Orange peel was present on bowls that were deep-drawn from low-carbon steel sheet. Orange peel is a cosmetic defect that occurs on parts produced by cold forming metal metal stock (e.g. sheet, plate, and wire). It is called orange peel because the surface has the appearance of the surface of an orange. In this case, circular pieces of cold-rolled and annealed steel sheet were deep drawn to form the bowls.
The orange peel was present on bowls produced from some batches of steel and not present on bowls produced from other batches of steel. Photos of the sides of bowls without and with orange peel are shown below.
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Failure analysis of the bowls was performed to collect information that would help identify the root cause of the problem. Composition analysis and cross-section metallography were performed on samples of bowls with and without orange peel.
The compositions were similar for the samples with and without the problem. The images below show the results of the metallographic evaluation. The bowl without orange peel had small grains throughout the cross-section, whereas the bowl with orange peel had large grains at the surface and small grains throughout the rest of the sample.
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.
As discussed in this article and this video, 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.
See the article Orange Peel for more information about the subject, including how to prevent it from occurring.
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Michael Pfeifer, Ph.D., P.E. is President of Industrial Metallurgists and a metallurgical engineering consultant with more than 30 years of experience in product design, manufacturing, failure analysis, and metallurgy training. He helps clients worldwide solve design and manufacturing problems and is the author of Materials Enabled Designs.
