I. PERU[ et al.: IMPROVING OF HOT WORKABILITY AND EXPANDING THE TEMPERATURE RANGE ...
485–491
IMPROVING OF HOT WORKABILITY AND EXPANDING THE
TEMPERATURE RANGE OF SAFE HOT WORKING FOR M35
HIGH-SPEED STEEL
IZBOLJ[ANJE VRO^E PREOBLIKOVALNOSTI IN RAZ[IRITEV
TEMPERATURNEGA INTERVALA VARNEGA PREOBLIKOVANJA
V VRO^EM ZA HITROREZNO JEKLO M35
Iztok Peru{1, Milan Ter~elj1, Matja` Godec2, Goran Kugler1
1University of Ljubljana, Faculty of Natural Science and Engineering, 12 A{ker~eva street, 1000 Ljubljana, Slovenia
2Institute of Metals and Technology, Lepi pot 11, 1000 Ljubljana, Slovenia
goran.kugler@omm.ntf.uni.lj.si
Prejem rokopisa – received: 2016-07-12; sprejem za objavo – accepted for publication: 2016-08-31
doi:10.17222/mit.2016.150
Increasing the hot workability and expanding the temperature working range for M35 high-speed steel (HSS) by industrial hot
rolling as well as by hot compression tests have been studied and achieved. The types of carbides for as-cast as well as for
wrought states were revealed by EBSD and the optimal soaking temperatures for both microstructural states were assessed. The
soaking temperature had a decisive influence on the characteristics of carbides that lead to improving the intrinsic hot
workability as well as to considerably expanding the temperature range of safe hot working down to 850 °C. The apparent
activation energy for hot working which amounts 639 kJ/mol as well as constants of the hyperbolic sine constitutive equation for
extended temperature range have been calculated. Industrial practice confirms the laboratory results.
Keywords: M35 HSS, hot workability, carbides, EBSD, activation energy
Na osnovi vro~ega valjanja v industrijskih razmerah ter s tla~nimi preizkusi v vro~em smo {tudirali in izbolj{ali vro~o
preoblikovalnost hitroreznega orodnega jekla M35 ter raz{irili temperaturni interval varnega preoblikovanja. Na osnovi posebej
zasnovane metodologije, smo ocenili optimalno temperaturo `arjenja pred vro~im preoblikovanjem, s pomo~jo EBSD, pa smo
dolo~ili tipe karbidov tako za predelano, kot tudi za lito stanje obravnavanega jekla. Temperatura `arjenja pred preoblikovanjem
mo~no vpliva na zna~ilnosti karbidov in posledi~no na vro~o preoblikovalnost ter temperaturni interval varnega preoblikovanja.
Na osnovi izmerjenih krivulj te~enja so bili za raz{irjen temperaturni interval izra~unani parametri sinus hiperboli~ne
konstitucijske zveze ter navidezna aktivacijska za deformacijo, ki zna{a 639 kJ/mol. Rezultati laboratorijskih testiranj so bili
uporabljeni in potrjeni v industrijski praksi.
Klju~ne besede: hitrorezno jeklo M35, vro~a preoblikovalnost, karbidi, EBSD, aktivacijska energija
1 INTRODUCTION
In order to increase the economy of tool-steels
production, both the improvement of their intrinsic hot
deformability and the extension of temperature range of
safe hot working are required. An improvement of the
intrinsic hot workability of tool steels is related to the
characteristics of carbides1–4 that on the other hand have
a large influence on the mechanical properties of tool
steels that are used for the manufacture of various tools
and dies usually subjected to high mechanical, tempera-
ture, chemical and tribological loads.5–7 Referring to the
scientific literature, it is known that initial deformations
at upper temperature limit, as well as final deformations
at lower temperature limit of hot working range, are
characterized by considerably decreased hot deformabi-
lity in comparison to temperatures within the mentioned
range.1–4,8–13 The decreased hot deformability at the upper
limit is attributed to the characteristics of carbides, i.e.,
to their type, size, shape, size- and/or spatial- distribu-
tions, fraction, melting point of eutectic carbides and/or
other phases, etc. Also at lower temperature limit charac-
teristics of carbides (i.e., with additionally precipitated
secondary carbides) are responsible for pore deformabi-
lity, where this is related also with the effect of a
decreased recrystallization rate. Moreover, the charac-
teristics of the carbides are also influenced by the casting
temperature and solidification rate.3,14–23 The appearance
of cracks on areas that are most exposed during
thermo-mechanical processing are thus related to similar
conditions as described above for both limits of the safe
hot working range. These areas are in combination with
increased tensile strains additionally subjected to
accelerated heating and/or cooling.24 Some recently
published works3–4 reported that by proper selection of
the process parameters prior to hot deformation the
increase of hot deformability can be achieved.
In the present study we report about the increase of
the hot workability of M35 high-speed steel (HSS)
which is used for making severely stressed drills, milling
cutters, formed tools, tap drills and cold forming tools,
where an increased toughness is required. So far a
temperature of 900 °C was considered as the lowest
MATERIALI IN TEHNOLOGIJE/MATERIALS AND TECHNOLOGY (1967–2017) – 50 LET/50 YEARS
Materiali in tehnologije / Materials and technology 51 (2017) 3, 485–491 485
UDK 621.78:621.77:669.15 ISSN 1580-2949
Original scientific article/Izvirni znanstveni ~lanek MTAEC9, 51(3)485(2017)
temperature for the safe hot working of HSS. But market
demands for increasing the mechanical properties as well
as for lowering the dimensions of semi-products (also
below # = 14 mm) consequently requires hot rolling at
lower final temperatures down to 850 °C. Therefore, the
aims of our research were to: (i) study the possibility of
extension of hot working range based on optimal selec-
tion of appropriate soaking temperature, (ii) to improve
the microstructure of final rolled profile, (iii) to deter-
mine the main characteristics of the carbides, and (iv) to
determine the activation energy for hot working consi-
dering an extended temperature range.
2 EXPERIMENTAL PART
The chemical composition in mass fractions (w/%) of
the studied M35 HSS was: 0.92C, 0.28Si, 0.35Mn,
4.52Co, 4.11Cr, 4.94Mo, 2,03V, 6.35W, 0.021P and
0.012S. Besides industrial hot rolling of ingot and billet,
three types of laboratory hot compression tests for M35
HSS were conducted. For these tests cylindrical speci-
mens with dimensions of #10 × 15 mm were cut from
as-cast ingot and from a square billet, i.e., in the wrought
state. The first type of (i) hot compression tests carried
out on Gleeble 1500D is related to assessment of an
optimal soaking temperature for as-cast and wrought
initial microstructural states, respectively, using a special
procedure developed by T. Ve~ko Pirtov{ek et al.4 To
determine the optimal soaking temperature or upper tem-
perature limit of safe hot-working specimens were
soaked for 2 h at selected temperature (beginning at 1200
°C) and then compressed at this temperature up to true
strain of 0.9 using constant strain rate of 1 s–1. The upper
safe hot-working temperature is then selected as the
highest temperature where no macro cracks on the
surface of the deformed specimens appeared as well as
appropriate microstructure, i.e., no oversized carbides
and grains, in the deformed specimen was obtained. To
determine the optimal lower safe hot working tempera-
ture, samples were soaked on determined high limit and
then cooled to low deformation temperature (at first to
900 °C, considered appropriate in industrial practice)
and compressed up to true strain of 0.9 using constant
strain rate of 1 s–1. If there were no cracks on the sample
surface, temperature was lowered for 10 °C and proce-
dure repeated until cracks were noticed on the deformed
surface. A safe lower hot deformation limit is then last
low deformation temperature without cracks on the
sample surface. Using this procedure also appropriate
microstructure in compressed samples should be ob-
tained, i.e., carbides growth during soaking should be
minimal, etc. The detailed description of the procedure is
given in 4.
The second type of (ii) hot compression tests is
similar to first type, i.e., including soaking at selected
temperature (beginning at 1200 °C), which was followed
by cooling and hot compression at a selected lower tem-
perature. In this way the influence of the soaking tempe-
rature on the shifting of upper limit of temperature range
of safe hot working was studied.
Furthermore, in order to study the mechanical and
microstructural response on impaired deformation
conditions, especially at lower temperatures, a third type
(iii) of hot compression tests were carried out, i.e., com-
pression in strain rates range 0.001–10 s–1 and tempe-
rature range 1150 °C to 850 °C. During compression
testing the samples were heated with a rate of 3° C/s to
the soaking temperature and holding there for 10 min,
which was then followed by cooling to the deformation
temperature with 2 °C/s and holding on deformation
temperature for 10 min. After deformation the samples
were quenched with water. The flow curves were
determined by considering the temperature compensa-
tion1 for higher strain rates. The microstructure and the
precipitation of carbides on deformed and quenched
samples have been studied and the apparent activation
energy for hot working for an extended temperature
range was calculated.
Optical microscopy (OM, Carl Zeiss AXIO
Imager.A1m) and a field-emission scanning electron
microscope (FE SEM JEOL 6500F) in combination with
the attached energy-dispersive spectroscopy (EDS,
INCA x-SIGHT LN2) and HKL Nordlys II electron
backscatter diffraction (EBSD) camera using Channel5
software analytical tools were applied for observation of
the microstructure and determination of the type of
carbides. The SEM was operated at 15 kV and a 1.5-nA
current for the EDS spot and map analysis as well as for
EBSD spot analysis, with a tilting angle of 70°.
Individual diffraction patterns were obtained with 4×4
binning and using 5–7 bands. For EDS analysis only 15
kV was used to get small analysing volume; however, it
was estimated to be approximately 1 μm3. Therefore,
measuring compositions of carbides we get also
information from the matrix and thus the iron amounts in
the carbides are much higher as it is. Also, the carbon in
carbides is increased due to carbon build up under the
electron beam.
3 RESULTS AND DISCUSSION
3.1 Initial microstructure
In Figure 1a the initial as-cast microstructures with
details in Figure 1b and denoted spots for EBSD
analysis for M35 HSS are presented. The microstructures
consist of martensite, primary, eutectic as well as
secondary carbides. In Figure 2c a Kikuchi pattern for
spot 2 (i.e., eutectic carbides) is given that reveals M2C
(Mo2C – Space group 60, Pbcn, Laue group 3 mmm)
type of carbides. Similar result was obtained also for
spots 1 and 4, while for spots 3 and 5 MC (VC - Space
group 225, F m-3m, Laue group 11 m-3m) type of car-
bides were revealed. Analysis of the distribution of Mo,
W, V, Cr reveals that eutectic carbides (spots 1, 2 and 4)
are mainly composed of Mo (Figure 1d) as well as of W.
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486 Materiali in tehnologije / Materials and technology 51 (2017) 3, 485–491
MATERIALI IN TEHNOLOGIJE/MATERIALS AND TECHNOLOGY (1967–2017) – 50 LET/50 YEARS
On the other hand, the primary carbides (spots 3 and 5)
are mainly composed of V (Figure 1e). Analysis of
chemical composition on spots denoted in Figure 2b as
well as type of carbides is given in Table 1. Thus eutec-
tic carbides are M2C type, while the primary carbides are
of MC type. The wrought microstructure is given in
Figure 1f. From this figure it is visible that the eutectic
carbides are broken down and the size of grains is
decreased in comparison to the microstructure given in
Figure 1a.
Table 1: Average chemical compositions of the analysed carbides on
spots denoted in Figure 1b in mass fractions, (w/%)
Tabela 1: Povpre~na kemijska sestava analiziranih karbidov na
mestih, ki so ozna~ena na Sliki 1b, v masnih dele`ih, (w/%)
C V Cr Fe Co Mo W Type
Spots 1,2,4 8.9 9.2 5.1 6.1 - 35.4 35.5 M2C
Spots 3,5 15.3 37.6 3.3 6.7 - 17.0 20.1 MC
3.2 Obtained optimal soaking temperatures and flow
curves
Using the procedure developed4 the optimal soaking
temperatures for as-cast and for wrought states were
assessed to be 1150 °C and 1160 °C, respectively. This
enables safe hot working within the temperature range
between assessed optimal soaking temperature and 850
°C. Our study also revealed, i.e., using the second type of
hot compression tests, that by using higher soaking
temperature (e.g., 1200 °C), at upper temperature limit of
safe hot working is lower by about 20 °C, while the
lower temperature limit is about 50 °C higher. Further-
more, it was found that at optimal soaking temperatures
determined in the present work and soaking time of 2h,
the carbides did not exhibit extensive growth. In Figures
2a to 2d flow curves (i.e., the wrought state) for the
temperature range 1150–850 °C and strain rate range
0.001–1.0 s–1 are given. Achieving the peak values with
an increasing of the strain rate and decreasing of the
temperature suggests that dynamic recrystallization has
taken place during the hot deformation, which was also
confirmed by the inspection of the microstructures.
The strain-rate and temperature dependences of the
peak stress were determined using the hyperbolic sine
Equation (1)25:
Z
Q
RT
A n= ⋅ ⎛⎝
⎜ ⎞
⎠
⎟ = exp (sin )
 	 (1)
where Z is the Zener-Hollomon parameter, R is the gas
constant, Q is the apparent activation energy for hot
working, and A, n and 	 are constants. The activation
energies as well as the constants in this Equation (1)
were determined using the procedure developed by G.
Kugler et al.26 The values of all constants and activation
energy are given Figure 3a. Thus the apparent
activation energy for hot deformation amounts to 639
kJ/mol. That is slightly higher in comparison to values
found in literature for other HSSs. Namely, C. A. Imbert
et al.2 obtained a value for the activation energy of 455
kJ/mol for M2 HSS, while J. Liu et al.1 obtained values
of 467 and 654 kJ/mol for the upper and lower tempe-
rature ranges, respectively, for T1 HSS. The comparison
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MATERIALI IN TEHNOLOGIJE/MATERIALS AND TECHNOLOGY (1967–2017) – 50 LET/50 YEARS
Figure 1: a) Initial microstructure of as-cast specimen from ingot with
detail and b) denotes spots for EBSD and c) EDS analysis Kikuchi
pattern for spot 2 (M2C type of carbides), d) distribution of Mo, e)
distribution of V and f) wrought microstructure
Slika 1: a) Za~etna mikrostruktura litega stanja za vzorce vzete iz
ingota z detajlom in b) ozna~ena mesta za EBSD in c) EDS-analiza
Kikuchijevih vzorcev za to~ko 2 (M2C tip karbidov), d) porazdelitev
Mo, e) porazdelitev V in f) predelana mikrostruktura
Figure 2: Flow curves for M35 HSS for temperature range 850–1150
°C and different strain rates: a) for strain rate of 0.001 s–1, b) for strain
rate 0.01 s–1, c) for strain rate 0.1 s–1, d) strain rate of 1 s–1, wrought
state
Slika 2: Krivulje te~enja za hitrorezno jeklo M35 v temperaturnem
obmo~ju 850–1150 °C pri razli~nih hitrostih deformacije: a) 0.001 s–1,
b) 0.01 s–1, c) 0.1 s–1, d) 1 s–1; predelano stanje
between the measured and calculated peak stresses is
given in Figure 3, where very good agreement was
obtained. A slight deviation can be observed at 850 °C
for the lowest strain rate, i.e., 0.001 s–1, which can be
attributed to an increased precipitation of secondary
carbides (Figure 4a) under these thermo-mechanical
conditions. Furthermore, more intensive precipitation of
the secondary carbides begins at temperatures below
1050 °C, while above it precipitation was almost not
observed (Figure 4b for the deformation temperature of
1100 °C).
Since for both microstructural states in the deforma-
tion chain, i.e., ingot hot rolling as well as billet defor-
mation, soakings were carried out at previous assessed
appropriate temperatures, small sized carbides were
expected in all the specimens deformed at 850 °C.
Namely, soaking at the appropriate temperature does not
lead to oversized carbides.4 Soakings before the defor-
mation of the ingot (as-cast state) were carried out at
1150 °C, while for a billet (wrought state) this tempera-
ture was 1160 °C. Very similar microstructures for strain
rates of 0.01 s–1 and above (for strain rate of 0.01 s–1,
Figure 5a) at a deformation temperature of 850 °C were
obtained. From Figure 5a it can be further visible that
small sized carbides are mainly below 3 μm, which also
indicates good mechanical properties for the further
application of the M35 HSS. At deformations at higher
temperatures, i.e., 1050 °C, slightly increased coarsening
of carbides can be observed at lower strain rates (Figure
5b) in comparison to higher strain rates, i.e., 1 s–1
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Figure 5: SEI micrographs for compressed specimens at different temperatures and strain rates: a) 850 °C and strain rate of 0.01s-1, b) 1050 °C
and strain rate of 0.01 s–1, c) 1050 °C and strain rate of 1 s–1
Slika 5: SEI mikrostrukture za tla~no deformiran vzorec pri razli~nih temperaturah in hitrostih deformacije: a) 850 °C in hitrost deformacije
0.01s–1, b) 1050 °C in hitrost deformacije 0.01 s–1, c) 1050 °C in hitrost deformacije 1 s–1
Figure 4: Precipitation of secondary carbides at lower strain rates;
intensive precipitation at lower temperatures (850 °C) and decreased
precipitation at higher temperatures (1100 °C)
Slika 4: Izlo~anje sekundarnih karbidov pri ni`jih hitrostih deforma-
cije; intenzivno izlo~anje pri ni`jih temperaturah (850 °C) ter manj
intenzivno izlo~anje pri vi{jih temperaturah (1100 °C)
Figure 3: Comparison between measured and calculated peak values
of the flow curves at different temperatures and strain rates
Slika 3: Primerjava med izmerjenimi in izra~unanimi vrednostmi za
maksimalne napetosti te~enja pri razli~nih temperaturah in hitrostih
deformacije
(Figure 5c). This can be attributed to the more time that
is available for an increase of their size.
For samples deformed at 850 °C the analyses of the
chemical compositions of the carbides were carried out
together with the determination of their types. These
results are shown in Figure 6 and Table 2. Typical
image of presented carbides in the matrix with denoted
points for chemical analysis is shown in Figure 6a.
Analysis of spectrum 5 reveals that Mo (19.2 %) and W
(36.7 %) are the main elements in carbides with relative
increased size that is further confirmed in Figures 6b to
6c, where distributions of both elements, respectively,
are presented. The amount of V in this case is relatively
low, i.e., 2.97 %. On the other hand, an analysis of spec-
trums 3 and 4 revealed that in these carbides V, i.e.,
26.21 % and 20.28 %, respectively, and Mo, i.e., 11.3
and 10.4 %, respectively, and W, i.e., 14.8 % and 13.7 %,
respectively, are the most important carbide-forming
elements. The distribution of V on these spots is given in
Figure 6d. The amounts of Fe for spectrums 3 and 4
were 28.7 % and 31.8 %, while for spectrums 1 and 2
these values were 92.9 and 67.1 %, respectively, but with
much lower amounts of V, Mo and W in comparison to
spectrums 3 and 5. Thus, on spots 1–2 the amounts of
these elements were 5.7 and 2.6 %, 5.8 and 6.4 %, and
7.1 and 7.4 %, i.e., for V, Mo and W, respectively. The
analysis further revealed that the carbide on spectrum 5
refers to the M6C type (Kikuchi pattern given on Figure
6e), on spectrums 1, 3 and 4 to MC type (Kikuchi pattern
given on Figure 6f), while on spectrum 2, this refers to
the M23C6 type. From the distribution of the chemical
elements in the carbides it can be concluded that in-
creased sized carbides refer to M6C type. A comparison
of the results related to the carbides for as-cast state
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Figure 7: Coalescence of carbides of different types (a-b)
Slika 7: Koalescenca razli~nih tipov karbidov (a-b)
Figure 6: SEI image of carbides distribution with denoted spots for:
a) chemical and EBSD analysis on deformed sample, b) distribution of
Mo, c) distribution of W and d) distribution of V, e) Kikuchi pattern
for M6C type (spectrum 5) and for: f) MC type of carbides (spectrum
4) deformation temperature 850 °C
Slika 6: SEI slika prostorske porazdelitve karbidov z ozna~enimi
mesti za: a) kemijsko in EBSD analizo vzorcev deformiranih pri
temperaturi 850 °C, b) porazdelitev Mo, c) porazdelitev W in d)
porazdelitev V, e) Kikuchijevi vzorci za M6C tip karbidov (spekter 5)
in za: f) MC tip karbidov (spekter 4)
Table 2: Chemical composition in mass fractions, (w/%) on denoted spots given in Figure 5a
Tabela 2: Kemijska sestava v masnih dele`ih, (w/%), na ozna~enih mestih, ki so podana na Sliki 5a
C V Cr Fe Co Mo W Type Type
Spectrum 1 10.3 5.7 4.1 62.9 4.2 5.8 7.1 MC VC
Spectrum 2 7.7 2.6 3.9 67.1 4.8 6.4 7.4 M23C6 Cr23C6
Spectrum 3 12.4 26.2 3.9 29.7 1.8 11.3 14.8 MC VC
Spectrum 4 14.4 20.3 3.6 31.8 2.1 10.4 13.7 MC VC
Spectrum 5 7.4 3.0 2.7 28.7 2.4 19.2 36.7 M6C Fe3W3C
(Figure 1 and Table 1), where M2C and MC types of
carbides were found, with those for deformed state
(Table 2), where three types of carbides, i.e. M6C, MC
and M23C6, were revealed, implies that M2C type of car-
bides were transformed into M6C and MC types.4 The
occurrence of the M23C6 type in the wrought microstruc-
ture can be attributed to the precipitation of secondary
carbides.
Here, it is worth to mention that also the coalescence
(unification) of different types of carbides was observed
in the sample deformed at 850 °C, i.e., predominately
M6C type (Figure 7a, spots 1-2) and MC type (Figure
7a, spot 3). This can be seen in Figures 7a to b. Thus in
the final microstructure of M35 HSS very complex
carbides are presented. Industrial practice3 confirms our
laboratory results that are related to improving the hot
deformability, expanded range of safe hot deformation
and size of carbides.
4 CONCLUSIONS
To increase the hot workability of M35 HSS indus-
trial hot rolling and hot compression tests were carried
out, which was supported with an analysis of the micro-
structure. From the obtained results the following con-
clusions can be derived:
• The appropriate soaking temperature depends on the
initial microstructural state of M35 HSS, where for
as-cast state this value is slightly lower in comparison
to the wrought state, i.e., 1150 and 1160 °C, respec-
tively.
• The selection of the appropriate soaking temperature
has a decisive influence on the characteristics of car-
bides, on the hot workability, and on expanding the
temperature range for safe hot working, i.e., at its
upper and lower limits. In consequence, this influ-
ences the mechanical properties of the final product.
The selection of the appropriate soaking temperature
for both microstructural states in process chain of hot
rolling led to a decreased size of the carbides in final
product, i.e., approximately 3 μm. Safe hot rolling up
to temperatures around 850 °C is thus assured, which
enable hot rolling of profiles with smaller cross-
sections.
• M2C and MC types of carbides were found in as-cast
microstructure.
• M6C, MC and M23C6 types of carbides were found in
the final microstructures of compressed samples that
deformed at 850 °C. Carbides of type M6C as largest
carbides reached a size of around 3 μm. W and Mo
are the main carbide-forming elements in these types
of carbides. Smaller-sized carbides refer to the MC
type where V is the main carbide-forming element.
• Unification of different types of carbides takes place,
i.e., predominately M6C and MC, which results in
very complex compositions of carbides.
• The calculated apparent activation energy for hot
working amounts to 639 kJ/mol.
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