|MadSci Network: Engineering|
Hi Adam, Lets take your questions in reverse order. Metals have crystaline structures. A grain is a single crystal in a metallic material. The grains form when the metal solidifies from a liquid. Metals have a tendancy to fracture or break along the grain boundaries. Metals with large elongated grains are usually not as strong as metals with smaller and more spherical grains. ASTM method E 112 provides a detailed explanation of how grains are measured. The following images are examples of different size grains with ASTM size ratings. The sample is polished and viewed through a microscope at 100 X magnification. Typically, an etch (chemical solution) is used to enhance the contrast between the grains and the grain boundaries. One method for measuring grain size is to draw a line of known length across a portion of the image and count the number of times a grain boundary crosses the line. The average grain diameter is the line length divided by the number of grain boundaries. ASTM method E 112 has a table for converting average grain diameter to grain size. Notice that the larger grains have a smaller ASTM grain size. If you require a more detailed explanation, I recommend an introductory course to metallurgy. Bob Novak Specialist -Process R&D Carpenter Technology
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