Welding of Magnetized steel. 6

[demsha]

Hello All,

I have a field situation that screw piles which are supposed to weld to caisson shoes are magnetized.
I understand that magnetized material not supposed to weld. What would be the maximum amount of magnetism that can allow in the material prior to welding?

can any one tell me, how can these screw piles got magnetized? These screw piles were fabricated in south of USA and transported by trucks to Canada.

Thanks in advance

 

[fegenbush]

Cold working can magnetize steel. It can also occur by a number of other mechanisms. In my experience there should be little problem making the necessary welds with qualified welders (persons, not equipment) and procedures.

If using SMAW, it may be possible to wrap the lead around the pipe to be welded. If the arc does not improve, change the direction of the wrap. This is best used with a DCEP welding procedure. As an alternative, you can use an AC welding procedure.

 

[WeldQA]

I have had some sucess with machines of 400A or over by adding a second Earth/ Ground/ Return. Then placing the second Earth/ Ground/ Return clamp so that it is in line with the weld direction; i.e: weld toward the clamp.

 

[SectionIX]

We've found that pipe joints stored in north/south orientation for extended amounts of time have a higher propensity to store unacceptale amounts of magnetism. Material grade, chemistry, in-service piping exposed to MFL via pigging operations adds to the problem. As a general rule, 20 gauss or more is the action level where options such as demag machines/wrapping welding leads acround the pipe become necessary.

 

[gtaw]

The earth is one huge magnet. Transporting long slender ferromagnetic materials in either the North to South or South to North direction can magnetize the members. The effect seems to be more severe when moving South to North and becomes more problematic as the material approaches the Earth's poles. The member is left with a Remnant Field, i.e., it is magnetized.

Wrapping the welding lead around the member is rarely an effective means of countering the Remnant Field unless the welding currant is AC. When using DC for welding, it is best to demagnetize the member before attempting to weld it. The problem is not as severe if the welding is performed near mid-length, but it can become so severe welding is very difficult if a butt joint at one end is required.

Demagnetization can be accomplished with a welding power source with AC or DC output. It is easiest if AC output is available and the member is not thicker than 1/2 to 3/4 inch. Heavier cross sections are easier to demagnetize using DC, but the process is more time consuming.

The member is wrapped with a number of turns of welding lead. The strength of the magnetic field produced is a function of the current times the number of turns. The field produced must be stronger than the Remnant Field that preexists in the member. If AC output is used to energize the system, start with the amperage set at the maximum value. Reduce the current to the minimum value while the system is energized. The welder used must have a continuous variable amperage control. Do not attempt to reduce the current while energized if the power supply uses taps to change the current. If the welding machine has "ranges", start with the highest range and progress to the lowest range. Then, using the lowest current range, repeat the process, but remove one turn of welding lead at a time until there is no turns left and the lowest current setting is used.

If a DC power supply is used, the process is similar, but the welding leads have to be switched back and forth to reverse the polarity as the current is reduced. It is best to energize the circuit, break the circuit, switch polarity, reduce the amperage, reenergize the circuit, break the circuit, switch the polarity, reduce the current, energize the circuit again, ...... Repeat the sequence using 20 to 30 repetitions, each time reducing the current and reversing the polarity.

Do not use the switch on the welding machine while it is under load to turn the machine off and on. connect one end of the welding lead that is wrapped around the member to a piece of scrap metal. Use the electrode to make and break the circuit. This prevents damaging the welding machine. The time to magnetize the member is nearly instantaneous. There is no need to energize the circuit for more than a couple of seconds. Strike the arc, break the arc, reduce the current (reverse the polarity if using DC output).

I have used this technique for over thirty years. It is very effective. If the steps are followed as outlined, the remnant field should be barely detectible using a Gauss Meter held at the end of the member once the process is completed. If more than one welder is welding on the member simultaneously, place the work piece connectors at the end opposite the welder. That causes the current from each welder to "Buck" each other and since the current can be thought of as vectors, cancel the effect of the counter current. If only one welder is welding on one end of the member, place the work piece connect near the location of the weld so the member is not remagnetized. Remember, passing current through a conductor also creates a magnetic field, it is circular to the direction of the current (Flemming's Right Hand Rule). The intensity of the magnetic filed is a function of the permeability time the current. So the strength of the magnetic field is reduced by using reduced current. Again, DC produces the most notable affects, so AC can be used to mitigate the strength of the magnetic field produced by the welding current.

Good luck.

Best regards - Al

 

[dik]

You should have no problem welding it, but if required to be magnetised, then you will likely lose any magnetism in the HAZ of the weld. High temperatures causes iron/steel to lose magnetism.

Dik

 

[gtaw]

A couple of things to remember:

Long slender materials such as bar, pipe, structural shapes should be stored with their longitudinal axis oriented in the East-West direction.

DO NOT wrap several loops of the welding leads around the member while welding. That is how your science teacher magnetized the nail (an electromagnet) in primary school to teach us about magnetism. The greater the number of "turns" and the higher the current, the stronger the resulting magnetic field.

When using multiple welders, each welding along a long member such as a girder, connect the work leads to the two opposite ends from where the welders are working. In other words, if welders A and B and welding on the Eastern end of the member, the work lead should be connected to the far end, i.e. the West end of the member. Meanwhile, welders C and D, both welding on the Western end of the member should have their work leads connected to the Eastern eand near welders A and B. The currents are bucking each other so no magnetic field is produced.

The heat produced by welding is localized. Only the portion of the member that is heated above the Curie Point (the weld and the HAZ) is momentarily demagnetized. The residual magnetic field returns as soon as the temperature drops because the entire member is magnetized, not just the area being welded.

You can check for remnant magnetism by suspending a paper clip at mid-length by a thread. Hold the paper clip close to the end of the member, if it is still magnetized, it will rotate so one end of the paper clip is aligned with the axis of the magnetized part.

Members that are weakly magnetized can be demagnetized with a sharp blow with a small sledge hammer. Again, if the strength of the magnetic field is strong enough that the hammer blow isn't sufficient, demagnetize it described in the earlier post.

Best regards - Al

 

[Mr168]

I once had a very large vessel that had the end prep become magnetized. It was 280 gauss (you could stick a GTAW wire to the bevel and it would hold). They were just able to get the root pass in when it was reduced to 10 gauss. We tried every cable wrapping and induction coil trick in the book to no avail, and eventually had to bring MPS in with their demag equipment. 15 minutes later, we were done.

 

[dik]

Thanks GTAW, I didn't realise that there could be problems with welding material that had become naturally magnetised.

Dik