Various low-hydrogen stick electrodes are used as filler metal for the shielded metal arc welding (SMAW) process. Low-hydrogen is the most popular type of stick electrode used in North America. Whether a seasoned welder or new to the trade, it’s good to understand how low-hydrogen electrodes work and why they are used.
Low-hydrogen stick electrodes have smooth arc characteristics, easy slag removal, good bead shape, higher deposition rates and most can be used in all welding positions. They are used in a variety of industries involved in welding. Bridge and building construction, offshore drilling, pressure vessels, pipelines and power generation are all good examples.
Any welding application involving sensitive base materials, such as higher strength steels, restrained joints or those subject to more stringent welding codes, will likely involve the use of low-hydrogen filler metal.
Low-hydrogen electrodes must be dry to perform properly. Moisture pickup can degrade weld quality in several ways. Excessive moisture can cause weld porosity, which may be visible porosity on the weld face or it may only be subsurface and require some type of non-destructive or destructive testing to see. High moisture in the electrode coating can also lead to excessive slag fluidity, a rough weld surface and difficult slag removal. Finally, excessive moisture in low-hydrogen electrodes leads to elevated levels of diffusible hydrogen, which, in turn, can lead to hydrogen-induced weld cracking and under-bead cracking issues.
Hydrogen is naturally absorbed into liquid metal (i.e., the molten weld pool) and it naturally comes out of solid metal (i.e., the solidified weld metal). Steel above its melting temperature naturally absorbs hydrogen. As the steel cools below its melting temperature and resolidifies (freezes), the hydrogen that is trapped inside the weld metal then wants to migrate or diffuse out, and over time, it does. However, when the initial diffusible hydrogen levels inside the weld metal are elevated, there is more potential for hydrogen-induced cracking, which typically occurs in the heat-affected zone (HAZ).
HAZ cracking is not a result of elevated levels of diffusible hydrogen only. In addition, the base material must have a more sensitive microstructure, such as high-carbon and high-strength, low-alloy steels, which are more susceptible to cracking due to their higher carbon and alloy content. Also, higher internal weld stresses must be present, such as with thicker steel sections or constrained plates. For example, the plates on a large ship are constrained and cannot move, creating stress.
Finding the source
Where does the hydrogen come from during welding? One source is from contaminants on the base material. Paint, oil, primer, mill scale and rust on the surface of the steel are all potential sources of hydrocarbons. They can produce hydrogen from chemical reactions as they are consumed in the heat of the welding arc and, thus, introduce hydrogen into the weld metal. Therefore, the material needs to be as clean as possible.
A second source of hydrogen is from the surrounding atmosphere – from the moisture in the air. Welding in very humid conditions could potentially introduce more hydrogen into the weld metal.
The third source of hydrogen is from moisture condensation on the electrode coating. Stick electrodes have a conductive steel rod at the core that melts and becomes the filler metal. This core rod is covered with a flux coating that also melts, protecting the arc and forming a protective slag covering over the weld. If moisture is allowed to condensate on this outer flux, it becomes bound to the porous flux, introducing hydrogen into the weld metal.
It should be noted that maintaining a certain diffusible hydrogen rating is only a concern with electrodes that have a flux, such as stick electrodes, submerged arc fluxes and flux-cored wires (condensation on the inner flux occurs via moisture through the wire’s seam).
Hydrogen, however, is not a problem with steel TIG rods, MIG wires or submerged arc wires because they do not have a flux. There can still be moisture on the filler metal from condensation, but it does not become bound to the steel or the copper coating on the steel electrode. It simply evaporates as it is heated in the arc. Therefore, all TIG rods, MIG wires and submerged arc wires are considered low-hydrogen electrodes, which some may not realize.
Low-hydrogen stick electrodes are conditioned in the manufacturing process to minimize the level of hydrogen in their coatings, thus reducing the opportunity for diffusible hydrogen to be deposited into the weld metal. A low-hydrogen stick electrode is identified by the coating type designator in its classification number. It may also have an optional maximum diffusible hydrogen designator after its classification number, as defined by the American Welding Society (AWS).
The “HX” designators indicate the maximum milliliters of diffusible hydrogen per 100 grams of weld metal deposited. H4, H8 and H16 are the typical designators. For stick electrodes, an H16 or lower designator is considered low hydrogen, but technology has progressed to the point that most low-hydrogen stick electrodes now carry an H8 or lower designator.
How to handle
Even though low-hydrogen stick electrodes start off from the factory with a coating low in hydrogen, they can quickly pick up additional hydrogen from condensation if not stored and handled properly. That is why they usually come in a hermetically sealed or air-tight container where they can be kept indefinitely.
Once the container is opened, standard low-hydrogen electrodes should only be exposed to open air for up to four hours. After that, the electrodes need to be stored in an air-tight, temperature-controlled container and held at an elevated temperature, per the filler metal manufacturer’s recommendations. This prevents condensation on the coating.
Lincoln Electric recommends for its low-hydrogen products that open cans and loose electrodes be stored in an air-tight container at 250° to 300° F (120° to 150° C). The most common containers for low-hydrogen electrodes are called rod ovens.
Some low-hydrogen stick electrodes are manufactured with moisture-resistant coatings and identified by the addition of an “R” to their AWS maximum diffusible hydrogen designator (e.g., E7018-H4R). While exposure time to open air for low-hydrogen electrodes is limited to approximately four hours, R-designated electrodes can potentially be exposed up to nine hours. And that is significant because they can be left out of the rod oven for an entire work shift. However, some code requirements may specify exposure limits different than these guidelines.
Low-hydrogen electrodes that are not sealed or stored correctly and have exceeded their open-air exposure limits can be reconditioned (redried) before use. This is done by raising the temperature in the rod oven to a specific level and drying the electrodes for one hour at that temperature. They should be spread out in the oven so each one can reach the drying temperature. The filler metal manufacturer provides specific reconditioning temperature recommendations (see Table 1).
Low-hydrogen electrodes should not be redried at temperatures higher than recommended, nor for several hours at temperatures lower than recommended. Also, non-low-hydrogen electrodes, such as cellulosic and rutile electrodes, should not be stored or redried at the same temperatures as low-hydrogen electrodes.
Low-alloy low-hydrogen electrodes should not be reconditioned more than three times. Any low-hydrogen electrode, whether carbon steel or low-alloy steel, should be thrown away if excessive redrying causes the coating to become fragile and flake or break off while welding or if there is a noticeable difference in arc performance.
Tips on techniques
In addition to making a weld deposit with a minimal amount of diffusible hydrogen, success with low-hydrogen stick electrodes also comes down to using the proper technique. This includes using a particular type of low-hydrogen electrode in its intended welding position, welding at the appropriate current level for a given type and diameter of electrode and using the correct travel speed. It also includes using the proper electrode angle, arc length and so on.
Recommended current settings, by diameter and polarity, can generally be found in the filler metal manufacturer’s product literature. Current is measured in amperage, or amps. As a starting point, welders should pick a current setting in the middle of the range.
Another rule of thumb for current settings with medium-coated electrodes, such as an E7018, is to multiply every one thousand of an inch of the electrode diameter by one amp. For example, a 1/8-in. electrode in decimal form to the thousands digit is 0.125 in. So the machine should be set around 0.125 x 1,000 or 125 amps. Conversely, a 5/32-in. electrode should be set around 0.156 x 1,000 or 156 amps. If the current is too low for a given diameter, the arc can be difficult to start and maintain. In addition, the weld bead will be humpy. If the current is too high, it can prematurely destroy the coating and cause defects in the weld (see Table 2).
When making stringer beads, stick electrodes generally have an optimal travel speed, which provides the best puddle control and bead shape. The weld size produced becomes larger as the electrode diameter increases (see Table 3).
With low-hydrogen electrodes, welders should always use a drag travel angle, leaving the slag behind the puddle, and maintain a short arc length. They should always keep the electrode close to the puddle and not “long arc” the electrode, as this can result in arc instability and weld porosity.
Note that when welding out of position with cellulosic stick electrodes (such as E6010), welders often whip the electrode out of the puddle and then bring it back. This whipping, long-arcing technique allows the lightly slag covered puddle to freeze before depositing more weld metal. However, this whip technique should not be used with low-hydrogen electrodes.
Typically, welders should make stringer beads or use a straight progression. For larger welds, several small, multiple-pass stringer beads generally provide better mechanical properties, particularly notch toughness, compared to fewer passes of large, wide beads. However, some situations may require welders to manipulate the puddle by using some type of weave technique, taking care not to weave more than 3/4-in. wide.
According to an article written by Joseph Kolasa, Lincoln Electric Welding School instructor and Joseph Murlin, SMAW consumable product manager, the first thing welders should do when starting a weld is to hold a very short arc length. Longer arc lengths greatly increase the possibility of getting arc-start porosity.
Also, the hot start control should not be set too high (on welding machines with this feature). It can generate a long arc length and prematurely melt the electrode coating, creating insufficient shielding and, ultimately, porosity in the weld.
A common problem with arc restarts when welding vertical up is “finger nailing” of the coating (i.e., when one side of the coating burns back farther than the other side, potentially causing porosity issues). This occurs when welders use too much of an upward angle. Many welders restart too high in the weld joint and then drag down to the crater. They can avoid these scenarios when restarting a stick electrode by starting about 1/4 in. to 1/2 in. above the previous weld. They should point the electrode directly into the joint, using no more than a 5° or 10° push angle.
Restarting a partially consumed low-hydrogen electrode after the tip has cooled can be difficult. A ball of slag naturally forms on the end of the electrode. This hard, brittle slag acts as an insulator, making it difficult to establish the arc. Most welders want to put the electrode in an electrode holder and bang it on the plate like a hammer. This can chip the electrode coating. Instead, they should take the electrode out of the holder and roughly rub the tip on the surface of the welding table until they get down to the steel core. This allows for a good electrical connection for arc starting without damaging the coating.
Many welders choose to use low-hydrogen stick electrodes because of their smooth arc characteristics, easy slag removal, good bead shape and higher deposition rates. They’re also chosen because they have all-position welding capability. However, having a better understanding of why, where and how to use low-hydrogen electrodes, as well as how to store and handle them, can also make welders even smarter. As the use of low-hydrogen electrodes continues to grow, this knowledge and skill set will make any welder even more valuable to the industry.
Types of Low-hydrogen electrodes
A common AWS low-hydrogen classification number is E7018-H4R. The specific classification number for a particular electrode tells a story about it. The key to these numbers is illustrated in Table 4.
The first two or three digits in the number indicate the minimum tensile strength of the weld metal. A carbon steel (mild steel) low-hydrogen electrode has a minimum tensile strength rating of 70 ksi, while most low-alloy low-hydrogen electrodes have a minimum tensile strength rating between 80 and 120 ksi. With the exception of a few 80-ksi cellulosic electrodes, all low-alloy electrodes are also low-hydrogen electrodes.
The second to last digit in the number indicates the recommended welding position. A “1” means all-position (flat, horizontal, vertical and overhead), a “2” means in-position (flat and horizontal only) and a “4” means vertical down only. Electrodes that can weld out of position or against gravity have a fast freezing slag system, come in smaller diameters and have lower deposition-rate capabilities. Whereas, electrodes that can only be used in position or with gravity have a slower freezing slag system, come in larger diameters and have higher deposition rate capabilities.
The last digit in the number indicates the type of electrode coating and recommended weld polarity. Electrodes ending in “5” (sodium-based), “6” (potassium-based) and “8” (potassium-based with addition of iron powder) are low-hydrogen electrodes. All of them are also considered to have a “basic” coating – compared to cellulosic (organic), rutile (titania) or iron oxide-based coatings.
Of the three types, the “8” coating is by far the most popular. An in-position only “28” type electrode has 50 percent iron powder added to its coating for maximum deposition rates and would be considered a heavy-coated electrode. The addition of iron powder increases the deposition rate capability of these electrodes, as the iron powder in the coating melts and becomes part of the weld metal, along with the core rod.
An all-position “18” type electrode has 30 percent iron powder added to its coating and would be considered a medium-coated electrode. While it still has a fast freezing slag system, the additional 30 percent iron powder in the coating gives the electrode maximum deposition rate capabilities when welding out of position. In addition, because an “18” electrode has a heavy slag system, it is not recommended for vertical down progression, as the slag may run ahead of the puddle and get trapped underneath. In the vertical position, an “18” electrode should only be used with vertical up progression.
Table 5 includes examples of various types of low-hydrogen electrodes with their specific AWS classification number and description and typical applications.