Forging Die Lubricants: Graphite or Synthetics?
The selection of the proper die lubricant is a major component in attaining a successful forging process. The primary function of a forging lubricant is for increased productivity and profitability of the forging company. Prevention of “stuck” forgings, maximizing die life, decreasing the number of rejects and reducing the load on forging equipment to prevent premature equipment failure can be the benefits realized by using the correct forging lubricant.
Prior to the industrial revolution, the demands placed on a forging lubricant were minimal. Simple forgings could be made with organic lube materials such as animal fats, coal, soapstone and crude oils. During the 19th century, we saw the beginning of impression-die forging done largely on hammers. Crudely refined mineral oils, sawdust, saltwater, solutions of fatty soaps and a mixture of oil with graphite flakes became the accepted lubricants as the 20th century approached.
With the introduction of forging presses, higher production rates and increasing forming pressures followed. Significant demands on tooling and the introduction of more complex part configurations started to elevate the cost of forging. During this time, more highly refined oils and graphite compounds were developed to meet the lubrication requirements. As production rates increased, so did the tooling temperatures. Oil and graphite mixtures used at the time were efficient. They were not capable of providing die cooling, however, and were detrimental to die life due to their explosive nature. In the early 1970s, water-based graphite forging lubricants became available, and more recently, a number of synthetic or non-graphite lubricants have been introduced.
Today, there are four main types of die lubricants used for hot forging: oil-based graphite, water-based graphite emulsions, water-based graphite, and water-based synthetics or non-graphite.
Water-based graphite forging lubricants make up the majority of the lubricants used in the industry, commanding approximately 60% of the market. Oil-based graphite lubricants command approximately 25% and synthetic or non-graphite lubricants are used by approximately 15% of the industry.
The use of oil-based graphite lubricants continues to decline for general forging processes. However, it is still required for press forging of complex large aluminum parts and specialized hammer forgings. For this reason, we will concentrate on the use of a water-based graphite lubricant versus a water-based synthetic or non-graphite lubricant.
Water-based graphite or water-based non-graphite?
To answer this question we must first examine a number of required lubricant characteristics.
• Lubricity – Graphite promotes metal flow and is more lubricious. The coefficient of friction of water-based graphite products is normally 0.4 or less (as measured by the ring compression test – Figures 2-4). Why is this? Graphite has a hexagonal lattice structure. Imagine a thin slice of a honeycomb placed at a certain distance over another slice, which in turn is placed over the next and so on, with each vertex of each hexagon representing a carbon atom. The layers of the lattice are bound to the adjacent layers by weak van der Waals bonds, which allow the layers to slide over one another easily, like a teetering pile of paper sheets (Figure 1). Thus, the quintessential properties of graphite – softness and slipperiness – that make for an excellent solid lubricant are obtained. When the workpiece comes in contact with the graphite film, the graphite slowly degrades into carbon dioxide (CO2). The graphite moves, allowing the metal to flow across the surface of the die. With most water-based non-graphite products, the coefficient of friction is 0.6 or higher. Water-based non-graphite forging lubricants form a film on the surface of the die, which degrades or sublimes into a gaseous state when in contact with the workpiece. This provides lift. Because of the decomposition, it does not flow across the surface of the die.
• Adhesion of the lubricant film – Graphite has more adhesion and produces better part surface quality. Water-based non-graphite products tend to wash off if over-applied into the die cavity. This makes it very difficult to put extra lubricant in a troublesome area of the die. Water-based graphite lubricants contain a binder that holds the graphite particles to all surfaces of the die and reduces the amount of overspray. Most non-graphite products, especially the newer polymer-type products, usually deposit a relatively soft film on the die surface. This can hold scale and affect the surface finish.
• Cleanliness – Graphite is easier to clean. There has been some thought that build-up of lubricant (overspray) on the cooler parts of the die can be eliminated by using a non-graphite lubricant. There is no justification for this. Many non-graphite lubricants will form a tenacious build-up in cooler areas of the die where multiple lubricant layers are deposited. This build-up will not be black, but in some instances in which typical soap-type lubricants (salt and acid) are used, it can rival concrete when it comes to removal. Newer polymer-type non-graphite lubricants leave a heavy oily film, resulting in difficulties in cleaning and safety.
• Visual – Graphite is easier to see. Non-graphite forging lubricants tend to leave a translucent or transparent film. This makes it difficult for the operator to visually tell whether the die has been properly lubricated. A dark-gray water-based graphite lubricant film is much easier to observe.
• Cost – Graphite is less expensive to use. The majority of non-graphite products are used at lubricant/water dilutions of 1:5, or 20% concentration. It is not unusual for a water-based graphite lubricant to be used at a 1:20 (lubricant/water), or 5% concentration. This produces a low ready-for-use lubricant cost. Lower dilution ratios also lead to higher freight, handling and storage costs.
• Tool wear – Graphite better protects tooling. Most non-graphite forging lubricants will not form a film on cooler tooling (under 200-300°F/93-149°C). This can lead to premature die wear, sticking, bent tooling, etc. This also means that some non-graphite lubricants cannot be used in flood-type applications where a great amount of cooling is required or it is not physically possible to use a spray application device. Most non-graphite forging lubricants vaporize quickly under forging temperatures and loads. Therefore, the longer the tool/workpiece contact, the greater the superiority of graphite. We must also consider diesel effect (decomposition of synthetic components under high temperature), where decomposition produces gases that lead to erosive wear.
• Carrier (dilutant) – Water-based graphite forging lubricants are designed to be diluted with potable water and are odor-free in most cases. Non-graphite forging lubricants, especially the soap-based products can produce very pungent odors if diluted using hard water (high iron content). Of course, the better the water used for dilution, the better the performance of any water-based forging lubricant.
• Agitation/mixing – Non-graphite products are more easily maintained in their ready-for-use state. Although easily mixed, water-based graphite lubricants require agitation to ensure a homogeneous mixture throughout the ready-for-use product when diluted. Non-graphite water-based forging lubricants are true solutions and generally require no agitation after mixing.
• Application equipment – Non-graphite products are easier to use with application equipment. If spray application and dilution equipment is not maintained properly, water-based graphite forging lubricants can cause plugging problems in lines and spray nozzles. This may result in downtime to clean the systems or, if not caught in time, produce rejected parts or premature die wear due to poor die lubrication.
• Waste treatment – Graphite is easier to waste treat. Due to the higher dilutions used with water-based graphite forging lubricants, they produce less sludge than water-based non-graphite lubricants. Water-based graphite lubricants generate a harmless solid particulate sludge that is readily accepted as landfill because the pigments are insoluble in water and are not considered water leachable. By comparison, most water-based non-graphite lubricants are usually not accepted as landfill and must, at least in part, be disposed of by burning.
• Health and safety – Graphite-based and non-graphite lubricants are generally safe to use and are not considered a health or safety issue. Water-based graphite lubricants and non-graphite lubricants can contain a number of chemicals, such as sulphonate emulsifiers, polymeric thickeners, sodium nitrate, sodium carbonate, sodium silicate, silicone oils, graphite, biocides, fungicides, etc. Die lubricants containing these chemicals come in contact with high-temperature parts and tooling during the forging process. At this time, the lubricant decomposes into a gaseous wet mist containing particulate, scale, dirt, etc. Regulations for particulate air concentrations in forging operations default to generic dust or total particulate standards such as 5.0 mg/m3. Some reductions in die-lubricant mist concentrations are possible with careful management of the application and proper fume exhaust systems. In most cases, die-lubricant manufacturers ensure no dangerous chemicals are used when formulating their products.
Whether you choose a water-based graphite or water-based non-graphite forging lubricant, performance lubricants can improve any type of forging. From simple parts in shallow cavities to intricate forms with exotic metals, there is no perfect lubricant for every operation. No single chemical formulation provides the ultimate in metal movement, longest die life, cleanest environment or lowest cost.
The primary factors to be considered when selecting the proper lubricant are:
- Forging temperature
- Die temperature
- Type of material being forged
- Type of forging equipment used
- Complexity of the part shape
- Rate of production
- Heating source
- Environmental regulations
In order to achieve the optimum lubricant performance and minimum lubricant cost, selection should always be done with the assistance of your lubricant supplier.
This article is featured in FORGE's 10th Anniversary: 10-Top Forging Industry Articles. Read the other featured articles and vote for your favorite!