FIELD OF THE INVENTION
The present invention relates to a process to prevent scale formation on bend
tubes during post forming heat treatment process. More particularly, the
invention relates to a method for prevention of scale formation on alloy pipes
bends used in boilers during post heat treatment process of the bends in heat
treatment furnaces.
BACKGROUND OF THE INVENTION
In the power boilers, the pipes made of alloy steels are subjected to bending in
an incremental bending machine at 1050°C to designed radius and angle.
Depending upon the size and specification of the bend, the bending parameters
are decided to carryout the forming process. One of the major bottle neck faced
in the process, is to remove the formed scale during post forming heat treatment
of the pipes in the heat treatment furnace. In the furnace, excess air present
inside the furnace during nomalising induces oxidation at an elevated
temperature of about 1050°C, which interalia forms hard scales on the bend
surface. According to prior art, the formed scales are removed by manually
chipping every inch of the surface area of the bend by a chipping hammer. As
this activity produces high decibel sound, it is to be carried out away from the
production centre which warrants extensive movement of the bends.
Accordingly, the process time increases and quality of the bend is degraded and
hence there is a need to propose an innovative method of scale removal and/or,
a method for prevention of scale formation.

OBJECTS OF THE INVENTION
It is therefore, an object of the present invention to a method for prevention of
scale formation on alloy pipes bends used in boilers during post heat treatment
process of the bends in heat treatment furnaces.
Another object of the present invention is to propose a method for prevention of
scale formation on alloy pipes bends used in boilers during post heat treatment
process of the bends in heat treatment furnaces, in which a coating of calcium
carbonate powder is applied on the surface of the bends.
A further object of the present invention is to propose a method for prevention
of scale formation on alloy pipes bends used in boilers during post heat
treatment process of the bends in heat treatment furnaces, in which said coating
in the alloy steel pipe bends is uniformly applied prior to bending process.
SUMMARY OF THE INVENTION
According to the invention, calcium carbonate powder commercially known as
chalk powder and containing 87% Calcium Carbonate is mixed with water in a
ratio of 3:2 to form a homogeneous solution and uniformly applied on the
surface of the pipe bends prior to placing the pipe bends into the furnace for
heat treatment. This coating prevents formation of hard scales on the surface of
the pipes during heat treatment process at an elevated temperature of 1050°C.

BRIEF DESCRIPTION OF THE ACCOMPANYING DRAWINGS
Figure 1 shows a prior art bend pipe undergone heat treatment without chalk
powder coating.
Figure 2 shows a pipe bend undergone post heat treatment cycle with Chalk
powder applied after bending, according to the invention.
Figure 3 shows the bend pipe taken out from furnace after heat treatment.
DETAILED DESCRIPTION OF THE INVENTION
Chalk powder is generally composed of carbonic acid and lime is mixed with
water in a ratio of 3:2 which dissolves easily at room temperature. This solution
is applied uniformly over the surface of a bend alloy pipe. The thickness of the
coating is between 500 microns to 1000 microns. The surface coating is applied
before carrying-out the bending process in the incremental bending machine.
Now the bend pipe is put inside the furnace for heat treatment.
Figure 1 shows a bend pipe without application of said coating taken out from
the heat treatment furnace after normalizing and tempering process. This
contains a high degree of hard scales formed on the surface because of the
chemical reaction of iron with oxygen available in the excess air used for
normalizing, thereby forming iron-oxide scale formed which are hard layers on
the surface of the pipes. These hard layers are difficult to remove by
conventional process like hammering in the surface of the pipe.

Figure 2 shows a bend pipe with the application of chalk powder on the outer
surface of the bend pipe by the said process. Now the coated bend pipe is fed
into the furnace for heat treat process consisting of heating, normalizing and
tempering.
Figure 3 shows the bend pipe of Figure 2, when taken out after the heat
treatment process. An in-situ micro examination of the pipe reveals that there is
no scale formation on the surface of the bend pipe as the coating prevents the
chemical reaction at the elevated temperatures. Non-destructive testings were
carried out on the bend to ensure both metallurgical and physical properties of
the bend. The in-situ micro test report of figure 3 shows that the above
described method applying chalk powder in hot condition on the straight pipe
before bending can reduce scale formation to a greater extent without affecting
the default nature of alloy material. The said process has numerous advantages
such as no chemical hazards less noise, less manpower, less process time, bend
cycle time reduction, less space to carry out the process and thereby increases
the quality of the bend.

WE CLAIM :
1. A method for prevention of scale formation on alloy pipes bends used in
boilers during post heat treatment process of the bends in heat treatment
furnaces, comprising the steps of :
- providing a tubular member according to pre-determined size, thickness,
and outer diameter for forming a tube bend of a boiler;
- loading the tubular member on an incremental bending machine and
assign the radius and angle of the bend to be formed;
- unloading the bend member from the machine and applying a coating of
calcium carbonate powder on the surface of the member;
- placing the coated member in a heat treatment furnace maintained at an
elevated temperature of 1050°C for heating, normalizing, and tempering,
wherein the coating is formed by mixing chalk powder in a ratio of 3:2 at
room temperature, and wherein the thickness of the coating is maintained
between 500 microns and 1000 microns.

2. The method as claimed in claim 1, wherein the coating is applied over the
entire surface of the tube bend at uniform thickness.
3. The method as claimed in claim 1, wherein the coating prevents any
chemical reaction of iron with oxygen available in the excess air in the
furnace.

4. A method for prevention of scale formation on alloy pipes bends used in
boilers during post heat treatment process of the bends in heat treatment
furnaces, as substantially described herein with reference to the
accompanying drawings.

ABSTRACT

The invention relates to a method for prevention of scale formation on
alloy pipes bends used in boilers during post heat treatment process of
the bends in heat treatment furnaces, comprising the steps of: providing
a tubular member according to pre-determined size, thickness, and outer
diameter for forming a tube bend of a boiler; loading the tubular member
on an incremental bending machine and assign the radius and angle of
the bend to be formed; unloading the bend member from the machine
and applying a coating of calcium carbonate powder on the surface of the
member; placing the coated member in a heat treatment furnace
maintained at an elevated temperature of 1050°C for heating, normalizing,
and tempering, wherein the coating is formed by mixing chalk powder in a
ratio of 3:2 at room temperature, and wherein the thickness of the
coating is maintained between 500 microns and 1000 microns.