Grain growth problems in stainless steel forgings

Q: We have problems with high-nitrogen stainless steel forgings. The parts are not typically nitrided,

... so the alloy is a straightforward high-nitrogen austenitic stainless (316L) (ISO5832-9), or ASTM 1586, 21Cr, 10Ni, 3.0Mn, 2.5 Mo. Previously, you have stated that minor amounts of cold or hot work can induce a duplex structure upon annealing. What is the mechanism for this? Is it strain-induced, in that the surface of the forging is subject to greater working, and thereby higher dislocation density, lower recrystallization temperature, which in subsequent annealing causes rapid grain growth?

A: The answer to your question about dislocation density is yes, and it actually can come from minor deformations at the surface, as in shot peening or tumbling. It leads to an amount of cold deformation that causes the subsurface material to recrystallize following the forging operation, which had already resulted in recrystallization during cooling following forging. This is usually the fine grain in the center of the forging. When annealed at a typical temperature for the alloy, the center grains grow some but the critically strained surface grains can recrystallize and grow rapidly. This could also be true if the forge shop does some coining to sharpen up the dimensions. If they do not anneal after coining or blasting or tumbling, then I am lost for an explanation for the pronounced duplex grain size.

You are correct to think that light reductions before high temperature annealing (usually done at 1,800°F/1,000°C, or so) can promote grain growth on stainless grades that are, for example, shot peened before annealing. This also can cause surface strains that may promote abnormal grain growth near the surface.

Remember, 316L stainless also shows a tendency to exhibit abnormal grain sizes after annealing due to the lower carbon content. Grain size in this grade is dependent on the amount of deformation during hot forging. I expect that the grain size would be reasonably uniform after forging with reasonably large reductions, depending also on the subsequent annealing cycles.

I suspect the forgings are shot blasted to clean them up, or possibly tumbled in a media which would cause surface deformations. This is often the procedure used for cleaning stainless steel forgings — but after annealing, not before. If there is no annealing after coining or blasting or tumbling, then again, I am lost for an explanation.

For more than 40 years H. James Henning held key technical positions in the forging industry, including as director of technology for the Forging Industry Association, and as president of Henning Education Services, a Columbus, OH, firm specializing in customized education and training in forging technologies.

Guidelines and recommendations offered in this column are based on information believed to be reliable and are supplied in good faith but without guarantee. Operational conditions that exist in individual plants and facilities vary widely. Users of this information should adapt it, and always exercise independent discretion in establishing plant or facility operating practice.

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