A lubricant plays a very important role in forging. It can reduce the forging load and make for smooth knockout. The tribo-simulator tests that have often been used as standard evaluation methods for metal forming lubricants includes the ring compression test and spike forging test. However, the forging of components such as flange bolt and constant velocity joint is more severe than a spike forging test or ring compression test. Thus, the development of an evaluation method for lubricants for use in more severe conditions is much awaited. We have developed a new evaluation tribometer using backward extrusion with a taper punch. The new device is called a “Taper Cup Tribometer.” The taper cup test can evaluate lubricity under large deformation conditions. In this test, the surface expansion ratio can be controlled by changing the punch stroke. The surface expansion ratio of the new taper cup tribometer is around 15 to 50, as calculated by fine element analysis. The results of lubricant evaluation in cold forging of carbon steel are as follows. Zinc phosphate has a lower forging load than other lubricants, making it clear that it is a superior lubricant. Oil, on the other hand, has a high forging load and poor lubrication characteristics. The taper cup Tribometer can evaluate the friction characteristics of lubricants. Furthermore, MoS2 shows a higher forging load than zinc phosphate in the region of higher surface area expansion ratio around 12. The forging load of oil is higher than zinc phosphate in the region of higher surface area expansion ratio around 4. In this way, the new taper cup tribometer can recognize the forging limit for the lubricant under the forging load.

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