Deep welding is a term applied to a shielded metal arc welding (SMAW) technique which utilizes higher welding speeds than conventional methods, uses the benefits of greater arc penetration to obtain the required weld strength, and thereby decreases the cost of the welding operation. When applied to fillet welding, it is often called deep-fillet welding.

Basic Idea:

For years, sound welds have been made by conventional methods in the accepted belief that deeper penetration was produced by slower arc speeds. In fact, however, faster speeds, within limits, result in greater penetration, while slower speeds tend to build up more of the weld metal on the surface. A fillet weld with greater penetration resulting from faster travel speed appears smaller, but its strength actually is as great or greater than the weld made at slow speed, which sacrifices penetration for buildup. Since increased penetration reduces the amount of deposited metal needed, the speed of welding can be increased without impairing the strength.

Travel Speed-Penetration

The key factor in applying arc force is making the arc travel fast enough to utilize the penetrating power of the arc. An analogy would be to squirt a stream of water through the nozzle of a hose to dig into the ground. The digging action of the stream of water is effective only when the stream is directed at the digging point in the dirt, not when directed into the pool of water that soon accumulates. To maintain the digging action, the of stream of water must be kept moving rapidly enough to stay ahead of the pool, because when it is directed into the pool, its force is expended in merely displacing and churning the water in the pool, not in digging into the ground.

The same principle can be applied in welding. When the arc is moved slowly, the pool of molten metal buffers the arc, and its force is expended in the molten pool instead of penetrating into the parent metal at the root of the joint. This molten metal merely flows along the joint under the weld without fusing to the parent metal below the depth of arc penetration. When the arc is moved forward rapidly enough, the arc force digs into the base metal and the result is good

penetration.

When conventional arc speeds are used, there is usually a small puddle of molten metal under the arc, dissipating the arc force and preventing full penetration. The limiting speed is usually the highest speed at which the surface appearance remains satisfactory. See Figure D-1 for a comparison of arc penetration at conventional and high travel speeds. Note that deposited weld metal is minimized.

Effect of Current

An increase in current increases the arc force, which increases penetration, just as an increase in the analogous volume of water through the same size hose nozzle increases the digging power of the stream of water. To use higher currents, larger size electrodes

may be needed. In general, the first indication of excess current is a poor surface appearance of the weld.

 

Effect of Arc Length

In a further comparison of the arc to the stream of water from a hose, to dig deeply into the dirt the nozzle must be kept as close to the ground as possible in

order to avoid letting the stream of water spread out into an ineffective spray. In welding, when a long arc is held, heat is dissipated into the air, the stream of molten metal from the electrode to the work is scattered in the form of spatter, and the arc force is spread

over a large area. The result is a wide, shallow bead instead of a narrow bead with deep penetration.

The advantages of deep-welding are: (1) less deposited metal, (2) increased rate of deposit, and (3) lower costs and simplified process.

Less Deposited Metal. By getting deeper penetration, the welded joint is comprised of more fused base metal and less deposited metal than in conventional welding. Since the deposited metal is relatively costly and the fused base metal can be utilized at practically

no additional cost (other than labor to make the weld), the deep-welded joint is made at a proportionately reduced cost.

Greater penetration also allows changing joint preparation from a V-butt in 3/8-inch plate to a plain square-edge butt joint, reducing the amount of filler metal deposited by about 50%. This, in turn, reduces labor by almost 80%.Figure D-2 shows the use of arc penetration to reduce plate edge preparation.

Travel Speed. On welds where penetration is the major consideration, such as square-edge butt welds and fillet welds made by deep-welding procedures, the travel speed is not proportional to the current, since the limiting factor for travel speed is the rate at which the slag will follow and cover the weld. Thus, the travel speed with this type of joint is determined by the slag covering characteristics of the coated electrode, rather than by the melt-off rate.

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