-
in
, B.
Leib
Insti
00 C
epa
land
of
con
l tra
ond
diag
the
sion
complete process chains are becoming increasingly moreimportant
since their use can reduce the extent of laborious
composition, the transformation kinetics change, possibly tosuch
a significant degree that the diagrams lose their validity
DOI: 10.1002/srin.200900132 steel research int. 81 (2010) No.
3The latter exist in the form of data-collections for a
largematerial parameters and thereby particularly of the
con-tinuous cooling transformation diagrams (CCT diagrams).
transformations occur, are described in the literature by,
forexample, Kaspar et al. [8]. For instance, for hot formingat 770
8C with a deformation of 1.25, an acceleratedsubsequent
heat-treatment using the heat of hot forming.This integrated
heat-treatment, which is investigated withinthe framework of the
Collaborative Research Centre 489 atthe Leibniz Universitat
Hannover [1], is characterised byflashless, near-net shape forged
components (see Figure 1).Therefore, the usual post-forge
processing of conventionalforging methods can be dispensed
with.
The remaining heat in the forged component following hotforming
is then used for the subsequent heat-treatment [2].Suitable
materials for precision forged, high-performancecomponents like
gear wheels or crankshafts are heat-treatable steels since, owing
to their carbon content, theycan be surface hardened during the
cooling from the hot-forming temperature.
The key to successfully experimentally or numericallypredicting
heat-treating processes is generally a knowledgeof the
microstructural and temperature dependency of the
both a retardation of the pearlite transformation, initiatedfrom
the austenite grain boundaries, as well as a fall in themartensite
start-temperature [5].
Within the framework of investigations on the through-heat
treatability of heavy forgings, Bandel and Haumer [6]were able to
identify the quantitative influence of theaustenites grain size on
the transformation behaviour ofdifferent steels. They observed that
increasing the austen-ites grain sizes leads to extended initiation
and totaltransformation times for the isothermal pearlitic
trans-formations (see Figure 2). Contrastingly, the
bainitictransformations were not influenced.
Thermomechanical influences on the transformationbehaviour.
Phase transformations in steel, which areretarded or accelerated by
thermomechanical processing,or changes in the temperature intervals
during which theseprocesses; as for example during precision
forging withexperimental investigations. Besides saving costs,
theirimplementation enhances an understanding of the processand
promotes the potential for optimisation. This is anadvantage
particularly during the realisation of new types of
under these conditions.
Influence of austenitising. The carbides dissolve withincreasing
austenitising temperature and duration afterwhich grain growth
commences. Enlarged grains causeMicrostructure Transformations in
TemperHot Forging Temperatures
F. Nurnberger1)*, O. Grydin1), M. Schaper1), F.-W. Bach1)
1) Faculty of Mechanical Engineering, Institute of Materials
Science,
Germany, [email protected]) Faculty of Materials
Processing Technology and Applied Physics,
Czestochowa University of Technology, Al. Armii Krajowej 19,
42/23) Faculty of Metals, Engineering and Industrial Computer
Science, D
of Science and Technology, Al. Mickiewicza 30, 30059 Krakow,
Po
* Corresponding author
Apart from reducing the processing energy, hardening and
tempering
promises shortened conventional process sequences. In this case,
the
literature can only be used as a rough approximation of
microstructura
firstly, the deformation influences the transformation kinetics
and sec
both deformation CCT diagrams and, for reference purposes,
CCT
treatable steels (34CrMo4, 42CrMo4, 52CrMo4, 51CrV4, 34NiMo6)
at
Keywords: Deformation CCT diagrams, heat-treatable steels,
preci
Submitted on 27 September 2009, accepted on 28 December 2009
Introduction
Procedures for the numerical design of process-steps or224 2010
Wiley-VCH Verlag GmbH & Co. KGaA, Weing Steels during
Continuous Cooling from
Koczurkiewicz2), and A. Milenin3)
niz Universitat Hannover, An der Universitat 2, 30823
Garbsen,
tute of Modelling and Automation of Plastic Forming
Processes,
zestochowa, Poland
rtment of Applied Computer Science and Modelling, AGH
University
near-net shape forged components from their forging heat
primarily
tinuous cooling transformation diagrams (CCT diagrams) found in
the
nsformations during the heat-treatment. The reasons for this are
that
ly, the deformation temperatures are comparatively high.
Therefore,
rams without deformation were determined for a selection of
heat-
heating temperature of 1200 8C and deformation levels of 0.3 and
0.6.
forging, integrated heat-treatment
number of common alloys [3] and [4]. However, dependingon
process factors such as austenitising conditions, hotforming or
charge influences, that is, deviations of chemicalheim
www.steelresearch-journal.com
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Materials Technology steel research int. 81 (2010) No. 3Figure
1. Gear wheel and pinion shaft of the Collaborative Research
Centre489,precisionforgedat
theInstituteofMetalFormingandMetal
Forming Machines, Leibniz Universitat Hannover.transformation of
austenite to ferrite and pearlite is shown inthe CCT diagram of a
hypoeutectoid steel owing toadditional nucleation points such as
lattice defects, slipbands and increased grain boundary densities
(seeFigure 3).If thereby the fraction of bainite formed at the same
coolingrate falls, then the corresponding microstructural hardness
islowered.
If high dislocation densities, such as those with austenitichot
forming below the recrystallisation temperature, aretransferred to
the bainitic or martensitic transformedmicrostructure, very fine
microstructures are formed [9].
Investigations on the heat-treatable steel 42CrMo4by Weise show
accelerated ferritic-pearlitic transforma-tions and correspondingly
increasing transformation
Figure 2. Influence of the austenites grain size, according to
SEP
1510-61 [7], on the transformationof28 NiCrMo74 steel to
thepearlite
stage [6].
Figure 3. Influence of hot forming at 7708C on the CCT diagram
of a0.45% C steel (strain 1.25) [8].
www.steelresearch-journal.com 2010 Wtemperatures with increasing
deformation at deformationtemperatures of 850 8C [10]. Whereas low
deformations(approx. 30%) show no effect on the bainite regions,
thebainitic transformation is translated to shorter times athigher
deformations of approx. 60%. Furthermore, a hardermicrostructure
can be observed with increasing deforma-tion. Despite reports of
various authors [8] and [1113],Weise could establish no lowering of
the martensitic start-temperature as a consequence of hot forming.
Kaspar et al.point out that, in the literature, increasing
martensite start-temperatures have been occasionally observed [14]
and [15].
Jost and Hornbogen investigated the dependency of themartensite
start-temperature on the reduction of platethickness of a FeNiCoTi
alloy [16]. Using transmissionelectron-microscopical analyses, they
were able to establishthat the martensite start-temperature fell
during deformationif homogenous dislocation structures are
produced. Atcomparatively low temperatures, at which
inhomogeneousdislocation structures like slip bands existed, they
were ableto establish a rise in the martensite start-temperature
withincreasing deformation.
Kruglova et al. investigated the influence of hot formingon the
transformation behaviour of low-alloyed low-carbonsteels [17].
According to the alloys composition, deforma-tion and austenitising
condition, they were able to establish asmall rise in the
transformation temperatures of ferrite topearlite, a lowering or a
rise in the bainite onset and end-temperatures as well as an
acceleration or retardation of thepearlitic or bainitic
transformations. Here, high austenitisingtemperatures of 1200 8C
cause, for example, a lowering ofthe Ar3-temperature and a widening
of the temperatureinterval of bainite formation. Slight changes of
the trans-formation regions are mainly depicted in the CCT
diagrams.A modification of the microstructure could often
beobserved such as, for example, the formation of needleshaped
instead of granular bainite on austenitising at1200 8C and
subsequently deforming at 920 8C. In regionsof critical deformation
(approx. 5%), partial grain growthled to inhomogeneous structures
of non-uniform sizedistribution.
Objective. The process of precision forging is charac-terised by
bulk deformation at comparatively high defor-mation temperatures of
approx. 1200 8C. The transformationkinetics of the employed alloys
on cooling from the hot-forming temperature is not yet known for
the prevailingprocess parameters. For this reason, the objective of
theauthors was to characterise the transformation behaviour
ofdifferent heat-treatable steels, which are suitable for
surfacehardening, by means of deformation dependent
continuouscooling transformation diagrams.
Experimental Set-up
The alloys 34CrMo4 (1.7220), 42CrMo4 (1.7225),50CrMo4 (1.7228),
51CrV4 (1.8159) and 34CrNiMo6(1.6582) were used to construct the
deformation CCTdiagrams. The chemical compositions of the
investigatediley-VCH Verlag GmbH & Co. KGaA, Weinheim 225
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steel research int. 81 (2010) No. 3 Materials Technologymelts
are given in Table 1, measured by glow dischargeoptical emission
spectroscopy (GDOES, type GDA 550,company Spectruma) and by a
N,O,H-analyzer (TCH600,company Leco).
To simulate the precision forging process, the materialspecimens
of the investigated melts (5 mm diameter, 10 mmlong) were heated
within 30 s to an austenitising temperatureof 1200 8C. Following
isothermal soaking for 600 s, acompressive deformation of 30% or
60%, as the case maybe, was carried out at a deformation speed of 1
s1.Following this, the samples were linearly cooled using
Table 1. Chemical composition of the investigated melts in
weight %.
Alloy 34CrMo4 42CrMo4 50CrMo4 51CrV4 34CrNiMo6
C 0.325 0.410 0.491 0.467 0.326
Si 0.289 0.336 0.212 0.223 0.263
Mn 0.577 0.701 0.647 0.845 0.588
P 0.007 0.011 0.005 0.006 0.001
S 0.003 0.025 0.004 0.017 0.004
Cr 0.945 0.998 1.039 1.015 1.433
Cu 0.286 0.380 0.219 0.223 0.260
Mo 0.132 0.171 0.133 0.013 0.126
Ni 0.098 0.191 0.092 0.083 1.469
Al 0.025 0.029 0.024 0.018 0.023
Nb 0.019 0.019 0.020 0.063 0.020
N 0.0092 0.0112 0.0114 0.0090 0.0076cooling rates in the range
from 80 K s1 to 0.02 K s1. Forreference purposes, diagrams were
constructed withoutdeformation. Approx. 15 dilatometer measurements
fordifferent cooling rates were taken for each deformation
CCTdiagram. In the depicted curves, the start and end
ofmicrostructure transformations were manually constructed.
The dilatometric measurements were carried out at theTechnical
University Czestochowa, Poland using a defor-mation dilatometer DIL
805 A/D manufactured by BahrThermoanalyse GmbH. The investigations
were carried outaccording to the Polish standard PN-68/H-04500
[18],which essentially corresponds to the German
Stahl-Eisen-Prufblattern SEP 1680 or SEP 1681 [19] and [20].Where
possible, the amount of the microstructures phaseswas determined by
analysing the brightness distribution(histogram) of one or more
micrographs. For cases of threeor more different coexisting phases,
such measurement wasnot always definitive. Hence, the amount of
phases wasestimated manually in a microscope by using
predefinedmicrographs, containing known amounts of differentphases,
for comparison. Thus regarding the microstructureconstituents
quantity, the percentage error in the measure-ment for each phase
can be assumed to be single-digit.
226 2010 Wiley-VCH Verlag GmbH & Co. KGaA, WeinA linear
cooling mode was selected for constructing thedeformation CCT
diagrams since the measurement time forconstructing continuous
diagrams is then considerablyreduced, compared to isothermal
diagrams, so that a largerparameter range could be investigated. If
knowledge of theisothermal transformation kinetics is required for
theprocess design, different methods are available in theliterature
to compute isothermal [21, 22] and [23] or idealDiagrams [24] from
the CCT diagrams.
Owing to the specimens small geometric size,
hardnessmeasurements were carried out at only three
differentpositions. That is, given hardness values are mean values
forthree indentations. The relative error of measurement
wascalculated according to Students coefficient proposed byGosset
[25]. A confidence probability of 90% was usedwhich resulted in a
mean value of the relative error regardingall measurements of about
6.1% of the hardness value. Theposition of the hardness measurement
was chosen accordingto [26] in the following manner: (i) The
specimens wereembedded, longitudinally ground until the layer at
the centre(rotational axis) was reached and then
subsequentlypolished. (ii) Positions for hardness indentation were
about1/3 of radius and 1/3 of specimen length.
Results
For reasons of clarity, but contrary to the German
Stahl-Eisen-Prufblatter guidelines, the deformation CCT dia-grams
depicted in the following are not provided withhardness values. For
the same reason, a range from 200 8C to900 8C was selected for the
temperature profiles. In additionto this, phase regions are
depicted separately even if thetemperature difference between the
phases is less than 50 K.
Based on the selected minimum cooling rate of 0.02 K s1,only a
few data points in part exist for the ferrite or, as thecase may
be, the pearlite regions so that the depicted curvestrajectories
represent an estimate. Lower cooling rates arenot technically
relevant for precision forging with integratedheat-treatment since
this cooling rate already corresponds toa cooling period of more
than 16 h to reach room temperaturefrom the deformation
temperature.
34CrMo4. The CCT diagram for the material 34CrMo4(see Figure 4,
full-line) shows ferritic, pearlitic, bainitic andmartensitic
components.
The critical cooling rate observed, i.e. the rate at whichjust
about 100% martensite is formed, is 15 K s1. Bainite isformed at
cooling rates of between 6 K s1 and 0.15 K s1.Ferrite and pearlite
can be established at cooling rates lowerthan 1 K s1 and 0.6 K s1,
respectively (see also Table 2).Between the regions of martensite
and bainite formationthere is a relatively large temperature
interval which is freeof transformations. Furthermore, there are
smaller trans-formation-free intervals between the bainite and
pearlite aswell as the pearlite and ferrite phase regions.
A deformation of 30% (broken-line) shows, just as muchan
increase to 60% (dotted-line), merely slight changes ofheim
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Materials Technology steel research int. 81 (2010) No. 3
HV 564 563 433 288 278 267 235 205 174
60% strain
F 5 10 15 22 22 23
P 10 30 78 78 77
2)tem
pera
ture
in
Cte
mpe
ratu
re in
C
ferrite
Martensit
Bainit
Perlit
martensite
bainite
pearlite
10 1 10 410 2 10 3 10 5
900
600
400
200
300
500
700
800
30 %60 %
time in s10 1 10 410 2 10 3
900
600
400
200
300
500
700
800
0 % 30 %
Ac1
Ac3
austenite
Figure 4. Deformation CCT diagrams for the material 34CrMo4
with
0%, 30% and 60% deformation at an austenitising temperature
of
1200 8C.the transformation behaviour except for a rise of
themartensite start-temperature.
This is also demonstrated by the small changes of hardnessvalues
for increasing deformation. A comparison of themicrostructural
components formed for different deforma-tions shows a slight
retardation in the bainite formation and,as a consequence, a
smaller fraction of pearlite for a coolingrate of 0.3 K s1 (see
Table 2 and Figure 5, right).
At this cooling rate, the pearlitic structures slightlydecrease
in size with the deformation level. In contrast, thealmost
identical microstructures are shown for the coolingrate of 0.6 K s1
and varied deformation levels forcomparison.
42CrMo4. The CCT diagram for the material 42CrMo4(see Figure 6,
full-line) shows ferritic, pearlitic, bainitic andmartensitic
components. The critical cooling rate observedis 6 K s1.
Bainite is formed at cooling rates of between 1 K s1 and0.07 K
s1. Ferrite and pearlite can be established at coolingrates from
0.07 K s1 and less (see Table 3). Between theregions of martensite
and bainite formation as well asbetween bainite and pearlite
formation there is a temperatureinterval which is free of
transformations.
In comparison to the CCT diagram, a deformation of
30%(broken-line) shows merely slight changes of the trans-formation
behaviour except for a rise of the martensite start-temperature. It
turns out that the ferrite and pearlite
www.steelresearch-journal.com 2010 WTable 2. Metallographically
determined microstructural com-ponents together with hardness
values measured on the specimensof the heat-treatable steel 34CrMo4
for various linear cooling rates(Ks1) and deformation levels (F:
ferrite, P: pearlite, B: bainite, M:martensite, HV: Vickers
hardness in HV10).
34CrMo4
K s1 30 15 6 1 0.6 0.3 0.15 0.07 0.04
0% strain
F 1 10 18 21 24 31
P 3 67 71 76 69
B < 1 < 1 80 99 87 15 81)
M 100 100 20
HV 556 543 463 301 272 240 243 198 165
30% strain
F 5 9 13 28 32
P 2 26 87 72 68
B 35 100 93 65
M 100 100 65 formation temperatures have a tendency to fall with
thedeformation in addition to initiating the transformationearlier.
However, owing to the small number of data values,an interpretation
of the ferritic and pearlitic formationsinfluence due to
deformation can only be approximated.
In comparison to the transformation behaviour of meltone, a
ferrite, pearlite and bainite formation appears to beretarded to a
later time.
A comparison of the microstructural components formedduring
deformation shows an increased bainite content at acooling rate of
1 K s1 following deformation and a lowfraction of pearlite at a
cooling rate of 0.07 K s1 (see Table 3andFigure 7 - right). The
microstructures hardness tends todecrease owing to the slightly
accelerated microstructuraltransformation due to the
deformation.
Figure 7 - left, clearly illustrates the reduction of
theaustenites grain size owing to the deformation by meansof the
extended martensitic structures at a cooling rate of15 K s1.
50CrMo4. The CCT diagram for the material 50CrMo4(see Figure 8,
full-line) shows ferritic, pearlitic, bainitic and
B 4 50 95 80 55
M 100 96 50
HV 581 557 432 287 280 274 225 184 184
1)Bainite within segregations2)Bainite not detected during
dilatometric measurements
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steel research int. 81 (2010) No. 3 Materials Technology
Figure 5. Microstructure of the heat-treatable steel 34CrMo4 for
the
cooling rates 0.6 K s1 (left) and 0.3 K s1 (right),
respectively. Above:Without deformation, middle: Deformation level
30%, below: Defor-
mation level 60%.
Figure 6. Deformation CCT diagrams for the material 42CrMo4
after
0%, 30% and 60% deformation, austenitising temperature 1200
8C.
228 2010 Wiley-VCH Verlag GmbH & Co. KGaA, WeinTable 3.
Metallographically determined microstructural compo-nents together
with hardness values measured on the specimensof the heat-treatable
steel 42CrMo4 for various linear cooling rates(Ks1) and deformation
levels. (F: ferrite, P: pearlite, B: bainite,M: martensite, HV:
Vickers hardness in HV10).
42CrMo4
K s1 30 15 6 1 0.6 0.3 0.15 0.07 0.04
0% strain
F 4 4
P 60 96
B 56 60 90 100 36
M 100 100 100 44 40 10
HV 727 704 732 467 385 347 322 313 253
30% strain
F 1 2 15
P 5 45 85
B traces 70 95 100 94 53
M 100 100 100 30 5
HV 686 699 690 455 356 329 309 317 216
60% strainmartensitic components. The critical cooling rate
observedis 15 K s1. Bainite is formed at cooling rates of between6
K s1 and 0.3 K s1. Ferrite and pearlite can be establishedat
cooling rates from 1 K s1 and less (see Table 4).
Between the regions of martensite and bainite formationas well
as between bainite and pearlite formation there is atemperature
interval which is free of transformations.
A deformation of 30% (broken-line) causes an increase ofthe
martensite start-temperature, a slight retardation of theferrites
and pearlites formation as well as an increase of thebainite
formation temperature. It appears that the ferrite andpearlite
formation temperatures have a tendency to fall witha deformation of
60%. In comparison to the transformationbehaviour of melts one and
two, a reduced temperatureinterval of bainite formation exists. A
reduced influence ofthe deformation on the microstructural hardness
isdemonstrated.
A comparison of the microstructural components formedfor
different deformations shows a reduced bainite contentfor a cooling
rate of 0.3 K s1 and an increased fraction ofpearlite (see Table 4
and Figure 9, right). By means of thelength of the bainite plates
in Figure 9, right, it is clear thatthe bainitic microstructure
exhibits smaller dimensions withincreasing deformation.
F < 1 1 2
P 2 8 95
B 65 90 98 98 91 3
M 100 100 35 10 2
HV 681 685 472 373 334 322 328 322
heim www.steelresearch-journal.com
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Materials Technology steel research int. 81 (2010) No. 3
Figure 7. Microstructure of the heat-treatable steel 42CrMo4 for
the
cooling rates 15 K s1 (left) and 0.07 K s1 (right) respectively.
Above:Without deformation, middle: Deformation level 30%, below:
Defor-
mation level 60%.
Figure 8. Deformation CCT diagrams for the material 50CrMo4
with
0%, 30% and 60% deformation, austenitising temperature 1200
8C.
www.steelresearch-journal.com 2010 WTable 4. Metallographically
determined microstructural compo-nents together with hardness
values measured on the specimensof the heat-treatable steel 50CrMo4
for various linear cooling rates(Ks1) and deformation levels. (F:
ferrite, P: pearlite, B: bainite,M: martensite, HV: Vickers
hardness in HV10).
50CrMo4
K s1 30 15 6 1 0.6 0.3 0.15 0.07 0.04
0% strain
F < 1 1 1 1 6 11
P 2 6 15 99 94 89
B 10 96 91 81
M 100 100 90 2 2 3
HV 747 703 682 316 320 340 307 236 192
30% strain
F 1 1 1 3 4
P 5 40 99 97 96
B 10 99 93 59
M 100 100 90 1 1
HV 704 712 653 312 318 330 299 241 20251CrV4. The CCT diagram
for the 51CrV4 material(see Figure 10, sketched in grey) shows no
ferriticmicrostructural components in the
metallographicallyprepared specimens so that the phase regions
martensite,bainite and pearlite are depicted in the deformation
CCTdiagrams. The critical cooling rate observed is 6 K s1.Bainite
is formed at cooling rates of between 1 K s1 and0.6 K s1. Pearlite
can be established at cooling rates from0.3 K s1 and less (see
Table 5). Between the regions ofmartensite and bainite formation as
well as between bainiteand pearlite formation are temperature
intervals which arefree of transformations.
A deformation of 30% (broken-line) shows an increase inthe
martensite start-temperature and in the bainitesformation
temperature. It appears that the pearlite regionhas a tendency to
be accelerated and its temperature intervalto be widened. No
tendency can be discerned that themicrostructural hardness value
increases owing to thedeformation. In comparison to the
transformation behaviourof melts one, two and three, a reduced
temperature intervalof bainite formation is exhibited.
A comparison of the microstructural components formedfor
different deformations shows a reduced bainite
60% strain
F < 1 1 2 5
P 7 52 98 95
B 4 94 90 37
M 100 100 96 6 3 101)
HV 692 734 652 343 318 332 301 217
1)Martensite within segregations
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steel research int. 81 (2010) No. 3 Materials Technology
Figure 9. Microstructure of the heat-treatable steel 50CrMo4 for
the
cooling rates 6 K s1 (left) and 0.3 K s1 (right) respectively.
Above:Without deformation, middle: Deformation level 30%, below:
Defor-
mation level 60%.
Figure 10. Deformation CCT diagrams for the material 51CrV4
with
0%, 30% and 60% deformation, austenitising temperature 1200
8C.
230 2010 Wiley-VCH Verlag GmbH & Co. KGaA, WeinTable 5.
Metallographically determined microstructural compo-nents together
with hardness values measured on the specimensof the heat-treatable
steel 51CrV4 for various linear cooling rates(Ks1) and deformation
levels. (P: pearlite, B: bainite, M: martensite,HV: Vickers
hardness in HV10).
51CrV4
K s1 80 50 30 15 6 1 0.6 0.3 0.15
0% strain
P 3 100 100
B 98 97
M 100 100 100 100 100 2
HV 715 736 802 735 727 404 349 360 338
30% strain
P 10 50 100 100
B 88 50
M 100 100 100 100 100 2
HV 746 717 730 729 741 384 385 361 328
60% strain
P 5 50 100 100
B 91 50
M 100 100 100 100 100 4 content for a cooling rate of 0.6 K s1
and an increasedfraction of pearlite due to deformation (see Table
5 andFigure 11 - right). The usual needle shaped bainiticstructure
is depicted in Figure 11 - left, for a cooling rateof 1 K s1 and
various deformations.
34CrNiMo6. In the case of material 34CrNiMo6, theselected
minimum cooling rates were too rapid to bringabout pearlite
formation so that the depicted phase regionsinvolve martensite,
bainite and ferrite (see Figure 12).
The critical cooling rate used above is 6 K s1. Bainitecould be
measured at cooling rates lower than 1 K s1 andforms the main
microstructural component up to coolingrates of 0.02 K s1. Ferrite
can be established for the firsttime at cooling rates below 0.04 K
s1 (see Table 6).Between the regions of martensite and bainite
formation aswell as between bainite and pearlite formation there is
atemperature interval which is free of transformations.
A deformation of 30% shows a slight increase of themartensite
start-temperature and a considerable temperaturejump of the
bainites formation temperature. In comparisonto the transformation
behaviour of melts one, two, three andfour, a widened interval of
bainite formation is shown at latertimes. Owing to the deformation,
the microstructureshardness tends to slightly increase.
HV 813 757 741 746 743 379 385 364 336
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Materials Technology steel research int. 81 (2010) No. 3
Figure 11. Microstructure of the heat-treatable steel 51CrV4 for
the
cooling rates 6 K s1 (left) and 0.6 K s1 (right) respectively.
Above:Without deformation, middle: Deformation level 30%, below:
Defor-
mation level 60%.
Figure 12. Deformation CCT diagrams for the material
34CrNiMo6
with0%, 30% and 60%deformation,austenitising temperature1200
8C.
Table 6. Metallographically determined microstructural
com-ponents together with hardness values measured on the
specimensof the heat-treatable steel 34CrNiMo6 for various linear
cooling rates(Ks1) and deformation levels. (F: ferrite, B: bainite,
M: martensite,HV: Vickers hardness in HV10).
34CrNiMo6
K s1 30 15 6 1 0.6 0.3 0.15 0.04 0.02
0% strain
F < 1 19
B 4 5 80 89 100 81
M 100 100 100 96 95 20 11
HV 590 570 606 566 521 428 378 323 228
30% strain
F < 1 28
B < 1 1 23 70 99 100 72
M 100 100 100 99 77 30 1
HV 639 613 597 567 582 447 370 338 261
60% strain
F 11
B 8 41 92 96 89
M 100 100 92 59 8 4
HV 582 598 571 532 448 401
Figure 13. Microstructureoftheheat-treatablesteel34CrNiMo6for
the
coolingrates0.6Ks1(left)and0.02Ks1(right)respectively.Above:Withoutdeformation,middle:
Deformation level 30%, below:Deformation level 60%.
www.steelresearch-journal.com 2010 Wiley-VCH Verlag GmbH &
Co. KGaA, Weinheim 231
-
The usual needle shaped bainitic structure in a basicmartensitic
matrix is depicted in Figure 13, left, for acooling rate of 0.6 K
s1 and various deformations. On theright in Figure 13, the
ferritic-bainitic microstructure formedunder the different
deformation conditions is shown for acooling rate of 0.02 K s1. The
microstructure of thetransformationally sluggish heat-treatable
34CrNiMo6 steelchanges only slightly in its composition and
structure owingto the deformation of the material.
Influence of the deformation on the martensite
start-temperature. A comparison of the CCT diagramswith and without
deformation show that the martensitestathde
The structures of, in particular martensite but also, in
part,those of bainite and pearlite are finer due to
thedeformation.
The investigations carried out show that the influence
ofhot-forming on the hardness value is smaller than that of aslight
variation in the cooling rate.
Onthebasisof thewideningofbainites formationinterval,tendencies
are discernible that the upper critical coolingrate increases due
to deformation.
Summary
Continuous cooling transformation diagrams in depend-ence of
increasing deformations were constructed at heatingtemperatures of
1200 8C to illustrate heat-treatmentsimmediately from the
hot-forming temperature. Thesediagrams show the influence of
deformation on the trans-formation behaviour of the investigated
heat-treatable steelsat deformation levels of 30% and 60%.
steel research int. 81 (2010) No. 3 Materials Technology
Figdeviation of the measured martensite start-temperatures
areapprox. 5% to 8%. As expected, the martensite start-temperature
tends to decrease with increasing carbon content(cf. Table 1).
Owing to the deformation, a tendency to widen thetransition
interval can be observed for cooling rates at whichbainite is
initially formed. According to this, it is to beassumed that the
upper critical cooling rate slightly increasesfor the alloys
42CrMo4 and 50CrMo4 or 51CrV4 owing todeformation.
Influence of the cooling rate on the hardness valueThe measured
hardness values for the investigated alloys
are depicted in Figure 15 as a function of the cooling rate.The
hardness values increase with the cooling rates;however, plateaus
of constant hardness can be discerned(at approx. 0.1 K s1 to 1 K s1
as well as greater thanapprox. 6 K s1). This correlates with the
states in which themicrostructure is almost exclusively bainitic or
martensitic(moderate or high hardness values, respectively).
Conclusions
Slight changes in the lines for the onset and ending of
themicrostructural transformation can be observed owing
todeformation. In this way, the temperature interval for
Figure 14. Influence of deformation on the martensite start-
temperature Ms of the investigated alloys.amo23rt-temperature
tends to increase with deformation for alle investigated materials.
According to material andformation stage, the increase is hardly
measurable and
unts to no more than 50 K (see Figure 14). The standard2 2010
Wiley-VCH Verlag GmbH & Co. KGaA, Weinhei42CrMo4 alloys owing
to the deformation. However, thepearlite content increases for the
50CrMo4 and 51CrV4steels.cAn increase of the martensite
start-temperatures withdeformation is established for each of the
investigatedalloys.A deformation leads, moreover, to changes in
thequantitative microstructural composition at cooling ratesfor
which pearlite initially forms. The microstructuralomponents of
pearlite are reduced for the 34CrMo4 andedeformation on austenitic
grain sizes insofar that nosignificant acceleration of pearlite
formation can bestablished. The alloy 51CrV4 is an exception to
this case.tbainite formation widens to higher temperatures for
thealloys 51CrV4, 34CrNiMo6 and 50CrMo4.The influence of
deformation on pearlite formation isdifficult to interpret owing,
in part, to the small number ofdata points. The comparatively large
time intervals up tohe onset of pearlite formation appear to offset
the effect oftigaure 15. Influence of the cooling rate on the
hardness of the inves-
ted alloys.34CrNiMo634CrMo4
regressionalloy51CrV450CrMo442CrMo4
regressionalloy100
200
300
400
500
600
700
800
1001010,100,01
rate in K/scooling
hard
ness
in H
V10m www.steelresearch-journal.com
-
The diffusion driven transformations and formed micro-structures
exhibit weak dependencies on a previousdeformation for the selected
testing conditions. For thediffusionless formation of martensite,
the martensitestart-temperature has a tendency to increase with
deforma-tion. The influence of deformation on the
microstructuralhardness value is smaller than that of a varied
coolingrate.
Acknowledgements
The authors would like to thank the German ResearchFoundation
(DFG) for their financial support of the workcarried out within the
scope of the Collaborative ResearchCentre 489, subproject A3, and
Dipl. Wirt.-Ing. Chr. Birr forhis scientific support.
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