Continuos Heating Transformation of Bainite To Austenite
Continuos Heating Transformation of Bainite To Austenite
Continuos Heating Transformation of Bainite To Austenite
J. R. YANG
Graduate Institute of Materials Engineering, National Taiwan University, Taipei 107 (Taiwan)
H. K. D. H. B H A D E S H I A
Department of Materials Science and Metallurgy, University of Cambridge, Pembroke.Street, Cambridge CB2 3QZ ( U.K.)
(Received December 27, 1989)
Abstract 1. Introduction
The transformation of a bainitic F e - M n - S i - C The success of the current spate of inter-
alloy into austenite has been studied using dila- national research [1-15] on the modelling of
tometry, transmission electron microscopy and microstructure and properties in welded steel
microanalytical techniques. The formation of fabrications depends to a large extent on the
austenite was investigated using two different start- availability of reliable phase transformation
ing microstructures, the first consisting of a mixture theory to facilitate the computer models. A prob-
of bainitic ferrite and residual austenite, and the lem of particular importance in this respect is
second of a mixture of tempered bainitic ferrite and related to the kinetics of austenite formation. A
carbides. Results from isothermal austenitization quantitative theory dealing with the nucleation
experiments confirm earlier work on a different and growth of austenite from a variety of starting
alloy, that because of the incomplete reaction conditions is vital, especially for the estimation of
phenomenon associated with bainite growth, there microstructure in multirun weld deposits, but also
is a large temperature hysteresis before the reverse for the heat-affected zones in the parent plates of
transformation to austenite becomes possible. single-run welds. The heat input associated with
Continuous heating experiments revealed an iden- the deposition of successive layers of weld metal
tical austenitization behaviour for both initial causes some or all of the underlying structure to
microstructures when the heating rate utilized was be reheated to temperatures where austenite for-
small. This is because any residual austenite then mation occurs. The new austenite then trans-
tends to transform into pearlite or to decompose forms during cooling to a microstructure which is
into ferrite and discrete particles of carbides before very different from the solidification structure
the sample reaches a temperature where austenite associated with weld deposits.
growth becomes thermodynamically feasible. As a part of a programme of research on the
Consequently, the two initial microstructures kinetics of austenite formation in steels, the work
become identical by the time Ty is reached A t reported here deals with austenitization from
faster heating rates the residual austenite remains bainitic microstructures. Many of the alloys used
stable during heating and then commences to grow in the deposition of high strength welds are
as the appropriate elevated temperature is reached. designed to transform into bainitic microstruc-
A slightly higher degree of superheating is found to tures as the weld cools from the austenite phase
be necessary in the absence of residual austenite in field, at rates typical of commercial fabrication
the starting microstructure, since austenite nuclea- practice. It also appears that acicular ferrite,
tion is then necessary prior to growth. Since the which is in general a beneficial phase in steel weld
excess superheating is rather small, the results deposits, is essentially intragranularly nucleated
indicate that nucleation does not appear to be a bainite [16], so that the general principles of
major hurdle to the formation of austenite in the balnite growth and dissolution should also be
alloy studied applicable to acicular ferrite. In some earlier work
time time
because isothermal transformation of austenite to
bainitic ferrite ceases prematurely, before the
residual austenite reaches its equilibrium com-
position, any reverse transformation to austenite
cannot occur until the sample is heated to an
elevated temperature where the residual austenite
1.Balnitic ferrite and austenite l.Balnitic ferrite and austenlte
itself enters the austenite phase field. This leads
to a large hysteresis in the forward and reverse
transformation temperatures for austenite, a
hysteresis which is not found when the starting
microstructure is instead a mixture of allotrio-
morphic ferrite and austenite [20].
2. Austenite layers thicken 2. Tempered balnlte
The theory which explains these phenomena
[17-19] also makes other predictions which have
yet to be verified. Hence the work reported here
focuses on the effect of the carbon content of
residual austenite, i.e. the austenite left untrans-
formed after the formation of some bainite, on
3.Nucleatlon & growth of austenlte
the reversion to austenite. An examination of a
starting microstructure of bainitic ferrite and Fig. 1. Schematic diagrams illustrating austenite formation
using different initial microstructures; step 1 of the heat treat-
residual austenite is also of relevance, because ment corresponds to the transformation of a fully austenitic
bainitic steels which contain high concentrations sample into a mixture of bainitic ferrite and carbon-enriched
of silicon are becoming prominent in industry, residual austenite. (a) Austenitization by reheating the
mixture of bainitic ferrite and residual austenite. (b) Starting
both as wrought alloys [21] and in the form of microstructure of carbides and bainitic ferrite generated by
high silicon cast irons [22]. The high silicon step 2, which is a tempering heat treatment. Austenite nuclea-
concentration retards the growth of cementite, so tion is therefore necessary during step 3 of the heat treat-
ment.
that a microstructure of just bainitic ferrite and
residual austenite can be obtained by transforma-
tion of austenite below the bainite start (Bs) tem- extension of the earlier research is that both iso-
perature [23]. If this microstructure is, without thermal and continuous heating experiments have
cooling from the isothermal transformation been carried out in the present work.
temperature, reheated rapidly to higher tempera-
tures, then the austenitization process requires no
nucleation of austenite, permitting a controlled
2. Experimentalprocedure
study of the growth of austenite [17-19], as illus- The steel used had the chemical composition
trated in Fig. 1. On the other hand, the mixture of Fe-0.43C-3.00Mn-2.02Si (wt.%) and was pre-
bainite and austenite can be tempered to force the pared using high purity base materials as a 20 kg
latter phase to decompose into ferrite plus car- vacuum induction melt which was hot worked to
bides (Fig. 1). Austenitization of the resultant rods 8 mm in diameter. The hot-worked samples
microstructure of ferrite and carbides would then were then sealed in quartz capsules (under a
require the nucleation of austenite, permitting a partial pressure of argon) and homogenized for 3
study of the role of austenite nucleation in influ- days at 1250°C before hot reduction to rods
encing overall transformation kinetics. A further 3.2 mm in diameter.
2.1. Dilatometry
*Throughout this paper the term "residual austenite"
refers to the austenite that is left untransformed after the All kinetic measurements were performed
growth of bainitic ferrite stops. using a Theta Industries high speed dilatometer,
101
equality of chemical potential in both the a and samples were heated at relatively low rates (0.06
phases [28-32]. The Ae 3' curve therefore defines and 0.15°Cs-1). Figure 4 illustrates how the
the maximum carbon concentration tolerated by decomposition of the residual austenite could be
austenite which is in para-equilibrium with ferrite. detected during heating via the expansion associ-
The TO curve (Fig. 2(b)) represents the locus of ated with the density change as ferrite and
temperatures at which austenite and ferrite of the carbides replace the austenite phase. This inter-
same chemical composition have equal free pretation is consistent with the fact that the
energy [33, 34]. It is thermodynamically impos- expansion observed at intermediate temperatures
sible for the austenite to transform to ferrite (less than 600 °C) during slow heating is absent
without a composition change if x~ exceeds the for the higher heating rates, since the residual
appropriate value given by the TOboundary. The austenite does not then have the opportunity to
TO' curve is calculated on the same basis as the TO decompose. For those cases where the heating
curve, but allowing for the 400 J mol- 1 of stored rate was slow enough to permit some austenite
energy in the bainitic ferrite [24] due to the shape decomposition during heating, the temperature at
deformation accompanying its growth. which decomposition was first detected to occur
The data presented in Fig. 2(b) confirm that increased as the heating rate increased (Fig. 4).
the formation of upper bainite stops prematurely, The reaction can be expected to be delayed (and
well before the carbon concentration of the resid- eventually suppressed) to higher temperatures,
ual austenite reaches the para-equilibrium Ae 3' since the sample spends less time in the range
phase boundary. This effect is known as the where austenite has a tendency to decompose, as
"incomplete reaction phenomenon" [33, 34] and the heating rate is increased.
is believed to arise because bainitic ferrite grows Metallography of samples which, during the
by diffusionless transformation, any excess car- heating part of the treatment, were quenched
bon being partitioned into the residual austenite from 600 °C confirmed that austenite decomposi-
rather quickly at some stage after transformation. tion was responsible for the perturbations
Since diffusionless transformation is not possible observed in the length vs. temperature curves
beyond the TO'curve, it is natural that the reaction from the continuous heating experiments. The
should stop when xr = xr0,. larger blocky regions of residual austenite were
Figure 3 illustrates the microstructure ob- found to decompose to pearlite (Fig. 5). The thin
tained by isothermal transformation to upper films of austenite within the sheaf microstructure
bainite. From the point of view of reverse trans- also tended to decompose but into a mixture of
formation, it is important to note the two different ferrite and discrete particles of carbides (Fig. 5).
morphologies of austenite present. The blocky This difference in behaviour between the two
regions of austcnite originate from the geomet- forms of austenite is expected on two grounds.
rical partitioning of the prior austenite grains by Firstly, the larger surface-to-volume ratio asso-
non-parallel sheaves of bainite. The thin films of ciated with the film morphology should lead to a
austenite are, on the other hand, confined to higher carbide nucleation rate, thus allowing the
individual sheaves and represent untransformed transformation to begin in many different places
austenite which is trapped between adjacent at the same time, and hence working against the
platelets of a sheaf. development of colonies of pearlite (each pearlite
colony consists of two interpenetrating ferrite and
3.1. Austenitization by continuous heating cementite crystals [35], which in two-dimensional
Isothermal transformation to bainite at 350 °C sections give an appearance of alternating lamel-
ceases after about 30 min at the reaction temper- lae). Furthermore, only a small amount of decom-
ature. If the sample is held at that temperature for position is necessary before the films decompose
only 10 min so that the amount of untransformed completely, so that there may not exist an oppor-
austenite is rather large, and is then heated at a trinity for the development of the cooperative
controlled rate to induce the growth of the resid- growth of ferrite and cementite of the kind neces-
ual austenite, the latter is found to decompose sary to establish a common transformation front
during heating to a mixture of ferrite and carbides and a pearlite colony.
before reaching the higher temperatures where In fact, it is also known that the carbon concen-
the formation of austenite can be studied. This tration in the residual austenite is not homogene-
was especially found to be the case when the ously distributed [36-40]. The films of austenite
193
Fig. 3. (a) Optical micrograph showing the upper bainitic ferrite and residual austenite following isothermal transformation at
350 °C for 120 min. The blocky regions of austenite appear as the white etching regions, the film austenite being too fine to
resolve, (b) Bright field transmission electron micrograph of a sample transformed isothermally at 350 °C for 120 min, high-
lighting the (grey) films of austenite between adjacent bainitic ferrite platelets. (c) Corresponding retained austenite dark field
image.
104
become isolated by adjacent platelets of bainitic 3.2. Different initial microstructures and bainite
ferrite and consequently may contain a relatively transformation temperatures
larger concentration of carbon, since any excess In order to investigate further the effect of
carbon dumped into the films cannot easily be carbon in the residual austenite on its growth
dissipated into the surroundings by diffusion. kinetics, specimens austenitized at 950°C for
This may also contribute to a higher carbide 10 rain were isothermally transformed to bainite
nucleation rate in the film austenite. In the experi- for 120 min ( i . e . longer than is necessary for the
ments described above, the samples were allowed reaction to cease), before heating to induce
to transform to bainite at 350 °C for only 10 min, austenite growth. Figure 8 shows the results
so that the fraction of austenite left untrans- obtained in the form of a plot of the austenitiza-
formed is relatively large and its enrichment with tion start temperature T~ vs. the heating rate for
carbon correspondingly small. Other experiments four different starting microstructures. Three of
were also carried out in which bainitic transfor- these microstructures consisted of mixtures of
mation was permitted for 120 min at 350 °C, a upper bainitic ferrite and residual austenite and
time period longer than is necessary to ensure the were generated by isothermal transformation at
maximum volume fraction of bainitic ferrite. 350, 380 and 410 °C respectively, the sample in
Figure 6 illustrates the dilatometric data from each case being held at the transformation tem-
experiments in which the samples were first trans- perature for 2 h to allow the reaction to proceed
formed isothermally to bainite at 350°C for to its maximum extent, after which the specimens
120 min and then heated above 350 °C at rates of were heated for the austenitization process. The
105
Fig. 5. Electron micrographs confirming the decomposition of austenite during heating at slow rates. The heat treatments given
are as follows: 10 min at 950 ° C - 10 min at 350 ° C - continuous heating to 600 °C (heating rate = 0.06 °C s ])~ quenched using
helium gas. (a) Electron micrograph showing a pearlite-like colony in the microstructure. (b) Electron micrograph showing the
discretely nucleated carbides.
400
[ ...................
500
] ................... I ...................
600 700
~ ................... I ...................
800
~ ..................
Fig. 7. Samples austenitized at 950 °C for 10 min, quenched to an isothermal transformation temperature of 350 °C and held
there for 120 min before continuously heating at a rate of 0.06 °C s ~, and interrupting the heating cycle by helium quenching
when the temperature To was reached. (a) TO = 600 °C, showing that the residual austenite has decomposed into a mixture of
ferrite and carbides. (b) TO = 700 °C, showing that the residual austenite has decomposed into a mixture of ferrite and carbides.
(c) To = 730 °C, showing the formation of a layer of austenite (which decomposed to martensite on quenching).
108
800
Fe- 043C- 3"OOMn-2'02Si
9 5 0 ~ . 10 mins -+-']50=C,2hr s ~ 610¢~, ~ 0 rain ~ c ontinuous h~.~firlg
10eC,2hrs
800"
9S0=C,10mins gS0°C,ghr~ ~ co~muous h e a t i n g
780-
t-J
*-.7 6 O-
as
c~
300 i
720- 0.0 ~02 ~0~ 66 o:o8 0110 0~2
MOLE FRACTION OF CARBON
Fig. 10. Phase diagram showing the equilibrium Aeg, para-
7001~ ~ . . . . . I~i . . . . . . i~) . . . . . . I~.~,., equilibrium Aeg', T0 and To' phase boundaries for
Heating Rate (*C/S) Fe-0.43wt.%C-3.00wt.%Mn-2.02wt.%Si alloy. The Ae3' , TO
and To' curves are calculated as in ref. 24, and the A e 3 curve
Fig. 8. The experimentally observed austenitization start is calculated as in ref. 41. $ is the average carbon concentra-
temperatures as a function of heating rate and starting micro- tion of the alloy. The horizontal construction lines refer to
structures. the varie~ of temperatures at which mixtures of bainitic
ferrite and residual austenite were generated by isothermal
reaction, and the intersection of the corresponding vertical
construction lines with the A e 3 curve define the temperature
at which austenite growth should begin in each case.
0-006
o.oo~-
~ 0'003-
0"002-
0
0.001-
0.000
O-OOS-
c
----~--- . . . . . . . . . . . . 750°C
~ o.004- 780°C
~ 0.003"
J 0"002-
o>
o 0.001.
i
0000 i i r i
Fig. 1 1. Dilatometric curves for isothermal austenitization experiments in which samples containing a mixture of bainitic ferrite
and residual austenite were rapidly heated to and held at the austenitization temperature indicated.
Fig. 12. Transmission electron micrographs illustrating the microstructures obtained after heating mixtures of bainitic ferrite and
residual austenite to a variety of isothermal austenitization temperatures. (a) Partial decomposition of austenite into carbide
(10 min at 950 °C ~ 2 h at 350 °C ~ 2 h at 690 °C ~ gas quench to ambient temperature). (b) Partial decomposition of austenite
into carbides ( l 0 min at 950 °C ~ 2 h at 350 °C ~ 2 h at 710 °C ~ gas quench to ambient temperature).
110
Fig. 13. Transmission electron micrographs illustrating the microstructures obtained after heating mixtures of bainitic ferrite and
residual austenite to a variety of isothermal austenitization temperatures. (a) 10 min at 950 °C ~ 2 h at 350 °C --' 2 h at 720 °C ~ gas
quenched to ambient temperature. (b) 10 rain at 950 °C ~ 2 h at 350 °C - 2 h at 750 C ~ g a s quenched to ambient temperature;
shows that the austenite (which transforms to martensite on cooling) films havethickened. (c) Optical micrograph showing the
microstructure after isothermal austenitization at 820 °C for 2 h. (10 min at 950 ° C - 2 h at 350 °C ~ 2 h at 820 °C ~ quenched
using helium gas). Regions of residual ferrite (light grey) etching are apparent in an otherwise martensitic matrix, indicating that
the austenitization process is incomplete.
111
Fig. 14. (a) Optical micrograph and (b) transmission electron micrograph illustrating the fully martensitic microstructure
obtained by prolonged annealing at 780 °C ( 10 min at 950 °C ~ 2 h at 350 °C ~ 168 h at 780 °C ~ water quench).
112
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since any austenite present in the starting condi- 16 H . K . D . H . Bhadeshia, in S. A. David and J. Vitek (eds.),
tion decomposes during heating. At higher heat- Advances in the Science and Technology of Welding,
ASM International, Metals Park, OH, 1989, pp.
ing rates the tempered bainite austenitizes at a 189-198.
slightly higher superheat, since unlike the micro- 17 J. R. Yang and H. K. D. H. Bhadeshia, in J. Y. Koo (ed.),
structure which already contains some residual Proc. Int. Conf. on Welding Metallurgy of Structural
austenite, new austenite has to be nucleated. Steels, Metallurgical Society of AIME, Warrendale, PA,
However, the excess superheat is found to be 1987, pp. 549-563.
18 J. R. Yang and H. K. D. H. Bhadeshia, in G. W. Lorimer
rather small, indicating that the nucleation of (ed.), Proc. Int. Conf., Phase Transformations '87, Insti-
austenite is not very difficult because of the high tute of Metals, London, 1988, pp. 203-206.
temperatures necessarily involved in such experi- 19 J. R. Yang and H. K. D. H. Bhadeshia, Mater Sci. Eng. A,
ments. 118 (1989) 155-170.
Finally, the results have all been backed by 20 K. Tsuzaki, K. Yamaguchi, T. Maki and 1. Tamura, Tetsu
to Hagane, 74 (1980) 1430-1437.
extensive metallographic studies, using both light 21 H . K . D . H . Bhadeshia, in P. H. Scholes (ed.), Steel Tech-
and transmission electron microscopy. nology International, Sterling, London, 1989, pp.
289-294.
22 K. B. Rundman, D. J. Moore, K. L. Hayrynen, W. J.
Dubensky and T. N. Rouns, J. Heat Treating, 5 (1988) 79.
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Park, OH, 1970, pp. 397-432.
The authors are grateful to Professor D. Hull 24 H. K. D. H. Bhadeshia and D. V. Edmonds, Acta Metall.,
for the provision of laboratory facilities at the 28(1980) 1265-1273.
University of Cambridge, and the Ministry of 25 H. K. D. H. Bhadeshia, J. Phys. (Paris), Colloq. C4, 43
Education, Republic of China for funding this (1982) 443-448.
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27 H . K . D . H . Bhadeshia, Met. Sci., 16 (1982) 167-169.
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