Saturday, 3 March 2018

Removing Residual Magnetism before Arc Welding.

Image result for welding
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.

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