Fracture failure analysis and prevention of the rod wheel-side half shaft manufactured from micro-alloyed medium-carbon steel

• Obvious hardness fluctuation in the transition zone between spline and rod. • The upsetting process led to the coarsening of the partent austenite grains. • Increasing the forging ratio effectively improved the grain non-uniformity. The newly developed rod wheel-side half shaft manufactured from micro-alloyed medium-carbon steel broke during the test. The fracture occurred in the thermal deformation affected transition zone between the rod and the spline. It is necessary to study its causes and prevention mechanisms. First of all in this paper, the uniformity of the whole rod micro-structure and its mechanical properties were investigated. No obvious abnormalities were found. The finite element model was used to analyse the stress distribution of the transition zone under an actual load. The transition zone had a significant stress gradient of 300 MPa. The tensile test proved that it was prone to quasi-dissociation fracture. Second, the micro-structure of the transition zone was analysed. The abnormal growth of the parent austenite grain (35 % higher than the rod) led to the growth of the martensite lath. It increased the high energy grain boundary by 6 % and caused hardness to fluctuate. Finally, it was determined that the growth of the parent grain size mainly resulted from the energy input and incomplete re-crystallisation introduced in the upsetting process. Numerical simulations and engineering tests showed that the method of increasing the forging ratio can effectively improve the uneven distribution of grains. After optimization, the recrystallization area in the transition zone was more than 85 %, and the grain uniformity and mechanical properties remained stable.