Failure modes and properties
required for forging dies are shown in Fig. 1 (below). When
applied load is too large, dies fail mainly by yield, early
stage cracking and chipping. Too high sliding on the surface and
insufficient lubrication result in wear and seizure. Thermal
fatigue causes heat checking. And repetitive stress causes
fatigue fracture. To prevent these failures die design, assembly
and operational conditions must be justified. From die materials
point of view, furthermore, it is essential to give high
strength and high toughness as well. But these two properties
are trade off each other.
As
shown in Fig.2 (above), representing the position of Daido’s
tool steels with the axes of toughness and strength, higher
strength steels inevitably show lower toughness. Then it is
required to develop steels to exhibit both high strength and
toughness at the same time.
Figure 3 (below) shows the
examples of failed forging tools. A ring die shows early stage
gross crack. This impact fracture phenomenon is caused by lack
of toughness. A small crack is observed on the corner of a cold
forging punch. This small crack is caused by fatigue behavior.
Warm forging punch for CVJ outer race shows wear caused by
surface softening during warm forging. To prolong die life it is
necessary to give high hardness fatigue strength and also
toughness. Figure 4 (below) shows another example of failed cold
forging punch. Scanning electron microscope fractograpy made it
clear that the coarse carbides were the cause of fatigue
fracture.
