Journal of Minerals & Materials Characterization & Engineering, Vol. 11, No.1, pp.69-83, 2011 Printed in the USA. All rights reserved
Effects of Au sten itis ing C ondit ions on th e Microst ructu res and Mechanical
Propert ies of Martensitic Stee l wit h Dual Matrix Struc tur e
K.A. Bello*, S.B. Hassan and O. Aponbiede
Department of Metallurgical and Materials Engineering, Ahmadu Bello University, Zaria,
* Corresponding Author:,
A new high strength steel with dual matrix structure and exceptionally high toughness plus
ductility have been produced by intermediate quenching of 0.22_wt% C microalloyed steel.
The treatment consisted of ini tial austenitization and rapid quenching of the steel samples t o
achieve a fully martensitic state followed by annealing in the intercritical (
) region of
730oC-810oC for the period of 30, 60 and 90_minutes. These samples were subsequently
quenched to obtain dual phase microstructure containing varying proportions of ferrite and
martensite constituents. The mechanical properties of the samples were measured according
to ASTM standard and their microstructures were analyzed by optical microscopy. The
experimental results show that martensitic dual phase (MDP) steel samples developed within
the intercritical temperature range of 770–790oC revealed finer martensite and precipitate-
free ferrite microstructure. The tensile and impact properties of the developed HMDP steels
increased with intercritical annealing (ICA) temperatures, with an optimum properties
obtained at 790oC mainly due to finer microstructure of the constituent phases and absence
of carbide precipitate that permit ease of dislocation flow. A further increase in the
intercritical annealing temperature beyond 790oC led to general decrease in the mechanical
properties. This is attributed to the formation of coarse structure in this region. The results
further show that with increasing intercritical treatment time from 30 to 90 minutes, the
general mechanical properties of the MDP steels were found to increase except at the higher
70 K.A. Bello, S.B. Hassan Vol.11, No.1
temperature of 810oC which showed decreasing values. In general, the tensile strengths and
ductility as well as the impact properties of the developed dual phase steel samples are
greatly improved with the intercritical heat treatment investigated.
Keywor d s : Intercritical temperature; dual phase steels; microstructure; microalloyed steel;
dual matrix
The interest for high strength Dual-phase (DP) steel sheets having excellent ductility has
increased significantly in recent years in various industries and in the automotive industry in
particular [1]. Obviously, the potential that has encouraged the development and adoption of
these steels in the industry are mainly motivated by their possibilities to improve productivity
and reduce auto body weight, and to increase the passenger safety at a competitive price often
equal or lower than that of conventional material solutions of similar strength level [1, 2].
Dual-phase steels are characterized by a microstructure consisting of martensite in a
polygonal ferrite matrix. The hard martensite particles provide substantial strengthening
while the ductile ferrite matrix gives good formability [2]. The martensite/ferrite dual phase
mixture acts like a particle-reinforced composite. The tensile strength of the composite can be
approximated by simple rule-of-mixture [3,4]: δDP = VFδF + VMδM, where δ is tensile
strength and V is the volume fraction of the phase and the subscripts DP, F and M indicate
the composite dual phase structure and the ferrite and martensite phases respectively. Both
the phase fractions and the properties of martensite can be adjusted by controlling the steel
chemistry and intercritical annealing (ICA) temperature in the ferrite plus austenite region
just prior to rapid quenching [4].
Review of the relevant literature shows that the strength of DP steel appear to increase by
increasing the amount of martensite, as the martensite is the harder phase [3-5]. This,
however, might be done at the expense of ductili ty and tou ghness and for t his reason such are
inappropriate for undergoing various deformation processes [5, 6]. Recently, the degradation
of ductility and impact toughness of DP steels has been receiving attention and this
Vol.11, No.1 Effects of Austenit ising Conditio ns 71
phenomenon has been attributed to the formation of coarse martensite phases as the
intercri tical anneal in g tem perature incre ases [ 3, 5 , 6] . It has be en shown that the ductility and
fracture toughness can satisfactorily be optimized by developing microstructures with fine
distribution of ferrite and martensite in DP steel either by introducing new alloy design or
adopting an appropriate heat treatment procedure [3, 6, 7]. On the basis of alloying design,
the new chemistries are low in carbon and rich in hardenability enhancing element such as
Mn and Si in optimum ratio [4, 7, 8].
The present study aims at developing high martensite dual-phase steels by an intermediate
quench (IQ) heat treatment technique. To these purposes, this paper present the effects of the
intercritical thermal treatment parameters (temperature and time) on the microstructures and
mechanical properties of 0.22 wt% C microalloyed steel.
2.1 Material and Heat Treatment.
Commercial microalloyed steel provided for this study was obtained in the form of 15mm
thick plate (hot-rolled condition). The actual composition as determined by chemical anal ysis
is given in table1. The tens i le an d i z od im pact sam pl es were cu t f rom th e pl ate an d m achi ned
to a standard section and then normalized at 950oC for 1h to remove the effect of prior heat
Table 1: Chemical Compositions of the Steel Used.
All the machined samples were subjected to intermediate quench heat treatment which
involves double quench operation. These samples were initially austenitized at 920oC in a
carbolite muffle furnace, soaked for 60minutes and then quenched in 9% iced brine solution
to obtain a fully martensitic structure which was used as the starting microstructure in this
investigation. The individual specimen was subsequently intercritically annealed at
temperatures between 730 oC and 810 oC and held for the period of 30, 60, and 90_minutes
72 K.A. Bello, S.B. Hassan Vol.11, No.1
respectively for each of the temperature regimes. These were finally quenched in oil to get
different proportion of dual phase structure of ferrite and martensite. Throughout these heat
treatments, the temperature of each specimen was monitored by a thermocouple.
2.2 Material Characterization and Testing
Microstructures of the heat-treated samples were examined by optical microscopy using a
standard metallographic preparation procedure (samples were etched with 2% Nital solution
after grinding and polishing). A wild M50 metallurgical microscope was used for the
metallographic characterization.
Following the microstructural studies, tensile and izod impact testing of all the heat-treated
samples were conducted per ASTM standard. Tensile testing was performed on round
specimens with 50mm gauge length and diameter of 12.5mm using Monsanto type W-
Tensiometer. Izod V-not ch impact tes ts were condu cted in accordan ce with AS TM standards
(BS.131) using a pendulum-type impact-testing machine with 186.1joules capacity and at a
velocity of 3.5m/s. All tests were carried out under laboratory air environment (32o) and
minimum of three samples were tested for each of heat treatment conditions investigated.
3.1 Microstructure
Upon quenching of the steel from the intercritical temperature in oil held at different times,
austenite transforms to martensite producing ferrite-martensite dual phase structure. Figures
1-5 show the optical microstructure of samples processed by intermediate quench (IQ) heat
treatment technique. The structures of martensitic dual phase (MDP) steel obtained show
uniform distribution of lath martensite in ferrite matrix. The distribution is similar to those
reported for conventional DP steels [10].
At lower intercritical annealing (ICA) temperatures, the microstructures of the MDP steel
specimens obtained reveal fine particles of undissolved carbide precipitates (black dots) as
seen in Figures (1) and (2). It is widely reported [6, 11] that these precipitates are formed
during the reheating process to the ICA temperature. The amount of carbide precipitates
Vol.11, No.1 Effects of Austenit ising Conditio ns 73
decrease as the intercritical temperature increases and such carbides are not present in
specimens intercritically annealed at 770oC, 790oC and 810oC, as in Figures (3) to (5).
For all the samples, the micrographs show martensite layers thickening on the ferrite grain
boundary until it forms a continuous network around the ferrite grains as the time increases
from 30 to 90 minutes for each of the increasing temperature. According to Barros et al. [9],
this is a result of the increase in the martensite content with intercritical treatment times.
Observation of the specimens prepared at lower ICA temperature also show coarsening of
ferrite structure while coarse martensite was found at higher ICA temperature. At ICA
temperature from 770 to 790 oC, the microstructure developed comprised mostly of finer
ferrite and martensite, as shown in fig. (3) and (4).
b) c)
Figure 1: Photomicrograph of MDP steels intercritically annealed at 730oC for (a) 30min (b)
60min (c) 90min. The structure showing distribution of martensite (dark),
undissolved carbide (black dot) and ferrite (light). .2%Nital etch.
74 K.A. Bello, S.B. Hassan Vol.11, No.1
b) c)
Figure 2: Photomicrograph of MDP steels intercritically annealed at 750oC for (a) 30min
(b) 60min (c) 90min.The structure showing distribution of martensite (dark),
undissolved carbide (black dot) and ferrite (light). .2%Nital etch.
Vol.11, No.1 Effects of Austenit ising Conditio ns 75
b) c)
Figure 3: Photomicrograph of MDP steels intercritically annealed at 770oC for (a) 30min (b)
60min (c) 90min.The structure showing distribution of martensite (dark) and
ferrite (light). .2%Nital etch.
76 K.A. Bello, S.B. Hassan Vol.11, No.1
b) c)
Figure 4: Photomicrograph of MDP steels intercritically annealed at 790oC for (a) 30min (b)
60min (c) 90min. The structure showing distribution of martensite (dark) and
ferrite (light). .2%Nital etch.
Vol.11, No.1 Effects of Austenit ising Conditio ns 77
b) c)
Figure 5: Photomicrograph of MDP steels intercritically annealed at 810oC for (a) 30min (b)
60min (c) 90min The structure showing coarse distribution of martensite (dark) and ferrite
(light). .2%Nital etch.
3.2 Tensile Properties
Figu res 6 -8 report tensile test results of IQ-treated MDP steel sample. It is apparent from the
results that the tensile properties data readily change with intercritical annealing treatment
temperatures. It was found that MDP steels show continuous yield behaviour and its proof
strength at 0.2% strain is taken as the yield strength. The absence of discontinuous yielding in
conventional ferrite-martensite steels has been explained in term of the high dislocation
density surrounding the martensite islands [12]. Beynon et al. [8] has further pointed out that
during the austenite to martensite transformation, a hard deformation resistance phase is
introduced into microstructure and due to the volume expansion that takes place during the
transformation of austenite to martensite, mobile dislocation are introduced into the
78 K.A. Bello, S.B. Hassan Vol.11, No.1
surrounding ferritic constituent matrix. The mobility of these dislocations is responsible for
the continuous yielding behaviour attributed to dual phase steel grades.
As seen from the figures, the tensile strength and yield strength (Fig. 6 and 7 respectively)
initially increase gradually with ICA temperatures, reaching maximum values at a
temperature around 790 oC and then decreases with a further increase in ICA temperature up
to 810 oC. From figure 8, it is evident that IQ-treatment does appear to significantly improve
the ductility of MDP steel as it display similar trend with the strength. However, this result is
unusual and opposed to the conventional fact that ductilit y generally decreases as the strength
According to Sudhakar et al. [13], it was reported that the increase in strength obtained in DP
steels is mainly attributed to (a) the load bearing capability of martensite in the DP structures,
and (b) the fact that deformation in the ferrite phase is constrained by martensite. It has been
further reported [11] that the yield strength and ultimate tensile strength of DP steels are
linearly related to the amount of martensite, which increases with ICA temperatures.
Recently, it has been observed that dual phase steel with finer microstructure exhibits higher
ductility [6]. On comparison, the result of the ductility obtained in this investigation appears
to agree with previous finding.
Figure 6: Yield strength of intercritically annealed MDP steel at various intercritical
temperatures and times.
Vol.11, No.1 Effects of Austenit ising Conditio ns 79
Figure 7: Tensile strength of intercritically annealed MDP steel at various intercritical
temperatures and times.
Figure 8: Percentage elongation of intercritically annealed MDP steel at various
intercritical temperatures and times.
This result also shows that tensile proper ti es o f M DP s teel i ncreas es over t he r an ge o f 73 0 oC-
790 oC as the intercritical holding time is increased from 30 to 90_min.
3.3 Impact Toughness
Figure 9 illustrate plot of fracture impact energy versus intercritical annealing temperature
(see Figure 9). It can be seen that impact energy value of the MDP steel increases as ICA
730 750770 790 810
Intercritical AnnealingTemperature (oC)
Elongation (%)
80 K.A. Bello, S.B. Hassan Vol.11, No.1
temperature increases and then decreases. This trend is similar to the one obtained for the
tensile properties. This situation is associated with the morphology of the microstructural
constituents in MDP steel samples. The lower tensile strength, ductility and toughness
obtained at lower intercritical temperature could be attributed to the relatively coarser ferrite
with carbide precipitates whereas for lower strength at higher intercritical temperature is due
to coarser martensite. Thus the optimum value obtained for all the mechanical properties
investigated occurred at intercritical temperature of 790oC, which corresponds to the
microstructure comprised mostly of finer ferrite and martensite. It, therefore, appears that
strength, ductility and toughness are associated with finer martensite and finer precipitate-
free ferrite that permit ease of dislocation flow owing to the absence of barriers such as the
undissolved carbides.
Figure 9: Impact strength of intercritically annealed MDP steel at various intercritical
temperatures and times.
The impact toughness of MDP steel samples increase with the intercritical time from 30 to
90minutes for each of the intercritical temperatures except at higher intercritical temperature
where further increase to about 8100C gave decrease in the properties of the steel as time
increases. This is probably due to the continued coarsening of martensite phase in this
intercritical region [9].
Vol.11, No.1 Effects of Austenit ising Conditio ns 81
From the results of this research work, the following conclusions can be drawn.
1) The as-received microalloyed steel samples respond well to heat treatment, both during
quenching and intercritical annealing treatments.
2) Intermediate quenching treatment results in substantial improvements in the general
mechanical properties of developed MDP steels.
3) Continuous yielding was found to occur for MDP steel samples intercritically annealed
in the range730oC –810oC due to the introduction of mobile dislocations i nto the surrounding
ferritic constituent matrix during rapid transformation of austenite to martensite.
4) Finer distribution of ferrite and martensite phases were obtained for IQ-treated MDP
steel samples treated within the intercritical annealing treatments temperature range of 770-
7900C. However, samples that were intercritically annealed at 810oC exhibited coarse
structure and the MDP steels heat-treated at lower intercritical annealing temperatures
(730oC-750oC) were found to contain coarse ferrite with carbide precipitates; the effect of
which lowered the properties of the material.
5) Increasing the intercritical treatment for the period up to 90_mins within the temperature
range of 730oC-750oC causes a rise in the tensile and impact strength
6) Increasing the intercritical annealing treatment temperature beyond 7900C reduces the
tensile and fracture toughness properties of the developed MDP steel.
7) The yield strength, tensile strength, % elongation and impact toughness of the developed
martensitic dual phase steel increased to a maximum value of 524N/mm2, 795N/mm2,
22.8N/mm2, and 107joules respectively when intercritically annealed at temperature of
790oC. This is attributed to the finer distribution of ferrite and martensite constituents as well
as presen ce of precipi tate-free ferrite in this region causing an ease of dislocation flow in the
material due to the absence of barriers such as undissolved carbides precipitates. The
82 K.A. Bello, S.B. Hassan Vol.11, No.1
investigation therefore shows that intercritical treatment accompanied with grain refinement
is strikingly successful in improving the mechanical properties of the developed MDP steels.
8) The general mechanical properties of the developed MDP steel were substantially raised
with intercritical annealing time from 30 to 90_minutes.
This work was carried out at Ahmadu Bello University and Obafemi Awolowo University
Metallurgical laboratories as part of KAB’s MSc thesis. The authors are grateful to the
management of Islamic Education Trust Minna, Nigeria for funding assistance. KAB’s
appreciate the special assistance rendered by Dr. L.E. Umoru and Dr. A. Oyetunji.
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