The wear and ultimate failure of forging dies is inherent to the metal-forging process. Accurapuls Canada Inc. offers a micro cold-forging (or machine hammer-peening) process that cold-works the entire surface of a die to harden, finish and/or maintain the die’s surface integrity.

Forging dies are prone to a number of failure mechanisms, and replacement or repair can be costly and time-consuming. For this reason, it is important to take every step possible to maximize die service life.

Forging dies experience two types of fatigue. The first is in cold forging, where repeated contact with a workpiece leads to mechanical fatigue. The second is in hot forging, where the forging die experiences thermal fatigue in addition to mechanical fatigue. These thermal stresses lead to cracks in the die surface. This phenomenon is commonly referred to as “heat checking.”

Another common cause of die failure is wear. This is especially true in open-die forging applications. In open-die forging, a heated billet is placed in the bottom (lower) half of a matching die set. As the top (upper) half of the die set is forced down onto the heated billet, the workpiece is deformed into the shape of the die as material and flash translate past the die surface. Two major factors that affect the wear performance of forging-die surfaces are surface hardness and roughness.

In this article, we will discuss these failures in more detail and how micro cold forging can deliver an effective and economical solution to these challenges.

Micro Cold Forging

Micro cold forging (MCF), also known as machine hammer peening, is an automated surface-treatment process employing an electromagnetically controlled peening hammer fixed to a CNC machine tool, robot or special-purpose machine. The CNC mill that performs final milling cuts is most often used, thus another machine or setup is not needed. Figure 2 shows the MCF hammer attached to a robot.

The machine tool manipulates the hammer toward the workpiece die surface in the same way it would a cutter, and the spherically tipped striker contacts and feeds across the surface, imparting rapid reciprocating impact force. Both the hit frequency (up to 500 hits per second) and impact force (up to 400 pounds per hit) are variable to optimize results. Figure 3 shows the hammer operating on a forging die.

One of our customers uses this process to finish and maintain their forging dies. In the past, when cracks or wear would occur on the surface of their dies, they would have to reface the die by removing up to 30 mm of material. However, they have now implemented a new approach.

Newly manufactured dies are peened before they are used in production. At periodic intervals when a die experiences wear in high-stress areas, it is polished by hand to remove the damaged material. After this, the damaged areas are re-peened and nitrided. Using this method, our client has effectively eliminated crack formation in their forging dies using MCF. They have reported their dies lasting 95% longer between repair intervals.

In another instance, a customer has increased the service life of their dies by as much as 10 times.

Hardness Improvements

Wear is a complex topic made especially hard to quantify and analyze by the fact that there are several mechanisms and types of wear. The most prevalent forging wear mechanism is abrasion. Abrasive wear is governed by the hardness ratio between the abrasive material (in this case, the workpiece) and the worn material (in this case, the die). If the die is harder than the workpiece, the wear rate of the die is greatly reduced.^[2]

Our process has been shown to increase surface hardness to depths of up to 1.4 mm.^[3] Hardness increases are inversely correlated with the hardness of the original workpiece. In other words, a forging die that is made of a very hard tool steel won’t experience the same hardness increase as a low-carbon steel die.

Even in hard steels our process can deliver noticeable hardness increases. For example, in hardened AISI 1045 steel, we have observed Vickers hardness increases as much as 9.9%, from 527 HV to 579 HV (51-54.1 HRC).^[4] AISI H13 tool steel, a typical steel used in hot-forging dies, is tempered from its original Vickers hardness of 177 HV to a hardness ranging from 370 HV to 560 HV (37.7-53 HRC), depending on the application.

Since MCF is a cold-working process, hardness increases are dependent on yield strength. H13 and 1045 steels that are quenched to high hardness have a similar yield strength and will respond similarly to our surface-finishing process. Based on this information, MCF is a viable option to increase the hardness and wear performance of your hot-forging dies.

Improving Fatigue Performance

Fatigue failures ultimately come down to material endurance limit. When the die material is repeatedly stressed beyond its endurance limit during forging operations, the material will eventually fail. This failure is often in the form of surface cracking, causing the die to produce out-of-tolerance parts with poor surface finish until it has been repaired.

In hot forging, not only do dies experience the mechanical fatigue associated with repeatedly hammering a workpiece, they also undergo thermal fatigue. When the die surface is repeatedly heated and cooled, the expansion and contraction cycles lead to internal stresses in the material. Crack formation as a product of thermal fatigue is commonly referred to as “heat checking.”

Engineers often estimate the endurance limit of steel alloys as being half of the material’s ultimate tensile strength. Strain hardening is a known way to increase the ultimate tensile strength of a material. The MCF process effectively strain hardens the entire surface of the workpiece. For this reason, it is an excellent way to increase the life of forging dies.

Optimal Surface Roughness

The surface roughness of forging dies is a tricky subject. If the surface of the die is too rough, the die will experience greater abrasive wear, leading to quicker failure and poor surface finish of the forged part. An excessively smooth die surface decreases lubricant retention between the die and workpiece. The optimal range for forging-die arithmetic mean roughness (Ra) is between 1.5 µm and 0.51 µm.^[5]

MCF can easily produce a surface roughness (Ra) between 0.3 µm and 1.54 µm by varying the parameters of the process, such as hammering force and striker-ball diameter.^[2] Figure 4 shows the transition in surface smoothness of a milled part after peening.

In another instance, one of our clients has even reported achieving a grade-A diamond finish (Ra ≤ 0.076 µm) using our process.

These improvements, combined with the consistency and repeatability of an automated process, makes MCF a very viable option for forging-die polishing.

Benefits of Automation

Surface-roughness specifications in forging dies are typically met by hand polishing the die. This method has a number of shortcomings. The first is the margin for human error. Even the most skilled die-polishing professionals can make mistakes and polish the die too much, leading to the final geometry of the die being out of specification. The hand-polishing process is thus inconsistent.

Hand polishing a forging die requires a very specific skill set, and capable personnel can be hard to come by. Airborne ceramic particulates from the polishing equipment also pose a health risk to personnel. Hand polishing is also a very time-consuming process.

One way to address the need for a highly refined surface finish is to machine the die surface as smooth as possible before polishing takes place. However, this method also has inherent limitations. First, such cutting operations require a great deal of machining time using the very best and expensive machine tools with high speeds, feeds, accuracy, rigidity and program processing capacity. Second, it requires specialty consumable cutting tools that actually “cut” the steel as opposed to simply sliding, scraping or dragging over the surface, which can lead to surface defects. Third, cutting operations simply cannot achieve perfect smoothness. Therefore, despite the best final milling practices, secondary polishing remains a necessary process for obtaining required surface roughness.

Automation of this polishing process through MCF is an excellent alternative to hand polishing or advanced surface milling to achieve the necessary surface smoothness specifications. The potential benefits and savings of introducing this process into forging-die finishing and repair have only begun to be discovered.

Lead author Brian Guild is VP, Communications Director for Accurapuls Canada, Inc. He may be reached at 780-445-0920 or brian@accurapuls-canada.com. Co-author Karan Billing is a technical analyst at Accurapuls Canada, Inc. He may be reached at 780-802-0125 or karanbillingeit@gmail.com. For additional information visit https://accurapuls-canada.com/.

References

1. M. Groover, Fundamentals of Modern Manufacturing, 5th ed. Hoboken, NJ: John Wiley & Sons, Inc., 2013, p. 446

2. Pintaude, Giuseppe & Sinatora, Amilton & Albertin, E. (2005). A review on abrasive wear mechanisms of metallic materials

3. V. Schulze, F. Bleicher, P. Groche, Y. Guo and Y. Pyun, “Surface modification by machine hammer peening and burnishing,” CIRP Annals, vol. 65, no. 2, pp. 809-832, 2016

4. B. Adjassoho, E. Kozeschnik, C. Lechner, F. Bleicher, S. Goessinger and C. Bauer, “Induction of Residual Stresses and Increase of Surface Hardness by Machine Hammer Peening Technology,” Annals of DAAAM for 2012 & Proceedings of the 23rd International DAAAM Symposium, vol. 23, no. 1, pp. 697-702, 2012

5. D. Nowak, “Investigation of Surface Roughness and Lay on Metal Flow in Hot Forging,” Master’s, Marquette University, 2014

6. Lechner, C., Bleicher, F., Haberson, C., Bauer, C. and Goessinger, S., “The use of Machine Hammer Peening technology for smoothening and structuring of surfaces,” Annals of DAAAM for 2012 & Proceedings of the 23rd International DAAAM Symposium, 23(1), pp. 331-336, (2018)