Surface finishing requirements for hot forgings

Nov. 27, 2006
Q: I'm searching for information-on an industry standard for surface finish, including nicks and dings, for a 4140 steel hot forging. A customer is causing us to "apple polish" every forged surface prior to shipping. This is costing us dearly, and without
A: Neither the FIA Forging Manual nor the FIA Forging Design Guide contains specifications, per se , on surface finish. The usual print notations (not true specifications) for forgings may go something like this: As-forged surfaces shall be smooth and free of scale and other pits. Shot-blasted surfaces are usually considered as adequate on rough forgings (that will be machined). On forged surfaces that will not be machined, the surface finish shall have a roughness equivalent to a surface finish of 500RMS or finer. It should also be free of scale and scale pits. I know of no specification for dings or depression-like pits. If they are proven to be within dimensional tolerances, dings would be a cosmetic problem only. Often, Brinell "dents" are acceptable in neutral locations (low or no stress areas). Beyond these kinds of notations, I would have to know the technical reasons for your having to furnish parts so heavily polished. For net or near-net forgings, the surface finish can be specified as smooth as the "apple polish" that you mention. In this case, the forgings are usually warm forged and sometimes coined to sharpen up selected dimensions, as well as improving surface finish.

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.