Fracture mechanism of AISI 1025 rod shearing in automatic multi-stage cold forging and critical shearing speed

Experimental and numerical studies on shearing processes that focused on automatic multi-stage cold forging (AMSCF) are conducted to reveal the mechanisms of the fracture phenomena that occur in the cold rod shearing process in AMSCF. First, AISI 1025 (treated for AMSCF, specifically called SWCH25F) coil-rod of radius 17.5 mm was straightened and cut by an AMSCF machine at operating speeds of 20–90 rpm to elucidate the effects of speed on the surface quality of the sheared billet. Second, a universal testing machine was used in combination with a special shearing apparatus for experimental cold rod shearing to measure the shearing load-time (or stroke) curve and examine fracture mechanisms at sufficiently low speeds. Fractographical observations showed that speed clearly affects the cold shearing of a straightened coil-rod. The surfaces of billets sheared at low speeds showed clear evidence of ductile fracture, whereas those sheared at normal speeds did not. The experimental results were used in thermoviscoplastic finite element (FE) analysis together with the flow behaviors of the material in the cold-forging temperature range to explain the brittle fracture of the ductile material during cold coil-rod shearing in AMSCF. The concept of a critical coil-rod shearing speed for acceptable surface quality is presented.