Microstructural characterization of controlled forged HSLA-80 steel by transmission electron microscopy

Materials Characterization 50 (2003) 305–315

Microstructural characterization of controlled forged HSLA-80

steel by transmission electron microscopy

Samar Dasa, A. Ghoshb, S. Chatterjeeb,*, P. Ramachandra Raoa,1

aNational Metallurgical Laboratory, Jamshedpur 831007, IndiabDepartment of Metallurgy, Bengal Engineering College, Deemed University, PO Botanic Garden, Howrah West Bengal 711-103, India

Received 15 July 2002; accepted 15 July 2003

Abstract

In this study, transmission electron microscopy (TEM) was used to evaluate the effect of controlled forging followed by

cooling at various rates on microstructure of an HSLA-80 steel. The observations demonstrate that water-quenched steel has

finer multiphase constituents of lath martensite, bainite and twined martensite, whereas air-cooling has resulted in a mixture of

bainitic ferrite, retained austenite or MA constituents along with some Widmanstatten ferrite. When the steel is cooled in sand,

the maximum volume fraction of polygonal ferrite (PF) was produced which, in turn, increased volume fraction of MA

constituents. Precipitation of fine q-Cu, Nb and Ti carbides and carbonitrides was observed and identified using energy

dispersive spectrometric analysis (EDS) and electron diffraction.

D 2003 Elsevier Inc. All rights reserved.

Keywords: Steel; EDS; TEM

1. Introduction alloying elements provide hardenability for transfor-

Development of HSLA-80 and HSLA-100 steels

have successfully replaced HY-80 and HY-100 steels

in naval applications due to their improved strength,

toughness at ambient and sub-ambient temperatures

along with good weldability and corrosion resistance

properties [1–5]. These steels are generally Cu-bear-

ing and alloyed with Ni, Mn, Mo, Cr, along with Nb,

Ti and V microalloying elements. Most of these

1044-5803/$ - see front matter D 2003 Elsevier Inc. All rights reserved.

doi:10.1016/j.matchar.2003.07.001

* Corresponding author. Tel.: +91-33-2668-4561; fax: +91-33-

2668-4564.

E-mail addresses: [email protected] (S. Das),

[email protected] (A. Ghosh), [email protected]

(S. Chatterjee).1 Present address: Benaras Hindu University, Varanasi 221005,

India.

mation of austenite into fine structures in heavy

sections. This structural homogeneity has enabled

these steels to be used in both offshore and onshore

pipeline and other structural applications. While a

significant quantity of these steels is being produced

through controlled rolling operations, forging has

become a competitive technique for producing such

steels. Forging is an isotropic deformation process

accompanied with high strain rate. Moreover, various

post-forging cooling rates can produce a variety of

microstructures, which ultimately result in different

combinations of strength and toughness in these steels.

The isothermal, continuous cooling transformation and

age hardening characteristics of ASTM 710/HSLA-80

steel have been studied by previous workers [6–10].

The microstructural characterization of as-rolled and

S. Das et al. / Materials Characterization 50 (2003) 305–315306

tempered HSLA-100 steels has also been reported

[11,12]. However, the detailed microstructural charac-

terization of HSLA forging steels has not been ex-

plored so far. In this present study, an HSLA-80 steel

has been subjected to a controlled forging operation

and subsequently cooled in three different mediums,

i.e. water, air and sand. The decomposition product of

the austenite as a result of controlled forging and

different post-forging cooling rates has been evaluated

using transmission electron microscopy (TEM). The

distribution of various precipitates was examined and

identified by energy dispersive X-ray spectroscopy

(EDS) and electron diffraction.

2. Experimental

The steel used in this study was supplied by Naval

Research Laboratory,Washington, DC, andUSA under

an INDO-US collaborative programme. The chemical

composition of the steel is shown in Table 1, which is

an HSLA-80 steel. Steel slabs of 50� 50� 300 mm

were reheated at 1200 jC and controlled forged in two

stages. In Stage I, 50% deformation was applied in

temperature range of 1100–1050 jC and in Stage II

another 50% deformation was given in temperature

range of 850–800 jC. Subsequently, the slabs were

cooled in sand, air and water. The operating temper-

atures were measured by embedding a thermocouple

into steel slabs. Optical metallographic samples, pre-

pared by conventional grinding and polishing techni-

ques, were etched with 2% nital solution and observed

in a light microscope. Volume percentage of different

phases were measured from optical micrographs using

image analyser. For the TEM study, thin slices were cut

by a low speed diamond saw and then subjected to

mechanical grinding. Three millimeter diameter discs

were punched and electrolytically thinned in Struer’s

Tenupol twin jet polisher using a mixture of 90% acetic

acid and 10% perchloric acid. The samples were

evaluated in a Philips CM200 TEM operated at 200

kV. Energy dispersive X-ray spectroscopy (EDS) mi-

Table 1

Chemical composition (wt.%)

C Mn S P Cu Ni Si Cr

0.05 1.0 0.001 0.009 1.23 1.77 0.34 0.6

croanalysis was carried out to determine the chemical

composition of various precipitates.

3. Results and discussion

Different cooling rates from same finish forging

temperature have produced a variety of transformed

microstructures in the final product, i.e. polygonal

ferrite, Widmanstatten ferrite, lath martensite, bainite,

interlath retained austenite and/or MA constituents.

The various transformation products of austenite

formed in different temperature ranges can be classi-

fied using TEM. Polygonal ferrite (PF) is a high

temperature decomposition product of austenite and

can be characterized by the presence of distinct grain

boundaries of polygonal or equiaxed shaped grains

containing a relatively low dislocation density. Acic-

ular ferrite is the highly dislocated non-equiaxed

ferrite that forms on continuous cooling in the tem-

perature range which is slightly higher than that for

the transformation to upper bainite. Bainite can be

characterized by the presence of highly dislocated

ferrite laths with precipitation of carbides at the

interlath or intralath region of ferrite. In upper bainite,

precipitation occurs predominantly at interlath loca-

tions, whereas in lower bainite the precipitates form in

the intralath region [13,14]. The term ‘‘granular bain-

ite,’’ particularly in low carbon steels, has been

described by Bramfitt and Speer [15] to include

aggregates of ferrite of any shape (acicular, polygonal

or featureless) together with retained austenite and/or

MA constituents. The martensitic transformation

product is generally differentiated by its transforma-

tion substructures, i.e. twin substructures (plate mar-

tensite) and dislocated substructures (lath martensite).

3.1. Optical microstructure of the steels at different

cooling rates

Optical microstructure of water-quenched sample

(Fig. 1a) shows a mixture of martensite and bainite.

Mo Ti Al Nb Carbon equivalent

1 0.51 0.03 0.025 0.037 0.69

Fig. 1. Optical micrograph of the steel cooled at different rates; (a)

35 jC/s (water-quenched), (b) 1.15 jC/s (air-cooled), (c) 0.68 jC/s(sand-cooled).

S. Das et al. / Materials Characterization 50 (2003) 305–315 307

Martensite of very fine lath size is observed in packet

form and comparatively coarser laths of bainitic ferrite

have also been noted. The optical micrograph of air-

cooled sample in Fig. 1b shows that the microstruc-

ture is predominantly bainitic ferrite. The acicularity

of fine bainitic ferrite laths was also observed. The

distributions of discrete black regions in the bainitic

ferrite matrix were either retained austenite or mar-

tensite/austenite (MA) constituents. The optical mi-

crograph of sand-cooled sample (Fig. 1c) shows that

the microstructure is predominantly ferrite along with

randomly distributed dark islands. Acicularity and

fineness of the ferrite is decreased, and a polygonal

and/or quasi-polygonal shape has evolved. The size of

discrete second phase particles in the ferrite matrix has

increased compared to that in water-quenched or air-

cooled samples.

3.2. TEM study

3.2.1. Microstructure of water-quenched (cooling rate

35 jC/s) steelWater quenching from the finish forging tempera-

ture of 800 jC has produced generally fine lath

structure of the steel. Fig. 2a shows a low magnifica-

tion bright-field TEM image of highly dislocated lath

martensite packets with flat interfaces. Dispersed

twined martensite islands were observed in the lath

(Fig. 2b). The corresponding dark-field TEM image of

Fig. 2c clearly shows the twinned martensite island in

the intralath position and also at the lath boundary. A

selected area diffraction pattern (SADP) is obtained

from one of these twinned martensite regions and is

shown in Fig. 2d. The schematic illustration of dif-

fraction pattern, Fig. 2e, clearly indicates the matrix

and twin spots along with double diffraction spots.

The matrix has [13̄1]M zone axis and twins to be

[1̄31̄]T indicating that twining has occurred along

(211)M/T plane. The fine twins are generally associat-

ed with high carbon plate martensite due to a signif-

icant enrichment of carbon in austenite. Regarding the

formation of interphase twined martensite Dunne et al.

[7] have speculated based on their experimental

observations that carbon-enriched austenite would

transform to a twin-related ferrite since this path

minimises the accommodation of strain energy gen-

erated by the formation of adjacent plate shaped

crystals of ferrite. Based on earlier observations

[7,16], it was found that the carbon concentration in

the twined martensite was greater than 0.4 wt.% C.

Speich [16] observed that the quenched structure of a

medium carbon steel containing 0.45–0.55 wt.% C

Fig. 2. Transmission electron micrographs of water-quenched steel. (a) Bright-field image shows packet of lath martensite. (b) Higher

magnification bright-field image showing twinned martensite islands. (c) Dark-field image of (b). (d) SADP obtained from one of these twined

martensite regions. (e) Schematic presentation of (d).


Fig. 3. Transmission electron micrographs of water-quenched steel. (a) Bright-field image shows sheaves of bainitic laths. (b) Bright-field image

showing upper bainite laths containing precipitation of cementite at the lath boundary (arrowed). (c) SADP from interlath region. (d) Schematic

presentation of (c). (e) An enlarged bright-field image of bainitic lath shows intralath needle-shape precipitation of carbide particles. (f) Dark-

field of (e).


Fig. 4. Bright-field TEM image of the air-cooled steel. (a)

Formation of Widmanstatten Ferrite (WF) saw teeth; (b) a closer

view of interface of WF where tiny precipitates appear to be

interacting with dislocations.


should be typically lath martensite with about 20%

twined martensite, though Wenpu et al. [11] have

observed 50% lath martensite and 1% granular twined

martensite in a quenched HSLA-100 steel. In this

controlled forged HSLA 80 steel, the volume percent

of lath martensite obtained is f 55 which is very

close to the observation reported by Wenpu et al. [11].

Approximately 40 vol.% bainite was observed in

the water-quenched steel. Fig. 3a shows a low mag-

nification bright-field TEM image of highly dislocated

upper bainite laths. These laths were up to f 2 Am in

width, which was coarser than the lath martensite size.

The bright-field TEM image in Fig 3b shows precip-

itation of a thin carbide particle at the lath boundaries

(arrowed). The SADP taken from this region is shown

in Fig 3c. The schematic illustration (Fig 3d) indicates

the carbide as cementite and the orientation relation-

ship between orthorhombic cementite and ferrite was

obtained as:

½011̄�aN½2̄01�c and ð011ÞaNð112Þc:

In the higher magnification bright-field TEM im-

age of bainite (Fig. 3e), needle-shaped precipitates

approximately 40–60 nm long and 4–8 nm thick are

observed in the intralath position. A dark band, which

may be an inclined lath boundary, is also visible. In

the dark-field TEM image of Fig. 3f, this dark band is

clearly revealed and it consists of numerous fine

carbide and carbonitride precipitates. Intralath nee-

dle-shaped precipitates are also clearly visible in the

dark-field image.

3.2.2. Microstructure of air-cooled (cooling rate 1.15

jC/s) sampleA change in microstructure was observed when the

steel was cooled in air from the same finish forging

temperature. The bright-field TEM image in Fig. 4a

shows the presence of Widmanstatten ferrite (WF) saw

teeth with a triangular-shaped grain. A reduced dislo-

cation content and low angle grain boundaries are

associated with the WF grains. A view of the interface

in Fig. 4b shows the presence of dislocation tangles

interacting with fine precipitates. Aaronson [17] has

described these type of perturbations at g/a interphase

region as the initial stage of development of Widman-

statten saw teeth. From their experimental evidence,

Thompson et al. [8,9] proposed that Widmanstatten

ferrite growth may be accompanied with precipitation

of Cu particles at the interface between the WF grain

and austenite. The mechanism of formation of WF is

still a matter of debate, but it is in general agreement

that formation of WF occurs at faster cooling rates and

at a lower temperature range than that for polygonal

ferrite. Thus, at the initial stage of air-cooling, austenite

has decomposed into WF and the growth has been


accompanied by interstitial as well as substitutional

atom diffusion. However, the structure of air-cooled

steel contains maximum volume fraction of bainitic

ferrite, which has formed at the later stage of air-

cooling. Fig. 5a shows a bright-field TEM micrograph

of bainitic ferrite comprised of acicular ferrite laths

along with an interlath thin film of retained austenite.

The dislocation density was higher than in WF formed

at the initial stage of cooling. The width of the bainitic

ferrite laths varies from f 0.4 to 1.5 Am, and the length

is in the range of f 4.4–5.4 Am. Some blocky darkly

imaging regions were identified as MA constituents. A

bright-field TEM image (Fig. 5b) shows one such

interlath darkly imaging region where fine microtwins

Fig. 5. Transmission electron images of the air-cooled steel. (a) Bright-

austenite and/or MA constituents. (b) Bright-field image shows fine twins

granular ferrite (GF).

are observed. In the dark-field TEM image (Fig. 5c),

the twins are clearly revealed.

The occurrence of granular ferrite during the con-

tinuous cooling transformation of austenite in ASTM

710 steel has been discussed by Thomson et al. [8], and

in the present study similar features have been observed

and are shown in the bright-field micrograph of Fig. 5d.

3.2.3. Microstructure of sand-cooled (cooling rate

0.68 jC/s) sampleThe bright-field TEM micrograph (Fig. 6a) of

sand-cooled steel shows polygonal ferrite with a

reduced dislocation density. The bright-field TEM

image of Fig. 6b shows the polygonal ferrite with

field image shows bainitic ferrite laths with intermediate retained

at interlath dark region. (c) Dark-field of (b). (d) Highly dislocated

Fig. 6. Bright-field TEM image of sand-cooled steel. (a) Polygonal

ferrite (PF), with a low dislocation density. (b) An enlarged view

showing polygonal ferrite with MA constituents.


dark chunky-shaped MA constituents. The volume

fraction of the second phase (i.e. retained austenite

and or MA constituents) has increased in the sand-

cooled samples.

Very slow cooling from the finish forging temper-

ature of 800 jC in sand has allowed sufficient time for

carbon diffusion in the high temperature range to

obtain the maximum volume fraction of polygonal

ferrite. A large volume fraction of ferrite is associated

with a substantial amount of carbon enrichment in the

austenite. From the CCT diagram of HSLA 80 steel

obtained by earlier workers [7–9], it has been ob-

served that after the completion of ferrite formation,

no further transformation occurs until the bainite start

temperature is reached. The occurrence of the classical

upper bainitic structure was observed for the faster

cooling rate (Fig. 3) where precipitation of cementite

occurred at the ferrite lath boundaries. But at the

slower cooling rates such as those for air- or sand-

cooling, dark chunky-shaped retained austenite and/or

MA constituents were randomly distributed in ferrite

matrix (Figs. 5 and 6b). The maximum extent up to

which the bainitic reaction can proceed depends on

the composition of residual austenite [18,19]. If the

free energy of austenite and ferrite of same composi-

tion are equal, then the bainitic reaction ceases. These

factors lead to the conclusion that after the completion

of polygonal ferrite formation when the equilibrium

carbon concentration between austenite and ferrite has

been reached, austenite has become stable. With

further decrease in temperature, the carbon-enriched

austenite either transforms into twin-related martens-

ite/MA constituents or remained as retained austenite.

3.2.4. Precipitation

One of the prime objectives of this TEM study was

to observe the precipitation behavior of fine micro-

alloying carbides/carbonitrides and q-Cu particles as a

result of the high strain induced in the material during

controlled forging and different post-forging cooling

rates. Faster quenching from finish forging temperature

of 800 jC has suppressed the precipitation except for

some fine carbides and carbonitrides that have been

observed to precipitate at the dislocation networks in

the bainitic lath as described earlier (Fig. 3). Compar-

atively slower cooling in air from the same finish

forging temperature has resulted in a larger distribution

of precipitates (less than 30 nm in size), preferentially at

the dislocation substructure as shown in Fig. 7a. In the

sand-cooled steel, the dark-field TEM image, Fig. 7b,

shows a distribution of fine precipitates (f 8–35 nm in

size) with some coarser precipitates (55–65 nm in size)

in ferrite matrix. The EDS spectrum taken from the

cuboid shape particle (arrowed) in Fig. 7b is shown in

Fig. 7c. The EDS spectrum shows the presence of

several elements, i.e. Ti, Nb and Cu in the particle

Fig. 7. (a) Bright-field TEM image showing precipitation of fine carbides and carbonitrides in the air-cooled steel; (b) dark-field TEM image

showing precipitation in the sand-cooled steel. (c) EDS spectra from the cuboid precipitate (arrowed in b). (d) Corresponding SADP from the

above region. (e) Schematic presentation of (c); (f) bright-field TEM image showing the presence of q-Cu precipitate. (g) Corresponding EDS

spectra from the q-Cu precipitate marked in (f).



which indicates that the particle is complex in nature

and a substantial amount of diffusion of interstitial as

well as substitutional atoms has occurred during slow-

cooling from finish forging temperature of 800 jC. AnSADP from this region shows ferrite reflections along

with precipitate reflections, Fig. 7d. The calculated

lattice parameter of this face-centered cubic precipitate

was 0.4481 nm, which correspond to an Nb(CN)

precipitate. From the schematic illustration of the

SADP (Fig. 7e), the orientation relation between ferrite

and Nb(CN) was obtained as

½3̄11�aII½01̄1̄�NbðCNÞ and ð011̄ÞaIIð200ÞNbðCNÞ

The bright-field TEM micrograph of Fig. 7f shows the

presence of a q-Cu particle (arrowed) in the ferrite

matrix in the sand-cooled steel. The EDS spectrum

obtained from this particle (Fig. 7g) confirms it as q-Cu.The precipitation behavior in a steel of similar

composition was studied by Mishra et al. [20]. They

observed precipitates nearly 100 nm in size located at

the lath boundaries after solutionizing and quenching

from 1100 jC. They had assumed these particles to be

the Nb-rich precipitate, but detailed TEM observations

had not been reported. In this present study, controlled

forging followed by cooling at different rates has

resulted in a more uniform distribution of finer precip-

itates: microalloy carbides, carbonitrides and q-Cu.

4. Conclusions

Controlled forging followed by cooling at different

rates in this HSLA-80 steel has produced the follow-

ing microstructural changes:

(1) Water-quenching from the 800 jC finish forging

temperature has resulted in low temperature

decomposition product of austenite, i.e. f 55

vol.% lath martensite, f 35–40 vol.% bainite

and some twined martensite, retained austenite

and/or MA constituents.

(2) Steel cooled in air was characterized by a

predominantly bainitic ferrite microstructure

along with fine precipitates of microalloying

carbides, carbonitrides, interlath retained austenite

and MA constituents. The lath size was coarser

than that of water-cooled steel. The high temper-

ature decomposition product of austenite, i.e.

Widmanstatten ferrite, was also observed in the

air-cooled steel.

(3) Sand-cooling has resulted in a maximum volume

fraction of polygonal ferrite associated with

precipitation of q-Cu, Ti and Nb carbides and

carbonitride precipitates. Some localized coarsen-

ing of precipitates has been observed. The size

and the volume fraction of second phase, i.e. MA

constituents and retained austenite, have increased

in sand-cooled steel.

Acknowledgements

The authors gratefully acknowledge the financial

support rendered by the Naval Research Laboratory,

Washington, DC, USA under INDO-US programme

for carrying out this investigation.

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Microstructural characterization of controlled forged HSLA-80 steel by transmission electron microscopy