Bhadeshia H.K.D.H., Honeycombe R. Steels: Microstructure and Properties

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158 CHAPTER 7 ACICULAR FERRITE

Fig. 7.3 Interference contrast micrograph showing the surface relief caused when a metallo-

graphically polished sample of steel is transformed to acicular ferrite (courtesy of Strangwood).

transformation. If is not therefore surprising that the concentrations of substi-

tutional alloying elements are unchanged during the growth of acicular ferrite.

The deformation which changes the austenite into acicular ferrite occurs on

particular planes and directions, so that the ferrite structure and orientation

are intimately related to that of the austenite. It follows that plates of acicular

ferrite, like bainite, must without exception have an orientation relationship

with the austenite. This is not necessarily the case when a transformation occurs

by a diffusional mechanism, because a grain of ferrite can easily grow into any

adjacent grain of austenite with which it happens to come into contact.

During isothermal transformation, the acicular ferrite reaction stops when

the carbon concentration of the remaining austenite makes it impossible to

decompose without diffusion. This implies that the plates of acicular ferrite

grow supersaturated with carbon, but the excess carbon is shortly afterwards

rejected into the remaining austenite. This of course is the incomplete reac-

tion phenomenon described in Chapter 6 for bainite, where the austenite never

reaches its equilibrium composition since the reaction stops at the T

′

curve of

the phase diagram (Fig. 7.4). The obvious conclusion is that acicular ferrite can-

not form at temperatures above the bainite-start temperature,and this is indeed

found to be the case in practice.

There are many other correlations which reveal the analogy between acic-

ular ferrite and bainite. For example, the removal of inclusions by vacuum arc

melting, without changing any other feature, causes an immediate change in the

microstructure from acicular ferrite to bainite. The same effect can be obtained

7.3 MECHANISM OF TRANSFORMATION 159

Fig. 7.4 Data from experiments in which the austenite is transformed isothermally to acicular

ferrite, showing that the reaction stops when the carbon concentration of the austenite reaches

the T

′

curve (courtesy of Strangwood).

by increasing the number density of austenite grain nucleation sites relative to

intragranular sites. This can be done by reﬁning the austenite grains to obtain a

transition from an acicular ferrite microstructure to one which is predominantly

bainitic (Fig. 7.5).

The opposite phenomenon, in which an inclusion-containing steel with bai-

nite can be induced to transform into an acicular ferrite microstructure is also

observed. This can be done by rendering the austenite grain surfaces ineffective

as nucleation sites, either by decorating the boundaries with a thin layer of inert

Fig. 7.5 Illustration of how the microstructure changes from one which is predominantly

acicular ferrite, to another which is mostly bainitic as the austenite grain size is refined.

160 CHAPTER 7 ACICULAR FERRITE

Fig. 7.6 The change from a bainitic (a) to an acicular ferrite (b) microstructure when the

austenite grain boundaries are eliminated as nucleation sites by decoration with inert layers of

ferrite (courtesy of Babu).

7.4 THE INCLUSIONS AS HETEROGENEOUS NUCLEATION SITES 161

allotriomorphic ferrite (Fig. 7.6) or by adding a small amount of boron (30 ppm).

The boron segregates to the boundaries, thereby reducing the boundary energy

and making them less favourable sites for heterogeneous nucleation. In general,

any method which increases the number density of intragranular nucleation

sites relative to austenite grain boundary sites will favour the acicular ferrite

microstructure.

7.4 THE INCLUSIONS AS HETEROGENEOUS NUCLEATION SITES

Many experiments show that inclusions rich in titanium are most effective in aci-

cular ferrite production (Fig. 7.7). A number of different mechanisms have been

proposed. It is rare, however, that the speciﬁc titanium compound responsible

for the observed effects is identiﬁed. This is because many of the compounds

have similar crystal structures and lattice parameters. When microanalysis is

used, elements such as C, N and O are either undetectable or cannot be esti-

mated withsufﬁcient accuracyto determine the stoichiometric ratio with respect

to Ti. In reality, the non-metallic inclusions tend to consist of many crystalline

and amorphous phases, so that it becomes difﬁcult to identify the particular

component responsible for nucleation of acicular ferrite.

There are now many results which prove that the inclusions responsible for

the heterogeneous nucleation of acicular ferrite are themselves inhomogeneous,

as illustrated in Fig. 7.8. The microstructure of the inclusions is particularly

important from the point of view of developing a clear understanding of their

role in stimulating the nucleation of ferrite.As an example,it is sometimes found

that the non-metallic particles in some submerged arc weld deposits consist of

titanium nitride cores, surrounded by a glassy phase containing manganese,

Fig. 7.7 Large change in the acicular ferrite content as titanium is introduced into a welding

alloy (after G. Evans, 1992).

162 CHAPTER 7 ACICULAR FERRITE

Fig. 7.8 Scanning transmission electron micrograph of a non-metallic inclusion in a steel weld

metal. The inclusion surface is very irregular, and it features many phases (courtesy of Barritte).

silicon and aluminium oxides, with a thin layer of manganese sulphide (and pos-

sibly, titanium oxide) partly covering the surface of the inclusions. Theinclusions

may therefore be a wide variety of oxides or other compounds, but some can

inﬂuence the development of microstructure during cooling.

7.5 NUCLEATION OF ACICULAR FERRITE

It has been demonstrated, assuming classical nucleation theory, that inclusions

are less effective in nucleating ferrite when compared with austenite grain

surfaces. Experiments conﬁrm this since ferrite formation ﬁrst begins at the

austenite grain boundaries. Furthermore, larger inclusions are expected to be

more effective since the curvature of the inclusion/ferrite interface will then

be reduced. This is conﬁrmed by experimental observations.

7.5.1 Lattice matching theory

Inclusions have long been used to control solidiﬁcation in aluminium alloys.

The aluminium melts are inocultated with particles in order to increase the

solid nucleation rate and hence produce a reﬁned grain structure in the fully

solidiﬁed condition. It is found that inclusions whose lattices match well with

solid aluminium are quite effective nucleating agents. This idea has been extrap-

olated to solid state transformations in steels, where it is argued that those

inclusions which show the best ‘lattice matching’ with ferrite are most effective

in nucleating the ferrite.

7.5 NUCLEATION OF ACICULAR FERRITE 163

Table 7.1 Some misfit values between different substrates and ferrite.The data are from a

more detailed set published Mills, A. R.,Thewlis, G. and Whiteman, J. A., Materials Science

andTechnology 3, 1051, 1987) and include all cases where the misfit is found to be less than

5%. The inclusions all have a cubic-F lattice and the ferrite is body-centred cubic (cubic-I)

Inclusion Orientation Plane of epitaxy Misﬁt (%)

TiO Bain {1 0 0} 3.0

TiN Bain {1 0 0} 4.6

γ-alumina Bain {1 0 0} 3.2

Galaxite Bain {1 0 0} 1.8

CuS Cube {1 1 1} 2.8

The lattice matching is expressed in terms of a mean percentage planar

misﬁt κ. To calculate κ, it is assumed that the inclusion is faceted on a plane

(hkl)

,and that the ferrite deposits epitaxially with its plane (hkl)

||(hkl)

with the corresponding rational directions [uvw]

, and [uvw]

being inclined at

an angle φ to each other. The interatomic spacings d along three such directions

( j =1, 2, 3) within the plane of epitaxy are examined to obtain:

κ =

100



j=1

cos φ − d

|/d

. (7.1)

Data calculated in this manner, for a variety of inclusions phases, are presented

in Table 7.1.

To enable the lattice matching concept to be compared against experiments,

it is necessary not only to obtain the right orientation relationship, but the

inclusion must also be faceted on the correct plane of epitaxy. Many com-

pounds, including some of the titanium oxides, show good matching with ferrite,

and indeed seem effective in nucleating ferrite. However, there are other com-

pounds, such as γ-alumina, which show good ﬁt but are ineffective nucleants. It

is likely that there is more than one mechanism which helps make a nonmetallic

phase a potent heterogeneous nucleation site.

7.5.2 Other possibilities

Other ways in which inclusions may assist the formation of acicular ferrite

include stimulation by thermal strains or by the presence of chemical het-

erogeneities in the vicinity of the inclusion/matrix interface. Alternatively, the

inclusions may simply act as inert sites for heterogenous nucleation. Chemical

reactions are also possible at the inclusion matrix interface (Table 7.2). Those

minerals which are natural oxygen sources are found to be very effective in

stimulating nucleation, probably by inducing decarburization in the adjacent

steel. This effect seems to be independent of the crystallographic nature of the

164 CHAPTER 7 ACICULAR FERRITE

Table 7.2 List of ceramics which have been tested for their potency in stimulating the

nucleation of ferrite plates Gregg, J. M., Bhadeshia, H. K. D. H., Acta Materialia 45, 739,

1997

Effective: oxygen sources Effective: other mechanisms Ineffective

TiO

, SnO

TiN, CaTiO

MnO

, PbO

TiO SrTiO

, α-Al

KNO

NbC

mineral, except in the ability of the mineral to tolerate oxygen vacancy defects,

or to thermally decompose. Ti

has the ability to cause a dramatic reduction

in the manganese concentration of the adjacent steel, and this in turn stimu-

lates nucleation since manganese is an austenite stabilizer. TiO is puzzling in

the sense that it is an effective nucleant and yet does not cause any pronounced

modiﬁcation of the adjacent steel. It does have a good lattice match with ferrite,

but so does TiN, which is not an effective nucleant.

7.6 SUMMARY

Bainite and acicular ferrite have essentially the same transformation mech-

anism, but their microstructures differ in detail because the former nucleates

at grain surfaces and hence grows in the form of sheaves of parallel platelets.

Acicular ferrite, on the other hand, nucleates intragranularly on non-metallic

inclusions, which are in effect point nucleation sites. The platelets of acicu-

lar ferrite therefore radiate from the individual inclusions, thus generating a

microstructure which is much more disorganized with adjacent platelets point-

ingin differentdirections.There aremany kinds ofnon-metallic inclusions which

are effective in stimulating intragranular nucleation, but some titanium com-

pounds are found to be particularly potent. The exact mechanism of nucleation

remains to be resolved.

Acicular ferrite grows without diffusion, but the excess carbon is not retained

in the supersaturated ferrite. It is partitioned into the residual austenite shortly

after growth. The transformation is accompanied by shear, and rather smaller

dilatational displacements which together with the reproducible orientation

relationship, the plate shape and lack of chemical composition change ﬁt a

displacive mechanism of transformation.

FURTHER READING 165

Bhadeshia, H. K. D. H., Acicular ferrite, Bainite in Steels, 2nd edition Institute of Materials,

pp. 237–276, 2001.

Bhadeshia, H. K. D. H., Models of acicular ferrite, International Trends in Welding Science and

Technology (eds David S. A. and Vitek, J. M.), ASM International, pp. 213–222, 1993.

Bhadeshia, H. K. D. H. and Svensson, L.-E., Mathematical Modelling of Weld Phenomena (eds

Cerjak, H. and Easterling, K. E.), Institute of Materials, London, pp. 109–182, 1993.

Chandrasekharaiah, M. N., Dubben, G. and Kolster, B. H., Atom probe study of retained

austenite in ferritic weld metal, Welding Journal 71, 247s, 1994.

Gourgues,A. F., Flower, H. M. and Lindley, T. C., EBSD study of acicular ferrite, bainite and

martensite, Materials Science and Technology 16, 26, 2000.

Grong, Ø., Metallurgical Modelling of Welding, Institute of Materials, London, 1994.

Grong, Ø. and Matlock, D. K., International Metals Reviews, 31, 27, 1986.

Nishioka, K. and Temehiro, H., Microalloying ’88, Chicago, 1988.

Sugden,A.A. B. and Bhadeshia, H. K. D. H.,Lower acicular ferrite, Metallurgical Transactions

20A, 1811. 1989.

Svensson, L.-E. Control of Weld Microstructures and Properties, CRC Press, London, 1994.

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THE HEAT TREATMENT OF STEELS:

HARDENABILITY

8.1 INTRODUCTION

The traditional route to high strength in steels is by quenching to form marten-

site which is subsequently reheated or tempered at an intermediate temperature,

increasing the toughness of the steel without too great a loss in strength. There-

fore, for the optimum development of strength, a steel must ﬁrst be fully

converted to martensite. To achieve this, the steel must be quenched at a rate

sufﬁciently rapid to avoid the decomposition of austenite during cooling to

such products as ferrite, pearlite and bainite. The effectiveness of the quench

will depend primarily on two factors: the geometry of the specimen, and the

composition of the steel.

A large diameter rod quenched in a particular medium will obviously cool

more slowly than a small diameter rod given a similar treatment. Therefore, the

small rod is more likely to become fully martensitic.With the exception of cobalt

and aluminium, the addition of common alloying elements to a steel usually

moves the time–temperature–transformation (TTT) curve to longer times, thus

making it easier to pass the nose of the curve during a quenching operation,

i.e. there is a reduction in the critical rate of cooling needed to make a steel

specimen fully martensitic. If this critical cooling rate is not achieved a steel rod

will be martensitic in the outer regions which cool faster but, in the core, the

slower cooling rate will give rise to bainite, ferrite and pearlite depending on

the exact circumstances.

The ability of a steel to form martensite on quenching is referred to as the

hardenability.This can be simply expressed for steel rods of standard size,as the

distance below the surface at which there is 50% transformation to martensite

after a standard quenching treatment, and is thus a measure of the depth of

hardening.

167