Very high stresses in production lead to wear and thermal fatigue. For best result use materials with very good hot strength like Bohler W303 Isodisc, Uddeholm QRO 90 Supreme, or Bohler W360 Isobloc. When high toughness is needed, use Uddeholm Dievar or Bohler W400 VMR.
In progressive forging a large number of symmetrical, precision-forged parts with forged weights of up to about 5 kg are produced. The fully automatic process involves supplying hot-rolled bars at one end of the line, heating them inductively, cutting them to the required size, shaping them in 3-4 stages and discharging finished forgings at the other end of the line. Depending on the weight of the forgings, production capacity is between 50 and 180 parts per minute.
Die parts used in the progressive forging, such as die, stem, stem holder, punch and counter punch-ejector are subjected to very high stresses.
As the production speed is very high, the die parts need to be water-cooled to protect them against overheating. Nevertheless, despite intensive cooling, the tool surfaces can be strongly heated, even by the brief contact, with the hot metal being forged.
As a result of this alternate heating and cooling the die parts are subjected to extremely high thermal fatigue. The degree of the thermal fatigue cracking constitutes a measure of the material life. An additional factor is the degree of hot wear of the material, which depends on the surface temperatures and the mechanical stresses on the die.
Die material properties
The required properties profile of the hot forming die and die parts are:
- High temperature strength and good temper resistance to withstand hot wear and thermal fatigue cracking.
- Good thermal conductivity to withstand thermal fatigue cracking.
- Good hot ductility and toughness to resist initiation and rapid spread of thermal fatigue cracking.