... I have observed excessive grain growth just one time, but do not think highly of this practice. The billets are tumble-blasted, then reheated, descaled, sent to the forging press. Also, do you have any examples of what an overheated or burned sample of low-carbon forging looks like?
A: Normally, I do not recommend that much reheating. However, because this is 8620 steel that eventually will likely be carburized, I believe that the overheating temperature is close to 2,400ºF, and as long as the reheats are below that temperature, I see no serious problem with 2-3 reheats — so long as the billets are cooled to ambient and blasted before reheating. Reheating warm billets by induction is a "no-no."
I have a good file on heating, and on causes of overheating, but no photos. This is unfortunate, because you are not the only one asking for such photos.
The main indication of overheating is what appear to be duplex grains. That is, after normalizing, small grains within a network of large grains that are almost like "ghost" grains. Often they are observable first before etching or, better still, after re-polishing after first etching. I found that the most reliable indicator was the fracture test, which will show faceted structure on the fractured surface. Some call this a "honeycomb" fracture ("fattened" ferrite grain boundaries). Remember, the carbon levels in these carburizing grades make it more difficult to recognize overheating on fracture testing, due to the high ferrite percentage. If they are heat treated before fracturing, the facets are more readily seen on the fracture surfaces.
Burning is seen readily on as-polished micro surfaces by rather distinct outline of oxides including MnS and lower melting constituents. I have seen many instances of overheating, intentional or otherwise. I did some testing of intentionally overheated steels years ago to show plant personnel examples of what can happen. Burning is something that 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.) Unfortunately, I no longer have any records of that work done in the 1950s, 60s and 70s when I was working in forge shops.
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.