By Sherry Picklesimer
— Reprinted with permission from Tooling & Production, August 1999.
Prolonging the life and performance of tooling can increase productivity and reduce tooling and tooling maintenance costs. To reduce wear and increase tooling life, many in the metalforming and shearing industries use tool treatment processes and coatings.
One of the tool treatment processes metalformers use is the Thermal Diffusion (TD) process. The TD process is a high-temperature tool treatment that diffuses a nonporous metallurgically bonded vanadium carbide (VC) layer into and onto the tooling surface, reducing wear and prolonging tool life. The VC layer is 0.0002" to 0.0003" thick and has a hardness of 3500 to 3800 on the Vickers hardness scale.
Because the VC layer is diffused into and onto the tool substrate, TD significantly reduces galling, wear, seizure, and corrosion and provides superior peel strength and adhesion strength. It extends the life and performance of dies, punches, and tooling used in the stamping and forging industries by 5 to 50 times or more.
TD is effective with air-hardening cold and hot working die steels such as A2, D2, and H13, high speed steels, and cemented carbides. Although cemented carbides are very hard, they still tend to gall and can benefit from TD. The steels should have a 0.3% or greater carbon content and tolerate temperatures of 1600°F and above. The tool treatment process is used in such industries as metalforming, appliance, die casting, wire making, and automotive stamping.
To this point, however, TD is not widely used in shearing type applications. There is a misconception that since the VC layer is so hard and brittle that it will not perform well in shearing applications. This means that many in the metalworking industry are not aware that TD can help them reduce wear and extend tool life.
Field tests show that even in very severe shearing conditions, for example, the production of safety belt buckles from HSLA steel, TD can help reduce wear and extend tool life in piercing shear applications.
In the test, the behavior of TiN-PVD coated D2 punches, VC-TD treated D2 punches, and uncoated D2 punches was observed. All punches were used in piercing, blanking a hard steel, austempered bainitic steel with oxide film, Hv 360. In the tests, after 10,000 hits, the cutting edge of shearing punches treated with TD showed only a steady wear of the VC layer. There was no large chipping and spalling. The tests further showed that TD helped reduce wear at both 15% and 5% die clearances.
- Steel coil size: 0.6299" (16mm) wide and 0.0394" (1mm) thick
- Coatings: Arc Evaporation PVD for TiN and TD for VC
- Dies: No coating
- Lubricant: Machine oil
- Stroke: 30/min
- Clearance: 5% and 15%
- Number of hits: 10,000
Test results revealed that the TD-treated punch showed reduced wear on the treated surfaces as well as on the face of the tooling where it was not treated with TD. As a result, the burr height on holes pierced by the TiN-coated punch is larger than on holes pierced by the TD treated punch.
Observation of the surface and cross section of the punches after 10,000 hits showed that the TiN coating at the cutting edge had locally spalled off and the steel substrate had been exposed. The TD treated punch showed no spalling. The TD treatment on the cutting edge of the punch wore, showing a very smooth contour. This means there was no chi pping or spalling of the VC layer even at the edge of the TD treated tooling.
In the tests, the wear of both the TiN-coated punch and the TD-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 Fig 1.
Results at 5% clearance
At 5% clearance, where the loading conditions on the punches was much more severe than at 15% clearance, the difference between TD treated tooling and TiN-PVD was 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 TD-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 TD-treated punch showed minimum wear. Microscopic observation of the TD-treated punch after 10,000 hits 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.
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, PM steels, and even cemented carbides rather than A2 and D2 be used. The higher strength substrate coupled with TD treatment can help reduce the types of damage discussed above.