Journal of Minerals & Materials Characterization & Engineering, Vol. 9, No.7, pp.621-633, 2010 Printed in the USA. All rights reserved
Mechanical Behaviou r of Duplex Ph ase St ructu res in a Me dium Carbon Low
Alloy Ste el
K.K. Alanemea ,b*, S. Ranganathana, T. Mojisolab
aDepartment of Materials Engineering, Indian Institute of Science, Bangalore, India.
bDepartment of Metallurgical and Materials Engineering, Federal University of Technology,
Akure, Nigeria
*Corresponding Author:
The mechanical behaviour of duplex phases produced in a medium carbon low alloy steel with
potentials for use as machine body parts and vehicle panels, has been investigated. A
representative composition of the steel (C: 0.3; Si: 0.28; Mn: 0.97; Cr: 0.15) was utilized to
produce ferrite martensite duplex phases of varied proportions by intercritical annealing
treatment. The tensile, hardness, and rotating bending fatigue behaviour of the structures were
studied; and optical and SEM microscopy utilized to characterize the microstructures and their
fracture characteristics. The duplex phase structures exhibited continuous yielding behaviour;
and were characterised by high strain hardenability, high tensile strength, total elongation,
toughness and superior fatigue strength (endurance limit) in comparison with the normalised
structure. The fatigue fracture was observed to be characterized by mixed mode of ductile
(dimple) fracture and intergranular brittle cleavage for the duplex structures. Superior tensile
and fatigue property combinations were better harnessed when treatment was performed at
7600C and 7800C in comparison to 7400C.
Keywor d s : duplex phase steel; intercritical annealing; fatigue behaviour; microscopy
The automobile and many other industries in the manufacturing sector are constantly in demand
of steel possessing a good compromise of toughness and plasticity at high strength levels [1].
The steel grades are usually required for the production of body parts of vehicles, trucks, and
machineries [1 3]. The steel grades commercially utilised for these applications are designed
with compositions usually containing micro-alloying elements like molybdenum, niobium,
622 K.K. Alaneme, S. Ranganathan, T. Mojisola Vol.9, No.7
nickel, and chromium among others, which function as strengtheners, phase stabilizers,
formability enhancers [4 6]. Intercritical annealing treatment has been adopted over the years
to produce dual phase str uctures in these steels whi ch consist of ferrite an d martensite/bainite [7
9]; and have shown superior combinations of strength, toughness and plasticity over
conventional high strength low alloy steels [10 11]. Mazinani and Poole studied deformation
behaviour of martensite in a low-carbon dual phase steel [5]; Sun and Pugh investigated the
properties of thermomechanically processed dual phase steels containing fibrous martensite
[6]; Xu et al researched on the mechanical properties of fine-grained dual phase low – carbon
steels based on dynamic transformation [10]. The findings of these researchers all point to the
superior strength and plasticity characteristics of dual phase steels over conventional high
strength low alloy steels. There have been also a great number of investigations on the role of
martensite proportion in influencing the tensile properties of dual phase steels [7, 11, and 14].
The amenability of dual phase steels for structural load bearing and dynamic stress applications
has also attracted research interest with some encouraging results [12 -13]. Ta yanc et al studied
the effect of carbon content on fatigue strength of dual phase steels in which they observed
superio r fatigue s trength of the dual ph ase steels over the as received samples and the limiting
effect of intercritical treatment temperature was highlighted [19]. Chakraborti and Mitra (2005)
studied LCF behaviour of two dual phase steels with lammelar morphology and found that the
volume fraction of martensite has an influence on the fatigue behaviour [20]. Hadianfard
investigated the low cycle fatigue behaviour and failure mechanism of a dual phase steel in
which the effect of high strain amplitude and low strain amplitude on damage mechanism in the
dual ph ase steel was reported [21] . But generall y most i nvestigation s in fati gue character istics o f
dual phase steels especially the fatigue mechanisms have generated divergent opinions judging
by the research findings of most researchers [19]. The present work is an effort to improve the
tensile and fatigue strength of a medium carbon low alloy steel grade with potentials for use as
engine seat and bumper protectors, and also for m achine body parts. The working conditions are
such that the parts are often subjected to static and dynamic stresses which make quasi static
(tensile) and fatigue stress considerations crucial for investigation if the use of the steel grade
among other choice materials is to be endorsed. A representative chemical composition of the
steel grade is used as test material for this research work.
The material for the investigation is a medium carbon low alloy steel as-supplied as cylindrical
rods of 16mm diameter. Chemical analysis to determine the composition of the steel was
performed spectrometrically. The chemical composition (in wt %) is as follows: C (0.3); Si
(0.28); Mn (0.97); P (0.0341); S (0.0021); Cr (0.15); Ni (0.035); Mo (0.0034); V (0.0012).
The rods were initially subjected to normalizing treatment to annul the thermal and mechanical
history of the steel. The normalizing treatment was carried out at 8600C for one hour in a muffle
Vol.9, No.7 Mechanical Behaviour of Duplex Phase Structures 623
furnace and then cooling in air. Cold rolling of the rods to approximately 50% of the original
diameter (8mm) was carried out before intercritical treatment was performed by first determining
the lower critical temper ature (Ac1) an d the uppe r critical temperatu re (Ac3) for the test material
follo wing empirical relation s in accordan ce with Gorni [ 15]. The t est pieces were th en treated at
intercritical temperatures of 7400C, 7600C, and 7800C; and were held for 30 minutes, followed
by quenching rapidly in oil to avoid the development of quench cracks which arise with the use
of water quenching for the above composition. Control samples were prepared by normalising a
set of the cold rolled samples at 8600C for 30 minutes and then air cooling.
2.1 Hardness and Tensile Testing
The macro hardness of the specimens was evaluated using a Rockwell Hardness Tester using a
‘C’ scal e ( HRC ). The s p eci men surfaces w ere i ni ti al l y poli sh ed us ing em er y pap ers an d d iam on d
– et han o l suspension to ensure a smooth surface is produced to allow for reliable determination
of the hardness values; also three four repeat tests were performed on each specimen and the
average taken as representative of the hardness attained for the corresponding treatment.
The tensile testing was performed at room temperature using a universal tensile testing machine.
Tensile test was performed using standard specifications in accordance with the ASTM E8M
91 standards [16]. The test was conducted on round specimens having a gage diameter of 5mm
and gage length of 27mm. All tests were performed at room temperature and at a quasi – static
strain rate of 10-3/s. Multi tests where performed for each treatment to ensure reliability of the
data generat ed.
2.2 Fatigue Tests
Fatigue tests were performed using a cantilever - type rotating bending fatigue testing machine
operated at 50Hz and 3000R.P.M. The fatigue specimens were prepared in accordance with
standard procedures having standard configurations of 5mm internal diameter, 6mm outer
diameter, and 6omm gauge length. Polishing prior to testing to achieve a smooth surface which
helps eliminate any potential surface discontinuity was carried out. Bending moments where
applied to each specimen and the number of cycles required to fracture the specimen was
recorded. The applied moment was continually reduced for successive specimens and the
corresponding cycles to failure recorded until a stress (moment) threshold was observed (after
108 Cycles) and no sign of crack initiation. The corresponding stress for each applied moment
was obtained by using the empirical relation –
S = 32M/Пd3 (2.1)
Where, S= Stress amplitude (MPa), M = bending moment (N-m), and d = diameter of the
specimen (mm).
624 K.K. Alaneme, S. Ranganathan, T. Mojisola Vol.9, No.7
2.3 Metallography and Fractography
The microstructural investigation was performed using a ZEISS Axiovert 200MAT optical
microscope. The specimens for the optical microscopy were polished using a series of emery
papers of grit sizes ranging from 500μm 1500μm; while fine polishing was performed using
polycrystalline diamond suspension of particle sizes ranging from 10μm 0.5μm with ethanol
solvent. The specimens were etched with 2%Nital solution for between 5 10 seconds before
observation in the optical microscope.
Fractography analysis of the fatigue fractured specimen surfaces was performed using a SERION
scanning electron microscope with the Secondary electron imaging performed using an applied
voltage of 15KV.
3.1 Heat-Treatment and Microstructure
Figure 1 show the microstructures developed from the intercritical treatment performed at 7400,
7600, and 7800C. From the micrographs, it is observed that the individual microstructures consist
essentially of ferrite (gray phase) and martensite (dark phase) but with varying volume fractions
of martensite. The volume fractions of martensite were estimated using Sigma scan pro image
analyzing software; and volume fractions of 36, 68 and 80% were obtained respectively for the
7400, 7600, and 7800C intercritical treatments.
Figure 1(a) Duplex Phase structure produced by intercritical Annealing at 7400C for 30minutes
then water quenching. The structure reveals a large proportion of ferrite (gray phase) and
martensite (dark phase)
Vol.9, No.7 Mechanical Behaviour of Duplex Phase Structures 625
Figure 1(b) Duplex Phase structure produced by intercritical annealing at 7600C for 30 minutes
then water quenching. The structure reveals blocky and fine grain distribution of ferrite (gray
phase) and martensite (dark phase)
Figure 1 (c) Duplex Phase structure produced by intercritical annealing at 7800C
for 30 minutes then water quenching. The structure reveals medium - sized and
fine grain distribution of ferrite (gray phase) and a large proportion of martensite (dark phase)
626 K.K. Alaneme, S. Ranganathan, T. Mojisola Vol.9, No.7
3.2 Tensile Properties
Figures 2 and 3 show the engineering stress –strai n and true st ress strain curves for the duplex
phase structures produced by treating at 7400, 7600, and 7800C. For the purpose of comparing
stress –strain behaviour, the stress-strain profiles of conventionally heat-treated normalized
structure is superimposed in the graphs. Observation of Figure 2 shows that the duplex phase
structures exhibited continuous yielding typical of traditional dual phase steel compositions
despite the large volume fractions of martensite especially for the 7600, and 7800C intercritical
treatment which yielded 68 and 80% respectively; while the conventional normalized structure
exhi bited as ex pected discontinuous yielding (presence of definite yield point). The continuous
yielding phenomenon has been explained by Bhattacharyya et al. [14] and Park et al. [17] to be
due to phase straining (plastic deformation) induced in the ferrite matrix, as a result of
accommodating the volume expansions associated with the austenite to martensite
transformation on quenching from the intercritical phase region. Thus when the duplex – ferrite
and martensite structure is subjected to tensile loading, plastic deformation commences
immediately (plastic deformation of the ferrite continues), resulting in the continuous yielding
behaviour observed in the duplex structures [18].
Figure 2: Engineering Stress – Strain curves of the Duplex Phase Steels
Vol.9, No.7 Mechanical Behaviour of Duplex Phase Structures 627
Figure 3: True Stress – Strain curves for the Duplex phase Steels
The tensile properties of the duplex phase structures are summarized in Table 1. The yield
strength and ultimate tensile strength increased with the volume percent martensite while the
percent elongation for all duplex phase structures ranged between 17% - 21.5%; and were
relatively higher in comparison with the conventional normalized structure which had percent
elongation of 15%. The increase in tensile strength and yield strength with increase in the
volume percent martensite is consistent with observations of Nobuyuki [4], Sun and Pugh [6],
and Kumar et al. [7]. The toughness values presented in Table 1 (estimated by calculating the
area under the engineering stress strain graph, Figure 2) revealed higher toughness values for
the 7600C and 7800C intercritical treatment (B760 and C780) which is consistent with the large
volume fractions of martensite. The conventionally normalised specimen is observed to possess
the least toughness. The generally improved toughness observed in the du plex ph ase s tru ctu res i s
attrib uted to the co mposite struct ure of ferrite an d martensit e which creates a s ynerg y of the soft
ferritic phase and the hard martensitic phase, which helps in increasing the materials resistance to
crack propagation and fracture [18].
628 K.K. Alaneme, S. Ranganathan, T. Mojisola Vol.9, No.7
Table 1: Tensile Properties of the Duplex Phase structures
Vm (%)
σu (MPa)
σy (MPa)
Strain to
failure, εf
3.3 Fatigue Properties
Figure 4 shows the S N curves for the duplex ph ase structures and the co nventional normalized
specimen. From the graph it is seen that the three duplex phase structures had fatigue strengths
superior to that of the normalized samples. This trend is similar to the observations of Alzari et al
[22] , Nakajima et al [ 23], Alp and W azzan [24]. The plot also reveals th at the specimen s treated
at 7600C had the highest endurance limit (247MPa) while the specimens treated at 7800C and
7400C had correspondingly lower endurance limits of 232MPa and 225MPa respectively. This
indicates that the fatigue strength of the dual phase steels increases with increasing martensite
content to a limit of approximately 68% martensite beyond which further increase in martensite
content yields a decrease in fatigue strength as observed for the case of the 7800C treated
samples with 80% martensite. The increase in fatigue strength observed at higher intercritical
temperatures is attributed to tougher martensites formed at higher intercritical temperatures, due
to the lower carbon content of the martensite formed at higher intercritical temperatures to obtain
higher volume percentages of martensite [25]. Tayanc [19] noted that increasing volume fraction
of martensite lowers the ductility of the d uplex phas e steels and lead s to a decre ase in t he fati gue
life. Also they noted that the fatigue life is a function of the strain distribution between the ferrite
and martensite components of the duplex phase steels which influences crack initiation and
growth. The SEM fractographs in Figure 5 show that the fatigue fracture mode is a mix ed mode
consisting of ductile (dimple) fracture and intergranular brittle cleavage. Figure 5a shows that the
proportion of ductile dimple features is higher for the 7600C in comparison to the 7800C Figure
5(b), which could account for the relatively higher fatigue strength achieved for the 7600C series .
The ductile dimples arise from the ferritic grains while the intergranular cleavage facets
emanates from the martensitic grains. Fatigue cracks are known to propagate at a faster rate in
brittle structures in comparison to more ductile structures –thus it can be argued that in the
duplex phase steels there will be more crack deflection, and branching in the more ductile duplex
phase st ructures at th e ferrit e martens ite i nterfa ces l eading to redu ced fati gue cra ck gro wth rat e
Vol.9, No.7 Mechanical Behaviour of Duplex Phase Structures 629
Figure 4: S – N Curves for the Duplex phase steels
630 K.K. Alaneme, S. Ranganathan, T. Mojisola Vol.9, No.7
Figure 5(a): Fatigue fractograph of the 7600C showing mixed fracture mode comprising of
ductile (dimple – like features) and intergranular brittle facets of martensitic grains
Figure 5 (b): Fatigue fractograph of the 7800C showing predominant intergranular facets of
martensitic grains and sparse distribution of ductile dimple – like ferritice grains indicative of
mixed mode of fracture.
Vol.9, No.7 Mechanical Behaviour of Duplex Phase Structures 631
Overall, it is observed that the best combination of tensile strength, yield strength, total
elongation, toughness, and fatigue strength (endurance limit) is obtained when treatment is
performed at 7600C and 7800C for 30 minutes before water quenching. Thus tensile properties
and fati gue resist ance can be eff ectivel y enhanc ed for the c arbon steel grades b y the adop tion of
this processing heat-treatment condition which makes it suitable for high strength and structural
The medium carbon low alloy steel with duplex phase structures consisting of ferrite and
martensite exhibited continuous yielding behaviour typical of traditional dual phase steel
compositions; and were interestingly characterised by high strain hardenability, high strength,
total elongation, toughness and superior fatigue resistance (endurance limit) in comparison with
the normalised structure which is representative of one of the more frequently utilised
microstructures for the carbon steel composition. These promising tensile and fatigue property
combinations were better harnessed when treatment was performed at 7600C and 7800C in
comparison to 7400C. The fatigue fracture mechanism was observed to be mixed mode of ductile
(dimple) fracture and intergranular brittle cleavage.
The lead author is grateful to the Association of commonwealth Universities (ACU) for the
award of its prestigious Wington Titular Fellowship in Engineering which made it possible for
him to perform a substantial part of this research at the Department of Materials Engineering,
Indian Institute of Science, Bangalore, India. The mentoring of Prof. U. Ramamurty and Prof. S.
Ranganathan throughout my stay in India is immensely appreciated.
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