Ryuta Onodera1, Michio Hirokado2 and Mineo Shimizu1
1Department of Metallurgy, Faculty of Engineering, Kyushu University, Fukuoka
Radial direction fracture, named concerning the fracture surface with a radial pattern, is a kind of tensile fracture of round bar specimen and often observed in tempered steels. The present work studied the morphology of the fracture surface and the condition of occurrence of the R-D fracture. Scanning electron microscopic observations showed that the R-D fracture surface consisted of shear fracture plane inclined by 45°ree to the tensile axis and vertical fracture plane parallel to the tensile axis. The microstructures of the shear and the vertical plane were dimple pattern and low ductility shallow dimple pattern, respectively. The R-D fracture occurred prominently in 0.35--0.45C steels tempered at 773--873 K and became less prominent with increase and decrease of carbon content and tempering temperature. Steels annealed, normarized and isothermally transformed showed the cup and cone fracture only.
These experimental results mean that it is necessary to the R-D fracture that spherical carbides of moderate size disperse properly in the specimen. Accordingly the cause of the R-D fracture might be attributed to rows of elongated voids which were formed about the carbides and arranged along grain boundaries of tempered martensites, because the vertical plane with high density of void would be liable to crack by the lateral stress induced at a macroscopic crack tip.
Both a quenched and a furnace cooled S35C specimen after tempered at 973 K for 1.8 ks showed the R-D fracture. On the contrary, both a quenched and a furnace cooled specimens tempered for 72 ks showed the cup and cone type fracture. These results suggest that temper brittleness does not play an important role in the occurrence of the R-D fracture.
radial direction fracture, ductile fracture, tempered carbon steel, fractography, tensile test
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