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In the present study,we have analyzed a rupture incident on converter trunnion in a certain factory and it adopts disc spring for buffer.Crack propagation paths indicate that this is caused by torsional fatigue.Detection results show large vibration speed of tilting mechanism appeared in the smelting process and vibration reduction performance of spring buffer is poor.Trunnion is made of 18MnMoNb low alloy steel,and bainite is its main structure.Martensite,lath bainite,ferrite and even widmanstatten transformations will occur during welding repair.Strict preheating and post weld heat treatment(PWHT)should be adopted to avoid welding cracks.Thermal cycle simulation shows preheat temperature higher than 210℃ can reduce lath bainite formation and refine microstructure effectively,the temperature of PWHT should exceed 600℃ to ensure the transformation of lath bainite to granular bainite.