Arc blow occurs in DC arc welding when the arc stream does not follow the shortest path between the electrode and the workpiece and is deflected forward or backward from the direction of travel or, less frequently, to one side.
First, let's examine some of the terms associated with arc blow. Back blow occurs when welding toward the workpiece connection, or the end of a joint, or into a corner. Forward blow is encountered when welding away from the workpiece connection, or at the starting end of the joint. Forward blow can be especially troublesome with SMAW iron-powder electrodes, or other electrodes that produce large slag coverings, where the effect is to drag the heavy slag or the crater forward and under the arc.
There are two types of arc blow - magnetic and thermal. Of the two, magnetic arc blow is the type causing most welding problems, so we will study that one first.
Magnetic Arc Blow
Magnetic arc blow is caused by an unbalanced condition in the magnetic field surrounding the arc. This unbalanced condition results from the fact that at most times, the arc will be farther from one end of the joint than another and will be at varying distances from the workpiece connection. Imbalance also exists because of the change in direction of the current as it flows from the electrode, through the arc, and into and through the workpiece.
Visualizing a Magnetic Field
To understand arc blow, it is helpful to visualize a magnetic field. Figure 3-37 shows a DC current passing through a conductor (which could be an electrode or the plasma stream between an electrode and a weld joint). Surrounding the conductor a magnetic field, or flux, is set up with lines of force that can be represented by concentric circles in planes at right angle to the direction of the current. These circular lines of force diminish in intensity the farther they are from the electrical conductor.
The concentric flux fields will remain circular when they can stay in one medium expansive enough to contain them until they diminish to essentially nothing . But if the medium changes (such as from steel plate to air), the circular lines of force are distorted and tend to concentrate in the steel where they encounter less resistance. At a boundary between the edges of a steel plate and air, there is a squeezing of the magnetic flux lines, causing deformation in the circular lines of force. This squeezing can result in a heavy concentration of flux behind or ahead of a welding arc. The arc then tends to move in the direction that would relieve the squeezing and restore the magnetic field balance. It veers away from the side of magnetic flux concentration. This veering is observed as arc blow.
Figure 3-38 illustrates the squeezing and distortion of flux fields at the start and finish of a seam weld. At the start, the magnetic flux lines are concentrated behind the electrode. The arc tries to compensate for this imbalance by moving forward which creates forward arc blow. As the electrode approaches the end of the seam, the squeezing is ahead of the arc, with a resultant movement of the arc backwards, and the development of back blow. At the middle of a seam in two members of the same width, the magnetic field would be symmetrical, and there would not be any back or forward arc blow. But, if one member should be wide and the other narrow, side blow could occur at the midpoint of the weld.
Understanding the Effect of Welding Current Returning Through the Workpiece
Another "squeezing" phenomenon results from the current returning back towards the workpiece connection within the workpiece. As shown in Figure 3-39, a magnetic flux is also set up by the electrical current passing through the workpiece to the workpiece lead. The heavy line represents the path of the welding current while the light lines represent the magnetic field set up by the current. As the current changes direction, or turns the corner from the arc to the work, a concentration of flux occurs at x, which causes the arc to blow, as indicated, away from the workpiece connection
The movement of the arc because of this effect will combine with the movement resulting from the concentration previously described to give the observed arc blow. The effect of the returning current may diminish or increase the arc blow caused by the magnetic flux of the arc. In fact, control of the direction of the returning current is one way to control arc blow, especially useful with automatic welding processes.
In Figure 3-40(a), the workpiece cable is connected to the starting end of the seam, and the flux resulting from the returning welding current in the work is behind the arc. The resulting arc movement would be forward. Near the end of the seam, however, the forward arc movement would diminish the total arc blow by canceling some of the back blow resulting from concentration of the flux from the arc at the end of the workpiece, see
In Figure 3-40(b), the work cable is connected to the finish end of the seam, which results in back blow. Here, it would increase the back blow of the arc flux at the finish of the weld. The combination of "squeezed" magnetic fluxes is illustrated in Figure 3-41(b). A workpiece connection at the finish of the weld, however, may be what the welder needs to reduce excessive forward blow at the start of the weld.
Because the effect of welding current returning through the workpiece is less forceful than concentrations of arc-derived magnetic flux at the ends of workpieces, positioning of the workpiece connection is only moderately effective in controlling arc blow. Other measures must also be used to reduce the difficulties caused by arc blow when welding.
Other Problem Areas
- Corner and Butt Joints with deep Vee grooves
Where else is arc blow a problem? It is also encountered in the corners of fillet welds and in weld joints which use deep weld preparations. The cause is exactly the same as when welding a straight seam - concentrations of lines of magnetic flux and the movement of the arc to relieve such concentrations. Figures 3-42 and 3-43 illustrate situations in which arc blow with DC current is likely to be a problem.
- High Currents
There is less arc blow with low current than with high. Why? Because the intensity of the magnetic field a given distance from the conductor of electric current is proportional to the square of the welding current. Usually, serious arc blow problems do not occur when stick electrode welding with DC up to approximately 250 amps (but this is not an exact parameter since joint fitup and geometry could have major influence.)
- DC Currents
The use of AC current markedly reduces arc blow. The rapid reversal of the current induces eddy currents in the base metal, and the fields set up by the eddy currents greatly reduce the strength of the magnetic fields that cause arc blow.
- Magnetically Susceptible Materials
Some materials, such as 9%nickel steels, have very high magnetic permeability and are very easily magnetized by external magnetic fields, such as those from power lines, etc. These materials can be very difficult to weld due to the arc blow produced by the magnetic fields in the material. Such fields are easily detected and measured by inexpensive hand - held Gauss meters. Fields higher than 20 Gauss are usually enough to cause welding problems.
Thermal Arc Blow
We've already examined the most common form of arc blow, magnetic arc blow, but what other forms might a welder encounter? The second type is thermal arc blow. The physics of the electric arc require a hot spot on both the electrode and plate to maintain a continuous flow of current in the arc stream. As the electrode is advanced along the work, the arc will tend to lag behind. This natural lag of the arc is caused by the reluctance of the arc to move to the colder plate. The space between the end of the electrode and the hot surface of the molten crater is ionized and, therefore, is a more conductive path than from the electrode to the colder plate. When the welding is done manually, the small amount of "thermal back blow" due to the arc lag is not detrimental, but it may become a problem with the higher speeds of automatic welding or when the thermal back blow is added to magnetic back blow.
Arc Blow with Multiple Arcs
Some recent welding process advances involve the use of multiple welding arcs for high speed and improved productivity. But, this type of welding can also cause arc blow problems. Specifically, when two arcs are close to each other, their magnetic fields react to cause arc blow on both arcs.
When two arcs are close and have opposite polarities, as in Figure 3-44(a), the magnetic fields between the arcs causes them to blow away from each other. If the arcs are the same polarity, as in Figure 3-44(b), the magnetic fields between the arcs oppose each other. This results in a weaker field between the arcs, causing the arcs to blow toward each other.
Usually, when two arcs are used, it is suggested that one be DC and the other AC, as shown in Figure 3-44(c). In this case, the flux field of the AC arc completely reverses for each cycle, and the effect on the DC field is small. As a result, very little arc blow occurs.
Another commonly used arrangement is two AC arcs. Arc blow interference here is avoided to a large extent by phase-shifting the current of one arc 80 to 90 degrees from the other arc. A so-called "Scott" connection accomplishes this automatically. With the phase shift, the current and magnetic fields of one arc reach a maximum when the current and magnetic fields of the other arc are at or near minimum. As a result, there is very little arc blow.
How To Reduce Arc Blow
If DC current is being used with the shielded metal-arc process - especially at rates above 250 amps - a change to AC current may eliminate problems.
- Hold as short an arc as possible to help the arc force counteract the arc blow.
- Reduce the welding current - which may require a reduction in arc speed.
- Angle the electrode with the work opposite the direction of arc blow, as illustrated in Figure 3-45.
- Make a heavy tack weld on both ends of the seam; apply frequent tack welds along the seam, especially if the fitup is not tight.
- Weld toward a heavy tack or toward a weld already made.
- Use a back-step welding technique, as shown in Figure 3-46.
- Weld away from the workpiece connection to reduce back blow; weld toward the workpiece connection to reduce forward blow.
- With processes where a heavy slag is involved, a small amount of back blow may be desirable; to get this, weld toward the workpiece connection.
- Wrap the work cable around the workpiece so that the current returning to the power supply passes through it in such a direction that the magnetic field set up will tend to neutralize the magnetic field causing the arc blow.
The direction of the arc blow can be observed with an open-arc process, but with the submerged arc process it is more difficult to diagnose and must be determined by the type of weld defect.
Back blow is indicated by the following:
- Undercut, either continuous or intermittent
- Narrow, high bead, usually with undercut
- An increase in penetration
- Surface porosity at the finish end of welds on sheet metal
- Forward blow is indicated by:
- A wide bead, irregular in width
- Wavy bead
- Undercut, usually intermittent
- A decrease in penetration
The Effects of Fixturing on Arc Blow
Another precaution the weld operator needs to be aware of with arc blow is its relationship to fixturing. Steel fixtures for holding the workpieces may have an effect on the magnetic field around the arc and on arc blow and may become magnetized themselves over time. Usually, the fixturing does not cause any problems with stick-electrode welding when the current does not exceed 250 amps. Fixtures for use with higher currents and with mechanized welding should be designed with precautions taken so that an arc blow-promoting situation is not built into the fixture.
Each fixturing device may require special study to ascertain the best way to prevent the fixture from interfering with the magnetic fields. The following are some points to note:
- Fixtures for welding the longitudinal seam of cylinders (Figure 3-47) should be designed for a minimum of 1-in. clearance between the supporting beam and the work. The clamping fingers or bars that hold the work should be nonmagnetic. Do not attach the workpiece cable to the copper backup bar; make the work connection directly to the workpiece if possible.
- abricate the fixture from low-carbon steel. This is to prevent the buildup of permanent magnetism in the fixture.
- Welding toward the closed end of "horn type" fixtures reduces back blow.
- Design the fixture long enough so that end tabs can be used if necessary.
- Do not use a copper strip inserted in a steel bar for a backing, as in Figure 3-48. The steel part of the backup bar will increase arc blow
- Provide for continuous or close clamping of parts to be seam-welded. Wide, intermittent clamping may cause seams to gap between clamping points, resulting in arc blow over the gaps.
- Do not build into the fixture large masses of steel on one side of the seam only. Counter-balance with a similar mass on the other side.
By understanding the mechanics of arc blow and how to correctly diagnose it in the weld, operators should be able to eliminate it from their applications and be able to create welds without the problems normally associated with arc blow.
Information from The Lincoln Electric Company
source : lincolnelectric.com