Protective coatings continue to play a major role in increasing productivity and reducing costs in hot forging and heat treatment. This paper presents details and successful case studies of three such protective coatings.
Japanese Cold-Welding Technology
Die, mold and tool wear are major reasons for production downtime and increased costs in most industries. A carbide coating to protect only the wear-prone areas of dies using Japanese cold-welding technology is a practical and economical technique that has proven to increase die, mold and tool life. This technique, though similar to welding, does not pose difficulties of smoke emission, pre- and post-weld heat treatment or skilled-labor requirements. It can also be carried out on the die without unloading it from the forging press.
When a forging die is in use, it is mandatory to keep it well-lubricated and maintain the die temperature, as required by the application. Protective coatings/lubricants are used to achieve these objectives. Graphite-in-water formulations have been popularly used as die lubricants until recently. Though effective as a lubricant, graphite is highly polluting and soils the surroundings. Effective white lubricants using environmentally friendly materials have been developed to eliminate graphite and its associated pollution. A substantial increase in die life and reduced pollution are possible by using white lubricants.
Oxidation and resultant scaling at high temperatures is caused during the heating of billets and ingots for forging and during heat treatment of formed components. Scaling leads to enormous losses by way of product rejections, reduced yield and the need for increased non-value-adding operations like shot blasting, grinding, pickling, etc. These parameters are becoming increasingly sensitive in open- and closed-die forging, especially of expensive steel grades, nickel-bearing alloys and aerospace forgings. Anti-scale protective coatings can be used to prevent or substantially reduce high-temperature oxidation and scaling.
These techniques can be easily adopted by all metal forges – large and small.
Using Japanese Cold-Welding Technology to Increase Die Life
Die, mold and tool wear are major reasons for production downtime and increased costs. Apart from using the most appropriate die steel, a few effective treatments can be administered to dies to increase their service life.
Most forging dies wear out in localized areas. The complete die impression does not bear wear and tear evenly and uniformly. Only sensitive portions of the die – edges, profiles that take the majority of the forging load, etc. – wear much faster than the rest of the die. Figure 1 highlights specific regions of excessive wear on a typical die.
A carbide coating applied using the Japanese cold-welding technique involves the application of a coating of tungsten carbide on selective wear-prone areas of dies. Cold welding is carried out as a preventive-maintenance technique on new dies. It is a surface-hardening technique similar to nitriding but is administered manually using the cold-welding equipment. The hardness of a tungsten-carbide layer deposited by cold welding on dies can surpass nitriding to reach hardness of more than 70 HRC.
There are a number of benefits of the cold-welding technique: skilled welding labor is not required; no fumes are generated so no air removal is necessary; dies need not be removed from the forging equipment; and pre- and post-welding thermal treatments are not necessary. Further implications of these benefits are that the nitriding of dies is not required because the tungsten coating is harder than the nitriding process. The process can be used in specific high-wear areas of the die. And, of course, the costs of die maintenance and downtime are decreased, while the service life of the die is increased.
The carbide coating is seen as a silvery, coarse coating on wear-prone die surfaces. Demonstrations of this technique have shown encouraging results (Table 1). There is no risk in terms of die/tool breakage or reduced life. The observed percentage of die/tool life increase has ranged from 35% in initial trials to as high as 120% in recent trials. Various parameters that contribute to success of this technique have been well-documented, leading to refinement of the technique.
Increasing Die Life Using White Lubricants
Die lubricants play an important role in achieving optimum die life. The use of cheap oils for die lubrication leads to very low die life and pollutes the forge shop. A water-miscible graphite-based lubricant is better than oils. However, graphite can be very messy to use, and, being a good conductor of electricity, graphite can damage the electrical systems of forging presses.
Many times, switching over from oil to water-based graphite or to synthetic lubricants is daunting. This is due to improper spray techniques, leading to low die life or die breakage (Figure 3).
A new generation of synthetic die lubricants is now available that is both clean and effective. Synthetic die lubricants have often proven better than graphite lubricants. However, synthetic lubricants must be used with a correct method of spraying, which requires customized spraying systems and spray guns, depending on the specific forging profile.
We ran some comparative tests on the production of a forging sleeve and a two-wheeler crankshaft, shown in Figures 4a and 4b. The results of these tests are shown in Tables 2 and 3.
Having performed studies on the forging sleeve and the two-wheeler crankshaft, we observed that a substantial increase in die life is possible by the use of environmentally friendly die lubricants when correct spraying equipment and spraying techniques are implemented.
Increasing Yield Using Protective Coatings
In the forging process, oxidation and resultant scaling lead to pit marks and rejections. They can lead to the use of operations like shot blasting, grinding, etc., which are costly and time-consuming.
Oxidation and scaling are a function of time, temperature and the thermodynamic affinity between oxygen and metal. Recent developments in highly oxidation-prone steel grades (like nickel-bearing and high-speed steels) and increasingly stringent customer demands do not allow for scale pits, uncontrolled decarburization and a poor surface finish.
Prevention against excessive oxidation and scaling can be done by using a protective coating, which is applied to billets or components before charging them into the preheat furnace. The anti-scale coating acts as a barrier between oxygen and metal. Care is taken to apply a uniform, impervious layer of coating by brushing, spraying or dipping.
The use of protective coatings on billets during heating for forging and again on forgings during heat treatment has proven to substantially reduce scaling, control decarburization, improve the surface finish and increase yield. Figure 5 shows substantially reduced scaling on billets. As a result, forged parts do not have scale pits and have an acceptable surface finish.
Forge shops are assured of increased productivity and substantially reduced costs in hot-forging and heat-treatment processes by using protective coatings like the cold welding of carbide on dies, synthetic die lubricants, and anti-scale protective coatings of billets and forged parts.
A number of forge shops in India and other countries have adopted these techniques to increase die and tool life, eliminate pollution and substantially reduce forging costs.