Hot forging involves controlled plastic deformation of heated metals and alloys into desired useful shapes. Modern civilization would survive without hot forging to produce innumerable critical component parts. At the same time, without suitable forging die lubricant, economical and effective closed-die forging is not possible.
In the particular case of closed-die forging, die costs amount to 10-15% of the total production cost. Any improvement in the forging die life would automatically result in improved productivity and profitability for the forge shop.
The following factors contribute to ensuring maximum life of a forging die:
• Correct die material.
• Correct die design, especially proper draft, corner radius and fillet.
• Appropriate heat treatment of forging dies; Using protective anti-scale coating to prevent scaling on critical surface areas of die; Nitriding of forging die.
• Reduction of friction during hot forging by the use of polished dies and suitable die lubricant.
• Correct method of application of die lubricant at appropriate intervals.
Requirements for Hot Forging Die Lubricants
The basic function of any lubricant is to reduce friction between two surfaces and to reduce wear. In the case of forging, a good hot forging die lubricant must have the following properties:
1. Reduce friction.Minimizing friction etween the workpiece and the forging die. Table 1 compares the coefficient of friction for various types of lubricants used in hot forging.
2. Reduce forging load.Figure 1 graphs the rate of reduction in the forging load that results from the use of graphite-in-oil-based die lubricant.
3. 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, such that it correctly fills the die cavity.
4. Function as a parting compound.Forging dies with a low degree of draft radius usually face problems with jobs sticking in the die. Special additives in the forging lubricant must ensure a mild gas generation to aid the ejection of the forging from the die after the part has been forged. However, the lubricant must not cause an explosive effect, nor generate smoke.
5. Serve as a barrier to heat transfer.By sealing temperature in the workpiece, the lubricant helps to maintain the correct die temperature. The die should not gain excess heat due to continuous contact with hot billets, as this will lead to faster die wear and cracking. At the same time, the die should not be subjected to rapid cooling due to excess lubrication, as this will lead to die chilling effect. Forging lubricant must help to maintain the correct die temperature by acting as a barrier to heat transfer from hot billet to forging die.
6. Prevent build-up in the die cavity.Preventing build up of lubricant residue in the die cavity will prevent subsequent problems, such as forging underfill.
7. Lubricant must be readily removable.Effectively removing lubricant residue from the workpiece is an important factor for product quality and process efficiency.
8. Avoid ‘explosion’ during forging due to rapid phase change.Sawdust, when used as a lubricant in hot forging, is known to prompt loud explosions, with sparks, thus releasing the forging from the die. Modern lubricants can achieve the same effect without the explosion and hazard.
9. The forging die lubricant must be eco-friendly.Preferably, biodegradable lubricants should be used. Use of polluting oils and additions like graphite in lubricants results in smoke and promotes unsanitary conditions in the forge shop and surroundings — which can cause further damage to the environment in general.
10. The hot forging lubricant must be economical.The choice of a product must not only be effective but affordable, in order to justify its use.