Cold Work - Powder Compacting
The proper tool steel selection for this application depends primarily on powder type, required density of green compact and part complexity. Abrasive wear, cracking and chipping are the most critical failure mechanisms to consider.
Powder compacting is an application where powder enters the die and is pressed into a green compact, this is similar to coining but with powder. The green compact is then sintered and can be sized to give tighter dimensions or improve the properties of the compacted part.
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 formed parts (out of tolerance or bad surface qualities). The tool has then 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 this / these failure mechanism(s)
- Match the steel choice to the length of the production run