高炭素鋼の各種変態組織と脆性破面

概要

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The brittle fracture surfaces of the isothermally transformed high carbon steel (0.7% C) were investigated. The various microstructures which were lamellar pearlite, degenerate pearlite, uppcr bainite, lowere bainite, and martensite were obtained based on the TTT diagram. The brittle fracture surfaces were prepared by Charpy impact test at -196℃. The direct correspondences between the brittle fracture surfaces and the microstructures were carried out in scanning electron microscope. The mode of quasi-cleavage surfaces was closely related to the decomposition of austenite to ferrite. The crystallographic orientation of the cleavage plane was determined as {100} by means of facet pit technique and goniomicroscope. The fracture facets of pearlite, degenerate pearlite, upper bainite and lower bainite corresponded to the domains in which the crystallographic orientation difference in respect to the cleavage surface {100} were small. Such the domains on the other hand, were not seen in the case of martensite. It seemed that the martensite plates itself formed the fracture facet. From this point of view, there was a great difference in fractographic appearance between high carbon martensite and low carbon lath martensite. It is probably due to the difference of the crystallographic behaviors in transformation.
1972-06-01