IFRF Combustion Journal
Article Number 200602 , September 2006
ISSN 1562-479X
High Temperature Oxidation of Steel in an Oxygen-enriched Low
NOX Furnace Environment
D. Poirier, E.W. Grandmaison*, M.D. Matovic1, K.R. Barnes2 and B.D. Nelson3
Department of Chemical Engineering
1
Department of Mechanical and Materials Engineering
Queen's University
Kingston, ON K7L 3N6
Canada
2
KB Technical Services, Inc
(formerly) Stelco Inc,
Research Manager
Stelco Inc.
P.O. Box 2030
Hamilton, ON L8N 3T1
Canada
3
Senior Researcher
Dofasco Inc.
P.O. Box 2460
Hamilton, ON L8N 3J5
Canada
*Corresponding author:
E.W. Grandmaison
Department of Chemical Engineering
Queen’s University
Kingston, ON K7L 3N6
Canada
tel.: (613) 533-2771
fax: (613) 533-6637
e-mail:
[email protected]
2
ABSTRACT
Steel scaling tests have been performed in a research furnace utilizing an oxygenenriched, low NOX, burner. This work was performed in conjunction with a study of
the combustion characteristics for the Canadian Gas Research Institute (CGRI) low
NOX burner. The furnace (a facility of the Centre for Advanced Gas Combustion
Technology (CAGCT)) was fired with the burner mounted in a sidewall
configuration similar to the geometry encountered in steel reheat furnaces. Scale
habit, intactness, adhesion and oxidation rates were examined for five grades of steel
over a range of stack oxygen concentrations (~0.8% - ~4.3%) and oxygen
enrichment levels (0 – 90%) at 1100°C. Steel grade had the largest effect on scaling
properties examined in this work. Within the tests for each grade, stack oxygen
concentration had the largest effect on the scaling properties while oxygen
enrichment level had only a small effect.
Key Words:
steel scaling, reheat furnace, low NOX burners, oxygen-enrichment.
3
BACKGROUND
Poirier et al. (2004) have recently presented results of an investigation of oxygenenriched combustion studies with the CGRI low-NOX burner.
A potential
application of this technology is in steel reheat furnaces where steel slabs and/or
billets are heated to temperatures of ~1215–1300°C. In this process, exposure to a
combustion product atmosphere inevitably leads to the formation of an oxide scale
on the steel surface. Results of steel oxidation tests, presented in this paper, were
performed to complement the oxygen-enrichment combustion work by subjecting
different steel grades to oxy-fuel combustion product atmospheres and a range of
stack oxygen levels commonly encountered in industrial practice.
There has been considerable interest in the problem of high temperature oxidation of
metals – textbooks on this subject include Birks and Meier (1983) and Kofstad
(1988). When steel is exposed to oxidation conditions above ~570°C, a multilayer
scale forms consisting of FeO (wustite), Fe3O4 (magnetite) and Fe2O3 (haematite)
with the wustite layer next to the steel surface and haematite at the gas-scale
interface. The oxidation rate can be estimated by the increase in scale thickness,
xscale, with time and this rate can also be expressed in terms of the mass gain, ∆mO,
per unit time, ∆t, for a sample of area A, Figure 1,
ρ scale
dx scale
∆m O ∆t
= K S/O
dt
A
In this expression, the mass gain, ∆mO, represents the uptake of oxygen in the steel
sample over a time period ∆t and KS/O is the stoichiometric ratio of the scale mass
per unit mass of oxygen in the oxide scale. Mass gain can often be measured in steel
scaling experiments and KS/O can be estimated from the Fe/O stoichiometry – the
mass ratios of the oxides, FeO/Fe3O4/Fe2O3 are typically 95/4/1 (Paidassi, 1958) and
the stoichiometry, FeyO, is often approximated by y ≈ 0.95, but the atomic ratio may
range from 0.88 – 0.95 (Engell, 1958).
4
Fe2O3/Fe3O4/FeO
~ 1%/4%/95%
Gas
Phase
Scale
x Scale
Steel
<< Fe/Fe++ transport
(O equivalence)
O2/O= transport >>
Phase boundary oxidant reactions:
O2, CO2 and H2O
Figure 1.
Mechanisms for the high temperature oxidation of steel.
The oxidation mechanism, Figure 1, can depend on (i) the transport of oxidant gas
from the bulk gas phase, (ii) phase boundary reaction(s) at the gas/scale interface, or
(iii) the diffusion of Fe cations to the scale/gas phase interface. In the first two cases,
the flux of oxygen, N
O=
, in the oxidation process does not depend on the scale
thickness, xscale, and we obtain,
N
O=
=
ρ scale dx scale
dt
K S/O
x scale ≈
N
O=
=
∆m O ∆t
A
K S/O
ρ scale
t
x scale ≈ k l t
i.e. a linear growth rate with a constant kl. The value of this constant will depend on
which mechanism is rate-controlling. In the third case, where diffusion of Fe cations
is the rate controlling step, the oxidation rate does depend on xscale, according to a
Fick’s law-type diffusion relationship,
5
N
O=
=
ρ scale dx scale
K S/O
dt
=
∆C Fe( + )
∆m O ∆t
≈ DFe( + )
A
x scale
where DFe(+) is the cation diffusivity and ∆CFe(+) is the cation concentration gradient
across the scale layer xscale. Integration of xscale over a time period t gives,
2
x scale
=
2 DFe( + ) K S/O ∆C Fe( + )
ρ scale
t
2
x scale
≈ kp t
i.e. a parabolic growth rate with a constant kp (first proposed by Tammann, 1920). A
quantitative theory for the parabolic oxidation process has been developed by
Wagner (1933) and experimentally validated in the early work of Himmel et al.
(1953) and Engell (1958) and more recently by Abuluwefa et al. (1996, 1997a,
1997b) and Omerod et al. (1997). In practice, the parabolic oxidation rate can break
down when the scale surface cracks and/or loses contact with the steel substrate.
The oxidant gases can then react at the underlying steel surface to form new scale
beginning with a rapid linear growth rate. This behaviour has been well documented
in the industrial tests reported by Abuluwefa (1992).
The steel scaling process in a reheat furnace depends on the gas composition
(Rahmel and Tobolski, 1965; Sachs and Tuck, 1970; Cook and Rasmussen, 1970),
temperature, steel surface characteristics and steel composition. In the present work,
scaling characteristics of five grades of steel were examined in a large-scale research
furnace at 1100°C with different combustion product atmospheres arising from the
use of oxygen-enrichment with the CGRI low NOX burner. The measurements
formed part of the work reported by Poirier et al. (2004) and were performed to
examine the effect of oxygen-enrichment level and stack oxygen concentration on
steel scaling characteristics.
Oxygen enrichment leads to different combustion
product environments with significantly larger proportions of CO2 and H2O along
with the excess oxygen commonly present in combustion systems. A reduction in
the ballast nitrogen with the oxidant feed also leads to lower gas velocities and
potential changes in mass transfer characteristics for a furnace environment with
oxygen-enrichment.
6
EXPERIMENTAL METHODOLOGY
The CAGCT furnace, Figure 2, served as the heating environment for steel samples
tested in this work. This furnace consists of two unequal size chambers separated by
a checker-work, brick end-wall. The first chamber is the main furnace cavity with
internal dimensions of 4.5 m long, 3 m wide and 1 m high (177 in. x 118 in. x 39
in.). The second chamber serves as an exhaust plenum with interior dimensions of
0.6 m long, 3 m wide and 1 m high (24 in. x 118 in. x 39 in.). The checker wall, 215
mm thick (8.5 in.), with an 8 x 3 array of openings, 75 mm x 115 mm (3 in. x 4.5
in.), separates these two chambers. The refractory lining for the furnace walls and
roof are ceramic fibre blocks, 305 mm (12 in.) thick and the furnace wall structure
and refractory is a combined 362 mm (14 in.) thick. Refractory wall surfacethermocouples, 0.254 mm (0.01 in.) dia. Pt/Pt-10%/Rh, are embedded about 5 mm
(0.2 in.) into the refractory walls, at positions T1 – T41 as shown in Figure 2. The
floor of the furnace consists of water-cooled panels to permit heat flux
measurements, but in the present tests the furnace floor was covered with 25 mm (1
in.) refractory blanket providing a near-adiabatic condition on this boundary of the
furnace.
A single burner rated at a maximum firing rate of 400 kW (1.4 Mbtu/h) was fired
from a sidewall configuration, 750 mm from the blind endwall of the furnace cavity.
The burner design, Figure 3, was a modified version of the CGRI burner described
by Sobiesiak et al. (1998).
The fuel and oxidant streams for the burner were
supplied through seven nozzles around the burner axis. The oxidant feed streams
consisted of concentric jets with oxygen supplied in the center jet and the air
supplied in the annular jet for each of the seven nozzles.
The oxygen nozzle
diameter and the air nozzle annulus were sized so that the momentum of the
combined oxidant stream would remain relatively constant with changing O2enrichment level for a constant firing rate. The air and O2 nozzle angle (10°), airnozzle annulus size, fuel jet angle (20°) and fuel nozzle diameter were maintained at
constant values for the results reported in this work.
7
Burner
Plenum
Wall
-362
0
146
797
890
1000
1203
1500
x
1797
2000
2110
3000
T1
586
T8
SP1
T5
T9
T2
1195
1805
T14
T21
T17
SP2
T6
T7
Furnace
Exhaust
T22
T3
T15
T10
SP3
T18
T23
SP4
2414
T11
T4
2854
Top
View
T20
T16
T12
Refractory
T19
T13
3362
453
-362 0
496
z
873
0
T26
127
T24
3996
4500
y
1004 1496 2004
T28
T25
500
2996
750
1362
1000
1047 1750 1995
T31
T33
2750
T35
T30
T27
T29
T32
T34
3496
5100 5462
4254
T38
T41
T37
T40
T36
T39
Side
View
Water-cooled floor panels
Figure 2.
The CAGCT research furnace showing position of the specimen
ports, SP1 --- SP4, used for insertion of the steel samples.
Refractory thermocouples are shown as T1 --- T41.
All
dimensions in mm.
UV scanner port
Pilot burner port
Air/oxidant nozzle
Fuel nozzle
Figure 3.
Front view of the CGRI low NOX burner. Air/oxidant ports are
concentric jets with the oxygen feed in the centre jet and air feed
in the annular jet.
8
The firing rate was adjusted to maintain a constant furnace temperature of 1100°C as
O2 enrichment and excess oxidant was varied. A four-hour warm-up period was
used to allow the furnace to reach quasi-steady state operating conditions. Oxygen
enrichment level, ψ O 2 , defined as


&O
m
2
 × 100
m
& O2 + m
& O2A 


ψ O2 = 
& O and m
& O A are the mass flow rates for the pure oxygen and oxygen
where m
2
2
associated with the air feed streams, respectively, was varied between 0% and 90%.
The stack oxygen concentration was examined at three levels, 0.8% (± 0.2%), 1.7%
(± 0.3%) and 4.3% (± 0.5%) by volume.
Five grades of steel samples with the chemical composition shown in Table 1 were
examined in this work: samples S1 and S2 were strip steel grades and P1, P2 and P3
were plate steel grades. Grade P1 is a common structural plate grade (ASTM
A36/CSA Gr. 44W), P2 is a high strength steel (Grade 70 CuNiCr-containing
weathering steel) and P3 is a high strength linepipe steel (API 5LX70).
The steel
specimens were nominally 12.7 x 63.5 x 100 mm (0.5” x 2.5” x 4”) and 630 g. The
steel samples were exposed to the furnace combustion product atmosphere by
hanging them through specimen ports (~10 cm diameter), SP1 – SP4, in the roof of
the furnace, Figure 2, adjacent to the exhaust plenum. Roof thermocouples T21 --T23 and the primary furnace thermocouple are located near these specimen ports to
provide an accurate estimate of the furnace temperature near these samples. In order
to perform the complete set of experiments outlined below, it was necessary to
suspend a series of up to 4 samples with 3 mm dia. stainless steel wire at one time
through each port.
Steel
grade
9
S1
S2
P1
P2
P3
Composition, % mass
C
0.060
0.060
0.190
0.150
0.050
Table 1.
Mn
P
S
Si
Cu
Ni
Cr
Mo
V
Nb
B
Ti
Al
Sn
Ce
0.270
0.650
0.400
0.030
1.300 0.007 0.008 0.260 0.009 0.008 0.031 0.002 0.030 0.002
0.014 0.035 0.002
1.280 0.010 0.008 0.270 0.300 0.350 0.260 0.006 0.050 0.033
0.013 0.022 0.002
1.420 0.009 0.002 0.290 0.014 0.019 0.170 0.240 0.002 0.076 0.0002 0.011 0.036 0.002 0.002
Chemical compositions of the five steel grades.
ASA
N
Ca
CEQ
Dl
0.033 0.006
0.420 0.760
0.020 0.005
0.471 1.140
0.034 0.007 0.0004 0.235
10
The scaling experiments were performed at a target temperature of 1100oC for
prescribed periods of time to monitor scaling rates for different grades of steel as
outlined below:
1. Two grades of steel, S1 and P1, were examined at four oxygen enrichment
levels, ψ O 2 = 0%, 25%, 50% and 90%, three stack oxygen concentrations
(0.8%, 1.7% and 4.3%) and oxidation times ranging from 1.8 ks (30 minutes)
to 14.4 ks (4 hours).
2. Two additional grades, P2 and P3, were tested at 2% stack oxygen
concentration, ψ O 2 = 0% and oxidation times ranging from 1.8 ks to 14.4 ks.
Replicate tests were performed for these conditions to provide an estimate of
the experimental error in this work.
3. All five grades of steel were tested for a 14.4 ks period, over the full range of
oxygen enrichment (0%, 25%, 50% and 90%) and stack oxygen level (0.8%,
1.7% and 4.3%) with the scale layer preserved for later morphology analysis.
The mass and surface area for each specimen were estimated by direct measurement
prior to exposure to the furnace atmosphere. Following the desired oxidation period,
the samples were removed from the furnace, quickly quenched in a N2 atmosphere
and weighed with the accumulated scale retained on the sample. For samples tested
under conditions (1) and (2) above, the scale layer was removed and the sample was
again weighed to provide an estimate of the steel mass loss. The scaling results were
then expressed in terms of the mass gain (oxygen uptake) or steel mass loss per unit
area of the steel samples. A full range of steel mass loss measurements were
obtained for the conditions noted above. The scale from most samples could be descaled easily but scale from some sample grades required substantial effort with a
hammer and scrapper.
Scale morphology work included samples of the five steel grades. Hedonic rankings
shown in Table 2 were used to assess the scale characteristics under three headings:
(i) surface characteristics, (ii) scale “intactness” and (iii) scale adhesion to the steel
substrate. Characteristics of subsurface scale and the scale/steel interface were
11
examined by photomicrography. Selected morphology samples were encased in
epoxy, sectioned (cut perpendicular to the sample face, straight through), mounted
and photographed under an optical microscope.
Some SEM work was also
performed on selected samples to give improved depth of field focus and, more
importantly, in backscatter-mode, to distinguish the boundaries, thickness and
morphology of iron oxide phases through the scale profile.
Scale surface characteristics
Scale “intactness”
Scale removal effort
Rank
Description
Rank
Description
Rank
Description
0
Smooth
0
Intact
1
Scale falls off with
very little effort
1
Few fine cracks
2
Removed with little
effort
2
Few fine flakes from
outer surface of scale
3
Removed with some
effort
4
Difficult to remove
A few nodules
along as-cast edge
A few nodules
randomly
distributed
1
2
3
Slightly mottled with
blisters and nodules
3
4
Heavily mottled
4
5
Porous
5
6
Table 2.
Some fine flakes
from outer surface of
scale
Loss of small bits of
thick scale from
sides after cooling
Loss of large
sections of thick
scale from sides after
cooling
Loss of large
sections of thick
scale from sides and
main faces after
cooling
Outline of the ranking system employed for assessment of (i) scale
surface characteristics, (ii) scale “intactness” and (iii) scale
removal effort (scale adhesion).
12
EXPERIMENTAL RESULTS
The results of this work included both qualitative and quantitative assessment of the
oxide scale formed on the five steel grades in a furnace combustion product
atmosphere at 1100°C with different stack oxygen concentrations and oxygen
enrichment levels. The qualitative results are presented in the form of hedonic
rankings, supported by photographic evidence, for the scale characteristics as well as
photomicroscopy and SEM analysis of samples at selected oxidation conditions. In
the present work, the steel samples entered the furnace in a “cold” state (ambient
laboratory temperature) and reached the furnace temperature within a ~5 minute
period. The exothermic heat effect associated with the scaling reactions would cause
a transient temperature spike in the scale and steel substrate. This transient would
equilibrate to the furnace temperature in about the same time period. Hence scaling
rates for the early stages of the steel sample heating process would not yield useful
information.
In the present work, the samples were exposed to furnace gas
atmospheres for a minimum period of 30 minutes before samples were extracted
from the furnace for analysis.
Figure 4 shows photographs of steel test samples: (i) before exposure to the furnace
atmosphere, Figure 4-A, and (ii) after furnace atmosphere exposure, Figure 4-B – 4H, showing surface and scale intactness at different operating conditions. Rankings
for the surface characteristics and steel intactness corresponding to the descriptions
noted in Table 2 are also presented in Figs. 5 and 6 in the form of grouped bar charts
to illustrate the effect of stack oxygen concentration and oxygen enrichment level.
The only factor appearing to significantly affect these properties was the steel grade
tested. The order of the steel grades in going from a smooth, intact scale to rough
and cracked, separated scale was S2, S1, P2, P3 and P1. The outward scale surface
characteristics did not appear to be linked to the steel chemical composition in any
obvious way. It appears that perhaps steel carbon content may have some influence
on scale habit and intactness, becoming less smooth and intact as carbon content
rises and the formation and escape of CO2 from the steel/scale interface increases.
13
A
B
C
E
F
D
G
H
Figure 4.
Steel samples – picture A is a steel sample before furnace
atmosphere exposure, pictures B – H show various grades
exposed to furnace conditions (1100°C) for 4 hours:
B: grade S2, ψ O 2 = 50%, 1.7 % stack oxygen: surface
characteristic = 0 and scale intactness = 0,
C: grade S2, ψ O 2 = 90%, 0.8 % stack oxygen: surface
characteristic = 1 and scale intactness = 0,
D: grade S1, ψ O 2 = 50%, 4.3 % stack oxygen: surface
characteristic = 2 and scale intactness = 1,
14
E: grade P2, ψ O 2 = 25%, 1.7 % stack oxygen: surface
characteristic = 3 and scale intactness = 2,
F: grade P1, ψ O 2 = 50%, 1.7 % stack oxygen: surface
characteristic = 4 and scale intactness = 4,
G: grade P1, ψ O 2 = 25%, 4.3 % stack oxygen: surface
characteristic = 4 and scale intactness = 5,
H: grade P1, ψ O 2 = 50%, 4.3 % stack oxygen: surface
characteristic = 4 and scale intactness = 6.
S1
Surface characteristics
4
ψΟ2
S2
0%
25%
50%
90%
3
0.8% 1.7%
2
0.8%
4.3%
4.3%
1.7%
1
0
0
1
2
0.8% 1.7%
4
3
P1
4
0
4.3%
1
2
3
P2
0.8%
1.7%
3
4
P3
0.8%
4.3%
1.7%
4.3%
2
1
0
0
Figure 5.
1
2
3
4
0
1
2
3
4
0
Stack O2, vol. %, w.b.
1
2
3
4
Ranking for the scale surface characteristics for the five steel
grades as a function of stack oxygen concentration (0.8%, 1.7%
and 4.3%) with oxygen enrichment level as a parameter.
Scale "intactness"
15
S1
6
5
4
3
2
1
0
ψΟ2
S2
0%
25%
50%
90%
4.3%
0.8% 1.7%
4.3%
0.8% 1.7%
0
1
2
3
P1
6
5
4
3
2
1
0
4
0
1
2
3
4
P3
P2
4.3%
4.3%
0.8% 1.7%
4.3%
1.7%
0.8% 1.7%
0.8%
0
Figure 6.
1
2
3
4
0
1
2
3
4
0
Stack O2, vol. %, w.b.
1
2
3
4
Ranking for the scale “intactness” for the five steel grades as a
function of stack oxygen concentration (0.8%, 1.7% and 4.3%)
with oxygen enrichment level as a parameter.
Scale adhesion to the steel surface following the reheating process is an important
factor in the steel industry. The effort required to remove the scale from the steel
substrate following exposure to the combustion product atmosphere and sample
cooling in an N2 atmosphere was subjectively ranked according to the degrees of
difficulty (ranking of 1 --- 4) shown in Table 2. A more complete set of tests was
performed for the S1 and P1 grades at three stack oxygen concentrations (0.8%,
1.7% and 4.3%), four oxygen enrichment levels (0%, 25%, 50% and 90%) and four
furnace exposure times (1.8, 3.6, 7.2 and 14.4 ks). The largest effect of any of these
variables was observed at exposure times of 14.4 ks and these results are shown in
Figure 7.
16
Scale removal effort
S1 steel grade
P1 steel grade
ψΟ 2
4
0%
25%
50%
90%
3
0.8% 1.7%
2
0.8% 1.7%
4.3%
4.3%
1
0
0
1
2
3
4
0
1
2
3
4
Stack O2 concentration, % w.b.
Figure 7.
Ranking for the scale removal effort for steel grades S1 and P1 as
a function of stack oxygen concentration (0.8%, 1.7% and 4.3%)
with oxygen enrichment level as a parameter.
The effect of steel grade is clearly the most important factor – the overall adhesion
rankings were 1.9 for the S1 grade and 3.7 for the P1 grade. At lower stack oxygen
concentrations theses samples tended to have a more adherent scale and the effort
required to remove the scale appeared to decrease with increasing oxygen
enrichment – a larger number of samples would have to be tested to confirm these
minor effects more accurately. Tests were also performed for the P2 and P3 grades
at 1.7% stack oxygen, ψ O 2 = 0% and four exposure times (1.8, 3.6, 7.2 and 14.4 ks).
These samples had a consistent adhesion ranking 3 for the P3 grade and 4 for the P2
grade at all four exposure times. Based on these results, it appears that factors
affecting steel scale adhesion, from most dominant to least dominant, were: Steel
grade > Furnace oxygen concentration level > Oxygen enrichment level > Furnace
exposure time. The effect of steel grade was much more pronounced than for any of
the other factors listed. The order of scale removal effort, from high to low effort,
for the various grades of steel tested was P2>P1>P3>S1. The scale adhesion results
17
can, in part, be related to the composition of the steels, Table 1. Surface enrichment
of elements, especially Ni, can result in surface enrichment and metal/scale
entanglement, Sachs and Tucker (1968). The P2 steel grade is highest in Ni content
and possesses the most tenacious scale. Other metal components, such as Cu, can
also influence scale characteristics. Steel grades P1, P2 and P3 all have somewhat
elevated levels of Cu, Cr and Mo. Figure 8 shows photographs of de-scaled steel
samples supporting these observations - a very clean removal of the scale for the S1
grade in the left photograph and a strongly adherent “subscale” characteristic of the
P1 grade in the right photograph.
Figure 8.
Photograph of a “clean” de-scaled S1 steel grade sample (left) and
a de-scaled steel sample showing residual porous “subscale” for
the P1 grade (right).
Photomicrographs (30x magnification) of the steel/scale interface for each steel
grade at 1.7% stack oxygen and 0% and 90% oxygen enrichment are shown in
Figure 9. A comparison of the samples at ψ O 2 = 0% and 90% shows that the oxygen
enriched cases appear to have somewhat less porous scale in the outer 1/2 - 2/3 of the
scale, a porous layer of scale close to the steel interface and a clearly visible thin
separation occurs along the entire steel interface. More detail of the steel/scale
interface is shown in Figs. 10 and 11 for the steel grades S1 and P2. Figure 10
shows photomicrographs of the interface at 400x magnification with a clear
18
separation for the S1 grade (least adherent scale) and some entanglement between
the scale and steel for the P2 grade (most adherent scale). Similar behaviour is also
depicted in the SEM (500x) pictures for these two grades in Figure 11.
A
B
C
F
G
H
D
E
I
J
Figure 9. Photomicrographs (30x) of the steel/scale interface for the five steel
grades exposed to combustion product atmosphere for 4 hours at
1100 °C and 1.7% stack O2. Top row of pictures, A – E, are for
grades S1, S2, P1, P2 and P3 at ψ O 2 = 0%. Bottom row of pictures, F
– J, are for grades S1, S2, P1, P2 and P3 at ψ O 2 = 90%.
S1
P2
Figure 10. Photomicrographs (400x) of the scale/steel interface for least
(S1) and most (P2) adherent scales. Both samples exposed
for 4 hours at 1100oC, 0% oxygen enrichment, and 1.7% stack
oxygen.
19
Figure 11.
SEM (500x) of the steel/scale interface for grades S1 (left picture)
and P2 (right picture) at 0% oxygen enrichment and 1.7% stack
oxygen. Exposed for 4 hours at 1100°C.
Quantitative estimates of the steel scaling rates were obtained at 1100°C, consistent
with the other trials performed in this work. Each sample was pre-weighed and a
surface area was estimated by direct measurement. The samples were then exposed
to the furnace atmosphere for times ranging from 1.8 ks to 14.4 ks. The samples
were then removed from the furnace, cooled in a N2 atmosphere, weighed with the
scale attached and weighed again with the scale removed. The steel mass gain (a
measure of the oxygen uptake in the oxide) and the steel mass loss (oxide material
removed) were estimated from these measurements. The steel mass loss data formed
the most complete set of data and these results are reported in this paper.
•
A series of four replicate tests were performed with the P2 and P3 steel
grades, exposing samples at times of 1.8, 3.6, 7.2 and 14.4 ks in the furnace
(1100°C, ~2% stack oxygen and 0% oxygen enrichment).
•
The primary series of scaling tests were performed with two steel grades, S1
and P1 (steel compositions were given in Table 1). For these tests, steel
specimens were exposed to a complete range of operating variables
associated with the overall oxygen enrichment studies. These included the
oxygen enrichment level (0%, 25%, 50% and 90%) and the stack oxygen
concentration (0.8%, 1.7% and 4.3%).
20
Mass loss estimates for steel grades S1, P1, P2 and P3 at 1.7% stack oxygen and
ψ O 2 = 0% are shown in Figure 12. In these graphs the steel mass loss, mL, is
normalized by the sample area, A. A linear relationship for mL/A or (mL/A)2 as a
function of time would indicate linear or parabolic oxidation rates respectively.
Results for each of the replicate tests for the P2 and P3 grades are also shown in
Figure 12. Estimates of the mean square pure error for the mL/A data were relatively
constant at si2 ≈ 0.00856; when expressed in terms of the square of the mass loss, the
6
4
S1
P1
P2
P3
2
0
0
5
10
15
(Steel mass loss), (mL/A)2, kg2/m2
Steel mass loss, mL/A, kg/m
2
mean square pure error was si2 ≈ 0.0462 (mL/A)2.
50
S1
P1
P2
P3
40
30
20
10
0
0
Time, ks
Figure 12.
5
10
15
Time, ks
Steel mass loss data for steel grades S1, P1, P2 and P3 at 1100°C,
1.7% stack oxygen concentration and 0% oxygen enrichment.
The results for each steel grade in Figure 12 indicate that a parabolic oxidation
mechanism,
(mL/A)2 = β0 + β1t
is an appropriate choice for oxidation times, t ≥ 1.8 ks, with the model parameter
estimates shown in Table 3. The operating conditions for these tests represent an
average or “nominal” reheat furnace operating condition with no oxygen enrichment.
At these conditions the lowest to highest scaling rates were in the sequence:
P1 and P3 grades (no significant difference between these two grades) < P2 < S1
21
Table 3.
Steel grade
β0, kg2/m4
β1 , kg2-s/m4
S1
-2.10
2.87
P1
-1.71
1.61
P2
-1.64
2.04
P3
-1.05
1.62
Parabolic oxidation rate parameters, β0 and β1, for different steel
grades at 1100°C, 2% stack oxygen concentration and 0% oxygen
enrichment.
A more extensive series of test results, Figure 13, were obtained for the S1 and P1
grades (grades with the highest and lowest scaling rates at the nominal test
conditions noted above) with steel mass loss estimated for different stack oxygen
concentrations (0.8, 1.7 and 4.3%) and oxygen enrichment levels (ψ O 2 = 0, 25, 50
and 90%). Beyond t = 1.8 ks, the overall scaling rate for each steel grade can be
expressed by the relations:
S1 grade:
(mL/A)2 = 2.68t – 2.16
P1 grade:
(mL/A)2 = 1.93t – 1.93
While these relations, depicting the solid lines in Figure 13, give a broad picture of
the scaling kinetics with a higher scaling rate for the S1 grade, some caution must be
used since the scatter in the data conceals effects of different operating conditions
and scaling mechanisms. The intercept and slope parameters, β0 and β1, for the
individual operating conditions are shown in Table 4. With increasing stack oxygen
concentration, there was an increasing tendency for the oxidation process to follow a
full parabolic mechanism – this is indicated by the decrease in the magnitude of the
intercept, β0, with increasing stack oxygen concentration.
22
40
(Steel mass loss) , (mL/A) , kg /m
20
2
2
S1
0
6
4
2
0
30
2
4
2
Steel mass loss, mL/A, kg/m
2
4
6
P1
0
5
10
Time, ks
0
30
20
10
0
15
S1
10
P1
0
0.8% Stack O2 1.7% Stack O2
5
10
Time, ks
4.3% Stack O2
ψO = 0 %
ψO = 0 %
ψO = 0 %
ψO = 25 %
ψO = 25 %
ψO = 25 %
ψO = 50 %
2
ψO = 50 %
ψO = 50 %
ψO = 90 %
2
ψO = 75 %
ψO = 90 %
2
2
2
2
2
2
15
2
2
2
2
ψO = 90 %
2
Figure 13.
Steel mass loss data for steel grades S1 and P1 at 1100°C, three
levels of stack oxygen concentration and oxygen enrichments
levels in the range of 0% -- 90%.
23
Stack oxygen
0.8%
0.8%
0.8%
0.8%
1.7%
1.7%
1.7%
1.7%
4.3%
4.3%
4.3%
4.3%
Table 4.
Steel grade S1
Steel grade P1
2
4
2
4
O2 enrichment β0, kg /m β1, kg /ks-m β0, kg2/m4 β1, kg2/ks-m4
0%
-3.43
2.70
-2.92
1.54
25%
-4.23
2.74
-3.79
1.73
50%
-5.76
2.81
-3.39
1.91
90%
-3.13
2.63
-3.44
2.17
0%
-2.10
2.86
-1.71
1.61
25%
-3.53
3.19
-2.08
1.81
50%
-1.47
2.76
-1.04
1.80
90%
-1.32
2.67
-4.24
2.09
0%
-0.52
2.08
-0.72
1.97
25%
-0.83
2.67
0.28
1.87
50%
-0.58
2.75
-1.78
2.17
90%
1.02
2.31
0.53
1.97
Estimates of the parabolic oxidation rate parameters (intercept =
β0 and slope = β1) for steel grades S1 and P1 at different furnace
operating conditions.
The effects of stack oxygen concentration and oxygen enrichment level are more
clearly demonstrated in Figure 14 where the steel mass loss is shown as a function of
stack oxygen concentration in the form of stacked bar charts. At each stack oxygen
concentration, stacked bars are presented for the steel mass loss at the four oxygen
enrichment levels. For each steel grade, the scaling rate at 1.8 ks increases with
increasing stack oxygen concentration (top graph in Figure 14), but this effect is not
significant at 14.4 ks (bottom graph in Figure 14). The effect of oxygen enrichment
can be assessed by observing trends in the groups of data presented at each stack
oxygen concentration. In most cases there is simply random variation in the steel
mass loss at each stack oxygen concentration suggesting that oxygen enrichment
does not have a large effect. A wider variation was observed in the scaling rates at
0.8% stack oxygen concentration and 1.8 ks (top graph in Figure 14) for the S1 grade
and to a lesser extent for the P1 grade. The lowest scaling rate in these groups of
data occurred at 50% oxygen enrichment for each steel grade and these tests were
performed at a stack oxygen concentration of ~0.6% while the other tests at this
stack oxygen concentration were performed close to, or above, the average value of
24
0.8%. Hence the scaling rate variability these conditions are attributable to the effect
of the stack oxygen concentration and the inherent difficulties in controlling the
furnace atmosphere at these conditions. There was a slight increase in scaling rates
with increasing oxygen enrichment levels for some conditions (S1 grade at 4.3 %
stack oxygen and 1.8 ks; P1 grade at 0.8 and 1.7% stack oxygen and 14.4 ks), but
this effect was not large within the scatter of the present data.
Steel mass loss (1.8 ks), kg/m
2
ψΟ
2
0%
25%
50%
90%
3
P1 steel grade
S1 steel grade
4.3%
4.3%
1.7%
2
0.8%
1.7%
0.8%
1
0
0
1
2
3
4
0
1
2
3
4
Stack O2 concentration, %, w.b.
ψΟ
Steel mass loss (14.4 ks), kg/m
2
2
0%
25%
50%
90%
10
8
0.8%
6
P1 steel grade
S1 steel grade
1.7%
4.3%
4.3%
0.8% 1.7%
4
2
0
0
1
2
3
4
0
1
2
3
4
Stack O2 concentration, %, w.b.
Figure 14.
Steel mass loss data for steel grades S1 and P1 at 1100°C as a
function of stack oxygen concentration (0.8, 1.7 and 4.3%) with
oxygen enrichment levels shown in the form of stacked bars. Top
graphs are for 1.8 ks steel exposure times, bottom graphs are for
14.4 ks steel exposure times.
25
CONCLUSIONS
The high temperature oxidation of different steel grades has been examined in an
oxygen enriched furnace environment at 1100°C. The effect of employing oxygen
enrichment, compared with more traditional combustion environments, was assessed
for five steel grades in terms of
•
Scale surface habit (ranked on a scale of 0 → 5 for a smooth → porous surface),
•
Scale intactness (ranked on a scale of 0 → 6 for an intact → heavily detached
scale),
•
Scale adhesion (ranked on a scale of 0 → 4 for little effort for removal →
difficult to remove), and
•
Scaling rates.
Photomicrographs with 0% and 90% oxygen enrichment conditions were also
performed to provide supporting evidence for these tests.
The only factor affecting scale habit and intactness was the steel grade; the
stack oxygen and oxygen enrichment levels did not significantly affect these
properties.
The steel grade was also the most important factor affecting scale
adhesion – steel grades with more elevated levels of Ni, Cu, Cr and/or Mo had a
more adherent scale.
Photomicrographs also indicated that the more adherent
samples had a porous scale and/or rough steel/scale interface. Those scales that were
easily removed had a clearly defined separation at the steel/scale interface.
The
scale also appeared to be less adherent with increasing stack oxygen concentrations
and the effort for scale removal appeared to decrease slightly with increasing oxygen
enrichment and increasing sample exposure times. These effects were much smaller
than the difference observed between sample grades.
The implication of these
results are that oxygen enrichment has far less effect on the scale properties than that
accounted for by differences due to the steel grade.
The scaling rate, expressed in terms of the steel mass loss (Fe loss), was
strongly dependent of the steel grade. The oxidation rates followed a parabolic
behaviour at 4.3% stack oxygen. At lower stack oxygen concentrations (0.8% and
1.7%), a lower initial oxidation rate was observed, followed by a parabolic behaviour
26
after the first 1.8 ks of exposure to the furnace atmosphere. Within the tests for each
grade, the parabolic oxidation rate was most strongly affected by the stack oxygen
concentration and the oxygen enrichment level only had a small effect on the
parabolic oxidation rates.
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27
ACKNOWLEDGEMENT
This work was performed under the U.S. Department of Energy (DOE) / American
Iron and Steel Institute (AISI) Cooperative Agreement DE-FC07-97ID13554,
Technology Roadmap Research Program for the Steel Industry. The support and
participation of Air Liquide Corporation, BOC Gases, Dofasco Inc., Fuchs Systems
and Stelco Inc. in this program is greatly appreciated.