Metallurgy and Welding 101: The Basics for Doing Hardfacing and Other Welding for Construction-Equipment Repair – Part 2

Continuation of an interview with arc-welding expert, Thomas J. Black, manager of hardfacing and high-alloy products for The Lincoln Electric Company in Cleveland, OH.

For arc welding, do contractors mainly use direct current (DC) or alternating current (AC)?

Black: Construction-equipment maintenance people almost always use DC in field arc-welding applications because it gives them much better control over the stick-welding process. Furthermore, there is a much lower probability of electrocution with DC, as the body is able to present greater resistance to the flow of electricity when the electric current is flowing through it in only one direction. Of course, electrocutions, even with AC, are fortunately quite uncommon.

Many construction companies also use DC power sources in their shops. A major reason for this is that these are often the same units they transport to the field. Thus, using the same power sources in shop and field gives them flexibility. It also allows them to use the same consumable electrodes for both shop and field applications. Finally, many construction companies will do stick arc welding in the field and semiautomatic-wire-feed arc welding in the shop, and such wire arc welding is done predominantly using DC power.

On the other hand, for shop-only arc-welding power sources, some construction companies use AC models because AC is more energy efficient and thus more economical, lowering electricity bills about 20% compared with DC power.

Besides stick welding, there are of course other types of arc welding, such as MIG welding. Please expound on what that is, where it is used, how popular it is, and its pros and cons.

Black: The acronym MIG stands for metal inert gas. In MIG welding, instead of the welder using a stick electrode, he uses a consumable solid-wire electrode made of the alloy material. The wire can be automatically and continuously fed to the welding gun, making this a fast way to weld.

In actual practice, many people have been misusing the term MIG welding, applying it to any kind of arc welding in which a wire electrode is fed continuously through a welding gun. But this is not correct. To speak correctly, MIG welding designates an arc-welding technique that employs a solid, small-diameter wire automatically fed through a welding gun and that uses an inert-gas shield to protect the molten weld area from the atmosphere.

In MIG welding, what is the purpose of the gas shield? What kind of gas is used? And what prevents the gas from floating away from the weld site?

Black: The purpose of the gas is to protect the molten weld area from the atmosphere. Specifically, if there were no inert gas covering the weld area to protect it, then nitrogen from the atmosphere would dissolve in the molten, high-temperature metal of the weld pool and, as the liquid cooled, become much less soluble in the liquid at lower temperatures and bubble out of the molten metal, creating a weld area that would be porous, frothy, lacelike, and weak. The weld area would be further weakened by the reaction of atmospheric oxygen with the molten iron, creating iron oxides. It is to protect the molten weld area from these harmful effects of both nitrogen and oxygen that a weld pool must be protected from direct exposure to the atmosphere – either by an inert gas or by a flux over the weld area.

As for the gases used to protect the molten weld area in MIG arc welding, typically they are carbon dioxide, a mixture of argon and carbon dioxide, a mixture of argon and oxygen, or a mixture of argon, carbon dioxide, and oxygen. Most MIG welding is done using some sort of argon mixture. Carbon dioxide gas is cheaper than argon but its use is more restrictive.

The source of the gas for welding is some sort of pressurized gas cylinder. A tube conveys the pressurized gas from the cylinder through the welding gun onto the weld area. Pulling the trigger on the welding gun activates a solenoid valve in the gas line, delivering gas over the weld area at the rate of 20–40 cubic feet per hour. As the old gas blanket drifts away from the welding area, it is continuously replenished with fresh gas.

Please expand further on the solid wire used in MIG welding?

Black: As I mentioned earlier, in MIG electric arc welding, the wire takes the place of the stick electrode. The wire is a consumable electrode, made of the particular steel alloy needed to make the specific weld. Such solid wire is available in diameters ranging from 0.02 inches, up to 0.0625 inches. The MIG wire feeder feeds wire off a reel through the welding gun continuously and automatically at a preselected feed rate. The operator controls starting and stopping with the welding-gun trigger.

The welder would of course select a solid wire made of the appropriate steel alloy for the welding task. In addition, the diameter of the wire chosen would depend upon the size of the power source. The lower the amps, the smaller the wire diameter that would be chosen and the slower the flow of the protecting gas. The higher the amps, the bigger the wire that could be used and the greater the flow of protecting gas.

So far you have discussed two main types of electric arc welding: stick welding and MIG welding using solid wire. Aren’t there other important types of electric arc welding that use tubular wire—so called metal-cored wire and flux-cored wire—instead of solid wire? Please discuss what they are, where they fit in, and their pros and cons.

Black: Metal-cored wire is a tubular wire that contains a metal alloy in the core of the wire. There is no flux material placed in the core; thus, the weld area has to be protected by an inert gas or some other means.

Flux-cored wire, on the other hand, is a tube of wire—diameters range from 0.035 to 0.125 inches—that contains both a metal alloy and a flux within the wire’s core. When melted during welding, the flux is released from the core, forming a protective coating over the weld beads. The flux protects the molten weld from the atmosphere during welding and, upon cooling and hardening, forms a shiny surface, enhancing the weld’s appearance.

Said another way, flux-cored wire welding is like doing stick electric arc welding “inside out.” In stick welding, the slag is a coating on the outside of the electrode. In welding using flux-cored wire, the slag is in the inside core of the wire.

Both solid-wire and metal-cored wire electric arc welding are used less in the field and in out-of-position applications since there is nothing to support the weld pool. Their use is mainly for flat or horizontal welding. With flux-cored wire welding, on the other hand, the flux supports the weld pool; thus, this self-shielded method is most suitable to field welding applications and is quite popular there because welders do not have to worry about a protective gas blowing away.

Electric arc welding using self-shielded flux-cored wire has become a very common method for field applications. Why its popularity? (1) There is no need to transport gas cylinders to the field, (2) there is no need to erect welding curtains to keep the wind from blowing into the weld area, and (3) there is no need to worry about protective gas blowing away. As I mentioned, the flux-cored wire is self-shielding: The flux contained in the core flows over the molten weld beads, protecting them from direct exposure to the atmosphere.

Nonetheless, in shop welding applications, solid-wire and gas-shielded flux-cored and metal-cored welding is extensively used. Indeed, some welders prefer gas-shielded welding in shop situations, especially with argon-gas mixtures; metal transfer is smoother, and there is less smoke.

Incidentally, the same electric power supply and other equipment can be used for both flux-cored wire welding and solid-wire MIG welding. In switching over from flux-cored wire to MIG welding, the worker does need to change both the wire and the welding gun, for the gun provides the arrangements for gas shielding.

What, then, are the most popular welding methods used in the field for the repair of construction equipment?

Black: Eighty percent of the welding on construction equipment in the field is done with either stick or self-shielded flux-cored electric arc welding. There is limited use of gas shielding in the field because gas shields can too readily blow away. The stick electrode and self-shielded flux-cored wire are self-shielded products: They contain the flux material that protects the molten weld either on the outer surface, as on the stick electrode, or in the wire’s core.

What about in the shop? What are the most popular welding methods there?

Black: Gas-shielded arc welding using solid wire or metal-cored or flux-cored wire is quite popular, accounting for perhaps 80% of shop welding. Furthermore, on the more sophisticated and extensive construction-equipment repair jobs–for example, welding repair work on tractor undercarriage sprockets and idlers and on cable drums and pulleys–there is a type of arc welding used not yet mentioned: fully automatic submerged arc welding. In this method, the gun feeds either a solid or tubular wire to the weld pool. A granular flux having the texture and appearance of kitty litter is either spread by hand, gravity-fed, or carried in a compressed airstream over the weld pool, forming a protective blanket. Since the arc is submerged beneath the flux layer, this method is called the submerged arc welding. The unfused flux material can be recirculated.

Submerged arc welding is popular in the shop for such applications for these reasons: a high-quality weld, higher productivity compared to other arc welding methods, no smoke, and no visible arc light, the arc being submerged under the granular flux—thus, welding hood and protective clothing are not needed.