microstructure transformations in tempering...

DOI: 10.1002/srin.200900132 steel research int. 81 (2010) No. 3

Microstructure Transformations in Temper
ing Steels during Continuous Cooling fromHot Forging Temperatures
F. Nurnberger1)*, O. Grydin1), M. Schaper1), F.-W. Bach1), B. Koczurkiewicz2), and A. Milenin3)

1) Faculty of Mechanical Engineering, Institute of Materials Science, Leibniz Universitat Hannover, An der Universitat 2, 30823 Garbsen,

Germany, [email protected]) Faculty of Materials Processing Technology and Applied Physics, Institute of Modelling and Automation of Plastic Forming Processes,

Czestochowa University of Technology, Al. Armii Krajowej 19, 42/200 Czestochowa, Poland3) Faculty of Metals, Engineering and Industrial Computer Science, Department of Applied Computer Science and Modelling, AGH University

of Science and Technology, Al. Mickiewicza 30, 30059 Krakow, Poland

* Corresponding author

Apart from reducing the processing energy, hardening and tempering of near-net shape forged components from their forging heat primarily

promises shortened conventional process sequences. In this case, the continuous cooling transformation diagrams (CCT diagrams) found in the

literature can only be used as a rough approximation of microstructural transformations during the heat-treatment. The reasons for this are that

firstly, the deformation influences the transformation kinetics and secondly, the deformation temperatures are comparatively high. Therefore,

both deformation CCT diagrams and, for reference purposes, CCT diagrams without deformation were determined for a selection of heat-

treatable steels (34CrMo4, 42CrMo4, 52CrMo4, 51CrV4, 34NiMo6) at the heating temperature of 1200 8C and deformation levels of 0.3 and 0.6.

Keywords: Deformation CCT diagrams, heat-treatable steels, precision forging, integrated heat-treatment

Submitted on 27 September 2009, accepted on 28 December 2009

Introduction

Procedures for the numerical design of process-steps orcomplete process chains are becoming increasingly moreimportant since their use can reduce the extent of laboriousexperimental 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 ofprocesses; as for example during precision forging withsubsequent 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 thematerial parameters and thereby particularly of the con-tinuous cooling transformation diagrams (CCT diagrams).The latter exist in the form of data-collections for a large

224 � 2010 Wiley-VCH Verlag GmbH & Co. KGaA, Wein

number of common alloys [3] and [4]. However, dependingon process factors such as austenitising conditions, hotforming or charge influences, that is, deviations of chemicalcomposition, the transformation kinetics change, possibly tosuch a significant degree that the diagrams lose their validityunder these conditions.

Influence of austenitising. The carbides dissolve withincreasing austenitising temperature and duration afterwhich grain growth commences. Enlarged grains causeboth 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 theaustenite’s grain size on the transformation behaviour ofdifferent steels. They observed that increasing the austen-ite’s 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 thesetransformations 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 accelerated

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Materials Technology steel research int. 81 (2010) No. 3

Figure 1. Gear wheel and pinion shaft of the Collaborative Research

Centre489,precisionforgedat theInstituteofMetalFormingandMetal

Forming Machines, Leibniz Universitat Hannover.

Figure 2. Influence of the austenite’s 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 a

0.45% C steel (strain 1.25) [8].

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

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temperatures 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 [11–13],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 alloy’s 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 investigated

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steel research int. 81 (2010) No. 3 Materials Technology

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 0.0076

melts 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 s�1.Following this, the samples were linearly cooled usingcooling rates in the range from 80 K s�1 to 0.02 K s�1. 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.


A 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 Student’s 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 s�1,only a few data points in part exist for the ferrite or, as thecase may be, the pearlite regions so that the depicted curves’trajectories 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 s�1. Bainite isformed at cooling rates of between 6 K s�1 and 0.15 K s�1.Ferrite and pearlite can be established at cooling rates lowerthan 1 K s�1 and 0.6 K s�1, 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 of



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.

Table 2. Metallographically determined microstructural com-ponents together with hardness values measured on the specimensof the heat-treatable steel 34CrMo4 for various linear cooling rates(Ks�1) and deformation levels (F: ferrite, P: pearlite, B: bainite, M:martensite, HV: Vickers hardness in HV10).

34CrMo4

K s�1 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 – – – – – –

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

B – 42) 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

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 s�1 (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 s�1 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 s�1.

Bainite is formed at cooling rates of between 1 K s�1 and0.07 K s�1. Ferrite and pearlite can be established at coolingrates from 0.07 K s�1 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


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 formation’sinfluence 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 s�1 following deformation and a lowfraction of pearlite at a cooling rate of 0.07 K s�1 (see Table 3andFigure 7 - right). The microstructure’s hardness tends todecrease owing to the slightly accelerated microstructuraltransformation due to the deformation.

Figure 7 - left, clearly illustrates the reduction of theaustenite’s grain size owing to the deformation by meansof the extended martensitic structures at a cooling rate of15 K s�1.

50CrMo4. The CCT diagram for the material 50CrMo4(see Figure 8, full-line) shows ferritic, pearlitic, bainitic and



Figure 5. Microstructure of the heat-treatable steel 34CrMo4 for the

cooling rates 0.6 K s�1 (left) and 0.3 K s�1 (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.

Table 3. Metallographically determined microstructural compo-nents together with hardness values measured on the specimensof the heat-treatable steel 42CrMo4 for various linear cooling rates(Ks�1) and deformation levels. (F: ferrite, P: pearlite, B: bainite,M: martensite, HV: Vickers hardness in HV10).

42CrMo4

K s�1 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% strain

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


martensitic components. The critical cooling rate observedis 15 K s�1. Bainite is formed at cooling rates of between6 K s�1 and 0.3 K s�1. Ferrite and pearlite can be establishedat cooling rates from 1 K s�1 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 theferrite’s and pearlite’s 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 s�1 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.




cooling rates 15 K s�1 (left) and 0.07 K s�1 (right) respectively. Above:


mation level 60%.

Figure 8. Deformation CCT diagrams for the material 50CrMo4 with


Table 4. Metallographically determined microstructural compo-nents together with hardness values measured on the specimensof the heat-treatable steel 50CrMo4 for various linear cooling rates(Ks�1) and deformation levels. (F: ferrite, P: pearlite, B: bainite,M: martensite, HV: Vickers hardness in HV10).

50CrMo4

K s�1 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 704 712 653 312 318 330 299 241 202

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


51CrV4. 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 s�1.Bainite is formed at cooling rates of between 1 K s�1 and0.6 K s�1. Pearlite can be established at cooling rates from0.3 K s�1 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 bainite’sformation 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






mation level 60%.

Figure 10. Deformation CCT diagrams for the material 51CrV4 with


Table 5. Metallographically determined microstructural compo-nents together with hardness values measured on the specimensof the heat-treatable steel 51CrV4 for various linear cooling rates(Ks�1) and deformation levels. (P: pearlite, B: bainite, M: martensite,HV: Vickers hardness in HV10).

51CrV4

K s�1 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 – – –

HV 813 757 741 746 743 379 385 364 336


content for a cooling rate of 0.6 K s�1 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 s�1 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 s�1. Bainitecould be measured at cooling rates lower than 1 K s�1 andforms the main microstructural component up to coolingrates of 0.02 K s�1. Ferrite can be established for the firsttime at cooling rates below 0.04 K s�1 (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 bainite’s 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 microstructure’shardness tends to slightly increase.



Figure 11. Microstructure of the heat-treatable steel 51CrV4 for the



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(Ks�1) and deformation levels. (F: ferrite, B: bainite, M: martensite,HV: Vickers hardness in HV10).

34CrNiMo6

K s�1 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

coolingrates0.6Ks�1(left)and0.02Ks�1(right)respectively.Above:Without

deformation,middle: Deformation level 30%, below:Deformation level 60%.

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100

200

300

400

500

600

700

800

1001010,100,01

rate in K/scooling

hard

ness

in H

V10

34CrNiMo634CrMo4

regressionalloy51CrV450CrMo442CrMo4

regressionalloy

Figure 15. Influence of the cooling rate on the hardness of the inves-

tigated alloys.

The usual needle shaped bainitic structure in a basicmartensitic matrix is depicted in Figure 13, left, for acooling rate of 0.6 K s�1 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 s�1. 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 martensitestart-temperature tends to increase with deformation for allthe investigated materials. According to material anddeformation stage, the increase is hardly measurable andamounts to no more than 50 K (see Figure 14). The standarddeviation 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 s�1 to 1 K s�1 as well as greater thanapprox. 6 K s�1). This correlates with the states in which themicrostructure is almost exclusively bainitic or martensitic(moderate or high hardness values, respectively).

Conclusions

– S

Fi

tem

23

light changes in the lines for the onset and ending of themicrostructural transformation can be observed owing todeformation. In this way, the temperature interval for

gure 14. Influence of deformation on the martensite start-

perature Ms of the investigated alloys.

2 � 2010 Wiley-VCH Verlag GmbH & Co. KGaA, Weinhei

bainite formation widens to higher temperatures for thealloys 51CrV4, 34CrNiMo6 and 50CrMo4.

– T
he influence of deformation on pearlite formation isdifficult to interpret owing, in part, to the small number ofdata points. The comparatively large time intervals up tothe onset of pearlite formation appear to offset the effect ofdeformation on austenitic grain sizes insofar that nosignificant acceleration of pearlite formation can beestablished. The alloy 51CrV4 is an exception to this case.
– A
n 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 microstructuralcomponents of pearlite are reduced for the 34CrMo4 and42CrMo4 alloys owing to the deformation. However, thepearlite content increases for the 50CrMo4 and 51CrV4steels.
– T
he structures of, in particular martensite but also, in part,those of bainite and pearlite are finer due to thedeformation.
– T
he investigations carried out show that the influence ofhot-forming on the hardness value is smaller than that of aslight variation in the cooling rate.
– O
nthebasisof thewideningofbainite’s 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%.

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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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