Hot forging dies are exposed to severe mechanical and thermal cyclic loading. This causes hot wear, cracks and plastic deformation in the die. To prevent this, use materials with good hot strength and ductility/toughness. Uddeholm Orvar Superior or Bohler W303 Isodisc are suitable in most standard hot forging applications. When higher toughness is demanded, use Uddeholm Dievar or Bohler W400 VMR. For best hot wear resistance, Uddeholm Hotvar or Bohler W360 Isobloc is recommended. In brass/copper forging, Uddeholm QRO 90 Supreme is a good choice.
In hot forging a heated up billet is pressed between a die set to a nearly finished product. Large numbers of solid metal parts are produced in aluminum alloys, copper alloys, steel or superalloys where irregular shapes need to be combined with good mechanical properties. The main methods of impression die forging are drop or hammer forging and press forging.
Typical die failures
Hot forging dies are exposed to severe mechanical and thermal cyclic loading, which puts high demands on the die material. Thus, there are some phenomena which restrict die life.
- Wear: Wear occurs when the work material plus oxide scale glide at high velocity relative to the impression surface under the action of high pressure. It is most pronounced at convex radii and in the flash land. Wear is increased drastically if the forging temperature is reduced.
- Gross cracking: Gross cracking may occur during a single cycle or, as in most cases, over a number of cycles; in the latter instance, the crack growth proceeds via a high-stress fatigue mechanism. Gross cracking is more frequent in hammer blocks than in press tooling, because of the greater degree of impact.
- Plastic deformation: This occurs when the die steel is locally subjected to stresses in excess of the yield strength. Plastic deformation is quite common at small convex radii, or when long thin tooling components, e.g. punches, are subjected to high bending stresses.
- Thermal fatigue cracking: This results if the surface of the impression is subjected to excessive temperature changes during the forging cycle. Such temperature changes create cyclic, thermal stresses and strains at the die surface, which eventually lead to cracking via a low-cycle fatigue mechanism (heat checking).
Die material properties
The properties profile required for tool steel in forging dies depends to some extent on the type of forging operation, on the work material and on the size of the part, depth of impression etc. However, a number of general characteristics will always be required in all forging operations. The particular die damage mechanism to which resistance is conferred by the various properties are given in parentheses.
- Sufficient hardness and ability to retain this at elevated temperatures - temper resistance (wear, plastic deformation, thermal fatigue cracking).
- Enhanced level of hot tensile strength and hot hardness (wear, plastic deformation, thermal fatigue cracking).
- Good toughness and ductility at low and elevated temperatures (gross cracking, thermal shock cracking, thermal fatigue cracking).
- It is important that the die steel exhibits adequate toughness/ductility in all directions.
- Adequate level of fatigue resistance (gross cracking).
- Sufficient hardenability (retention of wear resistance, etc., if the die is re-sunk)