Reason Analysis on Premature Cracking of Connecting Rod of Steel C70S6 in Fatigue Test

By means of chemical composition analysis,mechanical properties testing,macro and micro examination,the premature cracking reason of connecting rod of steel C70S6 in the fatigue test was analyzed.The results showed that the crack initiated at the bottom of V-shape screw thread around the lower end of bolt hole.There were tiny machining marks around thread manufactured by screw cutting tap;the content of S,Cr and V was unqualified;the porosity appeared in some parts of connecting rod,the premature cracking was caused by these factors together in fatigue test.Therefore,it was proposed that the processing technology of screw thread and the forging technology should be improved.