Fatigue Isotropy in Cross-Rolled, Hardened Isotropic-Quality Steel
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OTROPIC-QUALITY STEELS AND THEIR APPLICATION
THE downsizing of engine and transmission components and the increase of power are the perpetual challenges in the powertrain engineering of motor vehicles. The sustainability debate of the past years calls for increased efforts in this field. A promising step forward can be taken by improving the fatigue response of a component by the application of material optimized for isotropic behavior. Therefore, this study will focus on the fatigue performance of isotropic steel. Forged and rolled materials generally show directional behavior. That is because deformed materials suffer from anisotropy predominantly induced by the contained, deformed inclusions.[1–3] Mechanical anisotropy calls for the overdimensioning of the segments of a part, e.g., in rotationally symmetric components, in which the design has to follow the performance of the weakest sections. The teeth of a gearwheel carry a microstructural orientation within them, which changes around the circumference of the wheel (Figure 1). The test gearwheel in Figure 1 was horizontally forged; that means that the flow lines in the blank were perpendicular to the forging direction prior to deformation.[4] However, after the deformation C. TEMMEL, Researcher and N.-G. INGESTEN, Manager Forged Materials, are with Materials Technology, Volvo Powertrain Sweden, SE-405 08 Go¨teborg, Sweden. Contact e-mail: [email protected] B. KARLSSON, Professor, is with the Department of Materials and Manufacturing Technology, Chalmers University of Technology, SE-412 96 Go¨teborg, Sweden. Manuscript submitted June 26, 2007. Article published online March 12, 2008 1132—VOLUME 39A, MAY 2008
operation, the material flow lines appear parallel to the image plane of Figure 1 in the left and right sections of the gearwheel and perpendicular to the image plane in the top and lower sections of the gear. This also implies that inclusions are oriented differently in different sections of the gearwheel. Ultimately, the teeth in the sections with a detrimental material orientation perform less well. Gear fatigue test results indicated this already earlier.[5] Steel that can maintain mechanically isotropic behavior, even after a forging or rolling operation, would help to solve the problem. Such isotropic steels have been developed and promise inter alia better fatigue resistance.[6,7] Their development originates generally from roller-bearing steels and later from steels for highly loaded hydraulic parts, such as diesel injection nozzles, etc. What those steels have in common is that they need to have the highest possible cleanness.[8] They have also been optimized to meet the demands of gearbox design with respect to cost, availability, and performance.
A. Isotropic Steels Uniform, nondirectional mechanical properties of steel can most successfully be achieved by avoidance of inclusion-induced anisotropy,[1,3,9] which is a result of the elongated inclusions contained in the matrix. To attain such uniform properties, several characteristics have to be un
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