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Metallographic etching for carbide volume fraction of high-chromium white cast irons.
Mater Charact 1990 Jun; 24(4):379-386
This paper describes the preparation of samples for the rapid determination of carbide volume fraction in high-chromium white cast irons by the use of the zeiss/kontron scanning electron microscope-image processing system (sem-ips). Sample composition ranges were, in percent: 17.7-28.8 Cr, 2.0-3.15 C, 0.02-1.10 Mo, and 0.5-6.3 Mn; balance fe. It was discovered that the normal metallographic etches used to enhance the carbide phase did not provide uniform contrast over the 6 by 3 mm area required for image analysis measurements, thus preventing accurate analysis. This nonuniformity of contrast prevented the ips from forming the required black and white binary image that distinguishes the carbide from the matrix, allowing the carbide area to be measured. The images observed by the human eye differ from those formed by the high-resolution analytical tv camera because the camera sees light in a linear range of brightness, while the eye responds logarithmically. The eye also is sensitive to edges---in this case carbide boundaries. Nonuniform areas of brightness and/or contrast are compensated by the eye and the metallographic image that is observed has the carbide well defined. In fact, the image has nonuniform contrast (nonuniform etch) in the region of interest, a feature quite readily apparent to the camera or image analyzer. A search is underway to find an etch that uniformly attacks the carbide phase or the matrix, thus allowing the operator of the ips to set the threshold levels to separate carbide from matrix and measure carbide volume fraction.
Sampling; Chromium; Chromium compounds; Cast iron alloys; Sample preparation; Carbides; Scanning electron microscopy; Imaging techniques; Iron; Iron compounds; Metallurgy; Visual images; Lighting; Measurement equipment; Photoengraving; Light properties
OP; Journal Article
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Page last reviewed: September 2, 2020
Content source: National Institute for Occupational Safety and Health Education and Information Division