STRUCTURE

UDC 669.017.3:669.112.227.33

INVESTIGATION OF THE FINE STRUCTURE OF ACICULAR FERRITE.

PART 2

V. I. Bol’shakov,1 G. D. Sukhomlin,1 and V. I. Kuksenko1

Translated from Metallovedenie i Termicheskaya Obrabotka Metallov, No. 10, pp. 3 – 9, October, 2009.

Problems of classification, initiation, and structure of intermediate structures in low-carbon steels are dis-

cussed. The first part of the work (No. 8, 2009) was devoted to a detailed study of the morphological and crys-

tallographic parameters of such structures and explanation of the high strength and viscoplastic properties of

steels with structure of acicular ferrite. Crystallographic aspects of acicular ferrite and topics of dislocation

and carbide hardening in this structure are considered below.

Keywords: low-carbon steel, phase transformations, structure, bainite, acicular ferrite.

INTRODUCTION

Steels with structure of bainitic type possess high strength,

plasticity, and viscoplastic properties and are widely used in

the building industry. Formation of bainitic structures by oil

cooling after austenization ensures the required properties in

rolled products from sparingly alloyed steels without addi-

tional tempering. This makes investigation of steels with

bainitic structure important enough.

In the first part of the present work we described the re-

sults of a study of steel 10G2FB after decomposition at inter-

mediate rates by methods of light microscopy. We showed

that the main structural components after the mentioned kind

of treatment are acicular ferrite and hypoeutectoid (allotrio-

morphic) ferrite. In addition, we considered different systems

of classification of the products of intermediate transforma-

tion in low-carbon steels and discussed the topics of nucle-

ation of acicular ferrite and the geometrical parameters of

this structure.

The present paper is devoted to studying the fine struc-

ture of acicular ferrite in steel 10G2FB after decomposition

of austenite in the intermediate temperature range.

RESULTS AND DISCUSSION

Retained austenite. As we have noted, acicular ferrite

consists of parallel laths united into packets. Additional com-

ponents of the structure of a packet are layers of retained aus-

tenite forming as a result of an incomplete � � � transforma-

tion. According to the data of [1 – 3] the retained austenite

can have an orientation relation (OR) with bainite laths,

which is close to that of Kurdyumov – Sachs [4] or to that of

Nishiyama – Wasserman [5, 6]. We should note that these

orientation relations are close and pass from one to another

by formal rotation of the crystallographic lattice by 5.26°

around axis [011]�

� [111]�.

In some systems of classification of products of interme-

diate transformation [2, 7] the structure with retained austen-

ite between bainite laths is singled out as a specific kind. In

our work the terms “bainite” and “acicular ferrite” are used

independently of the presence or absence or retained austen-

ite over lath boundaries. According to some data [8, 9] re-

tained austenite creates a certain margin of ductility in steels

under deformation.

Figure 1a presents an electron micrograph of a region of

acicular ferrite. Deciphering of the microscopic diffraction

pattern of the given region shows that in addition to reflec-

tions of ferrite it contains reflections of retained austenite

(Fig. 1b ). The dark-background image in reflection (113) of

austenite given in Fig. 1c demonstrates vividly that it has the

form of discontinuous layers over lath boundaries of acicular

ferrite at a thickness of no more than 50 nm. Imposition of a

large circle of reflecting planes of the axes of ferrite and aus-

tenite zones onto a double stereo projection (Fig. 1d ) con-

firms the assumption that the laths of acicular ferrite and the

layers of retained austenite obey the Kurdyumov – Sachs ori-

entation relation. A number of microscopic diffraction stu-

dies have shown the absence of the Nishiyama – Wasserman

Metal Science and Heat Treatment Vol. 51, Nos. 9 – 10, 2009

461

0026-0673/09/0910-0461 © 2009 Springer Science + Business Media, Inc.

1Pridneprovie State Academy for Building and Architecture,

Dnepropetrovsk, Ukraine (E-mail: s.kuksenko@mail.ru).

relation in steel 10G2FB with structure of acicular ferrite in

contrast to high-alloy steels [10 – 13].

Crystallographic aspects of the structure of acicular

ferrite. F. B. Pickering [3] has suggested that the crystallo-

graphy of bainite should be explained under the assumption

that individual ferrite laths are formed according to different

variants of Kurdyumov – Sachs or Nishiyama – Wasserman

relations so that the orientation of laths inside one packet

correspond to rotation of the ferrite lattice over a normal to

one and the same close-packed austenite plane. Our studies

show that a packet of acicular ferrite in steel 10G2FB con-

sists of plane-parallel laths with crystal lattices oriented with

respect to the initial austenite in accordance with the

(011)�

� (111)�

and [111]�

� [011]�

Kurdyumov – Sachs OR.

This is partial evidence in favor of Pickering’s suggestion.

The mutual orientation is visually presented in Fig. 2a. The

laths of one packet have a common axis [011]�

perpendicular

to a common habit plane close to (111)�.

We have shown in [14] after analyzing the Kurdyu-

mov – Sachs OR that a packet of low-carbon lath martensite

can contain six possible mutual orientations of laths

(Fig. 2b ) each of which corresponds to a certain density of

coinciding sites on the boundary. Four paired variants with

respect to the density of the coinciding sites (�3, 11, 33, and

129) are possible within the concept of lattices of coinciding

sites. These theoretical computations have been proved ex-

perimentally.

Assuming that the acicular ferrite and the austenite obey

the same OR of Kurdyumov – Sachs as low-carbon marten-

site, we were interested in checking the presence of special

orientations in packets of acicular ferrite.

The special orientations were determined by analyzing

the microdiffraction patterns from neighbor laths and com-

paring the orientations obtained with the existing double ste-

reo projections for the presumed lattices of coinciding sites.

In order obtain reliable results we used a goniometer that al-

lowed us to rotate and incline the studied foils.

The photograph and the microdiffraction pattern pre-

sented in Figs. 3a and b match the orientation relation �3

and the concept of lattices of coinciding sites [15], i.e., a

twinning orientation exists between the laths. The micro-

diffraction pattern from the laths corresponds to the angular

462 V. I. Bol’shakov et al.

351

351

310

310

2 11

2 11

1 12

1 53

2 24

1 21013

101

111

0 31

1 21

2 11

1 12

101101

101

111 101

113

113

011

131

111

741

à b

ñ

d

Fig. 1. Microstructure of steel 10G2FB after austenization at 950°C and accelerated cooling at a rate of

15 K�sec: a) light-background image, � 15,000; b ) microdiffraction pattern, axes of zones [131] (Fe)�

and

[741](Fe)�

; c) dark-background image in reflection (113) of austenite, � 15,000; d ) double stereo projection

of the Kurdyumov – Sachs orientation relation.

relations of the double stereoscopic projection for �3 accu-

rate to 0.3° in the azimuthal direction.

We managed to determine all of the expected special

boundaries of �3, �11, �33, and �129. The twinning

boundary �3 is encountered most frequently due to its low

energy. A more detailed study is required for obtaining more

reliable statistical data. It should be noted that our data on

high-angle boundaries between laths in one packet contradict

the data of [16 – 18], where it is stated that a bainite packet

consists of laths separated by low-angle boundaries.

The mutual orientation of laths of acicular ferrite in

Fig. 4 corresponds to the lattice of coinciding angles �33.

The microdiffraction pattern (Fig. 4b ) from a whole group of

laths gives only two systems of reflections with axes of

zones [100]�1

and [111]�2

, which corresponds to special

boundary �33 between the laths. Approach from the stand-

point of the theory of lattices of coinciding sites allowed us

to have a new look on the microdiffraction pattern known, in

particular, from work [19]. A dark-background image in re-

flections belonging to different lattices (Fig. 4c and d ) shows

that the laths with crystallographic orientation alternate near-

est next. A scheme of this case in given in Fig. 4e. This fact is

explainable by the appearance of a field of stresses due to

formation of new laths. Alternation of lattices with opposite

orientations makes it possible to compensate the stresses ap-

pearing on the interface of neighbor laths and to lower the to-

tal energy of the system.

Investigation of the Fine Structure of Acicular Ferrite 463

(111)�

[0 1] [111]1 � ���

(011) (111)� ���

[111] [011]� ���[111] [110]� ���

(110)�

1-6

Kurdyumov – Sachs OR

5

2

1 4

6

3

à b

Fig. 2. Scheme of arrangement of planes in

formation of acicular ferrite in accordance

with one of the variants or Kurdyumov –

Sachs orientation relation (OR) (a) and six

variants of arrangement of crystal lattices of

ferrite on plane (111)�

in accordance with Kur-

dyumov – Sachs OR (b ).

2

1

à

cb100

100

1 14

1 23

1 32

1 41

1 23

122

2 11

1 22

122

211

100

1 22

1 11

1 32

1 11

511

211

2 11

0 22

0 11

011

1 01

2 11

1 11 312

101

100

101

213

112

123

101

211

111321000

110

011

111

110

231

121

132011

011

Fig. 3. Results of a study of mutual orientation of neighbor laths of acicular ferrite: a) region of acicular ferrite, � 15,000;

b ) microdiffraction pattern from laths 1 and 2, axis of zones [111]1

and [511]2; c) double stereo projection �3 with depos-

ited crystallographic orientation of neighbor laths of acicular ferrite.

Thus, the boundaries between laths are high-angle but

low-energy ones because they possess special mutual orien-

tations. The existence of specific orientation relations be-

tween neighbor laths is responsible for the diminished energy

of the lath boundaries, increases permeation of the boun-

daries by moving dislocations, and explains the compara-

tively high viscoplastic and plastic characteristics of the steel

with structure of acicular ferrite.

From the standpoint of methodology we should stress the

fact that full matching of the orientations of neighbor crystals

to the double stereo projection of the orientation relation in

both axial and azimuthal directions can be ensured with the

help of a goniometer, when the axes of the zones of the two

crystals correspond to coinciding poles on the double stereo

projection (for example, as in Fig. 3). Without the use of a

goniometer, cases of full matching are quite rare. Spe-

cifically, the axes of the zones can deviate by � 5°. However,

if the axes of the zones of two reciprocal lattices do not

match each other, we can judge on the correspondence be-

tween mutual orientations of neighbor laths by determining

their accurate mutual location in the reflection from the

planes in the azimuthal direction [20].

Carbide segregations. The properties of steel 10G2FB

with structure of intermediate type are affected by particles

of iron carbide and by segregations of special carbides of ni-

obium and vanadium. Iron carbides segregating during the

decomposition of austenite, which yields a structure of

acicular ferrite, are quite rare and do not affect noticeably the

properties of steels with such structure. Segregations of spe-

cial carbides are numerous and therefore raise considerably

the properties of the steel [21].

Steel 10G2FB bears microadditives of niobium and va-

nadium, which usually have the form disperse carbide

segregations at room temperature. Niobium and vanadium

carbides have an fcc structure of type B1 [22]. It is known

that the lattices of NbC and VC are conjugate to the ferrite

matrix through the Baker – Nutting relation [23], i.e.,

(100)fcc

� (100)bcc

, [100]fcc

� [100]bcc

. The presence of such

relation indicates that these carbides have segregated inside

laths of bainitic ferrite from the solid solution upon a de-

crease in the temperature or on the interface of the �- and

�-phases during the transformation.

Preliminary studies have shown that detection of reflec-

tions from NbC and VC carbides on microdiffraction pat-

terns or under electron microscope is problematic due to

their relatively small specific volume. In this connection we

resorted to obtaining a dark background from NbC and VC

particles, in accordance with which the specimen was in-

clined with the help of a goniometer until the axis of ferrite

zone [100] became parallel to the primary beam of electrons.

Then, with allowance for the parallel arrangement of vectors

[100]�

and [100]Nb(V)C

and for the known interfacial dis-

tances in the carbide [22, 24], we placed a small enough se-

464 V. I. Bol’shakov et al.

à

c d

b

e

�33

�33

�1

121

110

022

000

000

022

020

011

011

011

0 02

0 11

101

031031

022 022002

040

1 01

0111 10

011 121

(110)(111)

�

�

2��

(110)�1

Fig. 4. Results of a study of mutual orientation of neighbor laths of ferrite: a) region of acicular ferrite, � 15,000; b )

microdiffraction pattern obtained from the studied laths, axes of zones [100]�

and [100] corresponding to �33; c, d ) dark-back-

ground image in reflection of the lattice with axis of zone [111] and [100], respectively, � 15,000; e) drawing of mutual location

of laths in the studied packet.

lection diaphragm at the presumed place of occurrence of re-

flection [100]Nb(V)C

.

We established that many carbide particles were located

along lath boundaries and on sub-boundaries inside laths

(Fig. 5). This can be treated as a proof of nucleation of car-

bides on the already existing subgrain boundaries that have

formed by the time. Many of the detected special carbides

were located in the body of laths and were not associated

with subgrains. In both cases these carbides were effective

hardeners due to the process of precipitation hardening.

Studies of the fine structure of crystals with the help of a

high-resolution electron microscope by direct resolution of

atomic planes allowed us to obtain more detailed data on the

structure, sizes, and composition of special carbides in the

studied steel (Fig. 6); this is described in detail in [25].

It should be noted that coarse carbides promote genera-

tion of cracks of supercritical size and cause brittle fracture

[1, 17, 26, 27]. The data of our microscope studies show that

the segregations of special carbides are chiefly no larger than

20 nm in size. They are active obstacles in the way of mo-

ving dislocations and thus raise the strength properties pre-

venting considerable accumulation of dislocations and not

promoting crack nucleation.

Dislocation density. Dislocation density in products of

intermediate transformation is a result of the transforma-

tion-induced phase hardening. Other conditions being equal

the dislocation density increases upon increase in the cooling

rate of the metal and decrease in the temperature of forma-

tion of bainite laths. Growth in the dislocation density raises

the strength characteristics of the metal, but decrease in the

mobility of dislocations and formation of large dislocation

clusters upon growth in their number can lead to nucleation

of brittle cracks.

The elevated dislocation density in acicular ferrite deter-

mines to a great degree the high strength properties of the

steel. According to the results of the electron microscope

study the dislocation density in acicular ferrite after austeni-

zation and cooling at a rate of 15 K�sec was 2.5 � 1010 cm – 2.

This agrees with the data of other researchers who have mea-

sured the dislocation density in upper bainite by the x-ray

method and obtained a value on the order of 1 � 1010 cm – 2

[1, 28]. The dislocation density increases monotonically

upon increase in the cooling rate of the steel and in accor-

dance with decrease in the transformation temperature. In

this connection we may expect an elevated dislocation den-

sity in thinner laths formed at lower temperatures.

It is important to note that the parameters of plasticity

and toughness are affected substantially by the uniformity of

the distribution of dislocations, which lowers the possibility

of the appearance of peak stresses and of fixation of disloca-

tions by the atmospheres of interstitial elements.

Comparison of the results obtained with the data of [29]

gives us grounds to assume that carbon content of 0.1% is

quite sufficient for formation of saturated atmospheres near

dislocations and for their fixation. This fact affects the

strength properties of the metal positively and the plastic and

viscoplastic properties negatively. However, the results of

the determination of the impact toughness of steel 10G2FB

with structure of acicular ferrite (KCV + 20 � 1.65 MJ�m2,

KCV – 40 � 1.40 MJ�m2 ) allow us to state that the dislocation

density of about 1 � 1010 cm – 2 is not dangerous from the

standpoint of excess embrittlement of the steel.

Further strengthening due to growth in the dislocation

density seems to be possible in two ways, namely, either by

Investigation of the Fine Structure of Acicular Ferrite 465

à

b

Fig. 5. Microstructure of steel 10G2FB after cooling at a rate of

15 K�sec (a) and dark-background image in reflection of special

carbides (b ), � 15,000.

Fig. 6. Carbide particle (Nb, V)C in steel 19G2FB after austeniza-

tion and cooling at a rate of 15 K�sec, � 11,000,000.

increasing the rate of cooling of the rolled metal, which is

connected with technological difficulties under production

conditions, or by forming bainitic structures in decomposi-

tion of preliminarily strained austenite. Straining of the aus-

tenite causes formation of a great number of dislocations

with a tendency for the appearance of low-angle dislocation

boundaries that can be inherited by products of the interme-

diate transformation, in particular, by acicular ferrite

[29 – 31]. By controlling the parameters of thermomechani-

cal treatment (the temperature, the degree of deformation,

the cooling rate) we can ensure a favorable combination of

strength and viscoplastic properties, which is a quite promis-

ing technological and economical perspective. This means

that additional research is required for determining particular

parameters of the production processes under commercial

conditions.

CONCLUSIONS

A detailed staged study of the morphological and crystal-

lographic parameters has allowed us to explain the nature of

high strength and viscoplastic properties of steel with struc-

ture of acicular ferrite. It has been shown that in order to

raise the mechanical properties of low-carbon structural

steels it is acceptable to use accelerated oil cooling from the

austenitic range. Formation of intermediate structures in this

process ensures increase in the strength characteristics of the

metal at high enough levels of plasticity and toughness.

Acicular ferrite is singled out from the spectrum of interme-

diate structures of low-carbon steels as a structure providing

the combination of properties required from modern building

steels.

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