Austenitic Stainless Steel
The most common problem with tooling materials when working with austenitic stainless steel is adhesive wear, chipping and galling. Production lot size and steel thickness are important factors when choosing tool steel grades.
The basic problem with austenitic stainless steels is that the material is prone to adhering to the tooling surface when blanking first starts. During the blanking process the material gets harder through work hardening. This causes high stresses and may cause chipping or cracking. Chipping and cracking is especially severe when working with ¼ hard, ½ hard, full hard and thick steel. These issues can be reduced by choosing materials that are more resistant to adhesive wear and chipping.
Uddeholm Vancron 40, Uddeholm Vanadis 4 Extra Superclean™, Uddeholm Vanadis 6 and Uddeholm Vanadis 10 are especially good for long runs. Bohler K340 Isodur and Uddeholm Caldie are more appropriate for medium and short runs.
Failure mechanisms in cold work tooling
Due to cyclic mechanical loading and sliding contact between work material and tool surface, the active surfaces of the tool are successively damaged. The destruction of the tool will sooner or later lead to quality problems on the finished parts (out of tolerance or bad surface qualities). The tool then has to be exchanged (in case of total failure), reground or refurbished.
This maintenance procedure means production standstill and accordingly loss of productivity. It is therefore very important that the tools can resist the different types of tool failure mechanisms in order to achieve high productivity and economical production. The selection of the right tool steel is thus directly linked to the resistance of the actual tool failure mechanism for the application.
Common cold work failure mechanisms are:
- Wear - Results in a material loss from the tooling material and is related to the tooling materials hardness, carbide type and volume. Wear can also be related to the sheet material type and the process conditions.
- Chipping - Is related to the stresses in the process and the fatigue resistance of the tooling material
- Plastic deformation - Occurs if the process stresses are higher than the yield strength of the tool steel
- Cracking - Occurs when the process stresses are higher than the tensile strength of the tool steel
- Galling -Is a physical / chemical adhesion of the work material to the tool surface. The severity of galling depends on the surface finish and chemical composition of the tool steel and work material.
Method for tool steel selection
- Identify the dominant failure mechanism(s)
- Select a tool steel with properties that will overcome the failure(s)
- Match the steel choice to the length of the production run