In this second of two parts, we examine failures that occur in the tooling during forging. In Part 1, we presented the fundamental aspects of stress within a die that has been loaded during the forging process. In this article, we will describe a number of case studies, each of which provides an example of a die failure, the analysis that was done based on stresses in the die and determination of the root cause of the failure. It is our intent that, through these examples, the reader will be able to understand some of the reasons for die failures and methods that can be used to determine their root cause. This understanding, in turn, can be applied to help minimize future die failures.
There are a number of sound economic and engineering reasons why prevention of tool failures is important. Tooling costs, of course, are significant, but the total cost of die failure is much higher than the simple replacement cost. A tool’s failure is riddled with hidden costs above and beyond those of tool replacement. For example, there are costs associated with scrapped or rejected parts, rush charges on die replacements, lost energy inputs, late deliveries, lost customer goodwill, etc. Clearly, replacement of broken or worn dies is more expensive than just their replacement.
Operating a lean manufacturing operation can be very problematic if tool life is short or erratic. Delivery problems, for example, become routine when tooling problems occur unexpectedly. Additionally, the risk of safety problems is high in overstressed tooling, especially if the overloads lead to catastrophic die failures. When the ultimate strength of a die material is exceeded significantly, a catastrophic fracture may occur, resulting in injury.
Because of these economic and operating reasons, it is important to understand the root cause of any die failure and to institute corrective action to prevent it from recurring. Through the case studies examined here, we hope that you will be able to learn from the failures in other shops and benefit from their problems.
We looked at case studies that occurred during a period spanning two decades to illustrate theory and confirm that it “connects the dots” to real shop-floor tooling failures. These examples cover a range of die failures and solutions.
DIE CASE YIELDINGIn 1989, the late Alfred Lau presented a case at the North American Forging Technology conference in Orlando, Fla. A hot-forged turbine disk (Figure 1) was dimensionally out of tolerance after forging. The features on the inside were undersized, while the outside diameter was in tolerance.
An investigation showed that the punch (top die) was in tolerance. The die, which formed the outside diameter, was oversized in the forging contact region. A subsequent stress analysis revealed that the effective stress on the die insert was in excess of the yield strength at temperature, resulting in plastic deformation. The plastic deformation resulted in an oversized forging on the outside diameter, which was reduced (swaged) during ejection.
During the problem-solving period, quite a few alternative designs were analyzed. The production solution was simply to use a thicker die insert. After solving this problem, other like jobs were found to have similar issues and subsequently redesigned. The analysis of the redesigned die is shown in Figure 3. While there is still a high stress in the contact region, the through-wall strength prevents the die from yielding. Die life in production was extended more than tenfold until die wear became the failure mode.
DIE-INSERT FRACTUREFast forward to 1995, when we deployed die stress analysis on a die failure for a cold-formed automotive component. The die-insert fracture (Figure 4) during the first operation was the most significant failure. The monolithic die was shrink-fitted into a steel case. It fractured prematurely into two parts (sometimes called “wafering”). The low-cycle-fatigue fracture initiated on the inside, then propagated outward toward the outside diameter.
A stress analysis of the die insert showed a significant tensile stress in the failure region, perpendicular to the fracture. The die material was tungsten carbide. This material has excellent wear and stiffness characteristics, but it is prone to low-cycle-fatigue (LCF) failures when subjected to cyclic tensile stress. The maximum principal stress is shown in Figure 5. In this plot, the red represents tensile stress and the green is compression.
In order to solve this problem, the die insert was split longitudinally. The resulting analysis indicated a lower principal stress (Figure 7). In production, the LCF failure was no longer a factor, with die-insert life improvements of significantly more than tenfold!
M16 RECEIVER DIE FAILUREIn 2003, a PRO FAST project was conducted to study and resolve an issue with the dies used to hot forge an aluminum receiver. This premature die fracture limited production runs, delayed delivery and added costs to the process. Figure 8 shows the M16 rifle and the receiver forging assembly.
The initial symptom of the problem revealed itself as a rough area of raised material in the corner of the forging. Figure 9 shows the forging with the raised region of material. An investigation uncovered a crack on the die surface. Figure 10 shows the correlation between the raised material on the surface of the forging and the crack in the die.
An alternative design was developed using a pocket type of die holder with an insert. An interference fit was designed to keep the insert in a predominately compressive stress state during forming. Figure 13 shows a plot of the maximum principal-stress component in the redesigned die, which is predominantly compressive. The die holder is stressed below the yield strength with a reasonable safety factor.
TROUBLESHOOTING SUGGESTIONSBased on these examples, there are several suggestions that can be made to minimize the occurrence of forging-die failures. To prevent a failure, consider:
- Reducing the load on the die
- Increasing the strength of the die material
- Increasing the load-carrying ability of the die by redesign
- Increasing or enhancing the die support
SUMMARYIn this second portion of our two-part paper, we have presented several case studies of die failures. These provide specific examples of forging die failures. The proper analysis of these failures allowed the determination of the root cause for the failure. Once the root cause is known, a proper approach can be devised to delay or to avoid the failure in the future.
The support for this work from the PRO-FAST Program is appreciated. The PRO-FAST Program is enabled by the dedicated team of professionals representing both the Department of Defense and industry. These teammates are determined to ensure that the nation’s forging industry is positioned to meet the challenges of the 21st century. Key team members include: R&D Enterprise Team (DLA J339), Logistics Research and Development Branch (DLS-DSCP) and the Forging Industry Association (FIA). This work was originally prepared for the Forging Fundamentals 101 course by Scientific Forming Technologies Corporation and the Forge-It Team.
Co-author Dr. Chet Van Tyne is FIERF Professor, Department of Metallurgical and Materials Engineering, Colorado School of Mines, Golden, Colo. He may be reached at (303) 273-3793 or firstname.lastname@example.org. Co-author John Walters is vice president of Scientific Forming Technologies Corporation, Columbus, Ohio. He may be reached at (614) 451-8330 or email@example.com.
Note:Alfred Lau, whose work in the Die Case Yielding section is cited here, unfortunately passed away in February 2008. He held a number of technical positions at Wyman Gordon in Houston, Texas, since the 1980s and was a pioneer in applying die stress analysis to forging-die failures.