A case of mechanical anisotropy of steel after thermomechanical treatment
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A high degree of anisotropy of the mechanical properties of high-carbon steels subjected to high-temperature TMT has been observed. The anisotropy is manifested by the difference in the strength and ductility of steel tested under direct and inverse loads.
The “plasticizing” and “embrittling” effects are associated with the fact that shear along the slip planes developed during TMT is facilitated under direct loads and inhibited under inverse loads. In the latter case the direction of the forces is such as to promote the formation of submicroscopic cracks along planes with a particularly high concentration of defects and precipitated particles. This eventually leads to brittle fracture.
The way in which the anisotropy is manifested depends upon the nature of the stress state both during TMT and in service (or mechanical tests). When the service loads are direct (i. e., produce a stress state similar to that which obtains during TMT), an increase in the ductility and, possibly, the strength of the material is observed. Under sufficiently high inverse loads the material becomes brittle.
High-carbon steels subjected to high-temperature TMT have increased resistance to cleavage fracture under direct loads; when tempered at low temperatures, such steels can therefore undergo considerable plastic deformation before failing, fracture taking place by a shear mechanism at relatively high stress levels. The same applies to unalloyed high-carbon steels of the U8A type.
When isotropy of the mechanical properties of steel is aimed at, a TMT involving omnidirectional deformation should be applied so as to bring into play all the possible slip planes and hence ensure a more uniform distribution of dislocations, vacancies, and particles precipitated during treatment.
TMT involving deformation in torsion can be used as a method of strengthening machine parts operating under unilateral static loads (springs, torsion bars, etc.)
KeywordsAnisotropy Ductility Isotropy Brittle Fracture Slip Plane
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