Art Hedrick
Owner

Die Science: Selecting the Right Tool Steel Types

Tool steel selection is perhaps one of the most important factors to consider in die designing and building.

Tool steel selection is perhaps one of the most important factors to consider in die designing and building.

One big problem the die building industry faces is how to design and build the tool to form special materials while reducing die cost. Cutting and forming of ultrahigh-strength steels and superalloys, for example, demand tool steels with great toughness and wear resistance.

Die failures can be a common problem in working with these special materials. Considering all the important characteristics of a tool steel and selecting the right one can be the difference between die failure and success.

Tool Steel Performance

Performance, without question, needs to be considered. Regardless of the price, using a tool steel that does not perform adequately is wasteful and costly.

Tool steels fails in different ways. Some fail because of severe abrasive or adhesive wear, others fail because of cracking, chipping, or plastic deformation. Which tool steel is selected for a given application depends mainly on the failures that are expected to be most probable.

Tool steel selection requires more than just knowledge of the steel properties. The number of parts to b produced, as well as the type, thickness, and hardness of the work material, must be taken into consideration.

The basic idea is to chose a tool steel that eliminates all failures types except wear. Therefore, to choose properly, you first need to understand the types of failures that occur (see Fig. 1):

• Abrasive failure—Material being cut or formed sometimes contains elements that can erode the surface of the tool steel. High-carbon steel often contains hard particle oxide that erode the surface of the tool steel. Generally speaking, for forming harder, stronger materials, tool steel with high wear resistance is required.
• Adhesive wear—When the tool steel and the sheet material are not compatible or are metallurgically similar, micro welding or cold welding may be pulled out of the tool surface. Such as loss of tool material can result in significant wear of the tool surface. This is a common problem in the forming of stainless steel. For example, D-2, a high-carbon, high-chromium tool steel, typically is chosen for forming 409 and 439 stainless. The two main elements the comprise stainless are nickel and chromium. When the high chrome in the tool steel interfaces with the chrome in the stainless, adhesive bonding occurs. The solutions are to use a different tool steel or to coat the D-2 with a carbide coating. Remember that tool steel selection is not just about toughness and wear resistance, but also the interface friction that results during the forming or cutting process. 
• Cracking—Cracking tends to occur spontaneously and usually means that the die section needs to be repaired or most likely replaced. Cracks often result from improper grinding or machining methods. Typically, toolroom maintenance departments are not equipped with the proper grinding wheels to suit each steel type. Using a wheel that does not break down properly during grinding may result in burning of the tool steel. The excessive heat generated often results in heat checking or micro cracking of the surface, which can propagate into larger cracks. Cracking often results from excessive shock loading of the tool steel. Tool steel sections that have been machined with sharp inside corners or radio also are more prone to cracking. 
• Chipping—Chipping occurs when cracking is confined to an isolated area. The cracks often cause small fragments of the tool steel to fall off. Point chipping is a common problem with piercing.
• Plastic deformation—When the tool steel’s yield strength is exceeded, plastic deformation occurs. This can be the result of insufficient tool steel hardness or extreme impact force.