface of AISI304 stainless steel specimens with different strengths of 3 min/cm~2,6 min/cm~2,9 min/cm~2,and 12 min/cm~2 respectively,Experimental results show:After four different strengths of impact strengthening,the surface roughness has been significantly reduced,and the hardness,martensite content,and residual compressive stress have increased significantly.With the increase of impact strength,the surface roughness reaches the minimum when the impact strength is 9 min/cm~2.The surface hardness,surface and maximum residual compressive stress,and the surface martensite content are respectively when the impact strength is 9 min/cm~2,to reach maximum.The optimum impact strength parameter was optimized by experiments to be 9 min/cm~2.Based on this,the recovery behavior and stress relaxation characteristics of the ultrasonic impact specimen during long-term service were studied.The ultrasonic impact test specimens were subjected to different heating times at 250°C and 350°C under vacuum conditions to accelerate the experimental process to simulate the evolution of the microstructure and properties of the specimens subjected to ultrasonic impact during long-term service.The results show that the surface martensite content,surface microhardness,and surface residual compressive stress all show a significant decrease with a shorter holding time and then slowly decrease with increasing holding time at either 250°C or 350°C.Finally,it reaches a steady state,but it is still 85%,182%,and more than 150%higher than the non-impacted sample.The conclusion of the above experimental results is:Ultrasonic impact treatment is a very effective material strengthening method,but there is enhanced saturation effect,and has the best impact strength value.The best ultrasonic impact strength under this experimental condition is 9 min/cm~2.Although the effect of ultrasonic impact strengthening has a certain degree of degradation in accelerated experiments,the amplitude is not large,and the effect of ultrasonic impact strengthening still retains most of the impact-strengthening effects under the accelerated recovery conditions of heat treatment.This means that,after long-term normal temperature working conditions,the components of the ultrasonic impact treatment process are relatively slow in the process of tissue recovery and performance degradation,and their strengthening effects are reliable for quite some time,the UIT strengthening effect of AISI304stainless steel during long-term service has considerable stability and reliability. |
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