The shielded metal-arc welding process has been widely used for many years to weld pipe material It is still used, even in the most modem industries. The process is used with various types of alloy pipe materials in constructing power plants, oil refineries, chemical plants, nuclear power plants, and oil and gas transmission lines. Specifically it is used to make or join welds that meet the requirement of the industries’ respective codes and specifications for piping sys­tems and materials. These requirements cover various conditions such as high strength at elevated temperatures, corrosion and oxidation resistance, steels for cryogenic services, and high alloy steel that must resist chemical attack or require high tensile strength.

The ability of the steel to achieve its needed mechanical properties in the various environments, depends on proper steel making practices such as degassing, removal of impurities, hot rolling within the criti­cal range, grain control, and even controlling the cooling rate and heat treatment. When the welding is applied, the filler metal on the elec­trode must be formulated so that the completed weld meets the required strength and other properties for which the system was designed.

Shielded metal-arc covered electrodes are formulated in a way anal­ogous to process metallurgy. However, the challenge of formulating electrode coatings involves knowing that the filler metal will be trans­ferred in the intense arc heated column as fine particles to the base metai molten pool. Many of the materials used to make slag, in con­nection with refining metal in a furnace, are also used in fluxes for welding. Limestone, flourspar and rutile are used make steel. There are many other suitable minerals compounded to make a flux. For exam­ple, certain precautions must be taken to ensure protection during the transfer of the alloying element, and to guard against the pick-up of elements that could affect weld properties. Furthermore, weld metal is susceptible to oxidation when the metal is exposed to oxygen. Other problems include porosity and blow holes caused by the effusion of gases. These are some of the same problems that are experienced with ingots and casting during steel making.

Most flux-bearing arc welding electrodes contain additional elements to function as deoxiders, They are supplemented by adding alloying elements in the electrode coating. The molten pool is protected by deoxiders and fluxing agents that remove the oxide formation of the hot metal ahead of the arc. Numerous types of coated electrodes are available for welding various pipe alloy.

Electrode coatings have many functions, including:

Mild steel material has a low carbon content as well as small amounts of manganese and silicon. Welding such materia! offers little chal­lenge to its microstructure; in fact, the microstructure is basically per­lite, which is soft and ductile.

The appropriate electrode for welding this grade of material is the E - 6010 electrode. There are many variations of this basic electrode type. For example, the covering of the E-6020 electrode has a high iron - oxide content, as much as 30 percent. The amount varies substantially in the series, with little or no gas shielding provided by the covering. The primary purpose of this covering formulation is to provide a heavy layer on the metal, thereby obtaining a satisfactory operation with high current. The welder is then able to obtain a deep penetration in the butt and fillet welds in the horizontal and flat position. E-6020 electrodes

are used most often on relatively heavy sections where deep penetra­tion and high metal deposit rate is sought.

The E-6010 electrode has a thin layer of (flux) coating that barely covers the entire weld. This thin layer offers a higher viscosity than the heavy covering on other electrodes. It thus makes the E-6010 an elec­trode most suited for depositing root bead on pipe without dripping either inside or outside, once the root bead technique is adapted. The E-6010 electrode does have limits where it can be applied, as follows:

(1) It is effectively used for deposited root bead on carbon steel - uphill and downhill welding.

(2) It is limited to welding on low alloy steels because of its low content in alloying elements. However, it is sometimes used for depositing the root bead, which is then filled and capped with the low-hydrogen electrode, E-7018.

(3) Welding thinwall and low-to-medium carbon steel pipe with E-6010 is acceptable. But if the wall thickness exceeds half an inch, only the root bead is deposited by the E-6010, with fill ing and capping by the E-7018 electrode.

(4) E-6010 is considered a high cellulose electrode. Therefore, it generates a high level of hydrogen not accepted for welding high carbon and medium alloy steel.

(5) As a last resort, the E-6010 electrode can be used for depositing a root bead on high carbon steel, but not without preheating and maintaining interpass temperatures.

(6) Because E-6010 is considered a high cellulose electrode, the final gaseous product from the volatilization and combustion of the cellulose results in a covering of carbon monoxide, car bon dioxide, water vapor, and hydrogen. In turn, because of the high inducement of hydrogen, E-6010 is not used for depositing root bead on either medium alloy steel or steel with high tensile strength.

The electrode E-7047 has a low hydrogen type of coating. Because it is lightly coated, this electrode can be used for downhill welding, of root beads.

There are instances when a low or medium alloy pipe needs to be welded, and the root bead becomes a matter of concern because no other process is available and no electrode available other than E-6010 is available for depositing the root bead. Pick-up, also known as dilu­tion, becomes a factor. Pick-up relates to the welding of an alloy steel with a filler metal of a lower alloy content than the base metal content. In a multilayer weld, the first pass (root bead) naturally picks up the

greater amount of alloying elements from the base metal. When, for example, welding a chrome alloy pipe. The root pass deposit made with the available E6CH0 electrode will gain as much as 50 percent chromium due to dilution or pick up from the base metal.

The viability of this proceedure must be proven by the welder, as the root bead alone is deposited by this type of electrode. The welder must then prepare the surface of the root bead deposit by grinding and removing one third of its thickness, with smooth fusion edges on both sides of the root bead. The welder’s next objective is to use one elec­trode of reasonable size in diameter, and an adjustment in the current allowing penetratation of the root bead as deep as possible, but avoid­ing burn through the root bead.

A small crater should be apparent after the first electrode is con­sumed. The edge fusion will give the welder an indication as to the depth of penetration. The root bead can then certainly be enriched by the deposit from the second pass, using the correct electrode based on specifications. This procedure involves applying preheating tempera­ture and maintaining an interpass temperature, knowing that the base metal and the filler are both of alloy type, An experienced supervisor or experienced welder should help with this procedure. This points out that solving variations in welding is not always based on the procedure itself, but often on the skill of a well-trained welder who understands all that is required.

The electrode coating may carry supplemental alloying elements that are needed; the core wire is not being of the same composition as the base metal. Thus, it has become a practice to use a supplementary addition to the coating when manufacturing electrodes. In addition, the electrode coating has other functions. These include protecting the transfer of filler metal within the arc stream by a blanket of gases that keeps the oxygen out or dissolves the oxidizing gas to a less active one like Co2 or Co, which can also serve as a protective atmosphere.

The electrode coating is made up of compounds and elements that are responsible in their own way to protect the molten weld metal acting as either slag, deoxider, or fluxes.

The slag must be less dense than the weld metal for obvious reasons. It must not freeze at a higher temperature than the weld metal; other­wise, the slag will remain dormant or stagnate on the face of the bevel, disturbing the welding process. In addition, the viscosity of the slag (its ability to flow) has an important bearing on the quality of the weld. When the viscosity is low, the slag is watery. It will flow, leaving the hot metal exposed to the atmosphere, and thus to become oxide. If the viscosity is high, the slag will be very sluggish. This in turn becomes

a challenge to the welder, especially, when making welds in certain positions. When slag is formed on the weld, it will be impregnable to the gases from the atmosphere. Therefore, the weld metal will be pro­tected. However, manufactures of electrodes have taken these variables into consideration allowing the welder to choose the optimum electrode for the job at hand.

A deoxider is a oxygen setter. It serves in molten metal to dispose of oxygen and oxygen-bearing compounds or it remains in the weld pool as a safeguard in case oxygen enters. When oxygen does enters the molten weld pool, it reacts with manganese to form manganese oxide (MnO). As long as manganese is present, this process will continue rather than the formation of iron oxide. In turn, manganese oxide reacts with the carbon to form the Co bubbles that cause porosity (blow holes). If aluminum and silicon are in the steel or the weld pool, there will be no blow holes. The silicon and the aluminum should react with the oxygen to form oxide preferable to FeO. Because it has lower den­sity than the metal, the oxygen bearingslag will rise to the surface, join­ing the bulk of the slag.

During welding, the surface of the joint to be welded can develop an invisible oxide coating, even though it was brushed twenty-four hours earlier. While the weld is in progress, the heated metal ahead of the arc will be exposed to the atmosphere and the oxidation in progress. The object, then, is to protect the molten pool and to dispose of the oxide ahead of the arc by reducing them.

The method by which a flux deals with the oxide is by mixing or co­rn ingling with the oxide to form a slag that has a more favorable melt­ing point and viscosity. Therefore, the electrode coating is also a slag that forms a blanket on the weld surface, shielding the metal from oxi­dation and disposing of oxide that forms from inefficient shielding caused by poor manipulation or other reasons.

There are many individual components in the electrode coating. There are some that combine with other compounds and become chemically active shielding the molten puddle and display of oxide. For example, limestone is a component that can help produce Co and Co2 gases that serve as a protection to the arc area and the arc stream; it produces 40 percent of its weight in gaseous form; the rest becomes calcined, that is, it changes to a white powdery lime. This latter product of the welding process is an example of a fluxing agent. It is secured from limestone after calcination. Other components that could be added to this list are flourspar, sodium oxide, feldspar, ferromanganese, graphite, ferrosalin- ium, ferrosilison, chloride, and fluoride salt.

These agents melt and gather as a liquid blanket on all parts of the weld puddle except the arc spot. The flow of the fluxing agent can be seen as a slight watery substance, a blanket extending the edges of the puddle and dissolving the oxide. The principle function of the flux is to dissolve or dispose of the high melting oxide that may form despite the protection afforded by the gaseous shield around the arc.

To obtain high quality welds the SMAW process requires careful storage and handling of the electrodes. An experienced supervisor can determine whether or not the quality of the weld is at risk based on the condition under which the electrodes are maintained in the work place.

Too often the welder draws the conclusion that using high current is the best way to address the tiny holes that appear on the surface of the previous pass (weld). Yet when the surface is removed by using a grinder, the holes are in fact larger in diameter. Therefore, high current is not a remedy for such a defect. Gases such as Co and Co2, by them­selves or in combination with others, are very soluble in molten metal during welding. As the metal begins to freeze, gases in the weld seek to escape from the weld. As the heat dissipates or is conducted into the walls of the base metal, it also loses heat from the surface of the weld. If effusion of the gases from the weld is not completed before the weld puddle becomes solid, the gases will rise within the semi-molten weld, leaving what are called worm holes. Because of its viscosity at that stage, the metal will not be able to flow to fill these holes, which will remain as voids. If the surface of the weld is in a semi-molten stage, a portion of the gas will escape, and there will be the appearance of pin holes on the surface of the weld, known as porosity.

The coatings of properly baked electrodes are brittle. This coating can fracture quite easily during transportation to the work site, or when the electrodes are carelessly stored among tools such as chipping ham­mers and chisels. The cracks on the electrode coating can be so tiny that they cannot be seen by the naked eye. Often a welder will bend an electrode, to gain acess to a joint in constricted area. The coating then may be fracted or actually break-away from the core. As this damaged electrode is used the electrode coating can flake and fall into the weld, as well as to the ground. The problem then is that the electrode can be depleted of its alloying ingredient, which in turn will lead to inade­quate mechanical properties, oxidation and loss of corrosion resist­ance, especially if the steel happens to be subjected to high tempera­ture service. In addition, the weld will suffer from not having sufficient oxidizers, which is likely to cause porosity in the weld. Cracks due to rapid cooling shrinkage are another possibility.