Welding is used for pipes and tubes in the fabrication of boiler components like headers, panels, and coils.Arc welding processes, including gas tungsten arc welding, shielded metal arc welding, and submerged arc welding are used.The underlying principle of this entire arc welding process is an electric arc is struck between an electrode and base metal, whereas the heat input of electric arc is used for melting and joining metals.
The
raw material of the pipes and tubes used for fabrication of boiler
components, as mentioned previously at the manufacturing stage, are
finally
inspected
for quality by nondestructive magnetic particle examination and also
handled by magnetic cranes during transportation.Even after
demagnetization, some amount of residual magnetism will be present on
pipes and tubes and supplied as such.During fabrication, welding of
these residual magnetic pipes/tubes is a challenge.
Problems
during welding of pipes and tubes.
During
welding of pipes and tubes, an electric arc is produced between the
electrode and base metal to melt the metals at the welding point.This
electric arc consists of a stream of electrons.If a significant level
of magnetism is present in the pipes or tubes being welded, then
interaction takes place between the magnetic field and the electric
arc, which causes the welding arc to be deflected.This is known as
arc blow.Due to this wandering of the arc, the welder may not be able
to manipulate the arc resulting in welding defects like porosity,
incomplete fusion, and more.
Depending
upon the level of residual magnetism in steel, welding process such
as GTAW, SMAW, and SAW are more sensitive to arc blow.Arc instability
occurs in SMAW when the level of residual magnetism in steel is more
than 20 Gauges, and arc instability occurs in SAW when the level of
residual magnetism in steel is more than 40 gauss.
Magnetic
arc blow is more likely to occur with lower voltage arcs.Hence the
GTAW process,which has a low arc voltage of 10-15v, is more sensitive
and susceptible to arc blow.But GTAW is a common process for root
pass welding of pipes and tubes because it provides complete joint
penetration welding on one side.Therefore, it is mandatory to
demagnetize the residual magnetism developed in pipes and tubes to
less
than
10 Gauss before using the GTAW process.
Principle
and Method of Diamagnetism
Generally,
two types of demagnetization are available: electrical
demagnetization and thermal demagnetization.The electrical
demagnetization method subjects the magnetized test object to the
influence of a continuously reversing magnetic field that gradually
reduces in strength, causing a corresponding reversal and reduction
of the field in the test object.There are many types;
- AC Coil
- AC through current step down
- AC through current reactor decay
- DC through current reversing step down
- DC coil reversing step down
- AC yoke
- Reversing DC yoke
The
thermal demagnetization method heats the material above Curie
temperature, causing magnetic material to lose its magnetic
properties.It consists of
- Annealing above Curie temperature
- Preheating before welding.
Disadvantages
of the Diamagnetism Methods
Both
electrical and thermal demagnetization methods have certain
disadvantages that restrict their usage for industrial applications,
such as in boiler industries.The major disadvantages of using
electrical demagnetization are that it is only efficient for smaller
size components, and boiler components are larger size pipes and
tubes.Therefore, the only suitable method is thermal demagnetization,
although the annealing heat treatment demagnetization, although the
annealing heat treatment operation consumes more time in heating and
cooling cycles, and also power and fuel consumption for this process
is more costly.Combining this demagnetization of pipes and tubes with
other annealing operations may be more economical.
Principles
and methods of bridge piece technique
A
bridge piece is a small metal strip used to secure or fit up two butt
joint members in alignment for welding. This bridge piece is tack
welded on either side of the parts to be welded, securing them
alignment by keeping proper root opening and ID matching for making
sound weld metal.
When
two tubes or pipes having residual magnetism are edge prepared and
brought together for welding, the magnetic flux concentrates mostly
on the edges due to the nature of the magnetic field. On welding the
bridge piece to the tube or pipe by SMAW, the heat produced will
cause the tube or pipe edges to be raised to a temperature close to
the Curie temperature and reduce the magnetic flux at the edges,
enabling the use of GTAW.
- Select a bridge piece with a minimum leg length of 50 mm so as to have length welded by SMAW, causing more heat input.
- Select 3 to 4 bridge pieces, depending on the diameter of the pipes or tubes, to cover the circumferential length.
- Tack weld the bridge pieces on the pipes or tubes, as per the required alignment.
- Start welding the bridge pieces by SMAW process, probably 3.2 or 4 mm electrode with a slightly higher current of 150-160A.
- Make 1 or 2 weld passes to increase the heat input.
- Concentrate 2-3rd current on the bridge piece and 1-3rd current on the pipe to avoid damage to the pipe or tube.
- Carry the above method in all bridge pieces without time delay. Due to summation effect of welding heat input, the magnetic flux will be reduced at the edge of the piped or tubes, allowing for easy welding without arc blow.
- While welding the bridge piece onto a pipe or tube, the bridge piece is to be welded only on one side for easy removal after demagnetization.
- Immediately after welding the bridge piece, being root welding using GTAW.
- After completion, grind and remove these bridge pieces.
Conclusion
Although
various methods are available for demagnetization, they are more
restricted due to their applications and time-consuming process. The
bridge piece techniques is a fast and practical demagnetization
technique applied for welding of tubes and pipes having residual
magnetism. This method uses the basic thermal demagnetization
principle and is applied in a practical manner.
No comments:
Post a Comment