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Residual Stresses Associated with the Production of Coiled Automotive Springs

Cold coiling of high tensile steel rod into helical coil springs for the automotive industry is a new technique being implemented amongst spring manufacturers worldwide. To characterise this coil production process, the neutron strain scanning technique has been employed to non-destructively elucidate the influence production stages have on the tri-axial residual stress state. Samples investigated represented key production steps in the cold-coil forming process: Cold coiling; Tempering; Hot setting; Hot peening; Shot peening. Investigations revealed that the stress field was axi-symmetrical, that the dominant variation in all samples occurred along the hoop direction (helical circumference), whilst the radial and axial stresses are substantially lower. Accurate two-dimensional stress maps of the rod cross section have been compiled revealing key features associated with the cold coiling step. Comparison of the stress fields after each production step revealed altered stress values. The final shot peening process stage not only reduced stress concentrations at the internal bore, but contributed to the establishment of favourable surface residual stress conditions that enhance the fatigue life of the final product.