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Jun 04, 2023

Submerged arc welding tech tips and fundamentals

The submerged arc welding (SAW) process has been around for over 75 years, yet fundamental practices of SAW still are not well-understood in industry today. With most welding manufacturers offering

The submerged arc welding (SAW) process has been around for over 75 years, yet fundamental practices of SAW still are not well-understood in industry today. With most welding manufacturers offering high-speed inverters with advanced control over the waveform, there can be a temptation to ignore the fundamentals of SAW when either troubleshooting or increasing welding efficiency. Following are some of the most common SAW process tips and tricks that are succeeding in industry.

Using Non-optimal Electrode Diameter. When it comes to electrode diameter, bigger is not always better. Two different sized electrodes can both carry the same current, but they will behave differently in two specific ways that have an effect on the welding process.

The first is current density. Current density is the determining factor when considering melt-off efficiency. For example, at 600 amps, a larger 3/16-in.-diameter electrode is not running at its optimal current density. That means the melt-off rate is lower than with a smaller-diameter electrode carrying the same current.

The second is how current density affects the penetration profile. For a given current, a smaller electrode can produce a deeper penetration profile. This can be a disadvantage on thin materials, where the larger diameters actually reduce burn-through tendencies.

Incorrect Setup of Wire Straightener. Often operators do not set up the wire straightener to ensure that the wire is leaving the contact tip straight enough to prevent it from “wandering” during welding. Unlike an open arc welding process such as gas metal arc welding (GMAW), with SAW it is difficult for the operator to see if the electrode is tracking properly and not wandering off the desired placement in the joint.

This will be apparent as an inconsistent penetration profile on cut and etch. This is especially critical with metal-cored SAW electrodes. In some cases, a dual-plane wire straightener is required.

Inconsistent Contact-tip-to-work Distance. Contact-tip-to-work distance (CTWD) is the distance from the contact tip to the workpiece. CTWD is another variable that is obscured from the operator’s sight by joint design or a layer of flux, and may seem like the least significant variable, but this could not be further from the truth.

As CTWD changes, the resistive heating will alter the current required for melt-off of the electrode. In a constant-current mode, the wire feed speed will increase with the increase of CTWD. In constant-voltage mode, current will decrease with increase of CTWD. It’s important that this variable is correct when welding.

Often operators determine CTWD from where the flux nozzle is, but this is incorrect; it should be measured from the contact tip. In addition, operators who want to change the amount of flux coverage while welding often will raise or lower the wire feeder. In doing so, they inadvertently change the current or wire feed speed (depending on mode). For this reason, it is prudent to move the flux nozzle independent of the head, and as has been shown, keeping a consistent CTWD is key.

Incorrect Flux. Choosing a welding flux and wire combination is more complicated than selecting a GMAW or flux-cored arc welding (FCAW) wire. Different combinations can yield widely different weld deposits.

This is an illustration of a cross section of a typical submerged arc weld.

A few of the most overlooked and important questions to ask are:

- Will the weld be done with multiple passes or a single/limited pass? It is not recommended to use active fluxes (which contribute silicon and manganese) for multiple-pass welding as the increased level of Mn with subsequent passes can result in excessive hardness, excessive strength, and generally poor Charpy V-notch (CVN) toughness.

- What is the CVN requirement? Different flux/wire combinations will yield different welding properties. This requirement can vary from the typical -20 degrees F, - 40 degrees F and so forth, and must be accounted for when selecting welding flux.

- Is post-weld heat treating (PWHT) condition a requirement? This will make a significant difference in choosing the optimal flux/wire combination since typically the ultimate yield strength (UYS) and ultimate tensile strength (UTS) can fall below what is required for the classification after stress relief (SR).

- What exactly are you doing? What is the surface condition and the deoxidizer level required? For example, Lincoln Electric® manufactures at least six different fluxes that, when paired with an EM12K electrode, meet the requirement of F7A2-EM12K. However, they are all optimized for specific characteristics. It is best to consult a welding engineer, rather than just the AWS/CSA classification of the flux/wire combination.

Poor Storage and Handling of Flux. Lincoln Electric fluxes are low hydrogen H-8 or better. While no shop would store and expose an E7018 low-hydrogen stick electrode in an open shop environment for days on end, often the same shops will not have tight control in their standard operating procedure of the storage of SAW fluxes. It is best to follow the manufacturer’s recommendations.

Poor Recovery of Unfused Flux. It is acceptable to recover unfused flux, but care must be taken to ensure that contaminants are not introduced, such as grinding dust and bits of the whisk broom that is used to sweep up flux. It is best to use a vacuum flux recovery system.

As flux is recovered and reused multiple times, the particulate sizes slowly get finer. It is prudent to ensure consistent flux particulate size by mixing ideally 50 percent virgin flux in with the recovered flux. The best way to ensure proper mixing is to use flux hoppers that are capable of ratio-controlled mixing.

It is prudent to also use a magnetic separator and properly sized sifter to ensure that no metallic particles or mill scale is introduced into the recovered flux. These contaminants can cause defects such as porosity.

Porosity. Porosity is an easily identified issue in open arc welding, but it happens in SAW from time to time as well. If you have porosity, the first thing to check is moisture contamination caused by improper storage of the SAW flux.

This image shows the relationship between wire diameter and bead size. Welding DC+, 650 amps, 32 volts, with a travel speed of 24 IPM. Note the different penetration profiles for the three electrode diameters.

If that isn’t the cause, check for contaminants. Porosity often is caused by excessive off-gassing due to contaminants. Some examples of these gas forming contaminants are rust, paint, oil, mill scale, and sulphur. This is especially noticeable on the second side of a two-sided fillet weld since there is nowhere for the gas to escape but through the weld metal deposited on the other side. For example, it is critical that the mating surface of a butt or fillet weld is clean before fitting plates. If an operator is grinding tacks, he needs to ensure that grinding dust is not being accidentally introduced into the joint. Another potential cause, coming back to the recycled flux discussion, is when a recirculation flux recovery system is improperly used and the particle size degrades. Excessive fines can cause porosity issues.

Often increasing current and/or slowing weld travel speed can remedy the situation, since now there is more time for the puddle to be in a fluid state to ensure that the off-gassing makes it through the slagging system and past the weld face.

A less common cause of porosity is arc blow (nitrogen) porosity.

Insufficient flux coverage also can cause nitrogen porosity, but it should be pretty obvious to the operator since there would be an increase in arc flashing through flux/slag.

Although it is not recommended to weld over primer, weldable types such as zinc based preconstruction primer will require extra care in flux selection and welding procedure development. More active fluxes, which contribute more Si and Mn, should be selected.

Lack of Fusion. Often this is not caused by insufficient current to achieve desired penetration, but rather excessive weld metal for the travel speed or joint configuration. A too slow travel speed can cause the weld metal to roll forward past the arc, cushioning arc force into the base material. Care must be taken when welding in a V-groove, especially at the bottom where it is easy to have excessive weld metal for the joint cross section. This is obviously hard to notice in the SAW process.

In welding small-diameter roundabouts, this can be caused with insufficient offset of the head from top dead centre (TDC). Insufficient offset also can cause weld metal to roll forward out of the arc because of gravity. Conversely, excessive offset can cause weld metal to roll back into the arc. If an operator is experiencing molten flux spillage, it’s a good indication that the offset is not correct.

Slag Inclusions. Slag inclusions can show up in non-destructive testing and often are caused by insufficient travel speed or excessive current/WFS where weld metal can surge forward over the molten slag. As in the previous lack of fusion discussion, an operator can see this as well with improper offset of head from TDC. Both cases can result in slag entrapment. The operator can easily see this because the bead profile will be razor backed on centreline (insufficient offset) or concave (excessive offset).

Last, insufficient cleaning of slag from previous passes also can result in a slag inclusion.

Centreline Cracks. A centreline crack is a crack in the centreline of the weld bead, but not necessarily in the geometric centre of the joint of a multipass weld.

The effects of electrode position on circumferential welding are shown.

Possible causes include the following:

- Segregation crack – Low melting point constituents in the weld metal such as sulphur/copper/zinc/phosphorous or lead congregate toward the centreline since this is the last place the weld metal freezes. This hot cracking susceptibility can be predicted by the formula below. C , S , P, and Nb are hot cracking contributors, whereas Si and Mn are hot cracking resistors because they are deoxidizers.

UCS = 230 C + 190 S + 75 P + 45 Nb – 12.3 Si – 5.4 Mn – 1

- Width-to depth ratio – Narrow, deep beads are not ideal. The crack shown in the illustration here is caused by an excessive gap.

Ideally, the width-to-depth ratio of a weld should be 1.1-to-1.4. A penetration profile that is deeper than wide is not desirable.

- Bead shape and surface profile – Hat-shaped beads are also undesirable. They are the result of excessively slow travel speed.

A concave surface profile is not desirable because the surface of the weld metal is in tension.

Cracking in the Heat-affected Zone (HAZ): Often referred to as delayed cracking, cold cracking, or hydrogen-assisted cracking, these cracks generally are caused by excessively high residual hydrogen or sensitive material with a high carbon equivalent.

A common solution to HAZ cracking is to preheat the weldment to slow down the cooling rate, which allows more time for hydrogen to diffuse out. When required preheat is calculated, the key factor other than thickness is the carbon equivalent of the base material.

An operator also can ensure the hydrogen content in the flux is kept low via flux heaters and proper storage and the use of hydrogen scavenging fluxes.

This narrow, deep bead caused a centreline crack.

Transverse Crack. Not as common as other modes of cracking, transverse cracks typically are seen on high-strength, 110-KSI UTS material or greater. Transverse cracks also can be caused by high residual stress.

Even though transverse cracks typically occur because of excessive hydrogen levels in the HAZ, in some situations excessive hydrogen content can cause excessive hardness of the weld bead. In this less common scenario, the cracks resemble transverse cracks. In either case, a surface hardness test would be beneficial.

SAW can be complicated. But if operators stick to the fundamentals, many problems can be sorted out, and the benefits of SAW can be realized.

Ken Mui, P.Eng. is an application engineer, for Lincoln Electric, www.lincolnelectric.com. He can be reached at [email protected].