The use of Advanced High-Strength Steels (AHSS) in laser cutting, metal bending, metal rolling, and in the auto has created a new challenge, especially for stamping tools, and the methods used to produce them in metal fabrication.
Tool-Hardening in Retrospect
Various steels that can be hardened have been used as dies for stamping and forming purposes. These have been either forged, cast, or mill-formed. These days, less hard cast iron and cast steel is used for the dies to reduce their cost and weight.
Hardening of the die depends a lot on the size and shape of the die itself. Heating up the die and then quickly cooling it again, often called quenching, hardens it. Afterward, the steel is tempered to prevent cracking, which in turn increases toughness, ductility and extends its life.
Flame hardening used in stainless steel fabrication, aluminium fabrication, or steel fabrication for large dies with more complicated shapes focused on hardening the surface of the die, with special precautions being taken to extend the useful life of the die. Although special coatings are not amply available, chromium plating has been used to protect stamping dies with success for several years.
Due to AHSS becoming harder, the drawbacks of using chromium became more pronounced. Frequent wear and galling means that the dies need to be recoated with chromium after a few runs. This greatly lowers the efficiency of chromium, making it a more costly method preserving the condition of the die used for stamping AHSS. Moreover, slug damage makes repairs much harder on the chromium-plated surface. This damage is extended if the weaker material under the material is exposed for too long, as it wears out faster than the chromium covering it. Finally, compounds of chromium are among the 189 HAP (Hazardous Air Pollutants), which means chromium plating is potentially a significant health hazard.
Ion/Plasma Nitriding is a technique which involves treating the die in a vacuum vessel with heaters and a plasma generator. To achieve the nitriding effect, the die’s parts are covered using glow discharge plasma. This new technique, implemented in recent years, produces much harder dies. Moreover, nitriding does not introduce a new layer of protection but provides depth level hardness required to withstand the much greater impacted forces when forming AHSS, this prevents significant slug damage. Furthermore, it does not create any changes in the shape of the die as well since the process ensures that it is applied evenly in all areas of the die.
This technique produces reusable, harder dies, making them a feasible alternative.