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The
Development of High Hard and
Tough Matrix Type High Speed Tool Steels
By
SHUN-SUKE NAKAHAMA*1, YUKINORI MATSUDA*2, KUNIO NAMIKI*3, KOZO
OZAKI*4
5. Application
study and discussion
Based on the fundamental study
which had revealed the superiority of DRM steels to conventional
ones, we have moved to the next stage: application study to
forging dies. Examples of the application of DRIvI1 are shown in
Table 4 (below). DRMI shows longer life than MH85 and AISI H13
especially for dies the failure mode of which is crack.
Furthermore, DRM1 is effective to prolong the life of the dies
which have problems in the surface softening. The cause of the
die life improvement from H13 is the greater tempering softening
resistance by increasing the initial hardness. One of the
example applied to hot forging dies is sunmarized in Fig.29
(below). This front die made of AISI M2 is used for hot former.
The die failed by a crack which originated from the inner
surface of the die and propagated to circumference direction. A
crack was originally caused by fatigue and this crack propagated
to final fracture. DRM 1, having high fatigue strength, high
toughness and excellent heat characteristic, showed longer life.
Soft nitriding has further prolonged die life. The cause of the
improvement is mainly the decrease in wear on die surface. And
this prevents the abnormal local excess load which leads the
premature cracking.
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Table 5 shows examples
of the application of DRM2 indicating the longer life than
conventional MHS steel. Factors to prolong die life are prevention
of yielding and fatigue by increasing the strength with remaining
high toughness. An example is shown in Fig.30. This is a mandrel
used for cold forming and originally made of powder metallurgy
high speed steel. And the failure mode was fatigue fracture.
Although the new grade shows almost the same life with fatigue
fracture, it is superior to P/M steel from the viewpoint of
material cost.
Examples of the
application of DRM3 are shown in Table 6. Longer life has been
attained especially for the tools where AISI D2 and M2 fail with
chipping. This is due to higher toughness and fatigue strength of
DRM3 than these of D2 and M2.
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