Although I do not work at a forging house, I have visited and asked the same question. I believe the graphit is used as a releasing agent. when the heated metal is stamped, it will stick to the dye. the graphit helps release the forged part from the dye.
i had an issue with aluminum forged pieces that turned ugly after a hardcoat anodize process. The issue was identified as graphit contaminent in the surface of the forging. Graphit is not reactive to caustic or acidic solutions. At first we thought it was the forger who was causing the problem, but it was later identified as the extruding housing. the extruding housing also uses graphit. They use graphit on their cooling beds after material is extruded. This created a substantial problems because the graphit was forged into the part to a certain depth. It was basically unremovable.
As stated above graphite is a release agent for all types of forging, extrusion, stamping, or any metal work where a die form is used.
Besides being a release agent it's primary function is a lubricant the allows the metals to better conform to a die. It is essential in dies were the part has to have a shallow draft, closer to straight sides. The more intricate the die the more the lubricant aspect comes into play. It makes a lot of forgings and castings feasible that otherwise would couldn’t be made’.
The problems with graphite and aluminum forgings mention by skylerf can be minimized or in most case taken totally out of the picture with the proper choice of graphite lubricant/release agent with a proper application.
There is some science involved, but a also a hell of alot of black art and experience involved in the selection of graphite lubricant.
The main reason is the lubrication properties; to reduce friction. It has a low coefficient of friction and is normally platelet shaped (man-made).
Second for hot forging it has a higher operating temperature than the normal oil used for forging lubrication. Although there are oils that have a higher flash temp out there these days. In a mix of graphite & oil, the oil is the carrier.
Works well for a certain amount of sliding metal contact. With too much contact coupled with high temperature it burns into carbon and becomes more abrasive.
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