... I have observed excessive grain growth only one time, but do not think highly of this practice. The billets are tumble blasted, reheated, and descaled, and then go to the closed die. Do you have any micrograph examples of an overheated or burned low-carbon forging?
A: I do not recommend that much reheating. However, since this is 8620 steel that eventually will likely be carburized, I believe that the overheating temperature is close to 2,400°F. As long as the reheats are below that temperature, two to three reheats are not a problem — as long as the billets are cooled to ambient and blasted before reheating. But, do not use induction to reheat warm billets.
The primary indicator of overheating appear to be duplex grains. That is, (after normalizing) small grains within a network of large grains that are almost like "ghost" grains. They are often observable first before etching or, better still, after re-polishing after first etching. I found that the most reliable determinate was the fracture test that will show faceted structure on the fractured surface. There are some who call this a "honeycomb" fracture ("fattened" ferrite grain boundaries). Remember, the carbon levels in these carburizing grades make it more difficult to observe overheating on fracture testing, due to the high ferrite percentage. If they are heat-treated before fracturing, the facets are seen more readily on the fracture surfaces.
Burning is seen readily as rather distinct outline of oxides, including MnS and lower melting constituents, on as-polished micro surfaces.
My research indicates that burning is not recoverable by working (large deformations), unlike overheating. Impact testing will show how badly burned steel will become. (Charpy values can become very low).
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.