ARC BLOW
Another look, by Eric Waterfield
The following is mainly focused on the student welder, but may also prove helpful to the journeyman.
There is so much written about arc blow that it is difficult to come up with anything new; however, it seems that to find a really complete analysis of the problem in any one article that is written to cover all the situations met with by the welder is perhaps not possible. It is equally difficult to find the answers to all the questions because there are so many unique situations; but with an in-depth look at some of the causes of the problem, one may be able to find one's own solutions.
I am not about to attempt the reinvention of the wheel - my aim is to simply take out a few of the flat spots by including some of my own experiences.
A Magnetic Force
A common experience trainee welders face in welding school:
A butt weld is made on two plates of equal size, tacked together to form a square. An electric charge is passed through them creating a magnetic field equally on all the edges of the plates. Why then is the arc force disturbed only at the start and end of the joint? The arc interference is most severe at the edges, but is nonexistent at the centre of the weld.
It is seen that at the start of the weld (assuming the weld is proceeding from left to right), the arc blow will be less at the beginning of the weld than at the end. Thus the arc force will be from left to right, effectively in the same direction as the weld progression, but at the other end of the weld the arc blow will be from right to left - now the arc blow is in opposition and is much more of a problem than at the start.
An Analogy
Arc blow has been described as being an electronic wind - perhaps one could imagine and liken it to a windy day, and you are walking along a path...
At the start of your journey the wind is at your back and you are moving with it, effectively reducing its force. Then you reach a point in the journey - let us say halfway - where the wind seems to have stopped blowing, but toward the end of the path the wind picks up again. It is stronger and has changed direction, and now you are traveling against it, adding your speed to the force of the wind. Thus the force of the wind has suddenly increased in the opposite direction.
Thermal Force - Friend or Foe?
Another force called "thermal arc blow" is also effecting the arc, but with less force than the magnetic effect. This phenomenon is caused by electrons preferring to follow a heated path more than a cold one. At the start of the weld the hot electrode leaves behind hot weld metal, thus the electrons flow from the electrode to the hot weld metal in the opposite direction to the direction of the arc travel - see illustration. It can be seen that this has the effect of reducing the magnetic arc force by opposing it, but does not eliminate it completely.
However, at the end of the weld the thermal effect turns traitor and is now pressing in the same direction, joining forces with the magnetic arc force and helping it cause severe arc blow by creating a much stronger disturbance than at the beginning.
REDUCING ARC BLOW!
1 The positive effect of thermal arc blow can be maintained simply by reversing direction and welding away from the ends of the joint.
2 Weld in the same direction as the arc blow, always away from the start of the weld at the edge of the plate - for example, begin by making a short weld (large tack) at the opposite end of the joint (from right to left) with the weld direction toward the center of the joint. This is always good practice as it will also eliminate the crater often left at the end of the weld.
3 Keep a short arc, as it will reduce the area of the arc and trap the arc blow in the weld area.
4 Make use of the gas jet (arc force) from the end of the electrode by angling the electrode against the arc blow.
5 Do not use an electrode that is too large for the current - small diameter electrodes with higher current density handle the arc blow better.
6 Pre heat where possible, but ensure that the heat is distributed evenly on both sides of the joint.
7 The use of negative polarity is helpful if it is consistent with procedure.
8 If all else fails, use AC.
Other Causes of Arc Blow
Surrounding metal such as a plate welded on a steel bench which carries the welding current will also become magnetized in the same way as the welded plate. Placing the plate near the edge of the bench will add the magnetic effect to the plate.
Solution: Place the plate in the centre of the bench where magnetic force is neutral.
Steel alloyed with more than 0.1 percent carbon will become semi-permanently magnetic, and will transfer some magnetism to steel plate connected to it.
Other current sources such as other welding operations add magnetic fields to a steel structure, especially in situations where high amperage operations are carried out such as submerged arc.
A multitude of welders working in the same area can also produce arc blow.
Some industries may be using high current in their operations, not necessarily connected to the welding project. My most memorable experience was in 1969 when welding at a chlorine plant in Squamish BC. I was asked to weld a pipe that was situated on top of the chlorine cells. Producing chlorine is done by electrolysis, passing huge amounts of low voltage high amperage "Direct Current" through a brine solution.
The magnetic field was so strong that as I approached the cells, the length of two inch pipe I was carrying on my shoulder oriented its self with the magnetic field with such force that it almost knocked me down. Not only that, the spud wrenches I was carrying in my pants pockets also lined themselves up in the same way as the pipe causing me pain and "almost ruining me for wife!"
Trying to weld in these conditions led me to realize that one didn't necessarily need air for gouging - the arc force simply tore the molten weld metal out of the groove and it wasn't until the current to the cells was turned off that welding became possible.
I know that there are others who have had similar experiences!
