Electrode classification

The above shows the electrode classification system developed by the AWS, which is used for carbon steels and low alloy steels. The requirements for these electrodes are specified in the AWS standards A5.20 and A5.29, respectively.

To interpret this classification, let’s explore the significance of each character from left to right:

• The “E” designates electrode.
• The number in this position denotes tensile strength in tens of thousands of psi. For example, a 7 would signify a tensile strength of 70,000psi.
• A “1” in this position indicates the electrode is suitable for all welding positions. A “0” means the electrode should only be used in the flat and horizontal position.
• The “T” denotes the electrode’s tubular construction.
• The space following the dash may have any number between 1 and 14. This number indicates the electrode’s usability. Usability is a reference to several characteristics: the required polarity, if it is self shielded or gas shielded, and if it is suitable for single or multiple-pass welds.
• An “M” in this position indicates a mixed gas blend should be used. A “C” means 100% CO2 shielding gas.
• A “J” in this position means the electrode meets the requirements of improved impact toughness testing. This suffix is optional and may not be present.
• The next three characters, “HXX” are used for low hydrogen electrodes. H4, H8, or H16 indicate the electrode has less than 4, 8, or 16 milliliters per 100 grams of deposited weld metal. This suffix is also optional and will only appear if the electrode has low hydrogen requirements.

Carbon and low alloy steels are the metals most commonly joined with FCAW. However, FCAW can be used to weld several other metals, including stainless steels, cast irons, and nickel alloys. FCAW is also used in various surfacing applications, such as hardfacing. The classification for these electrodes have some similarity to the one discussed above, but each is unique and has specific suffixes and prefixes. If welding these metals, the welder must be sure to understand the electrode classification system properly to select the proper electrode. The AWS has created filler metal specifications for each material and welding process.

FCAW Electrodes

FCAW electrodes have a tubular construction. They consist of a metal sheath filled with a powdered flux. The metal sheath will typically be chemically similar to the base metal. The flux consists of various powdered metals and other elements and serves a variety of purposes:

• Shielding: Some flux elements will vaporize into a shielding gas in the heat of the welding arc
• Cleaning: Oxygen and Nitrogen are common impurities present in base metal. Elements like aluminum, silicon, and manganese present in the flux scavenge these impurities out of the weld.
• Slag formation: Some elements in the flux will form a slag that floats to the top of the weld and solidifies. This slag acts as a blanket which significantly slows the cooling rate of the weld, improving the finished mechanical properties.
• Arc Stabilization: The flux provides easier arc initiation and stabilizes the welding arc. This improves the transfer of metal droplets from the electrode to the weld pool, reducing spatter and increasing electrode efficiency.
• Alloying: Some elements in the flux will alloy themselves into the finished weld metal. This allows electrodes to be tailored to a variety of base metals and meet a wide range of mechanical and chemical requirements.

Some FCAW electrodes are suitable for single-pass welds only. These types of electrodes are designed for welding on extremely dirty or contaminated metal. They are very high in the deoxidizing and denitrifying elements mentioned above to accomplish this. Using these in multi-pass welds can result in a build-up of these elements at the intersection of weld beads when they are stacked up. This build-up is extremely detrimental to the mechanical properties of the weld.

If an electrode is classified as low hydrogen (indicated by the presence of the HXX suffix in the classification), there are special packaging and storage considerations. Low hydrogen electrodes are manufactured and tested to ensure strict limits on the hydrogen content of the finished weld metal. This is critical for steels that are susceptible to hydrogen-assisted cracking (abbreviated HAC, sometimes called underbead cracking). The electrode can be contaminated by hydrogen from ambient moisture in the air. For this reason, low hydrogen electrodes are often packaged in vacuum-sealed bags. In many welding applications there will also be limits on how long the electrode can be out of its packaging and exposed to the atmosphere, usually between 24 to 72 hours. When not in use, the electrode should be stored in a non-permeable container like a plastic bag. The electrode may also be stored in an electrode storage oven, these ovens are hot enough to drive away any humidity. In some cases, it is also acceptable to use an electrode oven to cook the moisture back out of an electrode that has been exposed to open air for too long.

Electrode diameter is of critical importance. FCAW electrodes are commonly available as small as 0.035” in diameter all the way up to ⅛” (0.125”). A good rule of thumb is that as base metal thickness increases, so should electrode diameter. This rule does not apply at a 1 to 1 ratio; a given diameter of electrode will be able to weld a range of metal thicknesses. But as metal thickness increases, so will the welding current requirements, this is necessary to achieve good penetration; larger diameter electrodes can operate better at these higher currents than smaller electrodes.

Another consideration closely tied to electrode diameter is welding position. Larger electrodes, with their higher current requirements, create a large and more liquid weld puddle. This makes larger electrodes more difficult to control generally and unsuitable for vertical or overhead welding, even if the electrode is classified for all-position welding. In this author’s experience, electrodes larger than 1/16” in diameter are unsuitable outside the flat or horizontal position. The primary advantage of larger electrodes is an increased deposition rate, that is, far more weld metal can be deposited in a given time than with smaller electrodes.

electrical stickout is also a critical factor in the operation of FCAW electrodes. Electrical stickout, sometimes called electrode extension or abbreviated as CTWD (Contact Tip to Work Distance), is (just that) the distance the electrode extends past the contact tip before it arcs to the work, where it generates the heat for welding. Earlier in the chapter, we discussed the fairly flat volt-amp curve that constant voltage welding power sources operate on.

The line of the volt-amp curve represents the possible outputs of the welding power source, at a given voltage and amperage setting. Where the output of the machine is on that line is directly related to electrical stickout. As electrical stickout increases, welding voltage goes up slightly while welding amperage decreases drastically. The inverse happens when electrical stickout is decreased. Because electrical stickout so dramatically affects welding amperage it must be carefully controlled by you, the welder.

Electrical stickout will be recommended by the electrode manufacturer. For FCAW-G (gas shielded) electrodes, this range usually falls between ½ to 1 ¼ inch, a relatively narrow range. While FCAW-S (self shielded) electrodes may have a range from ¾ inch to as long as 3 ½ inches, much larger. This discrepancy is the largest difference between gas and self shielded electrodes, besides the need for shielding gas. The reason for this is that some self shielded electrodes have flux which must be preheated to work properly once they combust in the heat of the welding arc. Once the welding circuit leaves the copper contact tip and enters the wire electrode, it experiences much more electrical resistance. This electrical resistance heats the electrode, and the resistance and heat increase with electrical stickout, accomplishing this preheating effect.

Shielding gasses

FCAW electrodes do not always require a supply of shielding gas, self-shielded electrodes produce their shielding from the combustion of powdered flux inside the electrode. When using gas shielded electrodes, however, the shielding gas selected can affect the characteristics of the arc and the finished weld. Regardless of the gas selected, the flow rate should be set between 30 and 45 CFH (cubic feet per hour)

Pure carbon dioxide, CO2, is probably the most widely employed shielding gasses for FCAW-G. CO2 promotes a globular transfer of metal from the electrode to the weld pool, producing an erratic arc with large amounts of weld spatter. It also produces a deep penetration profile. CO2 is a very efficient gas coolant, as it flows through the welding gun, it effectively cools the gun. Despite producing a harsh welding arc, CO2 is widely used because it is the least expensive shielding gas available. It should be noted that CO2 has an endothermic effect when it is depressurized; as it leaves the high-pressure gas cylinder it can ice up and freeze gas flow regulators. At flow rates above 25 CFH a heat source, such as a heat lamp or heater-equipped gas regulator, should be used, especially in winter conditions.

A gas blend of argon and CO2 is the other shielding gas commonly employed in FCAW-G. This blend is made up of 75-80% argon with the remainder being CO2. Argon has a higher ionization potential than CO2. Ionization potential is a reference to how much energy is required to positively charge the shielding gas which allows it to conduct the welding arc. The introduction of argon into the gas mix results in a much smoother arc with less weld spatter. This reduction in spatter increases electrode efficiency and operator appeal. An Argon/CO2 blend produces a flatter, typically better looking, weld bead but shallower penetration than pure CO2.

These two shielding gasses are used in almost all FCAW-G applications. An exception to this is a blend of 98% argon and 2% oxygen, which is used when welding some stainless steels. The shielding gas for any particular electrode will be recommended by the electrode manufacturer, and indicated by the M or C suffix in the electrode classification.

Attributions

1. Figure 9.14: SMAW Electrode Classification © American Welding Society, illustration by Nicholas Malara (SBCTC Illustrator) Used with permission from the rightsholder, the American Welding Society.