3. 90 V. Lesko6s'ek, B. Ule : Journal of Materials Processing Technology 82 (1998) 89±94
Fig. 1. Comparative hardness of carbides found in tool steels [2].
To improve the toughness of conventionally-manu-
factured high-speed steel M2, recent development [1] has
focused mainly on vacuum heat treatment procedures.
High-speed steels have better resistance to wear in
comparison to cold-work tool steels because of the
increased matrix and carbide phase hardness.
The carbide phase in high-speed steels increases the
wear resistance, which latter depends on the total vol-ume
of carbides and their hardness. The wear resistance in high-
speed steel is mainly determined by vanadium carbides
which have a micro-hardness of 2200 to 2400HV [2] (Fig.
1).
However, it must not be forgotten that high-speed steels
have a greater hot hardness. Even if the work-pieces are
placed into the tools whilst cold, the working tool surfaces
become hot because of the processing in the tool.
3. Experimental procedure
The test material was a conventional high-speed steel of
AISI M2 type of the same melt. The chemical composition
of the steel examined is listed in Table 1. The toughness of
high-speed steels is relatively low at the hardness to which
they are normally heat treated before use. It is therefore
necessary, in optimizing vac-uum heat-treatment
procedures, that the toughness test methods are capable of
responding to changes caused by the variations in
microstructure.
For this reason, recent investigations [1,3] have fo-cused
on the fracture toughness of high-speed steels.
Table 1
Chemical composition of the high-speed steel examined (in wt.%)
Fig. 2. The geometry of a cylindrical round-notched and pre-cracked
tensile specimens.
The geometry of cylindrical round-notched pre-cracked
tensile specimens, prepared according recommendations
[1,4] is shown in Fig. 2.
Accordingly to Grossmann's concept of a harden-ability,
the formation of uniform microstructure along the crack
front is possible because of the axial symme-try of
cylindrical specimens, in comparison with the CT-
specimens, where this condition is not ful®lled. For a
round-notched pre-cracked specimen, the stress inten-sity
factor is given by Heckel [4] as:
KI P:D3:2
( 1.27 1.72D:d) (1)
where d is the radius of the uncracked ligament after
fatiguing, P is the applied fracture load, and D is the outer
diameter of the cylindrical specimen. In order to apply
linear-elastic fracture mechanical concepts, the size of the
plastic zone at the crack tip must be small compared with
the nominal dimensions of the speci-men. The size
requirement for a valid KIc test is given by Shen Wei et al.
[5] as
D]1.5(KIc:sys) (2)
where sys is the yield stress of the investigated steel. This
requirement (Eq. (2)) was ful®lled in all of the present
measurements. The fracture surfaces of the cylindrical
round-notched and pre-cracked specimens were examined
in S.E.M. at low magni®cation.
As is shown in Fig. 3, the fatigue-crack propagation area
was sharply separated from the circular central part of the
fast-fractured area, so that diameter d of this area was
easily measured.
Material C Si Mn Cr Mo W V Co
AISI M2 0.87 0.29 0.30 3.77 4.90 6.24 1.18 0.53
4. V. Lesko6s'ek, B. Ule : Journal of Materials Processing Technology 82 (1998) 89±94 91
Fig. 3. Fracture surface of a cylindrical round-notched and pre-cracked
tensile specimen with the circumferential fatigue-crack propa-gation area
sharply separated from the circular central fast-fractured area.
Cylindrical round-notched tensile specimens with a
diameter of 10 mm were machined from soft annealed bars
with a Brinell hardness of 255. Specimens were fatigued to
produce a sharp circumferential crack at the notch root,
then austenitized in a vacuum furnace at
temperatures of 1050, 1100, 1150 and 1230°C, quenched in
a ¯ow of gaseous nitrogen at a pressure of 5 bar abs. and
double tempered 1 h at temperatures of 510, 540, 570 and
600°C, respectively.
4. Results and discussion
Metallographic examination of the specimens [1] shows
that the austenite grain size of all specimens that were gas
quenched from the austenitization tempera-ture 1050 and
1230°C was 21 and 8 SG, respectively.
The microstructure of the cylindrical round-notched
tensile specimens examined by S.E.M. at higher mag-
ni®cation is shown in Fig. 4, consisting of tempered
martensite and carbides.
The quantity of ®ne carbide particles decreases with the
increase of the austenitizing temperature. In addi-tion, it
was also noticed that at higher austenitizing temperatures,
particularly at 1230°C (the last column in Fig. 4), primary
carbide particles penetrate along the edges of the grains
because of partial melting and decrease of surface tension
of the matrix ± carbides in-terface. When specimens are
tempered above 500°C, the retained austenite is
`conditioned' and subsequently decomposes to martensite
on quenching [3]. The reac-tion is completed at 540°C,
although small amounts of austenite (less than 3%) remain
after tempering at 580°C. If retained austenite affects the
fracture tough-
Fig. 4. The microstructure of vacuum hardened and tempered specimens examined by S.E.M.
5. 92 V. Lesko6s'ek, B. Ule : Journal of Materials Processing Technology 82 (1998) 89±94
Fig. 5. The effect of the austenitizing and tempering temperature on the
fracture toughness and hardness of M2 steel (FT-fracture tough-ness; H-
hardness).
ness of M2 steel, the effect is small enough to be obscured
by secondary hardening. The fracture tough-ness would be
expected to increase with the increasing content of retained
austenite, as observed for under-aged microstructures
tempered below 500°C. In con-trast, specimens
austenitized at 1230°C, and thus containing larger amounts
of retained austenite than the specimens austenitized at
1050°C, have a lower fracture toughness. Obviously, other
factors such as the distribution, morphology and stability of
retained austenite may also be changing and have a
pronounced effect on the fracture toughness [3]. The
fracture tough-ness properties are summarized in Fig. 5.
Specimens austenitized at 1050°C were somewhat tougher
than those austenitized at 1230°C. The fracture toughness
decreases with increasing tempering temperature up to
540°C and then increases again with further increase in
tempering temperature. The fracture toughness is in-
versely correlated with hardness, as generally observed for
steels. Despite the non-standard specimens and pre-
cracking technique used, KIc values were repro-ducible and
in good agreement with other published data [3]. Fig. 5
shows that KIc increases with decreasing hardness, as
observed earlier [6]. In the temperature range of vacuum
heat-treatments examined, the hard-ness is dependent on
the secondary-hardening reaction. It is therefore postulated
that carbide particles play a primary role in determining the
fracture toughness of AISI M2 high speed steel.
The lower fracture toughness of specimens austeni-tized
at 1230°C can be associated with an increased carbon
content in the matrix and subsequently with a
more profuse precipitation of secondary carbides. The
enhanced volume fraction of carbide particles also re-duces
the fracture toughness by providing a preferential path for
crack propagation. This postulate is supported by the work
of Olsson [7], in a high speed steel the ®nal crack growth
occurring in the carbide ± matrix interfaces.
On the basis of the experimental results [1], it was
possible to draw the diagram shown in Fig. 6, where the
technological parameters of the vacuum heat-treat-ment,
mechanical properties and microstructure of the vacuum
heat-treated specimens are combined.
From the diagram in Fig. 6, it is evident that the fracture
toughness for the tempering temperatures 540, 570 and
600°C, respectively, increases with the decrease of
hardening temperature. This is in good agreement with
earlier observations [3,6].
On the other hand, for the tempering temperature of
510°C, it was found that the fracture toughness values are
very close, although slightly higher than for 600°C,
irrespective of the hardening temperatures. On the basis of
the curves in Fig. 6, it can be assumed that the high-speed
steel M2 hardened from low austenitizing temperatures and
tempered at 510°C can achieve an optimal combination of
hardness and fracture tough-ness. The relationship between
fracture toughness and austenite grain size, f.i. SG grade 8,
shows that at the tempering temperatures 510 and 600°C,
the obtained values KIc are 17.8 and 10.6 MNm 3:2
, a
difference that is quite important in practice. Different
fracture toughness at the same grain size (SG8,
1230:510°C) and virtually nearly constant fracture
toughness at the tem-
Fig. 6. In¯uence of austenite grain size on the fracture toughness of high
speed steel M2 (TT-tempering temperature, TA-hardening tem-perature,
HRc-hardness at 510°C).
6. V. Lesko6s'ek, B. Ule : Journal of Materials Processing Technology 82 (1998) 89±94 93
Fig. 7. Fine-blanking tool for a ratchet wheel.
pering temperature of 510°C, irrespectively of the
austenitizing temperature (1050 to 1230°C, SG21 to SG8),
con®rms the hypothesis that the austenite grain size is not
the only dominant parameter affecting the fracture
toughness of high-speed steel M2.
punches and dies were tested on a 250t triple-action
hydraulic press and compared with the ®ne-blanking tools
for ®ne-blanking ratchet wheels conventionally heat-
treated in a salt bath [1]. The ®nal hardness of the
conventionally heat-treated tools achieved after double
tempering at 600°C:1 h, was 58 to 59 HRc, depending on
the alloying. The tests [1] showed that higher working
hardness (61.5 HRc) and improved fracture toughness of
vacuum heat-treated punches and dies Ð particularly those
double tempered at 510°C Ð had a signi®cant affect on the
defects on the cutting edges. The vacuum heat-treated tool
life was greater by 15 ± 20%, compared to conventionally
heat-treated ®ne blanking tools tempered at 600°C.
None of the in-service dimensional instability due to the
later-transformed retained austenite was found at tool
testing. Namely, X-ray diffraction analysis showed that the
content of retained austenite did not exceed 1 vol.% in all
of the specimens [1].
5. Tool life tests
Long production runs have underlined the impor-tance
of improved ®ne blanking tool life, Fig. 7.
The qualities most commonly required from a ®ne-
blanking tool are wear resistance and toughness, which are
needed to maintain a keen cutting edge, combined with
suf®cient red hardness. In ®ne blank-ing, close
dimensional stability is essential. Since gross plastic
deformation of a ®ne blanking tool is not de-sirable in this
respect, the initial compressive yield stress is important. On
the basis of the experimental results shown in Fig. 6, it was
found that the opti-mum vacuum heat-treatment of ®ne-
blanking tools from high speed steel M2 needs at least two
pre-heat-ing stages (650 and 850°C respectively), a
variable hardening temperature (usually 1050 ± 1150°C)
and double tempering at the same temperature (510°C:1 h).
The life of a ®ne-blanking tool varies considerably
depending on the size and design of the blank, the type of
blanking steel, and the care and maintenance of the tool. To
establish tool life, three tools for a ®ne-blanking ratchet
wheel were selected.
The punches and dies were from high-speed steel M2.
The blanks, with a material gauge of 4 mm were from AISI
C 1045 blanking steel in a spheroidized-annealed
condition. The punches and dies were stress-relieved at
650°C:4 h and vacuum heat-treated. Depending on the
alloying and the condition of austenitization (1100°C:2
min), a ®nal hardness of 61.5 HRc and a ®nal fracture
toughness of 17.3 MNm 3:2
were obtained after double
tempering at 510°C for 1 h (Fig. 6). The vacuum heat-
treated
6. Conclusions
On the basis of a study of the in¯uence of vacuum heat-
treatment on mechanical properties and ®ne-blanking
performance of AISI M2 high-speed steel, the following
conclusions are drawn.
(1) The measurements of wear on the cutting edges of
punches and dies show that double tempering at 510°C:1 h
after vacuum hardening prolongs the life of a ®ne-blanking
tool for ratchet wheels by 15 ± 20%, compared to
conventionally heat-treated tools that were hardened at the
same austenitizing temperature and tempered twice at
600°C.
(2) The different fracture toughness at the same grain
size (SG8, 1230 ± 510°C) and the nearly constant fracture
toughness at the tempering temperature 510°C,
irrespectively of the austenitizing temperatures (1050 ±
1230°C), con®rm the hypothesis that the austenite grain
size is not the only dominant parame-ter affecting the
fracture toughness of high-speed steel M2.
(3) There is a marked improvement in ®ne-blank-ing
tool life when the tempering temperature is de-creased
from the currently recommended 600 ± 510°C. It is not the
type of heat-treatment process that sub-stantially affects the
tool life, but ®rst of all the proper choice of hardening and
tempering tempera-tures.
(4) The results presented, con®rmed by daily data,
justify the use of modern vacuum heat-treatment
technology and the use of the newest high-perfor-mance
high speed steels to achieve great improve-ments in the
lives of ®ne-blanking tools and overall economy.
7. V. Lesko6s'ek, B. Ule : Journal of Materials Processing Technology 82 (1998) 89±94 93
Fig. 7. Fine-blanking tool for a ratchet wheel.
pering temperature of 510°C, irrespectively of the
austenitizing temperature (1050 to 1230°C, SG21 to SG8),
con®rms the hypothesis that the austenite grain size is not
the only dominant parameter affecting the fracture
toughness of high-speed steel M2.
punches and dies were tested on a 250t triple-action
hydraulic press and compared with the ®ne-blanking tools
for ®ne-blanking ratchet wheels conventionally heat-
treated in a salt bath [1]. The ®nal hardness of the
conventionally heat-treated tools achieved after double
tempering at 600°C:1 h, was 58 to 59 HRc, depending on
the alloying. The tests [1] showed that higher working
hardness (61.5 HRc) and improved fracture toughness of
vacuum heat-treated punches and dies Ð particularly those
double tempered at 510°C Ð had a signi®cant affect on the
defects on the cutting edges. The vacuum heat-treated tool
life was greater by 15 ± 20%, compared to conventionally
heat-treated ®ne blanking tools tempered at 600°C.
None of the in-service dimensional instability due to the
later-transformed retained austenite was found at tool
testing. Namely, X-ray diffraction analysis showed that the
content of retained austenite did not exceed 1 vol.% in all
of the specimens [1].
5. Tool life tests
Long production runs have underlined the impor-tance
of improved ®ne blanking tool life, Fig. 7.
The qualities most commonly required from a ®ne-
blanking tool are wear resistance and toughness, which are
needed to maintain a keen cutting edge, combined with
suf®cient red hardness. In ®ne blank-ing, close
dimensional stability is essential. Since gross plastic
deformation of a ®ne blanking tool is not de-sirable in this
respect, the initial compressive yield stress is important. On
the basis of the experimental results shown in Fig. 6, it was
found that the opti-mum vacuum heat-treatment of ®ne-
blanking tools from high speed steel M2 needs at least two
pre-heat-ing stages (650 and 850°C respectively), a
variable hardening temperature (usually 1050 ± 1150°C)
and double tempering at the same temperature (510°C:1 h).
The life of a ®ne-blanking tool varies considerably
depending on the size and design of the blank, the type of
blanking steel, and the care and maintenance of the tool. To
establish tool life, three tools for a ®ne-blanking ratchet
wheel were selected.
The punches and dies were from high-speed steel M2.
The blanks, with a material gauge of 4 mm were from AISI
C 1045 blanking steel in a spheroidized-annealed
condition. The punches and dies were stress-relieved at
650°C:4 h and vacuum heat-treated. Depending on the
alloying and the condition of austenitization (1100°C:2
min), a ®nal hardness of 61.5 HRc and a ®nal fracture
toughness of 17.3 MNm 3:2
were obtained after double
tempering at 510°C for 1 h (Fig. 6). The vacuum heat-
treated
6. Conclusions
On the basis of a study of the in¯uence of vacuum heat-
treatment on mechanical properties and ®ne-blanking
performance of AISI M2 high-speed steel, the following
conclusions are drawn.
(1) The measurements of wear on the cutting edges of
punches and dies show that double tempering at 510°C:1 h
after vacuum hardening prolongs the life of a ®ne-blanking
tool for ratchet wheels by 15 ± 20%, compared to
conventionally heat-treated tools that were hardened at the
same austenitizing temperature and tempered twice at
600°C.
(2) The different fracture toughness at the same grain
size (SG8, 1230 ± 510°C) and the nearly constant fracture
toughness at the tempering temperature 510°C,
irrespectively of the austenitizing temperatures (1050 ±
1230°C), con®rm the hypothesis that the austenite grain
size is not the only dominant parame-ter affecting the
fracture toughness of high-speed steel M2.
(3) There is a marked improvement in ®ne-blank-ing
tool life when the tempering temperature is de-creased
from the currently recommended 600 ± 510°C. It is not the
type of heat-treatment process that sub-stantially affects the
tool life, but ®rst of all the proper choice of hardening and
tempering tempera-tures.
(4) The results presented, con®rmed by daily data,
justify the use of modern vacuum heat-treatment
technology and the use of the newest high-perfor-mance
high speed steels to achieve great improve-ments in the
lives of ®ne-blanking tools and overall economy.
