Simulation Analysis on the Fracture Failure of S2 Alloy Steel Screwdriver Bits

The microstructures and torsional fracture morphologies of S2 alloy steel screwdriver bits were characterized by scanning electron microscopy (SEM). The bits were oil-quenched from 830 °C and 860 °C and subsequently tempered at 150 °C and 170 °C under three carbon potential levels (0.35, 0.40, and 0.45). In parallel, Deform-3D and Ansys Workbench were employed to simulate and compare the microstructure evolution during quenching, the residual stress field after quenching and tempering, and the stress distribution developed under torsional loading. The results reveal that the non-planar fracture and low qualification rate of the bits arise from two independent but synergistic mechanisms: (1) insufficient austenitizing at 830 °C fails to produce fully uniform austenite, resulting in non-uniform martensitic microstructure and inhomogeneous hardness distribution after quenching; (2) low furnace carbon potential (≤0.35) causes surface decarburization and the formation of massive ferrite at the near-surface region, which acts as preferential crack initiation sites. Furthermore, the transformation stress generated during quenching, the residual stress remaining after tempering, and the stress concentration at tooth edges under service loading jointly promote crack initiation and propagation. A uniform, high-hardness tempered martensite microstructure is obtained when the bits are austenitized at 860 °C with the carbon potential strictly maintained between 0.40 and 0.45, held for 60 min before oil quenching, and air-cooled after tempering at 170 °C. This optimized heat-treatment route eliminates surface decarburization, ensures microstructural homogeneity, reduces residual stress, and enables the bits to fail by planar fracture under torsional load with 100% qualification rate.