 |
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
4. Special
features of DRM steels
(continued)
4.2 Properties
of DRM1 and DRM2
Heat treatment conditions of DRM1
and 2 are recommended as shown in Fig. 16. It should be noted in
quenching of DRM1 that quenching rate below 5000C should be
large to maintain high toughness by suppressing lower Bainitic
transformation. The austenitizing temperature for DRM2 should be
lower than 11 200C to prevent austenite grain coarsening
resulting in low toughness.
|
The tempering
hardness curves of DRM1 and DRM2 are shown in Fig. 17. DRM 1
shows higher hardness than that of hot work die steel AISI 1413.
DRM2 shows higher than conventional MHS steel, MH85.
Charpy impact values
of DRM steels are shown in Fig. 18. DRM 1 and 2 exhibit higher
values than those of cold work die steels such as AISI D2 and
Daido's DC53 and conventional MHS steel MH85. Microfractographs
taken from the Charpy impact specimens of DRM1 and MH85 are
shown in Fig.19. As we have intended, it is confirmed that there
observed coarse VC carbides at the crack origin in MH85, but no
ones in DRM1.
Fatigue test
results, the relation between the applied stress and the numbers
of cycles to fracture, are shown in Fig.20. Rotating bending
fatigue tests were carried out by using smooth specimens. DRM1
and 2 show higher fatigue strength than MH85. As confirmed in
Charpy impact specimens, microfractography also reveals that
there is no coarse carbides at fatigue crack origin in DRM1 and
2 as well.
Heat checking test
was conducted to evaluate the thermal fatigue properties. The
test methods are schematically shown in Fig.2 1. This is
accelerated test. A small specimen is repeatedly induction
heated up to 7000C and cooled by water. Then, a specimen is cut
at a half thickness and the cross section is ground and
polished. The number and the depth of cracks are measured. The
shallower crack and the less number of cracks mean the better
heat checking resistance. The test results are shown in Fig.22.
The maximum crack length of DRMI and DRM2 is shallower than that
of MH85 and AISI H13, which means the superiority of the new
grades to conventional steels.
Tempering softening
resistance is the other important property for forging die
steels. Test specimens, heat treated to the hardness 55 to 6OHRC
in advance, were held at 6500C for up to lOhr and the hardness
change was measured as shown in Fig.23. DRM1 shows higher
hardness than MH85 and H13. DRM2 is almost equivalent to MH85
but superior to H13.
<PREV
| TABLE OF CONTENTS |
NEXT>
|
