Fatigue anisotropy in cross-rolled, hardened medium carbon steel resulting from MnS inclusions
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INCLUSIONS AS A REASON FOR ANISOTROPY A. Anisotropy
FORGED materials generally show directional material behavior. This behavior is especially adverse when isotropic properties are required, as in, for example, most rotationally symmetric components. An example of such a component would be a transmission gear wheel that is die forged from a fraction of a billet. Anisotropy is introduced in the material by not only component forging and deformation operations but also earlier during the manufacturing process of the material such as, for example, continuous casting. The orientation of the billet is superimposed by the orientation created during either horizontal or vertical die forging of the material. Anisotropy of any kind, but especially mechanical anisotropy, arises from the four types of orientation of a microstructure that can be distinguished. Those four types are (1) crystallographic texture (preferred crystal orientation), (2) elongated grains, (3) banded (two)-phase structure, and (4) elongated inclusions.
inclusions as stress concentrators changes with their size and shape,[5,6] but also with their ability to bond with the matrix material.[7] An inclusion that cannot maintain bonding forces to the matrix material might behave just like a pore or crack within the material. An inclusion with proper bonding to the surrounding material is as a microstructural constituent less lethal to mechanical properties, and the material thus exhibits more ductility. It has been shown that inclusions are accountable not only for fatigue anisotropy, but they also dominate all mechanical anisotropy (such as tensile strength, ductility, fracture toughness, etc.).[1,3,4] Orientation of inclusions potentially governs directional behavior of a material.[1,7] To diminish inclusion anisotropy, several approaches can be considered. Anisotropy can be controlled by changing parameters such as shape, amount, and dispersion of inclusions. Those changes can be achieved by interalia variation of deformation temperature and deformation ratio of the material.[5,8] Also, changes in material composition chemistry might alter inclusions in a beneficial way (i.e., diminish the mechanical anisotropy).[3] Sulfides represent the largest indigenous inclusions in steel. Therefore they deserve special interest.
B. Inclusions Elongated inclusions have for fatigue anisotropy a major and superimposing effect because inclusions constitute with their characteristic as stress concentration factor a dominating site for fatigue crack initiation.[1–4] The criticality of
CORNELIUS TEMMEL, Researcher, and NILS-GUNNAR INGESTEN, Manager Forged Materials, are with Materials Technology, Volvo Powertrain Sweden, Go¨teborg, Sweden. Contact e-mail: cornelius.temmel@ gmx.net BIRGER KARLSSON, Professor, is with the Department of Materials and Manufacturing Technology, Chalmers University of Technology, Go¨teborg, Sweden. Manuscript submitted December 14, 2005. METALLURGICAL AND MATERIALS TRANSACTIONS A
C. Sulfides The most frequent and also most important sulfide inclu
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