Analysis and prevention of cracking phenomenon occurring during cold forging of two AISI 1010 steel pulleys
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I.
INTRODUCTION
RECENTLY, forging producers are increasingly using precision forging in which complicated parts can be formed directly in net shape or near-net shape in order to reduce cost. Particularly in cold forging, materials with high formability that can accommodate severe deformation are required; low carbon steels are widely used for these materials. However, cracks often form at the edges of parts where deformation is highly concentrated. These cracks are associated with the formation of adiabatic shear bands, and their rapid propagation results in defective parts.[1–4] Local concentration of plastic deformation may arise under dynamic loading, and this plastically localized region is called an adiabatic shear band. This shear band, developed by thermo mechanical instability, is reported to occur in titanium, aluminum, and copper alloys in addition to steels.[2,5–16] When shear bands are formed, load carrying capability of the regions is radically deteriorated—causing final failure.[17] For example, the formation of shear bands occurring during a very high-speed forming process, such as forging, induces defects in the products. Shear bands formed by ballistic impact readily cause failure of armor.[18–21] Thus, studies on the dynamic deformation behavior and on the formation of shear bands are essential in improving microstructure and dynamic properties, controlling high-speed forming process, and preventing cracking. DONG-KUK KIM and SUK YOUNG KANG, Research Assistants, and SUNGHAK LEE, Professor, are with the Center for Advanced Aerospace Materials, Pohang University of Science and Technology, Pohang, 790784 Korea. KYUNG JONG LEE, Senior Research Engineer, is with Sheet Products & Process Research Team, Technical Research Laboratories, Pohang Iron and Steel Co., Ltd., Pohang, 790-785, Korea. Manuscript submitted January 15, 1998. METALLURGICAL AND MATERIALS TRANSACTIONS A
However, systematic studies have yet to be made to explain the cracking phenomenon relative to microstructural parameters and their effect on formability. In this study, microstructural analyses were made on the cracking occurring at the top interior regions after cold forging of AISI 1010 steel pulleys. The cracking mechanism and the effect of microstructural parameters on dynamic deformation were also investigated so as to suggest the proper forging process to prevent cracking. Microstructures of the cracked regions, including adiabatic shear bands, were examined, and dynamic torsional tests were conducted using a torsional Kolsky bar. The test results were then correlated with microstructural analysis results. II.
EXPERIMENTAL
Materials used in this study are two kinds of AISI 1010 steels (0.1 pct low-carbon steels) having a ferrite-pearlite structure. The chemical compositions are listed in Table I. They were fabricated in rods 120 mm in diameter; one was processed by converter steel making and hot rolling, while the other was processed by electric furnace steel making and hot rolling. For convenience, the former is referred to
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