For forging, forming, shearing, pressure die casting and punching at elevated temperatures, hot work tool steels are used as they maintain their Wear resistance, mechanical properties etc. at these elevated temperatures.
Alloying elements like Chromium, Cobalt, Molybdenum, Tungsten, Vanadium and Nickel help in achieving properties required for Hot Work tool steels. One of the demanding uses of Hot work steel is in the area of Pressure die casting, where EFS(Extra Fine structure) and ESR(Electro slag refining) quality steels are used extensively. The following properties of the tool steels are of important consideration:
- High hot yield strength
- Lower thermal expansion coefficient
- High Charpy impact value
- High thermal conductivity
- Good weldability
The main problems encountered in a PDS die are Heat checks, Erosion and Crack. Right selection of tool steels can help minimize these problems and optimize die life. Another important use of Hot Work tool steels is in the area of forgings, done at elevated temperatures by use of vertical presses and Hammers. Press forging the pressure is applied fast unlike Hammer forging where the drop of hammer die is sudden and very fast. In case of latter, High toughness material is required to absorb this impact load while in Press forgings high strength hot work tool steels are recommended. Normally forging hammer die material is supplied in pre-hardened condition of 360-410BHN.
Extrusion is another important application of Hot work tool steels. This is a process to convert an Ingot or billet in pre heated condition to desired cross section or profile by forcing the metal through a die. Apart from the normal properties as in PDC die material, nitriding ability is also desired to avoid abrasive wear. Due to tough working conditions like high temperatures and pressure, the die life is reduced if proper care is not taken in choosing the die material. Normally AISI H11 and H13 are most popular extrusion die materials.