February 2023 Volume 5

EQUIPMENT & TECHNOLOGY

Forging Lubrication Tribology By Frank Sobnosky Jr.

Throughout our last three articles, we have covered the topic of ‘Troubleshooting and FAQ’. While solving practical forging lubrication problems is a deep and worthy endeavor that we will revisit; in this edition we want to detail a little more about forging lubricants themselves. While it may be understood why certain operations use a specific type of lubricant, we feel that there is much more to be covered on this subject. The tribology of lubricants during forging is a vast subject and we feel that forging professionals will benefit from a deeper understanding. Without further ado, let us get into the science of forging lubricant tribology! There are three primary forging lubricant types used for modern forging.These include graphite inwater, synthetic (salt solution), and graphite in oil. There are other types of forging lubricants used for specialty processes such as forging non-ferrous alloys using graphite and oil in water emulsions, but we will stick with the three highest volume types of products for this discussion. It is widely known that graphite based forging lubricants are used for difficult operations, but it is also important to understand how these products function. The reasoning ultimately lies in the tribology, or according to Webster, the “lubrication of interacting surfaces in relative motion”. This mediation between the two interacting surfaces, the die and tooling, ultimately dictates the choice of lubricant needed. Let us get a little deeper into these interactions.

The three modes of lubrication used in forging lubricants are well illustrated in the Stribeck Curve, shown in Figure 1. Graphite in water, the most common forging lubricant, is known as a boundary lubricant, or ‘dry-film’ lubricant.This means that when the lubricant is applied to the die at high temperatures, the water (carrier) is driven off immediately, and what remains is lubricating media (graphite) and organic binders that allow the lubricating media to stick to the die. As shown on the far-left side of the Stribeck curve, this dry film forms a thin barrier between the tooling and the workpiece, the graphite-based film serves as a lubricious medium that allows metal to flow to all parts of the die and form a properly forged part. In boundary lubrication, the carrier has no functionality between the die and workpiece other than delivering the lubricating media. Boundary lubrication has a high coefficient of friction, which is preferrable for near net, deep impression forging jobs. This careful coefficient of friction control allows metal to flow to all parts of the die in the proper amounts. Some may think that a lower coefficient of friction is always preferrable, but this is not always the case. These factors make graphite in water the choice for near net forging shapes with intricate geometries. An oil-based forging lubricant, usually containing graphite, works in quite a different manner. As the lubricant is applied to the die, the oil carrier is not driven off because its boiling point is much

Figure 1: The Stribeck Curve, showing different lubrication regimes that operate during forging for different types of lubricants.

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FIA MAGAZINE | FEBRUARY 2023