magnetism

Date 04-24-2008 01:08

Hello Darren;

It could be that the carbon steel has a remnant magnetic field from all the welding. Every time you pass a current through a conductor, you create a magnetic field. In this case the field strength is a function of the permeability of the steel times the welding current. The submerged arc process uses very high currents and produces strong magnetic fields. The carbon steel is going to stay magnetized, not as strongly as when the current was flowing while you were welding, but it can be relatively strong none the less.

You can try to demagnetize the vessel before attempting to weld again and you can use an AC electrode to minimize the field strength of the arc blow and the residual magnetic field.

You can try to demagnetize the vessel by wrapping welding lead around it. Use at last 10 turns. Hook the leads to an AC welding power supply and set the amperage as high as it will go. Use a large electrode to strike an arc on a piece of scrap to energize the coils around the vessel. You only have to activate the coil for a brief moment or two. Break the arc and reduce the amperage by 10 to twenty amps and repeat the process. Repeat the process ten to fifteen time, reducing the amperage by ten or fifteen amps each time until you are down around 30 to 50 amps. Then while the amperage is set low, remove one wrap of welding lead from the vessel each time you strike an arc.

The residual magnetic field should be nearly nonexistent when you are done.

Let us know if this helps.

Best regards - Al

Attachment: Demagnetize23-April-08.wmf (42k)

Attachment: Demagnetize23-April-08.dwg (25k)

By 357max

Date 04-24-2008 13:37

Use two welding machines. Use the AC weld output from a buzz box transformer welder and connect two work clamps and run AC through the part. Weld w/DC power source.

By 803056

Date 04-24-2008 21:02

I'm not sure I would do that. That is the same as a short which can overload the AC buzz box if you exceed the duty cycle.

By the way, I forgot to mention that you should wrap the leads around the vessel end to end first and demag it, then wrap the leads around the vessel as shown in the sketch. That should demag the vessel in all directions.

Best regards - Al

By 357max

Date 04-24-2008 21:45

A "dead" short would reduce the voltage to less than 5 volts. Good connections down to less than 2 volts. 200 amps output would have 400 to 1000 watts. Most machines will have a 20% duty cycle at rated load of 25 volts at 225 amps. That would be 5625 watt load. Therefore 1/5 the load would be a 100% duty cycle. A resistive load whether through the arc or through a connection.

By 803056

Date 04-25-2008 02:06

I've seen buzz boxes go up in smoke when welders tried to extend their duty cycle while welding at high amperage. Poof, a few flames, a lot of smoke, and then nothing!

A more realistic determination of the power used by the welder in a shorted condition is to measure the input voltage and input amperage. A single phase machine for home use, i.e., 225 amp buzz box, will require 220 volt at 60 amps to produce the maximum output. In a dead short condition, the machine can produce more than 225 amps, and it will toast the power supply if the duty cycle isn't adjusted for the actual current draw.

If you reduced the current to the point that the duty cycle is 100%, the current will not be high enough to produce an strong magnetic field by passing AC through the weldment as you suggest. The strength of the magnetic field produced is proportional to the current passing through the part. To buck the magnetic field produced by the welder using DC, the AC would have to be as high as the DC. However, only one half of the AC would buck the DC, the other half of the AC cycle would be additive to the DC thereby doubling the strength of the magnetic field on that half cycle.

However, I'm not totally adverse to your idea of using AC to counter the magnetic field produced by the DC. I would be willing to use the AC to energize several turns of welding lead wrapped around the part being welded. The theory is that two separate magnetic fields can't coexist in a part. The magnetic field produced by the AC would be longitudinal and would switch polarity 60 times per second. It would disrupt the circular field produced by the welder using DC. The beauty is that because the strength of the magnetic field is the product of the number of turns times the amperage, the amperage draw on the AC power supply could be relatively low and within the adjusted duty cycle of the power supply so as to prevent overheating.

I've never tried running AC through the wrapped welding leads while welding with DC, but it does sound feasible provided the field strength is not overwhelming and disrupts the DC welding operation. I do know that the solution I originally proposed works because I've done it many times and I've even performed the degaussing operations during welding operations if the residual magnetic fields became severe enough to upset welding operations. Energizing the wrapped leads with AC while welding with DC is an interesting concept.

Now, the trick is to actually try it.

Best regards - Al

By 357max

Date 04-25-2008 13:55

You are correct. When SMA welding at high amperages the voltage will be 20 - 35 volts (dependent on the electrode and diameter). So if 30 volts is measured with a 200 amp load that would be 6000 watts. That produces a lot of heat which could possibly melt the insulation on the transformer windings and with no insulation; there would be a short circuit and the machine would destroy itself.
Try it, connect an ac volt meter across the work and electrode; connect the leads in a dead short; turn the welding machine on. What does the volt meter read? Less than 2 volts; multiply the amperage times the volts and the wattage is less than 1000 watts. Machine would be over 100% duty cycle. From a constant current/variable voltage welding machine the amperage will only increase to the value selected by the amperage control. Its voltage will raise or lower itself in response to the resistance in the circuit. Welding arcs have a high resistance therefore the voltage will increase to produce the amperage selected. On the other hand; a constant voltage/variable amperage (GMAW) machine will reach out for the amperage to clear the short circuit. And that may be several hundred amps but the voltage will stay at the value selected.
Also measure the temperature of the transformer coils with an infrared temp stick. At 200 watts there is very little temperature rise.

By darren

Date 04-24-2008 22:40

apparently industry knowledge was not used in the solution. it was welded over and ever till there was a reasonably sound layer of carbon steel then backgouged on the stainless side and then re welded, i do not know the outcome yet, again apparently all the superstitious and "witch doctor mumbo jumbo" (yeesh what do you do ?) of industry knowledge was not considered. i however am interested in finding out the 2008 solution to such matters regardless of the 1940's solution that was applied.
i will keep you informed
thanks
darren