The use of high temperature and high pressure pipes and equipment is widespread in the chemical industry, but there is a common issue that arises during the installation and maintenance of large diameter, thick-walled pipes and equipment. This issue is magnetic bias blowing of weldments.
For instance, on July 23, 2010, the air return pipeline in Unit 1 of the compressor plant in the olefin air separation workshop underwent reconstruction. After the flange and pipe ports were connected and aligned, severe magnetic biasblowing took place, making normal welding impossible.
The magnetic biasblowing phenomenon manifests itself in several ways. During the initial welding at the root, the welding arc deviates towards one side of the metal pipe or the arc generates an explosion, making it difficult to initiate the arc normally.
In tungsten argon arc welding, the arc does not burn towards the weld metal at the tip of the tungsten electrode, but rather towards the argon nozzle.
Common degaussing methods and their principle analysis
Magnetic biasblowing is a significant problem in field welding construction. It can impact the stable combustion of the arc, result in incomplete penetration at the root of the weld, cause porosity between layers, and seriously affect the welding quality, requiring rework.
In severe cases, it may even make it impossible to ignite the electric arc, rendering the welding operation unfeasible.
The latter refers to the replacement of the air separation return pipeline.
The cause of magnetic deflection is the presence of a magnetic field around the weldment or the welding location.
There are various reasons for its occurrence.
However, it is evident that most cases of magnetic bias blowing are caused by the inherent magnetism of the metal used in the weldment, which results in magnetic bias blowing during welding.
In addressing the magnetic bias blow in the air return pipeline of Unit 1 in the air separation workshop’s compressor plant, we tried various methods, including electrode welding with magnetic induction, filling with high-conductivity degaussing material, and heating degaussing, but the results were not satisfactory.
To avoid delays in construction and the smooth start-up of the equipment, we ultimately used the physical reverse winding degaussing method, with the guidance of experts and the support of leaders at all levels of the plant.
The following is an overview and explanation of these commonly used degaussing methods:
Degaussing method of winding dc cable in reverse arc direction by using physical principle
This method involves creating a simple degaussing machine on site using limited and readily available materials, based on the principles of degaussing machines.
Based on our construction observations, we can determine that the magnetic field of the base metal causes the arc to regularly deviate during argon arc welding of the base metal with magnetic weldments.
This regular deviation is the basis for determining the direction in which the degaussing machine should be turned.
During manual argon tungsten arc welding, the power supply is connected using the DC positive method, meaning that the welding torch is connected to the negative pole of the power supply and the weldment is connected to the positive pole. This results in the flow of welding current from the torch to the weldment.
If the welding arc is considered a wire, and the right hand’s fingers are pointed in the direction of the arc deviation according to the right-hand rule, the direction indicated by the thumb is the “N” pole of the magnetic field generated by the base metal itself.
To eliminate the arc deviation, the welding cable is wound several times (depending on the number of test turns) around the welding joint in the opposite direction of the arc deviation, generating a reverse magnetic field. With increasing number of turns, the arc deviation will gradually decrease until the arc combustion returns to normal, indicating the complete disappearance of magnetic induction.
Winding the welding wire allows for simultaneous backing welding, demagnetization, and welding, making the process simple and efficient.
Practice has shown that the welding current remains unchanged during the wire winding process, and no adjustments to the welding current are necessary, resulting in a stable arc combustion.
When a magnetic welded pipe is detected, the arc is first ignited in the groove. The offset direction and angle of the arc are carefully observed, and the sound of the arc is carefully noted to determine the direction of the magnetic field and estimate the strength of magnetic induction.
The arc is then extinguished, the welding torch wire is positioned 30mm from the magnetic pipe side to the welding joint, and the corresponding number of turns are wound in the opposite direction of the arc deviation (the turns should be tightly and orderly arranged, without overlapping). The arc is then ignited again and the arc combustion is observed.
If the arc continues to deviate from its original direction, the number of turns of the winding wire should be increased until the arc burns normally. If the arc deviation direction is opposite to its original direction, the number of turns of the winding wire should be reduced until the arc burns stably.
Finally, the groove is cleaned with an angle grinder, and welding is performed with the winding wire on the pipe. The priming process usually keeps the arc combustion stable throughout, without affecting the welding operation or quality.
If, during the priming process, the arc gradually inclines in the opposite direction of the original deviation, the number of turns of the welding torch wire should be promptly reduced to adjust the arc to its normal state.
After the priming process is completed, all twisted welding wires are removed, and the welding operation and quality will not be affected during manual arc welding capping.
During the whole operation, pay attention to the following three points:
(1) Care must be taken to ensure the direction of arc deviation and the reversal of wire winding are correct, as incorrect actions will have an adverse effect.
(2) The number of winding turns of the welding torch wire must be appropriate to guarantee effective demagnetization.
(3) The winding position of the wire should not be too far from the edge of the groove, as this will affect the demagnetization effect.
This demagnetization method requires no additional equipment or materials, only the welder’s own tools, making it simple.
Demagnetization should be performed during welding, taking only 3 to 5 minutes from testing to actual welding.
Filling with high magnetic conductivity material, bypass drainage method
Before welding, a section of high magnetic conductivity material, such as silicon steel sheet or permalloy, is added to the gap in front of the part to be welded.
These materials have a much higher magnetic conductivity than air, causing magnetic lines of force to pass through the materials rather than the air gap. This leads to a reduction in magnetism of the parts filled with high magnetic conductivity materials.
By using this method, high magnetic conductivity materials can be welded by progressing and filling the gap.
However, the magnetic field lines between the remaining air gaps still affect welding to some extent, as the filler material cannot completely direct the magnetic field lines through the high magnetic conductivity material as a connector.
Therefore, the method of using high magnetic conductivity materials to reduce magnetic induction is only suitable for shielded metal arc welding. The use of high magnetic conductivity materials as backup materials is not common in the chemical industry, making it difficult to use them whenever they are needed.
Above the magnetic transition point, weld around the magnetic temperature area of the weldment
Another method is to heat the welded part to demagnetize the weldments on both sides of the crater before conducting welding.
However, this method has limitations for outdoor and large-scale welding.
The magnetic transition temperature for pure iron is as high as 768°C, making it challenging to reach and maintain such a high temperature during outdoor or large-scale welding.
Furthermore, for alloy steel, the magnetic transition temperature of the weldment is often close to or higher than the steel’s phase change temperature, which can easily degrade the base metal’s performance and make the working conditions for welders difficult.
The welding operation is challenging to master, leading to an increased rate of rework for the weldments.
Therefore, the method of welding by avoiding the magnetic temperature range of the weldment is only applicable in good welding and thermal insulation environments.
It is not feasible for welding large-diameter pipe fittings in our factory, especially CrMo alloy steel.
Magnetic field diverting method for welding magnetic conductive parts with welding rod arc welding
Experience with welding various pipe fittings during production and construction has shown that shielded metal arc welding is more effective in overcoming magnetic bias blow of the arc compared to tungsten argon arc welding. The magnetic properties of pipe fittings at the break have the greatest impact on welding and change with the distance from the break.
As a result, electrode arc welding is a feasible option for welding magnetic conductive parts, and the magnetic field can be induced 30mm away from the break of the two welding parts.
Based on years of experience, construction personnel have proposed a bold solution to the magnetic bias blowing issue through extensive analysis. They use the magnetic conductor of shielded metal arc welding to divert the magnetic field, balancing the force on the arc.
After multiple tests, experienced welders have found a successful method to divert the magnetic field: spot welding two steel plates at the top and bottom of the crater to ensure symmetry of the four magnetic steel bars.
After this, the influence of magnetic bias blowing on welding can be fully addressed through the use of subsection magnetizing, subsection back welding, and short arc welding.
Summary
In different scenarios, the constructors can choose the appropriate degaussing method based on the specific situation.
This simple method not only saves significant amounts of manpower and resources, but also ensures the completion of the construction period on time.
