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During hot rolling of steel inclusions are elongated

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orientation dependence. During hot rolling of steel, inclusions are elongated in the rolling direction and flattened in the rolling plane, forming an ellipsoidal shape. When traction is applied in the rolling plane along the prior rolling direction ( L orientation) or perpendicular to the rolling direction ( T orientation), very weak signals are observed. These signals apparently originate from cracks or disbonds over the minor axis of the ellipsoid. However, when the steel was tested so that the load was applied normal to the rolling plane ( ST orientation), copious acoustic emission was observed, presumably associated with fractures extending over the major axis. The sulfur content (number of inclusions), size and aspect ratio of the inclusions, together with the stress state, are the factors that control the acoustic emission from inclusions. The inclusion size and shape is determined by the solidification pathway and by thermomechanical processing after solidification. In heavily rolled steels, with sulfur contents greater than about 0.06 percent by weight, manganese sulfide fractures associated with short transverse loading can be a significant emission source. In many steels, crack growth occurs predominantly near welds where manganese sulphide has been melted and then reformed at interdendritic interstices during resolidification. If the sulfur content is high, large elongated inclusions are deposited. These can subsequently become the sites of discontinuities such as lamellar tears. Because discontinuity formation involves crack growth distances greater than 10 mm (0.4 in.), they are substantial acoustic emitters. Inclusions in Aluminum Alloys Numerous types of inclusions of varying ductility are found in aluminum alloys. Several researchers have found that only inclusions rich in iron ore fracture at ambient temperature. 99 These inclusions are found to give detectable signals and, as with signals from manganese sulphide in steels, the amplitude distribution of the acoustic emission scales with the size distribution of fracturing particles. Because of their smaller size and generally reduced aspect ratio, precipitates are less prone to fracture until very high stresses have been attained. Their small size, often less than 1 μm (4 × 10 –5 in.), makes them undetectable acoustic emitters. The exception has been in some specially heat treated steels containing large (5 μm [2 × 10 –4 in.]) spheroidal carbides. They are good emitters but are not often used in engineering alloys. Microvoid Coalescence Ductile fracture occurs by the sequence of processes depicted in Fig. 27 and outlined below. 1. During loading, cracks or disbonds occur at inclusions, causing a stress concentration in the matrix between inclusions. 2. Plastic deformation of ligaments between inclusions occurs, resulting in the growth of inclusion nucleated voids and an intensification of local stress.
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  • Fall '19
  • Nondestructive testing, Acoustic Emission, Acoustic Emission Testing

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