Journal of Mi nerals & Materials Characterization & Engineering, Vol. 9, No.7, pp.607-619, 2010
jmmce.org Printed in the USA. All rights reserved
607
Elucidation on Reactions Thermodynamics and Kinetics of OFC-A of
Steels
A.V. Adedayo1,2
1Department of Metallurgical Engineering, Kwara State Polytechnic, Ilorin, Nigeria
2 Materials Science and Engineering Department, Obafemi Awolowo University, Ile-Ife,
Nigeria
E-Mail: adelekeadedayo58@yahoo.com
ABSTRACT
The basic principles of Oxyfuel cutting of metals lie in rapid high-temperature oxidation of
the cut metal. Considerable proportion of the published work on the subject of oxygen
cutting, the details of the oxidation reaction are overlooked or confused. Most often, physical
characteristics of oxidized material is attributed to that of iron rather than iron oxide. The
analysis of the oxidation reactions pertinent to Oxyfuel cutting of steels has also been majorly
ignored. The oxidation process of iron and steel though similar in some respects, yet, in other
aspects, show significant differences. This paper presents experimental and theoretical
elucidation on reactions thermodynamics and kinetics of oxyfuel cutting processes of steel.
Six 10mm metallurgy steel rods of different wt% C were flame cut using different acetylene
and oxygen pressures. The composition of the steel rods used ranged from 0.16 wt% C to
0.33wt% C. Acetylene pressures used ranged fro m 3. 45 x 10-2 N.m-2 to 5.52 x 10-2N.m-2, while
oxygen pressure ranged from 2.76 x 10-1N.m-2 to 3.17 x 10-1 N.m-2. The result shows that the
cutting rates decreased with carbon content of the steel as a result of reduction of iron oxide
during decarburization reactions. Theoretical models of the thermodynamics and kinetics of
cutting process pertinent to steels are also discussed.
Keywords: Flame cutting, Oxyfuel, Oxyacetyl en e, Steel oxidation, Decarburization.
1. INTRODUCTION
OFC-A is American Welding Society (AWS) classification for oxyfuel gas cutting in which
acetylene is the fuel [1]. Oxy-fuel cutting are processes that use fuel gases and oxygen to cut
metals. French engineers Edmond Fouche and Charles Picard became the first to develop an
oxygen-acetylene machine in 1903.
608 A.V. Adedayo Vol.9, No.7
Oxy-fuel is one of the oldest welding processes, however, it is still widely used for welding
pipes and tubes, as well as repair work. In the developing worlds where incesant power
outage is a common phenomeneon, OFC has made arc-welding/cutting unlucrative. It is also
frequently well-suited, and favored, for fabricating some types of metal-based artwork.
Oxyfuel equipment is versatile, lending itself not only to some sorts of iron or steel welding
but also to brazing, braze-welding, metal heating (for bending and forming), and also oxyfuel
cutting. The low cost of OFC-A equipment is one of the main reasons for its use. In oxy-fuel
cutting (OFC), a cutting torch is used to heat metal to kindling temperature where the metal
starts to turn a cherry red. A stream of oxygen then trained on the metal combines with the
metal which then flows out as an oxide slag to creat the kerf [1-3] .The cut quality depends
on the torch tip size, type, and distance from the metal [1], the oxygen and preheat gas flow
rates and the cutting speed. However, the kinetics of the process will also depend on the
oxidation process of the cut metal.
The basic principles of Oxyfuel cutting (OFC-A) of metals, lie in rapid high-temperature
oxidation of the cut metal. Considerable proportion of the published work on the subject of
oxygen cutting, the details of the oxidation reaction are overlooked or confused [4]. For
example, it is not uncommon for the oxidized material to be attributed with the physical
characteristics of iron rather than iron oxide. The iron oxide produced as a result of oxidation
by OFC-A is not clear-cut elucidated. DeGarmo [3] believes Fe3O4 is formed. Masachuesset
Institute of Technology [5] identifies FeO, Fe2O3, and Fe3O4 as potential oxides that could be
produced. The analysis of the oxidation reactions pertinent to OFC-A of steels has, also been
majorly ignored. The focus of most literature has been on the Fe content of the ferrous alloy,
while the other contents are ignored. The oxidation process of iron and steel though similar in
some respects, yet, in other aspects, show significant differences. A practical example of
this is found during the construction of the steel skeleton for a flour mill in Nigeria in 2006. It
was planned that the period of execution of the job would be 6 months, based on the
understanding that mild steel (0.16w%C) would be used for the steel skeleton. The cost of the
project should be N15.23 million. However, the client purchased medium carbon steel
(0.33wt%C). The result was that the rate of job execution was much slower than expected.
The job would require about 14 months. Also more volumes of oxyfuel gases are required.
The cost for labour, accommodation, feeding, transport, and rented equipments had doubled.
Generally, cost became very high and over N13 million have to be added to initial N15.23
million. Definitely, there was problem. The client believed this was a gimmick by the
contractor to siphon fund. The client believed 0.33wt%C steel should cut faster since it has a
lower melting point.
In this present paper, a description of the mechanism of interactions of the different phases
involved in the reactions of slag forming process when steel is cut is provided. The role of
carbon in this process is elucidated. Theoretical models of the thermodynamics and kinetics
of cutting process pertinent to steels are also discussed. Similar theoretical models have been
applied in laser-oxygen assisted cutting [6-15].
Vol.9, No.7 Elucidation on Reactions Thermodynamics and Kinetics 609
2. METHODOLOGY
The hot-rolled 10mm metallurgy steel rods that were used in this study were procured from
the Universal Steel Rolling Mill, Ogba-Ikeja, Lagos, Nigeria. These were cut into 0.2m long
specimen. The compositions of the steel rods are given in Table 1.
Table 1: Composition of steels used.
SN Alloying elements (wt%)
C Si S P Mn Ni Cr Mo V W Fe
1. 0.16 0.18 0.0589 0.0288 0.6440 0.1030 0.1244 0.0114 0.0010 0.0007 Rest
2. 0.18 0.22 0.0410 0.0237 0.7160 0.1100 0.1250 0.0220 0.0020 0.0030 Rest
3. 0.25 0.15 0.0510 0.0419 0.3658 0.010340.0889 0.0177 0.0003 0.0032 Rest
4. 0.28 0.18 0.0523 0.0275 0.6247 0.1170 0.1306 0.0141 0.0013 0.0035 Rest
5. 0.32 0.18 0.0576 0.0367 0.6494 0.010640.1550 0.0141 0.0024 0.0037 Rest
6. 0.33 0.31 0.052 0.0274 0.7523 0.1110 0.17500.0170 0.0030 0.0040 Rest
At the required pressures, cutting torch is used to heat the steel rods surface to kindling
temperature where the steel rods starts to turn a cherry red. Oxygen is then released by
depressing the oxygen lever to react with the metal which then flows out of the cut (kerf) as
an oxide slag. The time taken to cut the rods is noted and recorded using a stop watch. The
cutting rate is calculated by finding the ratio of the diameter of rod to the total time taken i.e.:
Cutting rate = Diameter of rod / total time of cut
3. RESULTS AND DISCUSSION
Figures 1, 2, 3 and 4 show the variations of cutting rates with carbon contents at different
acetylene and oxygen pressures. From Fig. 1, the cutting rate was a maximum of 2.02
mm/min at 5.52 x 10-2 N.m-2 acetylene pressure and 3.17 x 10-1 N.m-2 oxygen pressure. A
minimum of 1.02 mm/min. was achieved at 5.52 x 10-2 N.m-2 acetylene pressure and 2.76 x
10-1 N.m-2 oxygen pressure. In Fig. 2, a maximum of 1.90 mm/min was achieved at 4.83 x 10-
2 N.m-2 acetylene pressure and 3.17 x 10-1 N.m-2 oxygen pressure for 0.16 wt% C steel. The
minimum of 0.91 mm/min was achieved for 0.33 wt% C steel at 4.83 x 10-2 N.m-2 acetylene
pressure and 2.76 x 10-1 N.m-2 oxygen pressure. Figure 3 shows that the maximum cutting
rate is 1.82 mm/min for 0.16 wt% C steel at 4.14 x 10-2 N.m-2 acetylene and 3.17 x 10-1 N.m-2
oxygen pressure. The minimum is 0.80 mm/min for 0.33 wt% C steel at 4.14 x 10-2 N.m-2
acetylene pressure and 2.76 x 10-1 N.m-2 oxygen pressure. Figure 4 has maximum cutting rate
of 1.70 mm/min. at 3.45 x 10-2 N.m-2 acetylene pressure for 0.16 wt% C steel. The minimum
of 0.70 mm/min. was achieved at 0.33 wt% C steel at 3.45 x 10-2 N.m-2 acetylene and 2.76 x
610 A.V. Adedayo Vol.9, No.7
10-1 N.m-2 oxygen pressure. Generally, the trend shows a decrease in cutting rates with
increase in carbon content.
0
0. 5
1
1. 5
2
2. 5
00.05 0.10.15 0.20.25 0.30.35
Carbon content (wt%)
Cutting rate (mm/min)
oxygen pressure (3.17 x 10-1) N.m-2
oxygen pressure (3.04 x 10-1) N.m-2
oxygen pressure (2.90 x 10-1) N.m-2
oxygen pressure (2.76 x 10-1) N.m-2
Figure 1: Variation of cutting rates with carbon content at 5.52 x 10-2 N.m-2 acetylene
pressure.
0
0.2
0.4
0.6
0.8
1
1.2
1.4
1.6
1.8
2
00.05 0.1 0.150.2 0.25 0.3 0.35
Carbon content (wt%)
Cutting rates (mm/min
)
oxygen pressure (3.17 x 10-1) N.m-2
oxygen pressure (3.04 x 10-1) N.m-2
oxygen pressure (2.90 x 10-1) N.m-2
oxygen pressure (2.76 x 10-1) N.m-2
Figure 2: Variation of cutting rates with carbon content at 4.83 x 10-2 N.m-2 acetylene
pressure.
Vol.9, No.7 Elucidation on Reactions Thermodynamics and Kinetics 611
0
0.2
0.4
0.6
0.8
1
1.2
1.4
1.6
1.8
2
00.05 0.10.15 0.20.25 0.30.35
Carbon content (wt%)
Cutting rates (mm/min
)
oxygen pressure (3.17 x 10-1) N.m-2
oxygen pressure (3.04 x 10-1) N.m-2
oxygen pressure (2.90 x 10-1) N.m-2
oxygen pressure (2.76 x 10-1) N.m-2
Figure 3: Variation of cutting rates with carbon content at 4.14 x 10-2 N.m-2 acetylene
pressure.
0
0.2
0.4
0.6
0.8
1
1.2
1.4
1.6
1.8
0 0.050.10.150.20.250.30.35
Carbon content (wt%)
Cutting rates (mm/min
)
oxygen pressure (3.17 x 10-1) N.m-2
oxygen pressure (3.04 x 10-1) N.m-2
oxygen pressure (2.90 x 10-1) N.m-2
oxygen pressure (2.76 x 10-1) N.m-2
Figure 4: Variation of cutting rates with carbon content at 3.45 x 10-2 N.m-2 acetylene
pressure.
3.1 Metal Oxidation Process
Generally, during hot rolling process, an oxide scale layer always develops at the metal
surface. Oxidation of steel at temperatures higher than 843K leads to three different iron
oxide layers: wustite (FeO), magnetite (Fe3O4) and hematite (Fe2O3), in oxygen content
increasing order, going from substrate to free surface. Below 843K, only the last two are
thermodynamically stable [16]. According to the literature, the relative thickness fractions of
612 A.V. Adedayo Vol.9, No.7
these layers between 973K and 1473K are 95% wustite, 4% magnetite and 1% hematite at
equilibrium [16, 17], although this balance can vary from one case to another. Depending on
temperature, time, atmosphere conditions and steel chemistry, energy barriers develop, which
must be overcome for an oxide to grow [16].
Kinetics of oxidation reactions of metals at high temperatures, are generally assumed to be
controlled by the rates of mass transport of reactants and products in the gas-slag interface,
slag-metal interface and slag phases [18]. The passage of oxygen from the gaseous phase
through the slag to the metal is generally believed to be as illustrated below Fig. 5 and 6.
Figure 5: Pattern of interactions during flame cutting
Vol.9, No.7 Elucidation on Reactions Thermodynamics and Kinetics 613
Figure 6: Reactions of slag forming process
However, Schwerdtfeger and Prange [19] have shown that slag-metal reactions involving
iron and/or silicon under industrial process conditions are very unlikely controlled by the
transfer kinetics of the reaction. Besides, whatever the means oxidation is being achieved,
cutting by OFC-A is achieved by melting of the slag. The cutting rate in OFC-A will depend
significantly on the rate of slag formation. Any reaction that retards formation of slag affects
the cutting rate. FeO is a significant proportion of products of oxidation. When ferrous metals
are cut by OFC-A, the process becomes one of rapid oxidation i.e. burning [1, 3, 5, 20]. The
retardation to the formation of FeO affects the cutting rate. Reactions below (equations 12 to
14) are unfavourable to the formation of FeO.
FeO + C Fe + CO (12)
FeO + CO Fe + CO2 (13)
CO2 + C 2CO (14)
614 A.V. Adedayo Vol.9, No.7
Min et al [21] and Kudrin [22] confirm reaction between iron oxide (FeO) and carbon (C),
while Min and Freuhan [23], Dogan [24] and Mori [18] elucidated on the reactions between
iron oxide and carbon monoxide (CO), carbon monoxide and carbon.
Generally, high carbon steels are more difficult to flame cut because of the reactions between
carbon, carbon monoxide and iron oxide [5, 25].
3.2 Thermodynamics of OFC-A of Steels
The general equation used to calculate ΔG (the change in Gibbs free energy) is :
STHG
Δ
Δ
=
Δ (15)
where ΔH is the enthalpy, i.e. the heat energy evolved by the reaction, T is the temperature in
K and ΔS is the change in entropy. Equation (15) above shows the temperature dependence
of ΔG, ΔH is also temperature dependent, as shown in the equation below:
+Δ=Δ
2
1
12
T
T
pTT dTCHH (16)
where Cp is the specific heat capacity at constant pressure.
During OFC-A, cutting flame only heats the metal to start the process; further heat is
provided by the burning metal. It is generally agreed that the exothermic reaction of oxygen
with the cut metal provides a considerable thermal input. The maximum reaction temperature
of the process will consist of the sum of the heat by the flame and that of the exothermic
reaction of oxygen with the cut metal. The steel flame cutting process consists of reactions as
follows:
Fe + ½ O2 FeO H1 = q1 (17)
[C] + 2[O] CO2 H2 = q2 (18)
[C] + [O] CO H3 = q3 (19)
While the heat by the flame is expressed as:
C
2H2 + O2 2CO + H2 H4 = q4 (20)
4CO + 2/3 H2 + O2 4/3CO2 + 2/3H2O H5 = q5 (21)
The total heat liberated by the OFC-A process of steel is given as the sum of heat of
equations (17), (18), (19) and those of equations (20) and (21) . The maximum heat input is
given as:
Vol.9, No.7 Elucidation on Reactions Thermodynamics and Kinetics 615
Q = q1 + q2 + q3 + q4 + q5 (22)
The relationship between the maximum heat input and maximum reaction temperature (Tm) is
expressed thus:
Q =
m
T
productpi dTCn
298
)( (23)
Where Cp is the specific heat capacity of products at constant pressure, ni is the mole
fractions of component i in the product. Thus;
Q = ++++
m
T
OHpOHHpHCOpCOCOpCOFeOpFeO dTCndTCndTCndTCndTCn
298
)()()()()( )....( 222222
(24)
Cp(CO2) = 44.14 + 9.04 x 10-3T – 8.58 x 105T-2 J/K/mol (25)
Cp(CO) = 28.45 + 4.184 x 10-3T – 0.46 x 105T-2 J/K/mol (26)
Cp(H2O) = 30.0 + 10.71 x 10-3T – 0.33 x 105T-2 J/K/mol (27)
Cp(H2) = 15.02 + 6.09 x 10-3T – 1.2 x 105T-2 J/K/mol (28)
Cp(FeO) = 43.18 + 6.86 x 10-3T – 1.15 x 105T-2 J/K/mol (29)
The total value of Q depends on the ratios of nFeO : nCO2: nCO . This ratio is a function of
concentration and activity of carbon and oxygen. Kudrin [22] and Bigeyev [27] showed that
there is increased tendency for formation of carbon dioxide at lower carbon concentrations.
Since Cp(CO2) is about twice the value of Cp(CO), higher values of nCO2: nCO will give higher
heat input. Also, cutting rate has been shown to increase with temperature [4]. This is
illustrated in Fig. 4. The energy available as heat from the FeO oxidation reaction at
atmospheric pressure suddenly rises in energy evolved above the boiling point of iron (Tb;
3330K). The reactions extinct at the dissociation temperature Td which is 3660K (see Fig 5).
616 A.V. Adedayo Vol.9, No.7
1900 2100 2300 2500
Temperature (K)
0
2
4
6
8
10
Cutting rate (m/min)
Figure 7: A typical relationship between cut front temperature and cutting rate [4, 27].
Figure 8: Variation of enthalpy and Gibbs free energy with temperature for FeO oxidation
reaction at atmospheric pressure [4].
Vol.9, No.7 Elucidation on Reactions Thermodynamics and Kinetics 617
Although the thermal energy generated by the oxidation reaction rises by approximately
350 kJ/mol (from 250 to 600) at the onset of boiling[4], the latent heat of boiling iron (347
kJ/mol) almost exactly matches this rise. Thus, if there is no increase in the proportion of iron
oxidized there will be no net increase to the thermal input to the cutting process. The
decarburization process retards iron oxidation, thereby reducing the proportion of iron
oxidized per time. Generally, high carbon steels are more difficult to flame cut because of the
reactions between carbon, carbon monoxide and iron oxide [5, 25].
4. CONCLUSION
Generally, the cutting rates decreased with carbon content of the steel. At the same oxygen
pressure, cutting rates have decreased up till about 65 pct with increase in carbon content of
about 0.17 wt%. The reason for the observed trend being the retardation to the formation of
FeO by decarburization reactions between iron oxide (FeO) and carbon (C), iron oxide and
carbon monoxide (CO), carbon monoxide and carbon.
ACKNOWLEDGEMENT
The author wishes to thank Dr John Powell of Lulea University of Technology. Lulea,
Sweden and Nottingham University, Department of Mechanical, Materials and
Manufacturing Engineering Nottingham, UK, for the help in providing relevant literatures
during the preparation of the manuscript. The efforts of Engr. A.U. Ayinla and Mr Dare
Amusa both of the Fabrication Unit, Institute of Technology, Kwara State Polytechnic, Ilorin,
Nigeria is gratefully acknowledged.
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