Hot work material is typically used for:
o Die casting
o Forging
o Extrusion
When a die casting die is hardened and tempered, some warpage or distortion normally occurs. This distortion is usually greater at higher temperature. |  |
This is well known, and it is normal practice to leave some machining allowance on the die prior to hardening. This makes it possible to adjust the die to the correct dimension after hardening and tempering by finish machining or grinding.
Distortion of the material can take place because of machining stresses. This type of stress is generated during machining operations such as milling and grinding. If stresses have built up in a part, they will be released during heating. Heating releases stresses, created through local distortion which in turn can lead to overall distortion. In order to reduce distortion while heating during the hardening process, a stress relieving operation can be carried out. It is recommended that the material be stress relieved after rough machining.
Any distortion can be adjusted during fine machining, prior to quenching. The size of the die or mould often decides the hardness of it, small moulds are 50 - 52 HRC and big ones are 45 - 48 HRC.
The amount of non-metallic inclusions and the hardness of the steel are some factors that have great influence on the machinability.
Typical failure mechanisms of hot work steels are:
Heat checking
Hot wear
Plastic deformation
Cracking
Corrosion
Optimum properties of the steel is:
Good hot hardness
Good tempering resistance
Low thermal expansion
Good toughness and ductility
Good thermal conductivity
|