A batch cracking failure occurred during the production of piston rods made of 45 steel of a specific model. The causes of cracking of the piston rods were investigated by means of fracture morphology observation, metallographic inspection, mechanical property testing and scanning electron microscopy analysis. The results show that the cracks on the piston rods are brittle cracks generated in the induction hardened layer during straightening. The straightening fracture is attributed to inconsistent moving speed during induction heating quenching of the piston rods, which leads to the formation of soft bands in the hardened layer. Consequently, the fluctuation ranges of surface hardness and effective hardened layer depth far exceed the standard requirements. In addition, the soft band areas are under residual tensile stress, and cracks initiate under the action of straightening stress. As a high-quality carbon structural steel, 45 steel features a wide controllable hardness range and excellent comprehensive mechanical properties after quenching and tempering treatment. Nevertheless, its poor hardenability necessitates surface quenching methods such as medium-high frequency induction quenching or flame quenching to achieve a balance between high surface hardness and favorable core toughness. For instance, the "superhardness" effect of high-frequency quenching can deliver a surface hardness of 62～66 HRC, which substantially improves the wear resistance and fatigue resistance of workpieces accompanied by low notch sensitivity. Such processes are widely adopted for components including crankshafts and piston rods. Current research on piston rods mainly focuses on fatigue life and modal analysis, and premature fracture failures of these components are mostly induced by inappropriate heat treatment processes.