The Study On Fatigue Performance For Welded Joints Of High Strength Steel By Ultrasonic Impact Treatment

To meet the requirement of high loading design, while considering the limits ofgeometric size and dead weight, the application of high strength steels has been muchmore emphasized. Low-alloy high-strength steel is widely used in automotive,shipbuilding, engineering machine, petrochemical industry and many other fields,because of its high yield strength and toughness, excellent performance at high andlow temperature.Fatigue fracture is one of main failure forms in practical engineering. It alwaysresults in huge loss of personal casualty, property and equipment, as it happenssuddenly under a low mean load and is hardly observed before failure. Although highstrength steel has so many outstanding features, the fatigue strength of its weldedjoints is not as good as expected. It is due to the surface welding defects, whichreduce or suppress the crack initiation period, which is highly dependent uponmaterial characteristics. Thus the largest part of fatigue life comes from the crackpropagation period, which is independent of mechanical properties. As a consequence,the fatigue strength of welded structures made out of high strength steel is not higherthan that of mild steel. The high strength steel has no advantage, unless to subject highmean load or the stress radio is great.Aiming at the above problems in the application of high strength steels, this paperstudies on the fatigue performance for welded joints of high strength steel treated byultrosonic impact treatment. This paper includes:Comparing the increasing degrees of the fatigue strength made out of differentstrength steels’ welded joints, it was found that there is no difference in fatiguestrength of as-welded joints of Q235, Q345and even S690QL, nevertheless theincreasing degree rises with the higher strength level when the joints treated byultrasonic impact treatment.For in-service welded structure, in this paper welded butt joints were treated byultrasonic impact treatment after cyclic loading for a while, then fatigue life testcontinued. Comparing this fatigue life with that of as-welded joints tested under thesame loading condition, it was found that ultrasonic impact treatment could improvethe remaining fatigue life of welded joints significantly, even make it as long as thejoints treated originally by ultrasonic impact treatment. Furthermore when treated at 50%N(which is the fatigue life of as-weld joints at95%survival probabilityassociated with a75%confidence level), a better improvement could be obtained thanthat treated at75%N and100%N.To further prolong the fatigue life of welded joints treated by ultrasonic impacttreatment, in this paper cruciform joints were re-treated every after a period of cyclicloading until fatigue fracture happened during the cyclic loading. But the result shows,this method does not benefit the fatigue performance obviously, only reduces thedispersion of the fatigue life.