DYNAMIC MATERIALS MODELING TECHNIQUE FOR
OPTIMIZATION OF WARM WORKABILITY OF ARMCO IRON
AND BINARY IRON ALLOYS
G. S. Avadhani
Department of Metallurgy, Indian Institute of Science, Bangalore 560 012, India.
ABSTRACT
Warm working characteristics of alpha iron, Fe-5Si and Fe-5Co alloys are studied in the
temperature range of 400-900C and strain rate range of 0.001-100s-1 using compression
tests. Processing maps are developed using Dynamic Materials Model (DMM). The
results show that alpha iron undergoes Dynamic Re-crystallization (DRX) with a peak
efficiency of power dissipation of 35% at 800C and 0.001s-1. Silicon addition increases
the efficiency of power dissipation in the DRX domain to 55% and this is attributed to a
decrease in the Curie temperature. Cobalt addition, on the other hand, increases the Curie
temperature of iron and shifts the DRX domain to higher temperatures. Alpha iron
exhibits flow instabilities in the temperature range of 400-700C when strain rate is above
10s-1. The flow instability in Fe-5Si alloy is more intense than in alpha iron, and the
instability regimes are wider in both Fe-5Si and Fe-5Co alloys. The Processing Maps thus
developed facilitate in avoiding these regimes during warm working of these materials.
INTRODUCTION
Processing of materials at temperatures in the range 0. 4 – 0.6 Tm (Tm is melting point
temperature) is termed as warm working. It offers the advantage of obtaining a
combination of increased strength and ductility in addition to good surface finish and
close dimensional tolerance. In view of these advantages, critical components for
automobiles are forged in the warm working regime(1).
Hitherto, processes are developed using trial and error techniques which are expensive as
well as time consuming and may not always lead to a successful solution or optimization.
In recent years, however, the trial and error techniques are replaced by modeling
techniques, which are developed on the basis of science based principles. The processing
map approach(2) developed on the basis of dynamic materials model(3) has been found to
be a very useful tool for understanding the hot deformation mechanisms in a variety of
materials and optimize the hot workability (4). In this approach, the power dissipation
efficiency  of the material is defined as:
= 2m/ (m + 1) ….. (Eq.1)
1-1
where m is the strain rate sensitivity of flow stress. The variation of  with temperature
and strain rate represents the characteristics of power dissipation occurring through
microstructural changes in the work piece, which constitutes a power dissipation map. A
continuum instability criterion (5) developed on the basis of extremum principles of
irreversible thermodynamics, which is given by:
 ln( m / m  1)
 m  0 …..(Eq. 2)
(  ) =
 ln 
is used to delineate the regimes of flow instability. The instability parameter (  ) is
evaluated as a function of temperature and strain rate and plotted to obtain an instability
map. It is superimposed onto the power dissipation map to obtain a processing map.
Deformation of pure iron in the warm working range at 1s-1 using high speed torsion was
studied(6) and the peak ductility occurring at about 860C was associated with a recrystallized structure during deformation. At lower strain rates, however the ductility peak
was observed(7) at about 800C. Dynamic re-crystallization (DRX) at lower stresses and
dynamic recovery (DRY) at higher stresses in hot torsion were reported by Glover and
Sellars(8). While diffusion controlled thermally activated glide was observed in alpha
iron and Fe-2.8 Si alloy in compression(9), one of the most important observation is the
effect of ferro-and para-magnetism on the creep deformation of alpha iron(10). It is
observed that the creep rates were lower and the activation energy for creep was higher in
the ferromagnetic regime due to lower diffusion rates which are also strongly influenced
by magnetic forces. Also, it has been shown that, re-crystallization behavior in pure iron
is affected significantly by Curie temperature(11).
The aim of the investigation is to evaluate the deformation behavior of alpha iron, Fe-5Si
and Fe-5Co alloys in the warm working range of temperature and in a wide range of
strain rate. It is well known that silicon addition to iron decreases the Curie temperature
while cobalt addition raises it(12). The study will therefore throw light on the influence of
magnetic forces on the deformation characteristics of alpha iron. The approach of
processing maps has been used in this study since the power dissipation characteristics are
sensitive to phase transformations and therefore will effectively reveal the influence of
ferromagnetism on deformation.
EXPERIMENTAL
The chemical composition of armco iron, Fe-5Si and Fe-Sco alloys and their initial grain
size are given in Table-1. Armco iron was forged at 900C and annealed for two hrs. at
750C followed by furnace cooling. The Fe-5Si and Fe-5Co alloys were hot forged at
700C, annealed at 875C and furnace cooled.
1-2
Cylindrical specimens of 10mm and 15 mm height were used for compression resting.
Care was taken to obtain closely parallel load bearing surfaces for the specimen. In
addition, grooves were provided on these surfaces so that effective lubrication (MoS2 up
to 700C and molten glass at higher temperatures) was ensured during compression. The
temperature of the specimen was monitored with the aid of a chromel/alumel
thermocouple embedded in a 0.8 mm diameter hole machined at half the specimen height.
This thermocouple was also used for the measurement of the adiabatic temperature rise in
the specimen during deformation. A computer controlled servo-hydraulic testing machine
(Custom built by DARTEC, UK) was used for the compression tests. The machine was
equipped with an exponentially decaying actuator speed, enabling constant true strain
rates in the range 0.001 – 100s-1 to be imposed on the specimen. Isothermal tests were
conducted by surrounding the specimen, platens and push rods with a resistance furnace.
The temperature was controlled to within  2oC. The adiabatic temperature rise was
recorded using a Nicolet transient recorder. The tests were conducted over a temperature
range of 400-900C at 100oC intervals for various true strain rates ranging from 0.001 to
100s-1. In each test, the specimen was compressed to about half of its original height and
the load-displacement data was obtained. These were converted into true stress-true
plastic strain curves using standard equations. The flow stress data as functions of
temperature, strain rate and strain were obtained from these curves and used to construct
the power dissipation maps. Similarly, the instability parameter was plotted as a function
of temperature and strain rate to get the instability map which was superimposed on the
power dissipation map to get the processing map for the materials investigated. The
deformed specimens were air cooled and examined using standard metallographic
techniques. Tensile testing was carried out on the sample in the DRX domain for alpha
iron using standard specimen dimensions to validate the results exhibited by the
processing map.
RESULTS AND DISCUSSION
Alpha iron
Typical true stress-true strain curves recorded at 500C and 800C at different strain rates
for alpha iron are shown in Fig. 1(a) and Fig. 1(b), respectively. At 500C, the material
shows strain hardening at all strain rates. At 800C, the curves corresponding to strain
rates of 0.001 and 0.01 s-1 showed steady-state behavior while at higher strain rates, strain
hardening is observed. The variation of flow stress () with temperature (T), strain rate
(  ) and strain () for alpha iron is shown in Table-2. The processing map obtained at a
strain of 0.5 for alpha iron is shown in Fig. 2. The maps obtained at other strains are
similar indicting that strain effect is not significant. The processing map of alpha iron
exhibit two domains.
The first domain occurs in the temperature range 600-850C and strain rate range 0.001 –
0.1s-1 with a peak efficiency of 35% at 800C. This may be interpreted to represent the
process of DRX on the basis of the shapes of the stress strain curves in the domain and
1-3
following observations on the variations of grain size, ductility and efficiency of power
dissipation with temperature.
Typical microstructure of -iron specimen deformed at 0.001s-1 & 800C (corresponding
to the peak in the DRX domain) is shown in Fig. 3(a). The variation of the average grain
diameter with temperature at a strain rate of 0.001s-1 is shown in Fig. 4(b). The grain size
increases with temperature up to the temperature for the peak efficiency (800C) beyond
which, there is an abnormal grain growth. The measured tensile ductility values at a strain
rate of 0.001 s-1 at different temperatures from the present investigation are plotted in the
Fig 4(a). The ductility data of Robbins et al.(7) at a strain rate of 0.5 s -1 is also shown.
Both the profiles show a ductility peak at 800C, which matches with the temperature for
peak efficiency in the DRX domain [Fig. 4(c)]. As the grain size increases, ductility drops
sharply beyond 800C. All these observations are typical of DRX (13,14) and confirm
that DRX occurs in this domain.
The lower efficiency value of DRX may be attributed to the magnetic domain structures
in alpha iron. The grain boundary migration which is essential for dynamic recrystallization is slowed down by the presence of magnetic domains below the Curie
temperature (-770 C). The migrating boundary has to overcome the strong electron spins
in the direction of magnetization. The efficiency of power dissipation is lower as some
energy is spent to reorient the magnetic domains by the migrating grains(15). This is also
the reason for higher activation energy observed for self diffusion of iron in ferrite
region(10). As the temperature is increased beyond the Curie temperature, the grain
boundaries can migrate uninhibited by magnetic domain structure giving rise to abnormal
grain growth, lowering the ductility and strength. The occurrence of DRX in alpha iron
was also reported by Glover et al.(8) on the basis of microstructural study and kinetic
analysis of hot torsion data.
The second domain occurs in the temperature range 400 –450C and strain rate range
0.001 – 0.01s-1 with a peak efficiency of 27% at 400oC and 0.001s-1, exhibits dynamic
recovery(14). In view of the BCC structure of -iron a large number of slip systems are
available which promote easy cross-slip of screw dislocations causing DRY. Thus the
lower temperature domain may be interpreted to represent dynamic recovery of -iron.
The processing map of alpha iron also exhibits flow instability regime in the temperature
range 400-700C when the strain rate is above 10s-1. Microstructural examination of the
specimen deformed at 500C and 100s-1 [Fig. 3(b)] showed that alpha iron exhibits flow
localization in this regime which should be avoided in processing.
Fe-5Si Alloy:
The processing map for Fe-5Si alloy is shown in Fig.5 at a strain of 0.5. The processing
map for this alloy is very similar to that for alpha iron (Fig. 2). The domain occurring
1-4
with a peak efficiency of 56% at 800oC/0.001s-1 may be interpreted to represent dynamic
re-crystallization(14).
In comparison with alpha iron, the peak DRX efficiency is higher by about 20%. The
DRX efficiency in Fe-5Si has increased because the Curie temperature is lowered by Si
addition. For a 10% addition of Si to iron, the Curie temperature decreases from 770C to
600C(12). The decrease in the magnetization of the Fe-Si alloys will enhance the grain
boundary migration compared to alpha iron. The instability regime in this alloy is also
similar to alpha iron which is not good for processing.
Fe-5Co Alloy:
The processing map for Fe-5Co alloy is shown Fig.6, which exhibits two domains. The
first domain in the temperature range 600-900C and strain rate range 0.001-1s-1 with a
maximum efficiency of 33% occurring at 900C and 0.01s-1 represents DRX of the
alloy(14). It is interesting to note that the DRX temperature corresponding to the peak
efficiency has increased to 900C in comparison with that in alpha iron and Fe-5Si
(800C) and the strain rate increased from 0.001 to 0.01s-1. Also the maximum efficiency
(33%) is similar to that in alpha iron (35%) but lower than in Fe-5Si alloy. These effects
may be attributed to the effect of Cobalt additions on the Curie temperature of alpha iron.
The Curie temperature of alpha iron increases from about 770C to about 900C with the
addition of 10% Co(12). Thus Cobalt additions strengthen the magnetic domains of alpha
iron, restrict the grain boundary migration and reduce the efficiency of DRX. Higher
temperatures are therefore required to achieve dynamic re-crystallization.
The other domain represents dynamic recovery of the alloy. The processing map of Fe5Co alloy also exhibits flow instability in a wider range of temperature and strain rate as
compared with alpha iron. All these instability regimes should be avoided in processing.
SUMMARY AND CONCLUSIONS
The warm working characteristics of alpha iron, Fe-5Si and Fe-5Co alloys were studied in
the temperature range 400-900C and strain rate range 0.001-100s-1. On the basis of the
flow stress data obtained as a function of temperature and strain rate in compression,
processing maps were developed, using Dynamic Materials Modeling technique. The
following conclusions are drawn from this investigation.
1. Alpha iron undergoes dynamic re-crystallization in the temperature
range600-850 C and strain rate range 0.001-0.1s-1 with a maximum
efficiency of 35% occurring at 800 C and 00.1s-1. At these conditions, the
ductility reaches a peak value.
1-5
2. The lower than expected efficiency value DRX in alpha iron is attributed
to the restrictive effect of magnetic domains to the migration of grain
boundaries.
3. Addition of silicon increases and that of cobalt decreases the efficiency of
power dissipation for DRX in alpha iron. This result is attributed to the
lowering and increasing the Curie temperature by Silicon and Cobalt
additions, respectively.
4. Alpha iron exhibits adiabatic shear bands in the temperature range 400700 C when the strain rate is above 10s-1. This instability regime becomes
wider in Fe-5Si and Fe-5Co alloys and should be avoided in warm
working of these materials.
REFERENCES
1. J.H. Reynolds and D.J. Naylor, “Microstructure and properties of warm
worked medium carbon steels”, Mater. Sci. Tech., 1988, 4, 586-602.
2. G.S.Avadhani, “Optimization of process parameters for the manufacturing
of rocket casings: A study using processing maps”, JMEPEG, 2003,12(6),
609- 622.
3. Y.V.R.K. Prasad, H.L. Gegel, S.M. Doraivelu, J.C. Malas, J.T. Morgan,
K.A. Lark and D.R. Barker, “Modeling of dynamic materials behavior in
hot deformation: Forging of Ti-6242”, Metall. Trans., 1984, 15A, 18831892.
4. Y.V.R.K. Prasad and S. Sasidhara, Eds. Hot Working Guide – a
compendium of Processing Maps, ASM Intl. Materials Park, Ohio, 1997.
5. Y.V.R.K.Prasad and T.Seshachryulu, “Modeling of hot deformation for
microstructural control”, Intl. Mater Rev. 1998, 43(6), 243-258.
6. R.A. Reynolds and W.J.McG Tegart, “The deformation of some pure irons
by high – speed torsion over the temperature range 700-1250 oC”, J.Iron
and Steel Inst., 1962, 200, 1044-1059.
7. J.L. Robbins, O.C. Shephard and O.D. Sherby, “Role of crystal structure
on the ductility of pure iron at elevated temperature”, J. Iron and Steel
Inst., 1961, 199, 175-180.
8. G. Glover and C.M. Sellers, “Recovery and re-crystallization during high
temperature deformation of alpha iron”, Metall. Trans., 1973, 4, 765-775.
9. J.L. Uvira and J.J. Jonas, Trans. Metall. Soc. “Hot compression of Armco
iron and silicon steel”, AIME, 1968, 242, 1619-1626.
1-6
10. S. Karashima, H. Oikawa and T. Watanable, “Creep deformation of iron
and its alloys in ferro-and paramagnetic temperature regions”, Acta Met.,
1966, 14, 791-792.
11. Yoshinori Murata and Masahiko Morinaga, “Re-crystallization behavior of
pure iron at curie temperature”, Scripta Mater.2000, 43, 509-513.
12. O. Kubaschewski : IRON-Binary Phase diagrams, Springer-Verlag, Berlin,
1982.
13. G.S.Avadhani, “Hot Deformation Mechanisms and Microstructural
Evolution during upset forging of: -Fe, Fe-5Ni, Fe-5Co & Fe-5Mo alloys
and Maraging Steel” Ph.D. Thesis, Indian Institute of Science, Bangalore,
India, 2001.
14. G.S.Avadhani, “Warm Working Behavior of Alpha Iron, Fe-Si, Fe-Co and
Fe-Ni Alloys: A Study using Processing Maps”, .M.Sc.(Engg.) Thesis,
Indian Institute of Science, Bangalore, India, 1996.
15. Alan Cottrell: An Introduction to Metallurgy p. 498, ELBS, UK, 1982.
LIST OF TABLES
Table 1
Chemical composition of armco iron, Fe-5Si and Fe-Sco alloys
and their initial grain size
Table 2
Flow stress values (in MPa) of Armco iron at different
temperatures and strain rates for various strains (corrected for
adiabatic temperature rise).
1-7
LIST OF FIGURES
Fig. 1
True Stress-True plastic strain curves for alpha iron obtained in
compression at (a) 500C and (b) 800C at different strain rates.
Fig. 2
Processing map obtained for Armco iron at a strain of 0.3. Contour
numbers represent percent efficiency of power dissipation. Shaded
region corresponds to flow instability.
Fig. 3
Microstructures of alpha iron in the DRX domain deformed at:
(a) 800C/0.001 s-1 and (b) 500C/100s-1 in the instability regime
showing localized flow
Fig. 4
(a) Ductility, (b) Grain size and (c) Efficiency of power dissipation vs
Temperature profile in the DRX domain, obtained for alpha iron.
Fig. 5
Processing map obtained for Fe-5Si alloy at a strain of 0.3. Contour
numbers represent percent efficiency of power dissipation. Shaded
region corresponds to flow instability.
Fig. 6
Processing map obtained for Fe-5Co alloy at a strain of 0.3. Contour
numbers represent percent efficiency of power dissipation. Shaded
region corresponds to flow instability.
Table 1: Chemical composition (Wt.%) and initial grain size of the materials used
Material
C
Mn
S
P
Si Co Fe Avg.Grain Dia.(m)
Armco iron 0.007 <0.03 <0.005 0.003 -- -- Bal.
118
Fe-5Si
0.007 <0.03 <0.005 0.003 5 -- Bal.
140
Fe-5Co
0.007 <0.03 <0.005 0.003 -- 5 Bal.
125
Table 2:
Flow stress values (in MPa) of Armco iron at different
temperatures and strain rates for various strains (corrected for
adiabatic temperature rise).
1-8
Strain
0.1
0.2
0.3
0.4
0.5
Temperature, oC
Strain
rate, s-1
400
500
600
700
800
900
0.001
0.010
0.100
1.000
10.00
100.0
261.8
322.3
452.0
475.4
387.1
386.0
143.8
185.7
256.9
336.3
361.7
377.4
86.1
92.1
144.8
206.0
285.8
316.7
48.6
64.1
88.9
121.2
176.9
226.5
22.8
34.6
50.9
71.8
102.2
135.7
20.1
25.0
36.9
52.5
72.9
88.5
0.001
0.010
0.100
1.000
10.00
100.0
299.0
368.3
507.1
536.2
482.0
452.4
166.1
209.4
282.7
376.9
415.6
438.1
97.0
112.2
164.3
225.9
319.4
364.5
54.2
73.1
100.1
135.8
196.6
261.9
24.4
37.5
58.0
82.9
115.5
163.6
21.3
28.2
41.6
60.9
83.1
108.7
0.001
0.010
0.100
1.000
10.00
100.0
326.5
394.7
537.7
581.8
525.0
526.7
185.3
224.1
299.0
395.5
437.3
482.6
103.6
121.5
176.4
238.2
332.7
389.6
54.9
77.6
109.2
147.9
205.7
285.9
24.8
39.1
60.5
90.0
126.9
177.6
21.9
29.5
43.5
66.4
92.3
120.7
0.001
0.010
0.100
1.000
10.00
100.0
334.3
414.1
556.0
604.7
563.2
572.1
200.0
235.4
305.9
400.8
447.3
502.6
106.4
128.1
185.7
244.8
335.4
403.9
56.5
80.4
115.6
156.5
214.2
295.4
24.1
40.2
63.3
94.3
133.6
185.7
22.4
30.3
44.3
67.6
95.7
128.0
0.001
0.010
0.100
1.000
10.00
100.0
345.6
425.4
582.2
622.3
611.7
628.1
209.4
244.0
317.1
404.8
453.8
516.3
111.0
131.0
193.0
247.6
335.5
395.1
57.9
80.2
120.8
161.4
217.9
290.4
23.6
40.3
63.1
95.9
137.2
176.6
22.7
30.7
44.6
68.8
100.4
129.6
1-9
Fig.1 (a)
Fig.1 (b)
Fig.3 (a)
Fig.2
Fig.3 (b)
1 - 10
Fig. 5
Fig.6
1 - 11