Hot forging involves the controlled deformation of heated metals and alloys into desired, useful shapes. In certain situations, the use of graphite-free lubricants can help reduce costs.

In closed-die forging the die typically amounts to 10-15% of the total forging cost. It stands to reason, then, that any improvement in die life would improve productivity and profitability in the forge shop. 

  We know that the following factors contribute to ensuring maximum die life: 

• Correct die material 

• Correct die design, especially proper draft, corner radius and fillet 

• Appropriate heat treatment of forging die, use of protective anti-scale coating to prevent scaling on critical surface areas of die or nitriding of forging die 

• Reduction of friction during hot forging by the use of polished dies and suitable die lubricant

• Correct application of die lubricant at appropriate intervals

Lubricant Properties

The basic function of a good hot-forging lubricant is to reduce friction and wear between two surfaces. A proper lubricant must have the following properties.

• Minimize friction between the workpiece and the forging die: Various lubricants provide differing levels of lubrication (and wear) of surfaces that come in contact with each other. The worst-case scenario is that of using no lubricant. Die wear in such a case is three to five times worse than if a graphite machine oil lubricant is used. 

• Reduce forging load: Figure 1 shows the reduced forging load by using a graphite-in-oil die lubricant. 

• Enable uniform metal flow to fill the die cavity: Especially in the case of critical forgings with low degrees of draft radius, die lubricants play an important role in enabling uniform metal flow so that the metal properly fills the die cavity. 

• Function as a parting compound: Forging dies with a low draft radius usually face problems of the job sticking in the die. Special additives in the forging lubricant must ensure a mild gas generation to aid the ejection of the newly forged part from the die. However, the lubricant must neither generate smoke nor be an explosive risk.  

• Serve as a barrier to heat transfer: This helps maintain the correct die temperature. If a die gets overheated from contact with hot billets, faster die wear and cracking can occur. Concurrently, the die should not be subjected to rapid cooling from excess lubrication, as this will lead to a die-chilling effect. The lubricant must help maintain the correct die temperature by acting as a heat-transfer barrier from hot billet to die. 

• Prevent lubricant buildup in the die cavity: This will lead to excess lubricant deposits in the die, resulting in dimensional inaccuracies and causing problems such as forging underfill. 

• Is readily removable from the workpiece.  

• Avoids “explosion” during forging due to rapid phase change: Sawdust, when used as a lubricant in hot forging, is known to cause a loud explosion with sparks and thus release the forging from the die. Modern lubricants can achieve the same effect without the explosion and hazard. 

• Must be environmentally friendly: Preferably, biodegradable lubricants are used. Use of polluting oils and additions like graphite in lubricants cause much damage to the environment due to smoke. They also take their toll on the forge shop floor and surroundings. 

• Must be of reasonable cost: The hot forging lubricant must be economical and justify its use. 

Application Methods for Die Lubricants

Die lubricants are typically sprayed automatically, sprayed manually or swabbed onto the hot dies. The accompanying image (above right) shows the lubricant being sprayed manually with a double-action spray gun. Many installations use automatic spraying systems that are timed with the stroke of the forging press. Deeper-cavity dies may require the use of a supplemental spray to ensure coverage throughout the entire die surface and cavity. 

  The correct amount of lubricant provides an optimum film in the die cavity to aid metal flow and to curtail heat transfer from the workpiece to the die. The excessive application of lubricant is wasteful, soils the work area and the workpiece, and pollutes the atmosphere. Excess lubricant may also deposit residue in the dies and cause problems like forging “underfill” and “lap.” 

  In typical applications, the cost of lubricant may be less than 2% of the total forging cost. The use of improper lubricants or their improper application, however, can lead to reduced die life, product rejections, rework, reduced productivity and customer dissatisfaction due to delayed deliveries, thereby upsetting forging production and increasing costs and overhead.

Types of Hot-Forging Die Lubricants

Owing to the development of a range of water-based lubricants, polluting oils and sawdust have been largely replaced by graphite lubricants. Purity, particle size and special lubricant additives are important factors in lubricants containing graphite. Graphite-based lubricants are popular throughout the world due to their low cost. Even though smoke pollution is absent during their use, however, certain problems are being faced by modern forging-press operators using graphitic lubricants. First, graphite particles can fly off and damage the electrical system. Second, graphite particles accumulate on the shop floor and pose a slipping/fall risk to personnel. Finally, hygiene factors due to the release of carbon monoxide and sulfur in the air are hazards that may occur during the use of graphite-based die lubricants. 

  As a result of these factors, environmentally benign graphite-free, water-soluble lubricants are gaining in importance. They do not require as much stirring during use as graphite lubes, and they may require no stirring at all. Also, the thickness of film formation on the forging die can be controlled by varying the dilution ratio of the forging lubricant with water. In the case of graphite-free forging lubricants, hazards like suspended particulate matter in air and the release of carbon monoxide and sulfur are substantially reduced.

  Biodegradable phosphate esters, soaps and organic substances are used in the development of eco-friendly lubricants. These additives also possess superior lubrication characteristics compared to graphite. Hence, in the case of small- to medium-sized forgings weighing up to 12 kg (26 pounds), graphite-free, water-soluble die lubricants have proven to perform either at par or better than graphitic lubricants. 

  The use of graphite-free die lubrication has proven less costly than graphitic lubricants. This is due to benefits such as increased die life and reduced need for constant die grinding due to reduced die wear. Also, maintenance downtime of die lubrication systems by the clogging of spray nozzles and the deposition of graphite inside tanks, spray pumps and automated lubricant-delivery pipes is eliminated by the use of graphite-free die lubricants.

How to Select the Right Type of Hot-Forging Die Lubricant  

Selection of the right type of hot-forging die lubricant is based on parameters like depth of die cavity, size and complexity of the forging, method of dispensing the lubricant onto the die, time required to complete one forging part, commitment to cost reduction, and pollution control. Though the best method of determining an appropriate lubricant and the dilution ratio of die lubricant to water is by field trials, general guidelines for selecting die lubricants for a range of forgings are provided in Table I. 

Summary

The use of the right type of die lubricant and the correct method of dispensing it are decisive factors in the success of closed-die forging. Specially developed non-graphitic, water-soluble hot-forging die lubricants hold significant potential toward hot-forging cost reduction and progress toward an environment-friendly forging operation. Such lubricants are proven to be effective in forgings weighing up to 12 kg (26 pounds).