Effects of carbides on fatigue characteristics of austempered ductile iron
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I. INTRODUCTION
AUSTEMPERED ductile iron (ADI) is widely used in industry because it offers a combination of low cost, design flexibility, high strength-to-weight ratio and good toughness, wear resistance, and fatigue strength.[1,2,3] Generally, ADI alloys possess mechanical properties that are superior to conventional ductile iron and cast iron but comparable to heattreated forged steels. The microstructure is dependent on both the composition of the cast iron and the exact parameters of the four stages in the production process. The process comprises the production of a ductile iron casting, austenitization (800 °C to 950 °C), subsequent quenching to a temperature (250 °C to 400 °C) suitable for the final stage, and isothermal transformation (austempering) of some of the austenitic matrix to other phases, prior to subsequent cooling to room temperature. The austempering heat treatment, in addition to the austenitization heat treatment, is of significance in determining the exact microstructure produced. The camshaft is a critical component in a combustion engine and advanced engine designs. The high strength, good wear properties, castability, and low cost of ADI make it particularly attractive for this application. However, due to a demand for improved engine performance, the service conditions for a camshaft have changed. This changes the demands placed on the camshaft material, requiring resistance to rolling (as opposed to sliding), fatigue resistance, and high strength and ductility. Contact fatigue is now a significant concern for manufacturers of this type of camshaft, and it is clear that both wear and fretting fatigue play an B. STOKES, formerly PhD Student, with the Materials Research Group, School of Engineering Sciences, University of Southampton, is Research Engineer, with Alaris Medical Systems, Basingstoke, United Kingdom. N. GAO, Senior Research Fellow, and P.A.S. REED, Reader in Structural Materials, are with the Materials Research Group, School of Engineering Sciences, University of Southampton, Southampton, SO17 1BJ, United Kingdom. Contact e-mail: [email protected] K.K. LEE, formerly PhD Student, with Image, Speech and Intelligent Systems, School of Electronics and Computer Science, University of Southampton, is Research Fellow, with the School of Information Systems, University of East Anglia, Norwich, NR4 7TJ United Kingdom. Manuscript submitted May 28, 2004. METALLURGICAL AND MATERIALS TRANSACTIONS A
important role. Fretting fatigue is a very situation-specific service problem, and the construction of appropriate tests is extremely complicated. Typically,[4] failure is due to the initiation and growth of very small cracks, which leads to surface pitting, resulting in further wear-induced failure. Therefore, the study of initiation and early growth of cracks and the effect of various microstructural features of ADI on such growth are particularly important. Building upon previous work,[5,6,7] this research further investigates the microstructure and related failure mechanisms of a specific A
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