A

ta22attc©

K

1

sfmtfaatbr

c(oC

0d

Journal of Materials Processing Technology 189 (2007) 107–113

Isothermal transformation of austeniteto bainite in high carbon steels

S.A. Sajjadi ∗, S.M. ZebarjadDepartment of Materials Science and Engineering, Engineering Faculty, Ferdowsi University of Mashhad,

Vakil-Abad Blvd., P.O. Box No. 91775-1111, Mashhad, Iran

Received 19 February 2006; received in revised form 15 December 2006; accepted 16 January 2007

bstract

Bainitic transformation occurs in a temperature range between the high temperature transformations such as ferrite and pearlite and the lowemperature transformations such as martensite. Bainite consists of aggregates of platelets or laths of ferrite separated by regions of phases suchs austenite or martensite. The aim of this research is determination of the kinetics of the austenite transformation over the temperature range50–500 ◦C. For this purpose, several specimens from a plain-high carbon steel were isothermally heat treated at different temperatures from50 to 500 ◦C for different holding times. The specimens then were studied using a transmission electron microscope. The results showed thatustenite, in the high carbon steel, transformed to bainite over the temperature range 250–475 ◦C, according to separate C-curves on the isothermal

ransformation diagram. It was shown that above ∼325 ◦C, austenite transformation starts with formation of upper bainite and below that temperaturehe transformation starts with formation of lower bainite. Also, in some regions in the temperature range 250–375 ◦C both upper and lower bainiteould be seen, simultaneously. 2007 Elsevier B.V. All rights reserved.

steel

ta[et

1

tltmab

eywords: Bainite; Kinetics; Isothermal transformation; C-curve; High carbon

. Introduction

The bainite transformation is still probably the least under-tood of all the austenitic decomposition reaction. Bainite isormed over a wide range of temperatures and has differentorphologies at different transformation temperatures. At upper

emperatures of the transformation range, it is distinguishedrom pearlite and known as upper bainite having a diffusivend lath mode of growth. At lower temperatures, it appears asplate-like structure resembling martensite with a displacive

ransformation and is known as lower bainite. The mechanismsy which bainite is formed from austenite are complicated andemain a source of considerable debate [1–11].

The formation of bainite may occur isothermally or duringontinuous cooling, and on a time–temperature-transformation

TTT) diagram, the bainitic and pearlitic temperature rangesverlay with bainite forming generally bellow the nose of the-curve. In many alloys and medium to high carbon steels,

∗ Corresponding author. Fax: +98 511 8793888.E-mail address: sajjadi@um.ac.ir (S.A. Sajjadi).

fnTtrb

924-0136/$ – see front matter © 2007 Elsevier B.V. All rights reserved.oi:10.1016/j.jmatprotec.2007.01.011

he overlap of the two C-curve is incomplete and the bainitend pearlite C-curves are clearly distinct. Kennon and Kaye12] confirmed that isothermal transformation diagrams show annvelope of two separate C-curves for austenite decompositiono pearlite and to upper bainite.

.1. Morphology

Upper bainite consists of laths of ferrite separated by cemen-ite oriented parallel with the long direction of the laths. Inower bainite structure ferrite appears platelet and the cemen-ite is formed within the ferrite at an angle of 55–60◦ to the

ajor axis plates [13]. Figs. 1 and 2 show respectively, opticalnd transmission electron microstructures of the two types ofainite formed at 450 and at 250 ◦C.

The transformation temperature at which lower bainite isormed is low therefore, carbon diffusion is slow so that it cano longer diffuse away adequately in front of the bainite ferrite.

hus, carbide precipitation within the bainitic ferrite plates is

he only way in which carbon content of bainitic ferrite can beeduced. Then, there are two kinds of carbide precipitin in lowerainite structure, within lower bainitic ferrite and between the

108 S.A. Sajjadi, S.M. Zebarjad / Journal of Materials Processing Technology 189 (2007) 107–113

e form

bt

1

dwabtifmiihtbiaa(f

amtofir

1

bTonirprks

Fig. 1. Photomicrostructures showing: (a) upper bainit

ainitic platelets. The microstructure becomes more refined ashe transformation temperature is reduced [11].

.2. Transformation kinetics

It is established that kinetics of upper bainite reaction areifferent from those of lower bainite. The change is associatedith the different nucleation and growth mechanisms, which are

ccompanied by a change in the morphology of upper and lowerainite. Also, it is suggested [14,15] that the rate-controlling fac-or for the formation of upper bainite is the diffusion of carbonn austenite whereas the diffusion of carbon in supersaturatederrite is suggested as the rate controlling process for the for-ation of lower bainite [15,16]. Both upper and lower bainite

n hypereutectoid steels nucleate predominantly at prior austen-te grain boundaries. Intergranular nucleation is infrequent inypoeutectoid steels but is a common occurrence in hypereutec-oid steels [17]. Also the carbide phase associated with upperainite is usually cementite [18] whereas the carbide phase thats observed in lower bainite depends on the transformation time

nd temperature as well as on the composition of the steel [19,20]nd may be �-carbide (Fe2.4C) η-carbide (Fe2C), or cementiteFe3C). It has been shown that the kinetics of the bainite trans-ormation change in the temperature range 300–350 ◦C [21]. In

[asc

Fig. 2. Transmission electron microstructures showing: (a) upper ba

ed at 450 ◦C and (b) lower bainite formed at 250 ◦C.

ddition to the chemical composition of steels, plastic defor-ation of austenite can influence the subsequent isothermal

ransformation of bainite. Larn and Yang [22] showed that theverall transformation kinetics of bainite became slower and thenal amount of bainite decreased in steels deformed in austeniteegion.

.3. Growth mechanisms

Upper and lower bainite have different growth mechanismecause the temperature range in which they form is different.he growth of upper bainite can be described as the movementf a ferrite/austenite interface of low coherency [15,23] whichecessitates some degree of self diffusion and leads to the rathermmobility of the interface. On the contrary, in the lower bainiteange the propagation of a more coherent interface becomesossible in which, little or no self-diffusion is required [15]. Thisesults in more rapid movement of the interface. The growthinetics of bainite in 0.5% C steels with different amount ofilicon have been investigated recently by Quidort and Brechet

1]. They proposed a model for precipitation of the temperaturend carbon content dependency of the growth rate. They alsohowed that silicon decreases the growth rate by suppressingementite precipitation.

inite formed at 450 ◦C and (b) lower bainite formed at 250 ◦C.

S.A. Sajjadi, S.M. Zebarjad / Journal of Materials Processing Technology 189 (2007) 107–113 109

Table 1Chemical composition of 1.2% C steel (wt.%)

C Mn P Si S Ni Cr Mo Cu Al Sn B

1 0

1

bhn{ic

hb

1

ifTtiio[

1

bhoftifeimah6tr

ita

aapastt

2

oTn

iAfat

aqmtts

farp

3

istaWbbf

b

.2 0.32 0.017 0.19 0.019 0.057

.4. Crystallography

It is established that the two bainites are regarded as havingasically different orientation relationship and habit plane. Theabit plane of bainite formed in high carbon steels in upper bai-ite range is near {1 1 1}� and in that lower bainite range is near1 1 0}� [24]. Shackleton and Kelly [25] proposed the follow-ng orientation relationship in lower bainite structures betweenementite and ferrite:

(0 0 1)θ||(2 1 1)α[1 0 0]θ||[0 1 1]α[0 1 0]θ||[1 1 1]α

The orientation relationship (KS) of laths of upper bainiteas been characterized in detail and to a high level of accuracyy Davenport [26] as follows:

[1 0 1]γ ||[1 1 1]α(1 1 1)γ ||(0 1 1)α

.5. The incomplete transformation phenomenon

Another difference between upper and lower bainite is thencomplete transformation of austenite to bainite which is aunction of reaction temperature and alloy composition [27].he extent of decomposition of austenite to bainite increases as

he isothermal transformation temperature decreases. Therefore,n upper bainite range the amount of untransformed austenites much more than in lower bainite range. Since the discoveryf this phenomenon numerous explanations have been reported28–31].

.6. Mechanical properties

The mechanical properties of lower bainite are superiorecause of the finer and more uniform carbide distribution [32],igher internal stress [33], high dislocation density [33], amountf carbon dissolved in bainitic ferrite [33,34], and fine bainiticerrite grains [35]. All of these characteristics are related tohe lower transformation temperatures at which lower bainites formed. Mechanical properties of steels transformed at dif-erent temperatures have been studied recently [11,36,37]. Haset al. [11] claimed that tensile strength for an eutectoid steelsothermally heat treated at 250 ◦C is about 1800 MPa whilst the

agnitude decreases to about 1600 and 1300 MPa as the temper-ture increases to 300 and 350 ◦C, respectively. Wang et al. [36]

ave studied the microstructures and mechanical properties of0Si2CrVA steel during isothermal transformation and reportedhat tensile strength was 1980 and 1880 MPa at 235 and 270 ◦C,espectively. Liu et al. [37] demonstrated that tensile strength

oitb

.097 0.018 0.12 0.01 0.007 0.0013

n lath-like upper bainite in a silicon steel was dependent onhe transformation temperature, so that at 400 ◦C the strength isbout 1200 MPa.

It is the purpose of the work presented in this paper to directttention to the isothermal transformation of austenite to bainitet temperatures between 250 and 500 ◦C in a hypereutectoidlain carbon steel with 1.2% C. To determine whether uppernd lower bainite are formed according to separate C-curves,tudy of the different morphologies obtained from the isothermalransformation at different temperatures by means of optical andransmission electron microscopes (TEM) was carried out.

. Experimental procedures

The 1.2% C plain carbon steel used in this investigation was in the formf 13 mm triangular rod and the chemical composition of the steel is given inable 1. The high carbon content allowed maximum time to quench past theose of the C-curve and so avoid diffusional transformation to nodular pearlite.

Specimens approximately 1.7 mm thick were cut from the rod. Each spec-men was drilled and attached to a length of temperature resistant wire.ustenizing was carried out under carburizing conditions in a front-loading

urnace at 1000 ◦C for 1 h to dissolve all carbides and to produce homogeneousustenite. The austenitic grain size resulting from this treatment was determinedo be ASTM 3 (125 �m).

The specimens were then quenched into a molten salt at a required temper-ture and isothermally transformed for various times. They were subsequentlyuenched into water at room temperature to transform the remaining austenite toartensite. Metallographic study of the transformed specimens was carried out

o identify the products formed at various temperatures. Care was taken to ensurehat any decarburization of the specimens was avoided in the microstructuraltudies.

Using optical microscopy, the morphology and the start and finishing timesor the upper and lower bainite transformation were determined for each temper-ture. Transmission electron microscopy using two-stage gold shadowed carboneplica specimens was employed to confirm identification of the transformationroducts.

. Results and discussion

It was observed from the study of the microstructures, result-ng from isothermal transformation of austenite, that the bainitetart, Bs, temperature, which is the maximum transformationemperature at which non-lamellar eutectoid mixture of ferritend cementite can be seen, is approximately 475 ◦C, Fig. 3.ith decreasing transformation temperature the morphology of

ainite changed from lath-like upper bainite to plate-like lowerainite. Fig. 4(a–d) show photomicrographs of bainite structuresormed by isothermal transformation at various temperatures.

Upper bainite transformation always started from grainoundaries, previous nucleated pearlite nodules (Fig. 5(a)), and

ther structural defects such as boundary surfaces of certainmpurity phases and subgrain boundaries. As the transforma-ion time increased, laths of upper bainite thickened along theoundaries. Thus, a relatively small number of nuclei of upper

110 S.A. Sajjadi, S.M. Zebarjad / Journal of Materials P

F4

bttcoewtaul

a

[Agi

spEmapfluu3

m5fttai

mt

ig. 3. Transmission electron micrograph showing upper bainite formed at75 ◦C, Bs temperature.

ainite could be seen in the interior of grains. On the other hand,he lower bainite transformation was roughly uniform throughhe volume of the austenite grains (Fig. 5(b)). Therefore, it can beoncluded that upper bainite transformation is more dependentn austenite grain size than the lower bainite reaction. Morozovt al. [38] demonstrated that upper bainite transformation curveas consistent with the effect of the austenite grain size on the

ransformation kinetics. They showed that, with decreasing theustenite grain size from 115 to 20 �m, the incubation time of

pper bainite transformation was reduced five times, whilst theower bainite transformation did not change.

The reason for the different morphologies of bainite has beenttributed to the different growth and nucleation mechanism

orlt

Fig. 4. Photomicrographs showing upper bainite formed a

rocessing Technology 189 (2007) 107–113

39]. Fig. 6 reveals the typical internal structure of upper bainite.s it is seen, carbide particles have precipitated parallel to therowth direction of the bainitic ferrite. The upper bainite shownn this micrograph was formed at 400 ◦C.

Lower bainite consists of aggregates of plates of ferrite,heaves, formed in parallel groups and separated by residualhases which form subsequent to the growth of bainitic ferrite.ach sheaf is comprised of numerous subunits. Fig. 7 shows theicrostructure of subunits within plates of lower bainite formed

t 250 ◦C. It can be seen that in this structure the bainitic ferrite islate-like and fine cementite plates have precipitate within theerrite unidirectionally at an angle of about 55–60 ◦C to theirong axes. Lower bainite plates have a more regular and lentic-lar shape in the plane of the specimen surface than those ofpper bainite. Fig. 8 shows a lower bainitic plate transformed at50 ◦C.

The study of two stage replicas made from specimens isother-ally transformed at different temperatures between 250 and

00 ◦C led to the conclusion that there are separate C-curvesor upper and lower bainite. The bainite start and bainite finishimes at each temperature were determined by estimation of theimes taken for the isothermal transformation of 3% of bainitend 97% of diffusional products. These curves, shown in Fig. 9,ntersect at temperature ∼325 ◦C.

Above ∼325 ◦C, austenite transformation starts with for-ation of upper bainite and below that temperature the

ransformation starts with formation of lower bainite. From

bservation of the replicas and Fig. 9, it is expected that in someegions in the temperature range 250–375 ◦C both upper andower bainite can be seen simultaneously. To explain the exis-ence of different bainites at the same temperature, Morozov et

t: (a) 450 ◦C, (b) 400 ◦C, (c) 350 ◦C and (d) 250 ◦C.

S.A. Sajjadi, S.M. Zebarjad / Journal of Materials Processing Technology 189 (2007) 107–113 111

Fig. 5. Photomicrograph showing: (a) upper bainite formed at 400 ◦C and (b) lower bainite transformed at 300 ◦C.

alnbed

Fl

ammations at the same temperature is the nucleation mechanism

Fig. 6. TEM micrograph of upper bainite transformed at 400 ◦C.

l. [38] suggested that different nucleation sites for upper andower bainite results in a difference of transformation mecha-isms which in turn, causes the different morphologies. Upper

ainite forms on austenite grain boundaries with activationnergy of nucleation which is comparable with the boundaryiffusion, whereas lower bainite forms through the volume of

ig. 7. TEM micrograph showing microstructure of subunits within plates ofower bainite.

oto

Ft

Fig. 8. TEM micrograph of lower bainitic plate transformed at 350 ◦C.

ustenite grains with a different activation energy. Therefore, theain difference between the upper and lower bainite transfor-

f the bainitic ferrite phase, whilst the growth mechanisms inhem can be the same. In other words, the lower energy barrierf diffusion motion of iron and carbon atoms on the boundaries

ig. 9. The 3 and 97% transformation IT curves for start and finish transforma-ion of austenite to upper and lower bainite.

112 S.A. Sajjadi, S.M. Zebarjad / Journal of Materials Processing Technology 189 (2007) 107–113

F rmed

odpttfsC

Fd

rtcae

ig. 10. TEM microstructures showing upper (U) and lower (L) bainite transfo

f the austenite grain than in the volume makes it possible forifferent transformation mechanisms to operate at the same tem-erature during nucleation on boundaries (upper bainite) and inhe interior of the austenite grain (lower bainite). Fig. 10 showshe upper and lower bainite formed at the same temperature atour different points in that range. The combination of the two

eparate C-curves for upper and lower bainite gives the single-curve shown in Fig. 11.

ig. 11. The 3 and 97% transformation IT curves of the high-carbon steel asetermined with metallography.

tait

4

1iT

at: (a) 250 ◦C, 1 h; (b) 275 ◦C, 1100 s; (c) 300 ◦C, 1400 s; (d) 375 ◦C, 100 s.

A point of interest is the change in the rate of the bainiteeaction (activation energy) below 350 ◦C which is attributed tohe change from upper to lower bainite. This can be used as ariterion for differentiating between the two structures and givesrather more rapid reaction than would be anticipated from an

xtrapolation of the kinetics of the upper bainite reaction to loweremperatures. It is shown [40,41] that change of the kinetics isccompanied by a change in the morphology of the bainite ands related only to the change of the growth mechanism ratherhan nucleation mechanism.

. Conclusions

The bainites formed during isothermal transformation of a.2% C steel in the temperature range 250–500 ◦C have beennvestigated by optical and electron metallographic techniques.he results obtained are as follows:

(i) Upper and lower transformation corresponds to separate C-curves which intersect at about 325 ◦C on the IT diagram.

(ii) The bainite transformation start temperature, Bs, is approx-◦

imately 475 C.

(iii) Lath-like upper bainite forms at temperatures between 475and 250 ◦C.

(iv) The upper bainite consists of fine ferrite laths separated bycementite parallel to the growth direction of the ferrite.

ateria

(

R

[

[

[[[[[[[

[

[[

[[

[[

[

[

[[[[[[

[[

[

[[

S.A. Sajjadi, S.M. Zebarjad / Journal of M

(v) Plate-like lower bainite transforms at temperaturesbetween 375 and 250 ◦C.

(vi) Lower bainite forms as plates containing fine cementiteparticles precipitated internally at an angle of about 60◦ tothe long direction of the plate.

vii) The upper and lower bainite C-curves intersect each otherat approximately 325 ◦C.

eferences

[1] D. Quidort, Y.J.M. Brechet, Acta Mater. 49 (2001) 4161.[2] G.R. Purdy, M. Hillert, Acta Metall. 6 (1984) 823.[3] H.I. Aaronson, W.T. Reynolds, G.J. Shiflet, G. Spanos, Metall. Trans. 21A

(1990) 1343.[4] Y. Ohmori, T. Maki, Mater. Trans. JIM 32 (1991) 631.[5] H.K.D.H. Bhadeshia, Bainite in Steels, The Institute of Materials, London,

1992.[6] M. Hillert, Metall. Trans. 25A (1994) 1957.[7] M. Hillert, ISIJ 35 (1995) 1134.[8] M. Kang, M.-X. Zhang, M. Zhu, Acta Mater. 54 (2006) 2121.[9] J. Liu, C.P. Luo, Mater. Sci. Eng. A 438–440 (2006) 153.10] X.J. Jin, N. Min, K.Y. Zheng, T.Y. Hsu, Z. Xu, Mater. Sci. Eng. A 438–440

(2006) 170.11] K. Hase, C. Garcia-Mateo, H.K.D.H. Bhadeshia, Mater. Sci. Eng. A

438–440 (2006) 145.12] N.F. Kennon, N.A. Kaye, Metall. Trans. A 13A (1975) 975.13] K.J. Irvine, F.B. Pickering, JISI 188 (1958) 101.

14] T. Ko, S.A. Cottrell, JISI 172 (1952) 30.15] J. Barford, JISI (1966) 609.16] S.V. Radcliffe, E.C. Rollason, JISI 191 (1959) 56.17] G.R. Speich, M. Cohen, Trans. AIME 218 (1960) 1050.18] A.E. Austin, C.M. Schwartz, ASTM 55 (1955) 623.

[[

[

ls Processing Technology 189 (2007) 107–113 113

19] S.J. Matas, R.F. Hehemann, Trans. Met. Soc. Am. Inst. Min. (Metall.) Eng.221 (1961) 179.

20] V. Franetovic, M.M. Shea, E.F. Ryntz, Mater. Sci. Eng. 96 (1987) 231.21] R.H. Goodenow, S.J. Matas, R.F. Hehemann, Trans. AIME 227 (1963)

651.22] R.H. Larn, J.R. Yang, Mater. Sci. Eng. A 278A (2000) 278.23] V.F. Zackay, H.I. Aaronson, Decomposition of Austenite by Diffusional

Processes, Interscience Publishers, New York, 1962.24] M. Oka, H. Okamoto, K. Ishida, Metall. Trans. A 21A (1990) 845.25] D.N. Shackleton, P.M. Kelly, Iron & Steel Inst.—Special Report 93, 1965,

p. 126.26] A.T. Davenport, The Crystallography of Upper Bainite, Republic Steel

Research Rep. on Project 12051, 1974, p. 1.27] H.I. Aaronson, The Mechanism of Phase Transformations in Crystalline

Solids, Institute of Metals, London, 1969, p. 270.28] F. Wever, K. Mathieu, Mitt. Kaiser-Wilhelm-Inst. Eisenforsch 22 (1940) 9.29] H.J. Wiester, Arch. Eisenhuttenwes 18 (1944) 97.30] J.M. Wheatley, R.G. Baker, Br. Weld. J. 10 (1963) 23.31] J.R. Bradley, H.I. Aaronson, Metall. Trans. A 12A (1981) 1729.32] R.W.K. Honeycombe, F.B. Pickering, Metall. Trans. 3 (1972) 1099.33] F.B. Pickering, Transformation and Hardenability in Steels, Climax Molyb.

Co., Chicago, 1967, p. 109.34] P. Vasudevan, L.W. Graham, H.J. Axon, JISI 190 (1958) 386.35] F.B. Pickering, Physical Metallurgy and Design of Steels, Applied Science

Publishers Ltd., London, 1978, p. 101.36] T.S. Wang, X.Y. Li, F.C. Zhang, Y.Z. Zheng, Mater. Sci. Eng. A 438–440

(2006) 1124.37] C. Liu, Z. Zhao, S.D. Bhole, Mater. Sci. Eng. A 434 (2006) 289.38] O.P. Morozov, V.M. Schastlivtsev, I.L. Yakovleva, Phys. Met. Metall. 69

(2) (1990) 146.39] O.P. Morozov, Fiz. Metal. Metalloved. 57 (1984) 142.40] O.P. Morozov, V.M. Schastlivtsev, I.L. Yakovleva, Fiz. Metal. Metalloved.

64 (1987) 1136.41] O.P. Morozov, V.M. Schastlivtsev, Fiz. Metal. Metalloved. 65 (1988) 375.