Submerged arc welding 121: how it works, fluxes and where it is used
Number 121 in ISO 4063 is submerged arc welding with a solid wire, SAW for short. The arc burns under a layer of granular flux, a head feeds and guides the wire, and the welder becomes the operator of a machine. That is why 121 turns up on long welds and thick plate, not on gates and railings.
Short answer
121 is submerged arc welding with a solid wire: a bare wire melts in an arc buried under flux, and the flux, once molten, turns into slag that shields the pool. The process is mechanised, gives deep penetration and high productivity, but suits almost only the PA and PB positions and straight or circular welds. It is run by an operator qualified to ISO 14732, not by a welder holding an ISO 9606-1 certificate.
How the process runs
Wire from a reel passes through drive rolls into the head and the contact tip. Flux pours from a hopper just ahead of the arc or around it. Part of it melts into liquid slag that covers the pool, keeps the air out, shapes the weld face and slows cooling. The rest stays loose and can be vacuumed back, sieved and reused. Once cold, the slag crust lifts off by itself or is chipped away.
With the arc covered there is no flash, almost no spatter and no shielding gas for the wind to blow away. The current is high and the current path in the wire short, so penetration is deep and the bead wide. The power source runs on direct or alternating current, and the head travels on a rail or carriage, or stays put while the workpiece is rotated.
121, 122 and 125: the sub-numbers
The whole of group 12 in ISO 4063 is submerged arc welding. The number depends on the electrode:
| No. | Process | Abbreviation |
|---|---|---|
| 12 | Submerged arc welding | — |
| 121 | Submerged arc welding with solid wire electrode | SAW |
| 122 | Submerged arc welding with strip electrode | SAW |
| 125 | Submerged arc welding with tubular-cored electrode | SAW |
Solid wire, 121, is the most common. Strip (122) gives a wide, shallow bead and is used, among other things, for cladding; a tubular-cored wire (125) changes the weld metal composition through its core. ISO 4063 has further numbers in group 12 for variants with several wires or added powder; the full list is in the ISO 4063 table.
Where it is used
The process pays off where the weld is long, straight or circular, and the metal thick: beams fabricated from plate (the fillet welds between flange and web), longitudinal seams of large-diameter pipe, tanks and boilers rotated on rollers, tower sections, ship structures. On short, winding welds and on site, the time to set up the machine eats the whole gain.
The flux stays on the joint by gravity, so the range of positions is narrow: PA for butt welds and PB for fillets. PC needs special measures, and vertical and overhead are out. The other positions are covered on the page welding positions; usually the part is turned so that the weld lies in PA.
How it differs from 135 and 136
| Feature | 121 | 135 / 136 |
|---|---|---|
| Shielding | flux and slag | gas (136 adds slag) |
| Arc | hidden | visible |
| Guidance | head, operator | torch in the hand |
| Positions | PA, PB | all |
| Penetration and productivity | deep, high on a long weld | lower, but more flexible |
| Wind | hardly a problem | blows the shielding gas away |
| Extras | flux to dry and recycle | cylinder and regulator |
The three gas-shielded numbers are compared on the page 135 vs 136 vs 138; flux-cored wire has its own page, flux-cored arc welding 136. The semi-automatic basics are on MIG/MAG welding.
Fluxes: fused and agglomerated
Flux is not an extra for the wire but half of the pair. ISO 14174 classifies fluxes by manufacturing method and chemical type. Fused fluxes are glassy, brittle grains that absorb little moisture and keep a stable composition. Agglomerated fluxes are powder bonded into grains; they can carry alloying additions and deoxidisers into the weld metal, but they absorb moisture and have to be dried. Mixed types exist as well. The composition decides how the bead looks and how tough the weld metal is under impact.
Damp flux means porosity in the weld and hydrogen, which promotes cold cracking. Drying temperature and time are given in the maker’s data sheet; we do not quote a single figure here, because it differs from flux to flux. The rule is to keep flux in sealed bags, use or re-dry opened ones, and sieve recycled flux free of dust and slag.
Wires and designations
For non-alloy and fine-grain steels, the wire and flux pair is described by ISO 14171.
An example of how the designation is built (not a recommendation):
S 46 4 AB S2Si. S is the SAW process, 46 the strength group of the weld metal,
4 the impact test temperature, AB the flux type (aluminate-basic to ISO 14174) and S2Si the
wire. Flux and wire are chosen as a pair and written in the WPS together: a change of flux
brand is a change of procedure.
Typical defects
- Porosity: damp flux, rust and oil on the edges, too thin a flux layer; see porosity in a weld.
- Lack of fusion and penetration: wrong travel speed, wire position relative to the groove; see lack of fusion.
- Slag inclusions: in multi-run welds when the groove is narrow and slag stays in the corner between runs.
- Hot centreline cracks: when the bead is deep and narrow; see weld cracks.
- Undercut: travel speed or current too high.
Acceptable sizes are set by ISO 5817, and the names and numbers of imperfections are collected in the ISO 6520-1 table.
Safety compared with an open arc
A covered arc means less radiation, spatter and fume than with MMA or MAG, but not none. What remains: fume from the flux and wire (ventilation is still needed; see welding fume), hot slag and flux, high open-circuit voltage and heavy currents, moving heads and rotators, heavy parts on the crane. Flux grains on the floor roll like ball bearings. Eye protection and protective clothing are still required for any task where the arc may show; see personal protective equipment.
Qualifying the operator and the procedure
A welder’s ISO 9606-1 certificate does not extend to the machine. The operator who sets up and monitors mechanised or automatic welding is qualified to ISO 14732; the standard allows more than one route, including recognition through the procedure test. The 121 procedure goes into documents like any other: a preliminary pWPS, its tests to ISO 15614-1 and the WPQR record, and on the shop floor a WPS with the process number, the wire and flux pair and the position. The manual test is described on the page welder qualification test EN ISO 9606.
Frequently asked questions
What does the 121 welding process mean?
Under ISO 4063, 121 is submerged arc welding with a solid wire electrode, SAW for short (UP in Germany). The arc burns between a bare wire and the workpiece under a blanket of granular flux, so it cannot be seen, and liquid slag seals the weld pool from the air. It is a mechanised or automatic process used for long welds and thick plate.
In which positions can submerged arc welding be done?
Practically only flat (PA) and, for fillet welds, horizontal-vertical (PB). The flux rests on the joint under its own weight and would simply fall off in the vertical or overhead position. PC is possible only with special flux retention, and the usual answer is to rotate the part on a positioner so that the weld lies flat.
Is an ISO 9606-1 certificate enough for submerged arc welding?
No. ISO 9606-1 certifies a welder working by hand or with a semi-automatic gun. A person who sets up and runs a mechanised welding head is a welding operator, qualified to ISO 14732. The procedure itself is qualified separately, most often to ISO 15614-1.
Do you need a helmet for submerged arc welding?
The arc is buried under flux, so there is far less radiation and spatter than with an open arc. Eye protection is still needed when setting up the head, at the start of the weld and wherever the flux layer is thin and the arc breaks through. Hot slag and flux also call for gloves and protective clothing.
How does 121 differ from 135?
In shielding and mechanisation. In 135 gas shields the pool and the welder guides the torch by hand in any position. In 121 flux and slag shield it, a head on a carriage or boom moves the wire, and welding is mainly in PA and PB. In return 121 gives deep penetration and high productivity on long welds.
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