FIELD OF THE INVENTION
The present invention is directed to alloy coated BOF stack tubes panel and to a
process of producing thermal spray coated BOF stack tubes directed to improved
performance and extended operational life. More particularly, the thermal spray coating
process according to the present invention is directed to providing selective alloy
composition and optimized spray parameters such that the stack is protected from damage
caused due to severe thermal fatigue in combination with erosion and corrosion of tube wall
from both sides by flue gas and dust particles, tending to wear out the tubes walls and
avoiding getting thinner and thinner and finally punctured causing water leakage needing
converter prolonged shut down for weld repairing of tubes. Advantageously, each of the
welded panels are given stress relieving treatment to avoid distortion/deformation during
thermal spray coating treatment. Importantly, the coating process parameters are
optimized with respect to number of passes, wire feed rate, current, voltage and air
pressure to achieve strongly bonded layer for producing micro crystalline or amorphous
coating of desired thickness. The coated panels/pipes in BOF stacks according to the present
process are estimated to work without any deterioration and leakage problem for more than
8-10 years, and thus having potential for wide industrial application with distinct advantage
of lowering downtime of BOF converter and improvement in productivity.
BACKGROUND ART
It is well known in the art of steel making in large steel plants that, the BOF stack
essentially carries effluent gases (C02, CO, H2) with floating fine dust having particle size in
the range of 400-500 p.m, at a temperature over 1000°C generated during steel making
operations. The stacks are fitted with water cooled pipes along the circumference to cool the
hot gases flowing upwards inside the stack avoid over heating of furnace shell walls. A
typical length of a stack is usually about 40m in length and 2.5 m in diameter, containing
130 cooling pipes. These seamless pipes are 50 mm dia and 5 mm wall thickness usually
made of carbon steel. The outer surface of these pipes are exposed to hot converter gases
with lot of abrasive dust particles causing erosive wear on outer side, frequent heating and
cooling of tube material causing thermal fatigue due to cyclically repeating thermal gradient
and hot corrosion from inner side due to cooling water which flows inside the tubes.
During the BOF operation stack tubes are seriously affected due to the severe
thermal fatigue/shock, combined with erosion/corrosion of tube wall from both sides such
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that the walls of tubes gets corroded and thinner, gradually in course of their sustained use.
As a consequence of this the tubes gets punctured in the long run, causing water leakage
needing BOF converter shut down needing about 8 to 10 hrs for weld repairing of every
damaged tube. Thus BOF stacks with older tubes are subjected to frequent converter
shutdown for prolonged duration adversely affecting the productivity of steel melting shops.
Most vulnerable zone in BOF stack that is prone to erosion and corrosion leading to water
leakage being 4 m long lower part and bent areas with 30 to 40 tubes and the expected
working life of tubes in this zone is 1.5 to 2.0 years, for safe operation.
In recent developments directed to solve the problems of BOF stack pipe leakages, the
carbon steel pipes are replaced at an interval of 18-24 months in some large steel plants.
Attempts have also been made for the pipes with weld surfacing with IMi-22 Cr-9 Mo-3.5 1Mb
type nickel based high alloy are reported to work for 8-10 years. However, this alloy is
expensive and the method of enhancing the life of BOF stack by depositing such alloy by
weld surfacing on tube walls of BOF stack is a costly affair.
There had been therefore a persistent need for developing a cost optimized process for the
protection of tubes in BOF stack for withstanding the adverse operating conditions in the
BOF operation and ensure longer safe operating life for the tubes reducing the frequency of
break down of converters due to tube leakage in BOF stack caused due to erosion/corrosion
by flowing corrosive gases or fluid/cooling water, such that the uninterrupted production in
BOFs in SMS shop is ensured in any steel plant and thereby improving productivity, reduce
costs and save involvement of complexities in maintenance/repair of leakage or
replacement of tubes in BOF Stack.
OBJECTS OF THE INVENTION
It is thus the basic object of the present invention to develop high performance alloy
deposited BOF stack tubes and a process for protective coating/layer on tube walls of BOF
stack avoiding damage due to the corrosion/erosion due to contact with the flowing flue
gases or cooling water in contact with the tube wall, by thermal spray alloy coating on tubes
panel of BOF stack.
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A further object of the present invention directed to said method of developing a process for
providing protective alloy coating on tube surface wherein residual stress free welded tube
panels are obtained by fabrication and stress relieving and in straightened condition to avoid
any warpage during thermal spray coating operation.
A further object of the present invention directed to said method of developing a process for
providing protective alloy coating on tube surface wherein said process involves selection of
cost effective alloy wire for arc spray metallic plasma coating with micro crystalline/
amorphous structure and thermally conductive for high service life in BOF hood.
A still further object of the present invention directed to said method of developing a
process for providing protective alloy coating on tubes panel of BOF stack, wherein 10
panels of 8 tubes in 6m length were welded together with steel strip as a wall between the
pipes to produce air tight panels to be welded finally inside the stack for making round
chamber, followed by a specific stress relieving treatment to avoid distortion/warpage
during thermal spray coating treatment.
A still further object of the present invention directed to said method of developing a
process for providing protective alloy coating on tubes panel of BOF stack, wherein said
process involves selection of said thermal spray coating technology and optimization of
coating parameters for achieving surface coating with high hardness, low friction coefficient,
high temperature corrosion and abrasion resistance, low porosity, high density and with
high bond strength and about 450-500 im in thickness.
SUMMARY OF THE INVENTION:
The basic aspect of the present invention is thus, directed to Alloy coated BOF stack
tubes panel comprising:
surface coating with selective alloy of composition comprising:
Si= 1.4 to 2.0 %;
Cr.= 25-30 %;
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Mn.= 1.50- 2.00%;
B= 3.00- 4.50 %; and
Fe = Balance, providing micro crystalline or amorphous coating about 450-500 um
in thickness , said alloy being thermally conductive and possessing high hardness
with wear and corrosion resistance and having large volume percentage of hard
chromium borides in metal matrix.
Another aspect of the present invention is directed to Alloy coated BOF stack tubes panel
wherein each said panel comprise plurality of tubes welded together with steel strip as a
wall between the tubes adapted to provide air tight panels for welding inside the stack
for making round chambers.
A still further aspect of the present invention directed to said Alloy coated BOF stack
tubes panel wherein said coating comprise thermal coating, preferably twin arc wire
plasma spray coating and /or any other similar coating process.
According to yet another important aspect of the present invention directed to a process
for producing alloy coated BOF stack tubes panel comprising:
providing said panels having plurality of tubes welded together with steel strip to
therby provide the air-tight panels ;
subjecting the panels to a selective alloy based thermal coating involving a selective
alloy of composition comprising:
Si= 1.4 to 2.0 %;
Cr.= 25-30 %;
Mn.= 1.50- 2.00%;
B= 3.00- 4.50 %; and
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Fe.= Balance, providing micro crystalline or amorphous coating about 450-500 um in
thickness , said alloy being thermally conductive and possessing high hardness with
wear and corrosion resistance and having large volume percentage of hard chromium
borides in metal matrix.
A still further aspect of the present invention directed to a process for producing alloy
coated BOF stack tubes panel wherein the welded panels are subjected to a stress
relieving treatment adapted to avoid distortion/warpages during thermal spraying
treatment.
A still further aspect of the present invention directed to a process for producing alloy
coated BOF stack tubes panel wherein the panels are surface coated with Twin Wire Arc
Plasma Spraying with said selective alloy.
A still further aspect of the present invention directed to a process for producing alloy
coated BOF stack tubes panel wherein the coating process is optimized in terms of
number of passes ,wire feed rate, current, voltage and air pressure for providing micro
crystalline or amorphous strongly bonded coating.
A still further aspect of the present invention directed to a process for producing alloy
coated BOF stack tubes panel wherein said spraying parameters comprise:
spraying parameters comprising,
Spray rate 3.65 kg/hr/100 amps,
Coating thickness/pass 5.0 mils,
Wire consumption 1.2 kg/m2/100 m,
Atomizing air pressure 50 psi,
Amperes 100-200, and
Arc load voltage 35v.
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A still further aspect of the present invention directed to a process for producing alloy
coated BOF stack tubes panel wherein said spraying parameters comprise:
physical parameters comprising:
Wire size 1.6 mm,
M. P. 1200 °C ,
Deposition efficiency 70%,
Bond Strength 40 MPa,
Coating Density 6.7 gm/cc, and
Micro hardness 595 DPH ( HRC- 55).
The present invention and its objects and advantages are described in greater details with
reference to the accompanying non limiting illustrative figure and examples:
BRIEF DESCRIPTION OF THE ACCOMPANYING FIGURES:
FIG 1: Process Flow Chart for manufacture of coated stack tube panels, according to the
invention.
DETAILED DESCRIPTION WITH REFERENCE TO THE ACCOMPANYING FIGURE AND
EXAMPLE:
The present invention is directed to a method for achieving thermal spray coated high
performance carbon steel tube panel for BOF stacker for longer operating life avoiding
puncture or leakage frequently causing interruption in production and longer down time for
replacement /repair. The present process involves fabricating tube stack comprising 10
panels of 8 tubes in 6m length that are welded together with steel strip as a wall between
the pipes to produce air tight panels to be welded finally inside the stack for making round
chamber for BOF operation. These welded panels are subjected to a specific thermal stress
relieving treatment to relieve locked up stresses developed during welding and to avoid
distortion/warpage during subsequent thermal spray coating treatment. The important
activities involved in the manner of implementation of the present invention directed to
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achieve a method of thermal spray coating of selective alloy composition on the tube panels
of BOF stackers comprised the following basic considerations:
(i) Assessment of service requirements in terms of temperature, class of wear and
corrosion
(ii) Analyzing reasons for failures of stack tubes and its impact on BOF operation,
(iii) Selection of appropriate coating material and cost effective coating technology
(iv) Preparation of welded tube panels for coating. Optimization of coating
parameters and developing process for thermal spray coating without any
distortion in welded panels of 6 meter long 8 carbon steel tubes,
(v) Application of wear/corrosion resistant coating on panels
(vi) Installation and evaluation of service performance in BOF Stacks.
The process flow diagram with different steps involved in the process development
is given in accompanying Figure 1.
The manner of implementation of the thermal spray alloy coating of BOF tube panel stacker
to achieve the desired properties of corrosion and wear resistance avoiding tube leakage for
enhanced operating life, is illustrated with the help of the following non limiting exampie:
EXAMPLE:
The thermal spray coating with alloy according to an embodiment of the present invention
involve surface coating of tube panels fabricated following the steps of the preceding
paragraph applying the process of Twin Wire Arc Plasma Spraying, wherein the deposited
selective special alloy composition comprising :
Si in the range of 1.4—2.0 wt % , Cr in the range of 25-30 wt %, Mn in the range of
1.50-2.00 wt % , B in the range of 3.00-4.50 wt% and Balance is Fe by wt. percent.
The thermal spray coating process, according to the present invention has been optimized
for producing selective micro crystalline or amorphous coating of about 450-500 urn
thickness. The alloy coating so deposited on tube surfaces is thermally conductive and
having the desired properties of high hardness with wear and corrosion resistance due to its
low friction and high hardness due to large volume percentage of hard chromium borides in
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metal matrix. The coating with these wires can be applied using a variety of methods similar
to thermal coating processes producing dense and tough coatings with high bond strength.
However, in the present invention twin arc wire plasma spray coating process is used after
optimizing the coating parameters in terms of number of passes, wire feed rate, current,
voltage and air pressure in order to achieve desired strongly bonded uniform coating in
preferred thickness having favored hardness, wear and corrosion resistance properties for
BOF application with longer operating life.
The optimized Spraying Parameters used in the alloy coating process are as follows:
Wire size 1.6 mm;
Spray rate 3.65 kg/hr/100 amps,
Coating thickness/pass 5.0 mils;
Wire consumption 1.2 kg/m2/100 m;
Atomizing air pressure 50 psi;
Amperes 100-200,
Arc load voltage 35v;
The resulting alloy deposition is tested and found to possess Physical Properties achieved in
the deposited layer of alloy coating as follows:
Deposit efficiency 70%;
Coating Density 6.7 gm/cc;
M. P. 1200 °C;
Bond Strength 40 MPa;
Micro hardness 595 DPH (HRC- 55)
INSTALLATION OF COATED PANEL IN BOF STACK AND PERFORMANCE TRIALS:
According to an embodiment of the present invention, the coated panels have been installed
in three BOF converters in wear prone zone for trials of observation of performance
achieved. These panels have been inspected twice over a span of one year and no
significant wear is detected. It is estimated, based on the physical properties achieved as
given above, that the coated pipes will work without any deterioration and leakage problem
for more than 8-10 years.
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All the 10 coated panels have been fitted in different converters as detailed below, the
surface of panels have been inspected two times at intervals of six months and no
detectable wear or erosion found on coated surface. All panels found to be in good condition
without any distortion/erosion without any leakage.
Observations of Comparative performance of Ordinary carbon tube versus thermally spray
alloy coated tubes, on the basis of above trials:

Numbers
of Panels
Fitted in
stacks BOF
Converter
Number Surface
Inspection
during Service Wear on
Coated Tubes Wear on
Older Tubes
3 1 none - -
5 3 After 6 months No
measurable
Wear 0.50 mm
2 2 After 12 months 0.20 -0.30 mm 1.2 mm
It is thus possible by way of this invention to obtain a process for the thermal spray coating
of selective alloy composition on surface of stack tubes of BOF hoods subjected to problems
of corrosion and/or wear by flue gases inside the pipes or the cooling water circulation
through the stack in BOF converters. The invention is directed to achieve the desired
objective of wear/corrosion resistance of the BOF stack tube following the selection of Cost
Effective Alloy composition in form of Wire with high performing coating, fabrication of
welded panels with specific stress relieving treatment to produce residual stress free panels
to avoid any warpage/ distortion during thermal spray coating, selection and optimization of
coating process for achieving high quality metallic coating on tube surface with favored
properties comprising good heat conduction, substantial resistance to thermal fatigue,
high-temperature abrasive wear and corrosion resistance and high bonding strength to
resist thermal spalling during thermal fatigue while in service. The invention is therefore
capable to provide favored longer operating life of the tube panels of BOF converters in
integrated large steel plants and the present process is advantageously adapted to eliminate
puncture/water leakage problem with enhanced service life of the pipes ,resultant reduced
converters down time and thus improving productivity of the SMS shop in steel plants.
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WE CLAIM:
1. Alloy coated BOF stack tubes panel comprising:
surface coating with selective alloy of composition comprising:
Si = 1.4 to 2.0 %;
Cr.= 25-30 %;
Mn.= 1.50- 2.00%;
B = 3.00- 4.50 %; and
Fe.= Balance,
providing micro crystalline or amorphous coating about 450-500 urn in thickness ,
said alloy being thermally conductive and possessing high hardness with wear and
corrosion resistance and having large volume percentage of hard chromium borides
in metal matrix.
2. Alloy coated BOF stack tubes panel as claimed in claim 1 wherein each said panel
comprise plurality of tubes welded together with steel strip as a wall between the
tubes adapted to provide air tight panels for welding inside the stack for making
round chambers.
3. Alloy coated BOF stack tubes panel as claimed in anyone of claims 1 or 2 wherein
said coating comprise thermal coating preferably twin arc wire plasma spray coating
and /or any other similar coating process.
4. A process for producing alloy coated BOF stack tubes panel as claimed in anyone of
claims 1 to 3 comprising:
providing said panels having plurality of tubes welded together with steel strip to
there by provide the air-tight panels ;
subjecting the panels to a selective alloy based thermal coating involving a selective
alloy of composition comprising:
Si= 1.4 to 2.0 %;
Cr.= 25-30 %;
Mn.= 1.50- 2.00%;
B= 3.00- 4.50 %; and
Fe.= Balance, providing micro crystalline or amorphous coating about 450-500 urn in
thickness , said alloy being thermally conductive and possessing high hardness with
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wear and corrosion resistance and having large volume percentage of hard chromium
borides in metal matrix.
5. A process for producing alloy coated BOF stack tubes panel as claimed in claim 4
wherein the welded panels are subjected to a stress relieving treatment adapted to
avoid distortion/warpages during thermal spraying treatment.
6. A process for producing alloy coated BOF stack tubes panel as claimed in anyone of
claims 4 or 5 wherein the panels are surface coated with Twin Wire Arc Plasma
Spraying with said selective alloy.
7. A process for producing alloy coated BOF stack tubes panel as claimed in anyone of
claims 4 to 6 wherein the coating process is optimized in terms of number of passes,
wire feed rate, current, voltage and air pressure for providing micro crystalline or
amorphous strongly bonded coating.
8. A process for producing alloy coated BOF stack tubes panel as claimed in anyone of
claims 4 to 7 wherein said spraying parameters comprise:
spraying parameters comprising:
Wire size 1.6 mm,
Spray rate 3.65 kg/hr/100 amps,
Coating thickness/pass 5.0 mils,
Wire consumption 1.2 kg/m2/100 (am,
Atomizing air pressure 50 psi,
Amperes 100-200 and
Arc load voltage 35v.
9. A process for producing alloy coated BOF stack tubes panel as claimed in anyone of
claims 4 to 8 wherein said spraying parameters comprise:
physical parameters comprising:
M. P. 1200 C ,Deposit efficiency 70%, Bond Strength 40 MPa, Coating Density 6.7
gm/cc and Micro hardness 595 DPH ( HRC- 55).
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10. Alloy coated BOF stack tubes panels, its process of manufacture and converters
installed with such coated panels substantially as hereindescribed and illustrated
with reference to the accompanying examples and figures.


Dated this 20th day of February, 2008

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Thermal spray alloy coated BOF stack tubes panels directed to improved performance and
extended operational life. More particularly, the thermal spray coating process is directed to
providing selective alloy composition and optimized spray parameters to protect the BOF
stack tubes from erosion and corrosion of tube wall, by flue gas and dust particles, causing
water leakage resulting prolonged shut down of converter for repair/replacement of tubes.
Each stack tubes panel is surface coated applying preferred Twin Wire Arc Plasma Spraying
process, with special alloy comprising selective wt percent of Si, Cr Mn, B and rest Fe, after
specific stress relieving treatment to avoid distortion/deformation during thermal spray.
Importantly, the coating process parameters are optimized with respect to number of
passes, wire feed rate, current, voltage etc, for producing micro crystalline or amorphous
coating of desired thickness in the range of about 450-500 μm, providing thermally
conductive layer of high hardness with wear and corrosion resistance. The coated
panels/pipes in BOF stacks are estimated to work without any deterioration and leakage
problem for more than 8-10 years and thus having potential for wide industrial application.