Bhadeshia H.K.D.H. Bainite In Steels. Transformations, Microstructure and Properties

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11 Other Morphologies of Bainite

Upper and lower bainite are established terms describing microstructures

which can easily be distinguished using routine microscopy, and whose

mechanisms of formation are well understood. There are, however, a number

of other descriptions of steel microstructures which include the word `bainite'.

These additional descriptions can be useful in communicating the form of the

microstructure. But this must be done with care, avoiding the natural tendency

to imagine a particular mechanism of transformation, simply because someone

has chosen to coin the terminology.

11.1 Granular Bainite

Of all the unusual descriptions of bainitic microstructures, granular bainite is

probably the most useful and frequently used nomenclature. During the early

1950s, continuously cooled low-carbon steels were found to reveal micro-

structures which consisted of `coarse plates and those with an almost entirely

granular aspect', together with islands of retained austenite and martensite,

Fig. 11.1 (Habraken, 1956, 1957, 1965; Ridal and McCann, 1965; Habraken and

Economopolus, 1967). Habraken and coworkers called this granular bainite

and the terminology became popular because many industrial heat-treatments

involve continuous cooling rather than isothermal transformation. The energy

generation industry in particular uses enormous quantities of bainitic micro-

structures generated by allowing large steel components to cool naturally

(Chapter 12). Granular bainite is supposed to occur only in steels which

have been cooled continuously; it cannot be produced by isothermal trans-

formation.

The coarse ferrite plates referred to earlier, do not really exist. They are in

fact, sheaves of bainitic ferrite with very thin regions of austenite between the

sub-units because of the low carbon concentration of the steels involved

(Leont'yev and Kovalevskaya, 1974; Josefsson and Andren, 1989). Hence, on

an optical scale, they give the appearance of coarse plates (Fig. 11.1a). Many of

the original conclusions were reached from microstructural observations

which were not of suf®cient resolution to establish the ®ne structure within

the sheaves of bainite. Indeed, evidence of this interpretation of so-called

coarse plates appeared in the literature as early as 1967 when thin foil TEM

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observations were made by Habraken and Economopolus, revealing the ®ne

bainitic ferrite platelets within the sheaves.

A characteristic (though not unique) feature of granular bainite is the lack of

carbides in the microstructure. The carbon that is partitioned from the bainitic

ferrite stabilises the residual austenite, so that the ®nal microstructure contains

both retained austenite and some high-carbon martensite. Consistent with

observations on conventional bainite, there is no redistribution of substitu-

tional solutes during the formation of granular bainite (Tenuta-Azevedo and

Galvao-da-Silva, 1978).

The extent of transformation to granular bainite is found to depend on

the undercooling below the bainite-start temperature (Habraken and

Economopolus, 1967). This is a re¯ection of the fact that the microstructure,

like conventional bainite, exhibits an incomplete reaction phenomenon.

The evidence therefore indicates that granular bainite is not different from

ordinary bainite in its mechanism of transformation. The peculiar morphology

is a consequence of two factors: continuous cooling transformation and a low

carbon concentration. The former permits extensive transformation to bainite

during gradual cooling to ambient temperature. The low carbon concentration

ensures that any ®lms of austenite or regions of carbide that might exist

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Fig. 11.1 Granular bainite in a Fe±0.15C±2.25Cr±0.5Mo wt.% steel: (a) light micro-

graph; (b) corresponding transmission electron micrograph (after Joseffson, 1989).

between sub-units of bainite are minimal, making the identi®cation of indivi-

dual platelets within the sheaves rather dif®cult using light microscopy.

Finally, it is interesting that in an attempt to deduce a mechanism for the

formation of granular bainite, Habraken (1965) proposed that the austenite

prior to transformation divides into regions which are rich in carbon, and

those which are relatively depleted. These depleted regions are then supposed

to transform into granular bainite. The idea is the same as that of Klier and

Lyman (1944) and has been shown to be thermodynamically impossible in

steels (Aaronson et al., 1966a).

11.2 Inverse Bainite

Ferrite is the dominant phase in conventional bainite; carbide precipitation

when it occurs is a secondary event. In the so-called `inverse bainite' which

is found in hypereutectoid steels, it is the cementite which is the ®rst phase to

form (Hillert, 1957). A central plate-like spine of cementite grows directly from

austenite (Hehemann, 1970) and then becomes surrounded by a layer of ferrite

(Fig. 11.2). The term `inverse' re¯ects the fact that, unlike conventional bainite,

cementite is the ®rst phase to precipitate from austenite.

The mechanism of the transformation is virtually unknown; there is no

evidence that the growth of the ferrite occurs by a coordinated movement of

atoms, and no crystallographic or chemical composition data. Judging from the

shape alone, the ferrite probably forms by a reconstructive transformation

mechanism. It is premature to classify the transformation as bainite.

11.3 Columnar Bainite

`Columnar bainite' is a description of a non-lamellar aggregate of cementite

and ferrite, the overall shape of which is like an irregular and slightly elon-

gated colony (Fig. 11.3). The distribution of cementite particles within the

colony is rather peculiar, the majority of needle-shaped particles being aligned

to the longer dimension of the colony. This latter region is surrounded by a

layer of a different microstructure, in which the coarse cementite particles meet

the austenite/ferrite interface edge on (Nilan, 1967). The structure is normally

observed in hypereutectoid steels (Greninger and Troiano, 1940; Vilella, 1940;

Jellinghaus, 1957; Speich and Cohen, 1960) but has been found in lower carbon

steels transformed at high pressures (Nilan, 1967). It may be relevant to point

out that the eutectoid composition is shifted to lower carbon concentrations by

hydrostatic pressure.

The microstructure can be obtained at transformation temperatures compar-

able with those associated with conventional bainite, but there is no invariant-

plane strain surface relief accompanying the growth of `columnar bainite'. It is

Other Morphologies of Bainite

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Bainite in Steels

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Fig. 11.2 Inverse bainite in a hypereutectoid steel: (a) light micrograph; (b) trans-

mission electron micrograph (after Farooque and Edmonds).

probable that columnar bainite is more akin to pearlite than bainite, but further

investigations are needed to make any sensible decisions about the mechanism

of growth.

11.4 Pearlitic Bainite

In steels containing strong carbide-forming elements, it is possible to obtain

pearlite, in which the carbide phase is an alloy carbide (such as M

) instead

of cementite. The alloy pearlite can form at temperatures above B

,orsome-

what below that temperature but only after holding at the transformation

temperature for very long time periods (usually many days). On the scale of

light microscopy, the pearlite etches as dark nodules (Fig. 11.4), but the

colonies tend to have crystallographic facets rather than the nicely rounded

Other Morphologies of Bainite

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Fig. 11.3 Electron micrograph, obtained using a replica technique, showing a

colony of `Columnar Bainite' in an Fe±0.82C wt.% following isothermal transfor-

mationat2888 C and at a pressure of 30 kbar (after Nilan, 1967).

colonies of normal pearlite. This is probably a re¯ection of the orientation

dependence of the interfacial energy of the alloy carbide.

Because of this faceting, transmission electron microscopy observations can

be misleading. The crystallographically faceted nodules of pearlite at a high

resolution give the appearance of parallel ferrite plates with intervening car-

bides, a microstructure on that scale similar to upper bainite. The terminology

`pearlitic bainite' given to this transformation product is misleading. There is

gross partitioning of substitutional solutes during the transformation, there is

no surface relief effect, the carbide and ferrite phases grow cooperatively, and

there is no reason to associate this microstructure with bainite.

11.5 Grain Boundary Lower Bainite

Bainite nucleation in most steels occurs heterogeneously at the austenite grain

boundaries. The nucleation rate of lower bainite can be large at temperatures

close to M

; the austenite grain surfaces then become covered by lower bainite

sub-units (Fig. 11.5). The rate at which carbon partitions from supersaturated

Bainite in Steels

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Fig. 11.4 Microstructure of the so-called `pearlitic bainite', which is really just a

pearlite with alloy carbide (in this case M

) instead of cementite: (a) light micro-

graph; (b) transmission electron micrograph.

ferrite is slow when transformation is at such low temperatures. Therefore, the

sub-units are able to form in arrays without any intervening austenite

(Bhadeshia and Edmonds, 1979a). These layers of sub-units have the overall

form of allotriomorphs, but there is no doubt that they form individually.

The microstructure has caused some concern in the context of 300M, which is

an ultrahigh-strength steel used in the quenched and tempered condition

(Padmanabhan and Wood, 1984). The alloy has a very high hardenability ±

10 cm diameter sections can be made martensitic by air cooling from the

austenitisation temperature. However, optical microscopy revealed the sur-

prising presence of allotriomorphs, which on detailed examination turned

out to be the grain boundary lower bainite described above.

11.6 Summary

Granular bainite is basically ordinary bainite generated by continuous cooling

transformation of low-carbon steels. The mechanism of inverse bainite is

unclear, but it involves the formation of cementite as the primary phase. It is

not clear whether the ferrite, when it eventually forms and engulfs the cemen-

tite, forms by a reconstructive or displacive mechanism.

Whilst there is some doubt about the mechanism of inverse bainite, the terms

columnar and pearlitic bainite are undoubtedly misnomers and are best

avoided. Columnar bainite is simply an aggregate of cementite and ferrite

Other Morphologies of Bainite

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Fig. 11.5 The microstructure of grain boundary lower bainite: (a) light micrograph;

(b) transmission electron micrograph.

which grows by a reconstructive transformation mechanism. Pearlitic bainite is

simply a crystallographically faceted alloy pearlite.

At high supersaturations, arrays of lower bainite sub-units can rapidly dec-

orate the austenite grain surfaces, giving the appearance of allotriomorphs.

This `grain boundary lower bainite' is much harder than allotriomorphic fer-

rite, and hence is easily distinguished.

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12 Mechanical Properties

12.1 General Introduction

Many years elapsed after the work of Davenport and Bain before the commer-

cial exploitation of bainitic steels. There were dif®culties in obtaining fully

bainitic microstructures in sizable samples of steel. It has long been recognised

that the in¯uence of bainite on the mechanical behaviour of a steel is dif®cult to

understand because of the inability to attain fully bainitic microstructures at all

transformation temperatures, a consequence of the incomplete reaction phe-

nomenon (Hehemann et al., 1957). Isothermal transformation to bainite was

considered impractical on a commercial scale, continuous cooling being the

preferred heat treatment. Furthermore, continuous cooling at a rate greater

than ' 50 K s

1

during transformation was also believed impractical. In these

circumstances, lean steels gave mixed microstructures of allotriomorphic fer-

rite and bainite, whereas richly alloyed steels transformed only partly to bai-

nite, the remaining microstructure consisting of martensite and retained

austenite. It was not until low-alloy, low-carbon steels, containing small

amounts of boron and molybdenum to suppress proeutectoid ferrite were

developed that the potential for commercial exploitation became realistic

(Irvine and Pickering, 1957).

Boron is effective in retarding proeutectoid ferrite formation but has a neg-

ligible effect on the bainite reaction, allowing bainitic microstructures to be

obtained over a wider range of cooling rates. The segregation of boron to the

austenite grain boundaries leads to a reduction in their energy, thereby making

them less favourable as sites for the heterogeneous nucleation of ferrite. The

reason why the effect is more pronounced for allotriomorphic ferrite than for

bainite has not been investigated, but it may be associated with the fact that for

bainite, which grows in the form of sheaves of small platelets, the vast majority

of platelets nucleate autocatalytically after the initial formation of some plate-

lets at the austenite grain boundaries (Chapter 6). Boron thus increases the

bainite hardenability. The level of other alloying additions can, in the presence

of boron, be kept low enough to avoid the formation of martensite. Steels of

typical composition Fe±0.0033B±0.52Mn±0.54Mo±0.11Si±0.10C, wt% were

found to yield fully bainitic microstructures with very little martensite during

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normalising (i.e. air cooling from the austenitising temperature), and permitted

the characterisation of the mechanical properties of bainite in isolation.

Many investigations of the mechanical properties fail to recognise that the

microstructures studied were not fully bainitic. In the discussion that follows,

attention is restricted to cases where the microstructure has been characterised

thoroughly, and where it plays a signi®cant role in determining the mechanical

properties.

12.2 The Strength of Bainite

The strength of bainite can in principle be factorised into components consist-

ing of the intrinsic strength of pure annealed iron (

), substitutional solid

solution strengthening contributions (

), strengthening due to carbon in solid

solution (

), and a variety of microstructural components including disloca-

tion strengthening, particle effects and grain size effects. Thus,

  



 

 k



L



1

 k



1

 C



0:5

12:1

where 

is the dislocation density and  the average distance between a

cementite particle and its two or three nearest neighbours. From measurements

done on martensite, k



is approximately 115 MPa m; assuming that the

cementite particles are spherical and of a uniform size, k

is given approxi-

mately by 0:52 V



MPa m, where V



is the volume fraction of cementite (Daigne

et al., 1982). Dislocation theory for body-centred cubic metals gives C

 0:38

b ' 7:34 Pam (Keh and Weissmann, 1963). The carbon and substitutional

solutes are listed separately because their solid solution strengthening contri-

butions vary differently with concentration. For carbon, the strengthening var-

ies with the square root of concentration (Speich and Warlimont, 1968;

Christian, 1971), whereas for the substitutional solutes there is a direct relation-

ship (Leslie, 1982). Equation 12.1 illustrates the form of the relationships, it is in

practice dif®cult to decipher the microstructural contributions because para-

meters such as grain size and particle spacing cannot be varied independently.

Figure 12.1 illustrates the magnitudes of the terms involved, together with

some typical data for a fully bainitic microstructure.

12.2.1 Hardness

The hardness of bainite increases linearly with carbon concentration, by

approximately 190 HV per wt% (Irvine and Pickering, 1965). This contrasts

with a change of about 950 HV per wt% in the case of carbon-supersaturated

martensite. The austenitising temperature does not in¯uence the hardness

unless it is not high enough to dissolve all the carbides (Irvine and

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