November 2022 Volume 4

MAINTENANCE

Forging Lubrication Troubleshooting and FAQ By Frank Sobnosky

In this edition of Forging Lubrication Troubleshooting and FAQ, we will continue investigating practical solutions to everyday lubricant related to metalforming challenges. Last time, we discussed part non-fill and excessive, abrasive die wear. Today, we will cover a major root cause of metalforming problems, which is that properly controlling tooling temperature is essential to lubrication. While engineering controls exist to control temperature, the lubricant also dictates a great deal of that action. So, without further ado, let us problem solve! There are two main parts of every metalforming die lubricant. These are the lubricating medium, either graphite or synthetic (organic salt), and the carrier. The carrier can be oil, water, or a combination of both. While it may seem that the carrier’s role is less important than the lubricating medium, the opposite is true. Each of these has their specific role to play, and each is crucial to the success of the forging process. While the lubricating medium parts the die and workpiece as well as controls the coefficient of friction between the two surfaces, the carrier is crucial to die temperature control. If this temperature is not properly controlled, then die life will suffer, even if the lubricating medium is correctly chosen and in the correct quantity.This applies in cases both when the temperature is too high or too low. Now, we will analyze each of these scenarios and specific examples. When die temperatures are too high, lubricant related issues can occur. The most common of these issues that we see are die hot spots. Thanks to sophisticated systems which measure temperature profiles of specific regions of the die, forgers can analyze how the temperature profile affects their product quality. Sometimes, they see spots of the die that are significantly hotter than the desired die temperature, which can cause lubricant to dry incorrectly. This is often seen in a ‘bubbling’ effect on the die surface. Applying lubricant to a die which is above the boiling point of the carrier causes the fluid to not properly form a film to coat the die in that spot. Uncovered die regions during forging lead directly to die wear and a lack of metal flow in that area. All of this goes back to the carrier, and its ability to control the temperature on the die. Not only does this occur when the ratio of water to lubricant is incorrect, but also when the lubricant spray is not uniform and not aimed at the correct areas of the die cavity, these hot spots can occur. While this is seen in all metalforming process, it is especially true when complex like crankshafts or axle tubes are forged. Other, similar issues, are heat checking and cracking. When a die is not cooled enough or cooled too quickly, it can cause a physical change in material properties and lead to stress cracking in the die. When a

proper balance of lubrication and temperature control is not struck, costly issues occur.

Figure 1: Die temperatures have a significant effect on the effectiveness of a metalforming lubricant. While the lubricating media has a significant role to play, without the correct carrier it is unable to function. Similarly, when there is too much water and not enough lubricant applied to the entire die cavity, there are opposite, but just as serious issues that can occur. When there is too much carrier and not enough lubricating medium applied to the die, the dies are cooled too much, leading to the opposite effect of 'bubbling'. Dry film boundary lubricants are designed to dry at specific temperatures, and when these are not met, films do not form as designed leading to a lack of lubrication. Additionally, lubricating media serve as an insulator, keeping valuable heat in the die. This leads to a decrease in

FIA MAGAZINE | NOVEMBER 2022 32