High Strength Steel
The hardness and tensile strength of high strength steels require tooling materials with high wear resistance and chipping / cracking resistance. These requirements are even more important with the ultra high strength steel (martensitic) grades used today. With ultra high strength steel there is a need for tooling materials that are outstanding in every category.
HSLA (High Strength Low Alloy): Yield strengths range from 43-100KSI. These materials get their strength from small additions of elements like niobium and titanium.
Dual Phase (DP): Tensile strengths range from 72-145KSI. The microstructure is made of two phases, one being ferrite and the other being martensite. The ferrite gives the material its formability and the martenstie gives the material its strength and hardness.
TRIP (Transformation Inducted Plasticity): The newest of the high strength steels work harden as they are formed typical tensile strengths are 72-100KSI
Martensitic: Tensile strengths are typically above 145KSI and are the hardest of the high strength steels to work because of high hardness and low formability, they are typically called ultra high strength steels.
When blanking high strength steels, the clearances between punch and die must be increased compared to carbon steel sheets/plates. This is due to the higher stresses needed to penetrate the hard sheet/plate. If the clearance is too low, there is a tendency to have increased galling and chipping. If the clearance is too large cracking problems may occur.
Tool steels that have superior performance when blanking high and ultra high strength steels are:
Powder Metallurgical tool steels - Uddeholm Vanadis 4 Extra and Uddeholm Vanadis 6
Double melted conventional tool steel - Uddeholm Caldie and Bohler K340 Isodur
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 to then to be exchanged (in case of total failure), reground or refurbished in some way.
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