Orange peel was present on bowls that were deep-drawn from low-carbon steel sheet. It was present on bowls produced from some batches of steel and not present on bowls produced from other batches of steel. Failure analysis of the bowls was performed as part of the effort to determine the root cause of the problem.
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Orange peel is associated with the grain size of a metal. Therefore, cross-section metallography was used to examine the microstructure of bowls with and without the defect. The images below show the results of the analysis. The bowl without defects had small grains throughout the cross-section. The bowl with orange peel had large grains at the surface.
For a given amount of deformation, there is a limit to the maximum grain size before orange peel appears. In this instance, the grains at the surface were too large for certain batches of steel sheet.
The grain size of metal is controlled through a combination of cold rolling and annealing. Proper control of annealing temperature, annealing time, and cooling methods after annealing are critical to obtain the desired grain size. The top surface of the sample with orange' peel was not properly cooled, which allowed the grains at the surface to grow too large.
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The roughened surface of large grained metals after forming is called orange peel because the surface has the appearance of the surface of an orange. See the article Orange Peel for more information about the subject, including how to prevent it from occurring.
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