Mild Carbon Steel
All types of failures are possible when blanking soft carbon steels. Some influencing factors are production lot sizes, steel thickness and complexity of the part being produced. Most types of steel can be used for blanking mild carbon steels the ruling factor of tool steel selection is total production costs.
Concerns of chipping increase with the thickness,hardness of the work material and complexity of the finished part. The risk of adherence (micro-welding) between the tool and the work material also increases with thicker, harder and more complex work materials and finished parts. The micro-welding causes decreased life of the tooling because of adheisve wear and galling. Increased thickness could also cause cracking of the punch during ejection because of the high forces required to eject the punch.
Tooling materials with high degree of cleanliness and well distributed carbides in a homogenous matrix help reduce this, see examples of grades below:
- Powder Metallurgy tool steels Uddeholm Vanadis 4 Extra, Vanadis 6, Vanadis 10
- Double melted conventional tool steels Uddeholm Caldie and Bohler K340 Isodur
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