As advanced high-strength steels are being deployed for automotive parts, the demands on the tooling used in the stamping of these parts are greatly increasing, observes Ed Severson, technical manager, New Business Development, Bohler-Uddeholm (www.bucorp.com ; Rolling Meadows, IL). Severson points out, “In the past, selecting a tool to improve performance was more straight-forward. If the tool is wearing out, use a more wear resistant tool steel. If the tool is chipping or breaking, use a tougher tool steel. This simple guideline for improving the tooling material is not as easy in today’s highly demanding applications.” Powdered metal (PM) steels are being used more frequently for fabricating the tools, but Severson explains that there are some serious considerations that have to be taken into account with the production of those materials if the tools are to have requisite performance characteristics.
Essentially, Severson explains, “Tool steels are composed of a matrix, carbide volume, and non-metallic inclusions.” The carbides are a function of the steelmaking process and are beneficial as regards wear resistance. The carbides are not, Severson says, as beneficial when it comes to the toughness and fatigue properties of the tool steel. The inclusions, too, are a function of the steelmaking, and include oxides, silicates, aluminates, and sulfides. These tend not to be as beneficial as the carbide and also detrimental to toughness and fatigue properties. According to Severson, both carbides and inclusions act as fracture initiation sites.
So with carbides, the objective should be to achieve uniform distribution in the matrix of smaller particles in the material. Similarly, the size and volume of the inclusions need to be reduced, as well.
Bohler-Uddeholm ran a series of tests to determine the effects of inclusions on impact toughness and fatigue strength. In both cases, inclusions were found to have a deleterious effect. For example, the conclusions reached in the fatigue testing analysis are:
- The primary failure mode in the standard cleanliness material was inclusion based. The primary failure mode in the cleaner material was a combination of inclusion and carbide based.
- The cleaner material can withstand higher overall stress levels before failure.
- The cleaner material can withstand a higher number of cycles before failure, with more tests surviving the full stress cycle.
Or that the cleaner the tool steel, the better the performance (i.e., higher stress loads and longer stress cycles, in this case).