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Journal of Mi nerals & Materials Characteriza tion & Engineering, Vol. 9, No.9, pp.787-794, 2010
jmmce.org Printed in t he USA. All rights reserved
Effect of Heat Treatment on Fe–0.3%P–0.14%C Alloy
Yashwant Mehta1*, K. Chandra2, Rajinder Ambardar1 & P. S. Mishra2
1Department of Metallurgical Engineering, National Institute of Technology, Srinagar,
Hazratbal, Srinagar, J & K– 190006, India
2Department of Metallurgical and Materials Engineering, Indian Institute of Technology,
Roorkee, Roorkee, Uttarakhand -247667, India
*Corresponding Author: firstname.lastname@example.org
Modern iron and steel industry is based on the iron – carbon diagram. However, a major
problem associated with Fe-C alloys is that they corrode and cause losses to the tune
of 5% of GDP to the world. Ancient Indian iron (Fe-P-C) artifacts like the Delhi Iron
Pillar have withstood atmospheric corro sion for about 1600 years. Phosphoro us and carbon
are used as the alloying elements for strengthening iron and imparting corrosion resistance
to it. Therefore, there is a need to understand the Iron-Phosphorous-Carbon alloy system and
to develop strong and corrosion resistant, iron products. In the present study, Fe-0.3%P-
0.14%C alloy is subjected to heat treatment schedule by varying the rate of cooling after 30
minutes of heating at 800 oC. Microstructural characterization is carried out on the heat
treatment samples. The studies reveal a higher concentration of carbides in the form of
pearlite formed at the grain boundaries of the ferrite grains in the air cooled samples. As per
the literature, carbon pushes phosphorous from the grain boundaries to the grain interior by
site competition. Micro-hardness studies on the test samples indicate that hardness of the
phases formed at the grain boundaries is higher as compared to the hardness of the interior
of the ferritic grains.
Keywords: Phosphoric Iron, Microhardness, Microstructure, Heat treatment.
Ancient and medieval India has made a tremendous i mpact on the world scene through many
spectacular achievements in iron and steel technology. An exemplary monument of those
times is the Delhi Iron Pillar s tanding at Mehrauli village, near Qutab Minar, on the outskirts
of Delhi. The Delhi pillar has long evoked the curiosity of metallurgists, particularly for its
788 Y. Mehta, K. Chandra, R. Ambardar & P. S. Mishra Vol.9, No.9
large size and excellent state of corrosion resistance. The composition of the pillar iron is
comparable to that of low carbon steel and shows a wide range as C (0.03–0.28), Si (0.004–
0.056), and P (0.114–0.48) .
The enhanced cathodic reactions due to the presence of slag particles at the grain boundaries
can lead to the formation of passive protective film of phosphate, possibly in the glassy
amorphous state conferring, thereby, high corrosion resistance to the pillar . The steps
leading to the formation of the protective film include i) initial rust for mation, ii) increase in
critical current density and the formation of the phosphate (FePO4) on the matrix, and finally
iii) the extension of the phosphate film over the slag inclusions .
Ancient archaeological phosphoric irons, from several different parts of the world,
containing between 0.05 to 0.5 wt.% phosphorus and levels of up to 1 wt.% have also been
detected [2 & 4]. Phosphorus increases the yield strength, ultimate tensile strength and
hardness, but decreases both elongation, and reduction in area at failure. Very high
phosphorus contents promote brittle behaviour . Phosphorus causes solid solution
strengthening, of the same order as interstitial carbon and nitrogen . It also results in
marked work hardening in iron when cold worked . Phosphorus at levels around 0.1 wt. %
is known to improve the strength and deep drawability of sheet steel used for automotive
applications. The carbon contents are maintained at < 0.01 wt. % to obtain high formability in
this application [7 & 8]. Phosphoric irons can be easily hot forged  and also cold worked
under suitable conditions, like low strain rates and with geometries that avoid stress
concentration . This has been explained partly to be due to stabilization of ferrite at high
temperatures and the known fact that the hardness of ferrite drops rapidly below that of
austenite at high temper atures .
Metallographic examination of a phosphoric iron, etched with nital, so metimes produces
a watery shimmer or wrinkled appearance in the ferrite grains. If a phosphorous alloy
which has been quenched from the dual phase ferrite – austenite region is etched with
nital, distinct boundaries are visible at prior phase interfaces, and these are attribut ed
to sharp changes in phosphoric content. Metallographic analysis of archaeological
artifacts reve als that th e lines of the ghosting patterns correspond to local changes in
phosphorous content . When the Fe-P alloy is heated upto the duplex phase region,
austenite starts forming. On cooling to room temperature, the austenite transforms to ferrite.
As the rate of diffusion of phosphorous in austenite is less than that in ferrite, the
kinetics of the diffusion controlled austenite - ferrite reaction in the dual phase region
is slow as compared to the reverse reaction. Thus, the growth of ferrite from an austenite
matrix is limited by the diffusion of phosphorous into the austenitic bulk. At slow
cooling rates, the phosphorous inhomogeneity of the duplex microstructure is maintained. A
very long heat treatment at a high temperature in the fully ferritic region results in
homogenization of the structure .
In an attempt to determine the boundaries of the dual phase loop, Haughton quenched a
series of iron phosphorous alloys after annealing at temperatures in the range of 900-
1050 oC. . A short heat treatment in the dual phase range results in the formation of
Vol.9, No.9 Effect of Heat Treatment on Fe–0.3%P–0.14%C Alloy 789
needles of austenite and heating at a higher temperature promotes spheroidization of the
austenite needles .
2. EXPERIMENTAL PROCEDURE
2.1 Sample Preparation
Melting of iron scrap was carried out in an induction furnace of 300 kg capacity. Ferro-
phosphorous, graphite, ferro-silicon, and aluminum shots were added to the melt for alloying
and de-oxidizing purposes. After melting, the molten metal was cast in a sand mould and
allowed to cool for 48 hours. Subsequently, the risers and runners were cut off and the casting
was subjected to rough grinding. The dimensions of the solidified plate casting were obtained
were 400 mm × 400 mm × 40 mm. The casting was examined for any surface defects. The
chemical composition of the casting is given in Table 1. The casting was cut vertically and
perpendicular to the bottom of the casting into approximately 10 mm slices to prepare the
specimens of requisite dimensions i.e. 8mm×8mm×10mm for the present study.
Table 1: Chemical composition of the cast alloy
Alloy P C Si Mn Cr Ni Al Cu W Fe
0.14C 0.281 0.145 0.1820.206 0.15 0.026 0.0680.03 0.025 98.887
2.2 Heat Treatment
Two sets of samples (each set comprising five samples) were subjected, separately, to a heat
treatment schedule at 800 oC in a vertical tubular electr ical furnace for 30 minutes. After the heat
treatment, the two sets of samples were subjected to two different rates of cooling i.e. water
quench and air cooling.
The details are given in Table 2.
Table 2: Heat treatment schedule
Sample type Temperature Time Rate of cooling
Set 1 800 oC 30 minutes Water quench
Set 2 800 oC 30 minutes Air cool
2.3 Metallographic Analysis
Subsequently, the samples were prepared for metallographic analysis and microhardness
After final polishing, the samples were etched by using 2% NITAL and the micro-structural
examination of the samples was carried out.
790 Y. Mehta, K. Chandra, R. Ambardar & P. S. Mishra Vol.9, No.9
2.4 Microhardness Evaluation
The micro-hardness of the different phases was measured using UHL VMHT Micro-
Hardness tester with a Vickers indenter.
2.5 Heat Treatment Temperature
The Fe-C diagram (Fig.1a) shows that the ferrite to austenite transformation starts at 723 oC.
While the Fe–P diagram (Fig. 1b) indicates that this transformation of ferrite to austenite
commences at 911 o
C . Since the alloy is of the ternary type the transformation
temperature will lie somewhere between 723oC and 911oC. When the Fe–0.3%P–0.14%C
alloy is heated to temperatures beyond the transformation temperature, austenite phase is
formed and starts growing into the ferrite grains from the grain boundaries in the form of
plates. When the alloy is cooled to temperatures below the transformation temperature the
austenite transforms to ferrite. In order to ascertain the transformation temperature it was
decided to heat treat the alloy at 800 oC and study its microstructure at 800 oC. If the phases
which are formed due to the decomposition of austenite are found in the microstructure, one
may conclude that the transformation temperature lies below 800 oC.
Fig.1 (a) Iron-carbon equilibrium diagram (b) High temperature loop of Fe–P Phase
Vol.9, No.9 Effect of Heat Treatment on Fe–0.3%P–0.14%C Alloy 791
3. RESULTS AND DISCUSSION
3.1 Metallographic Analysis
The micro-structural studies of the set of test samples of Fe-0.14%C-0.3%P alloy, which
were heat treated at 800oC for 30 minutes and water quenched, revealed that there was a
network of ferrite plates surrounding ferrite grains. The austenite phase is formed at the grain
boundaries of the ferrite grains when the alloy is heated above the transformation temperatur e
. Carbon is an austenite stabilizer and therefore, concentrates in it while phosphorous
being a ferrite stabilizer, concentrates in ferrite. On cooling the alloy, the austenite plates
which grow at the ferrite grain boundaries transform into ferrite plates. The difference
between the two kinds of ferrites is in their composition. The ferrite plates are richer in
carbon, whereas the ferrite grains are richer in phosphorous. The microstructures of Fe-
0.14%C-0.3%P alloy heat treated at 800oC for 30 min, water quenched, and etched with 2%
Nital are shown in Figures 2a and 2b.
Fig. 2. Microstructure of Fe-0.14%C-0.3%P alloy heat treated at 800 oC for 30 min, water
quenched and etched with 2% Nital. (a) Magnification 100 X (b) Magnification 1000
The micro-structural studies of the set of test samples of Fe-0.14%C-0.3%P alloy, which
were heat treated at 800 o
C for 30 minutes and air cooled, revealed small pearlite grains
surrounding large ferrite grains. When the alloy is heated above the transformation
temperature, austenite phase is formed at the grain boundaries of the ferrite grains. On
cooling the alloy, the austenite plates which grow at the ferrite grain boundaries transform
into small pearlite grains. The small pearlite grains are richer in carbon, whereas the large
ferrite grains are richer in phosphorous. The microstructures of Fe-0.14%C-0.3%P alloy heat
treat ed at 800 oC for 30 min, air cooled and etch ed with 2% Nital are presented in Figures 3a
Y. Mehta, K. Chandra, R. Ambardar & P. S. Mishra Vol.9, No.9
Fig. 3. Microstructure of Fe-0.14%C-0.3%P alloy heat treated at 800 oC for 30 min, air
cooled and etched with 2% Nital. (a) Magnification 100 X (b) Magnification 1000 X
3.2 Scanning Electron Microscopy
Fig. 4 represents the scanning electron micrograph of the alloy indicating the presence of
ferrite plates and ferrite grains. The ferrite plates are formed when aust enite plates transform
on cooling. The presence of porosity in the micrograph is due to the improper control of
pouring and casting.
Fig. 4. SEM micrograph of Fe-0.14%C-0.3%P alloy heat treated at 800 oC for 30 min, water
quenched and etched with 2% Nital.
3.3 Microhardness Evaluation
The micro-hardness of each phas e in each hea t treated sample was deter mined by using UHL
Micro-Hardness tester with a Vickers in denter. On an average 3-4 measurements were taken
on different areas of each phase of each sample. The micro-hardness of a specific phase in a
specified set of samples which was subjected to a specified rate of cooling has been reported
as a range of micro-hardness numbers (Table 3). It is observed that the hardness of the set of
water quenched samples is far greater than those of the air cooled samples. Further, the
Vol.9, No.9 Effect of Heat Treatment on Fe–0.3%P–0.14%C Alloy 793
hardness of pearlite grains and ferrite plates (prior austenite) is high er than that of the ferrite
Table 3: Ranges of Vickers Micro-Hardness of Various Phases of Fe–0.3%P–0.14%C alloy
Type. Type of coolin
Phase Range (Hv) of
Set 1 Water quench Ferrite grains 229-265
(prior Austenite) 432-461
Set 2 Air cool Ferrite grains 191-218
Pearlite grains 263-297
On the basis of the present work the following conclusions can be drawn:
i. The ferrite-austenite transformation temperature of Fe-0.3%P-0.14%C ternary alloy
lies between 723 oC & 800 oC.
ii. Carbon diffuses preferentially into the austenite regions at 800 oC & causes the
pearlitic transformation to occur on cooling in air cooled samples. Small pearlite
grains are found surrounding the large ferrite grains.
iii. In water quenched samples, the austenite regions at 800 o
C transform into ferrite
plates on cooling. The ferrite plates are found surrounding the large ferrite grains.
iv. It is observed that the hardness of the set of water quenched samples is far greater
than those of the air cooled samples.
v. The hardness of pearlite grains and ferrite plates (prior austenite) is higher than that of
the ferrite grains.
It is emphasized that a ll the experiments carried out on one specific Fe-P-C alloy and are not
necessarily valid for other alloys.
The authors wish to thank Shri Ajay Aggarwal, owner of Vaishnav Steel Private Limited,
Muzaffernagar, India for making the castings of the Fe-0.3P-0.14C alloy for research
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