Users in the metal forming industry know that the Thermal Diffusion (TDX) tool treatment process greatly reduces wear and significantly increases tool life in comparison to PVD coating. Many in the shearing industry, however, are not aware that TDX can reduce wear and extend tool life in shearing and piercing applications.
Some are under the misconception that the extreme hardness of the TDX layer (approximately 4,200 Vickers) makes it brittle so that it cannot perform well in shearing applications. This is not true. The .0002–.0003-inch thick TDX layer adheres so strongly to the substrate that it assumes the toughness of the substrate.
Tests show that even in very severe shearing conditions, such as the production of safety belt buckles, TDX works. After 10,000 hits, the cutting edge of shearing punches treated with TDX showed only typical wear of the TDX layer.
In the test, the behavior of D2 punches PVD coated with TiN, D2 punches TDX treated with Vanadium Carbide and uncoated D2 punches was observed. All punches were used in piercing, blanking a hard steel, austempered bainitic steel with oxide film, Hv 360. [I. Aoki, K. Hirohito, M. Iino, Journal of Japan Society of Technology of Plasticity, vol. 30, No. 342, 1997].
Test results revealed that the TDX treated punch showed reduced wear on the treated surfaces as well as on the face of the tooling where it was not treated with TDX. As a result, the burr height on holes pierced by the TiN coated punch was larger than on holes pierced by the TDX treated punch (Figure 1).
In the test, the wear of both the TiN coated punch and the TDX treated punch was less than that of the uncoated punch. The degree of edge wear after 10,000 hits at both 15% and 5% clearance can be observed in Figure 2.
- Steel Coil Size: .6299 inch (16 mm) wide and .0394 inch (1 mm) thick
- Coatings: Arc Evaporation PVD for TiN, and TDX for VC
- Dies: No coating
- Lubricant: Machine oil
- Stroke: 30 per minute
- Clearance: 5% and 15%
- Number of Blankings: 10,000
Results at 5% Clearance
At 5% clearance, where the loading conditions on the punches is much more severe than at 15% clearance, the difference between TDX treated tooling and TiN-PVD is even more significant.
The closer, 5% clearance produces more severe loading on the punches. Increased force is needed to push the blank into the die and to withdraw the punch from the hole. The longer shearing length also generates more punch side wear. Even under these severe conditions the TDX treated punch showed much less wear than the TiN coated punch or the uncoated punch.
The forces encountered with close clearances, especially with hard work materials, can cause other types of damage as well. Here, too, the TDX treated punch showed minimum wear. Microscopic observation of the TDX treated punch alter 10,600 shots at 5% clearance did, however, reveal damage from cracking and loss of the VC layer on the flank surface of the punch about 0.15mm above the cutting edge (see Figure 3).
This type of damage is formed only when substrates are deformed plastically by an applied stress that is larger than the compression yield strength of the substrate materials. In this instance, the plastic flow and cracking in the substrate were evident.
To address the problems created by severe conditions, it is recommended that higher strength substrate materials, such as high speed steels, super high speed steels and even cemented carbides rather than A2 and D2 be used. The higher strength substrate coupled with TDX treatment can help reduce the types of damage discussed above.