Tool builders and tool users know that all types of carbides are extremely hard. This fact leads many to think that carbide coatings on steels are very brittle and diminish the impact and fatigue strength of a substrate. This is not true.
When you combine the excellent surface properties inherent in diffused carbide and the high strength inherent in good substrate steels, you get outstanding surface properties – namely high wear resistance on the surface and substrate toughness.
This is especially true of the carbide coating infused on tool steels by the Thermal Diffusion (TDX) tool treatment process. During the TDX treatment, the diffused carbide layer adheres so strongly to the steel substrate surface that the carbide layer and substrate behave as one body. The TDX Process is the only tool treatment process that ensures this high adhesion and performance.
Test results support the fact that carbide-coated tooling is tough, not brittle. In the following test, a TDX treated punch was compared to an uncoated punch. Both punches were A2 steel, .133 inch (3.4 mm) perforating punches. Both were used to perforate .125 inch (3.2 mm) thick carbon steel plate. Figure 1 shows the cross sections of the punches.
Figure 1a depicts the TDX treated punch after 101,500 hits. The TDX coated surface shows wear at the punch edge, but is also still visible on the sides of the punch. If the carbide layer had chipped off, neither the steady coating wear nor the smooth contour of the exposed cutting edge would be visible. Reduction of the punch life is minimized by the TDX layer during resharpening.
Figure 1b depicts the uncoated punch after 16,100 hits. On the uncoated punch severe wear and chipping of the cutting edge are evident. Punch life will be dramatically reduced when resharpening occurs due to the wear or damage on the cutting edge and down the sides of the punch. The tests showed that the uncoated punch could take only about 1/6 of the hits the TDX treated punch endured.
Thus, the advantage of using TDX treated punches is that a high number of resharpenings is possible. TDX treated punches incur smaller end damage during use and, thus, require less material removal at the cutting edge (Figure 1). TDX also reduces side surface damage from galling. Smaller end damage and reduced galling mean less resharpening resulting in dramatically increased tool life.
The advantages of TDX hold true even when hard product materials such as stainless steel, spring steel, high strength steel as well as heavy gauge and double thickness materials are being punched. Even when working with these hard materials, TDX increases the number of hits before and between regrinding(s) up to twenty times or more.
Another example of the extreme toughness of the diffused carbide layer is that TDX treated steel does not fail under mechanical loading. In bending and fatigue tests, cracking of the layer prior to cracking of the fully hardened steel substrates is rarely observed.
Because of the metallurgical bonding of the TDX layer and its thickness of 3 to 5 µm, tool failure is not a result of the TDX layer “chipping off”. It can be caused by the chippling or fracture of the substrate itself – especially if imperfectly hardened substrates or extremely brittle substrates were used. Be sure to use the right substrate for your application. When substrate steels with great toughness and adequate compressive strength are TDX treated, you get high wear-resistant, tough tooling that withstands hit after hit.