A corrosion-resistant overlay can be deposited with several welding processes, and the choice is usually made early: when the fabricator quotes the job, or when the procedure is written. After that the process is fixed for the qualification, the tooling and the schedule. It is also the choice that sets how hard dilution will be to control on every layer that follows.
This article is about that choice. It compares the four process families used for cladding (GMAW, hot-wire GTAW, strip cladding and laser cladding) by the questions that actually decide it: part geometry, area, the chemistry required at depth, heat sensitivity and qualification. It is written for welding and quality engineers planning an overlay job. What weld overlay is, and the codes that qualify it, are covered in the weld overlay and cladding glossary entry. How to monitor dilution and bead overlap once deposition starts is covered in overlay cladding: dilution, bead overlap and setting a monitoring alert rule.
Key takeaways
- Start from the part: geometry and area to clad rule out some processes before dilution is even considered.
- The requirement is chemistry at a stated depth, not a dilution percentage. The process choice sets how many layers it takes to get there.
- Wide, shallow or small melt pools tend to dilute less than a concentrated arc. The qualification coupon, not the process name, proves the result.
- Each process has its own drift modes during deposition, so what is worth monitoring changes with the process.
- Figures for dilution and deposition rate belong to the qualified procedure for the alloy pair. This article gives criteria, not numbers.
Start from the part, not the process
Four questions narrow the choice quickly:
- Where is the surface? Outside of a shell, inside a large vessel, inside a small bore, a nozzle, a flange face, a valve seat. Access for the torch, the strip head or the laser optics comes first.
- How much area, and in what runs? Long uninterrupted runs on a large surface favour wide beads. Many short runs, edges and local repairs favour a smaller, more controllable pool.
- What must the finished surface contain, and at what depth? The specification states the chemistry at a depth below the finished surface. The glossary entry explains why this, and not dilution on its own, is the criterion.
- How sensitive is the base metal? Thin walls, parts prone to distortion and base metals with heat-input limits restrict how much energy each pass can deliver.
Write the answers down before comparing processes. They decide more than any process datasheet does.
GMAW overlay
GMAW, often in pulsed or controlled short-circuit modes, is the overlay process closest to everyday fabrication. It works in position, reaches medium-sized bores with the right torch, and runs on the kind of equipment many fabricators already own.
Where it fits: general-purpose overlay on shells, components and repairs, especially where a dedicated cladding cell is not justified.
Watch for: the arc digs into the base metal more than the low-dilution processes below, so the first layer is usually the most diluted and a second layer is common. Bead overlap and travel speed decide coverage, and spatter and arc instability show up directly in the deposit.
Hot-wire GTAW overlay
GTAW with a filler wire pre-heated by its own current separates the arc energy from the filler delivery. The arc melts a controlled pool, and the hot wire adds filler without adding much arc energy to the base metal.
Where it fits: bores and internal surfaces reached by orbital or internal heads, valve and pipe internals, and jobs where a clean, low-spatter deposit and fine control are worth a slower process.
Watch for: the wire current and wire feed have to stay balanced with the arc. A drift in either changes how the wire melts into the pool, and so the bead shape and the fusion with the previous bead. Bead pitch along the surface is the coverage variable.
Strip cladding: submerged arc and electroslag
Strip cladding feeds a wide metal strip instead of a wire. With submerged arc strip cladding the arc runs under a flux. With electroslag strip cladding the strip melts in a conductive slag pool and there is no arc at the strip tip. Both lay a wide, shallow bead in long runs.
Where it fits: large, flat or large-diameter surfaces such as vessel shells, heads and tube sheets, where many square metres have to be covered.
Watch for: the energy is spread over a wide bead, which tends to keep dilution lower than a concentrated arc. Which of the two variants a job uses is part of the procedure and is proven on the qualification coupon. The overlap between neighbouring strips, the strip edges, flux or slag handling and the start and stop of each run are where coverage defects collect. Small bores, tight radii and short runs are a poor fit.
Laser cladding
Laser cladding melts wire or powder with a laser beam. The melt pool is small and the energy is concentrated, so the fused zone in the base metal can be kept thin.
Where it fits: heat-sensitive or distortion-prone parts, local areas, edges and components where a thin, controlled deposit is the requirement.
Watch for: powder or wire delivery, focus and stand-off distance decide the deposit. The small pool also means small defects, such as lack of fusion between tracks, are hard to see after the fact. The process needs a laser cell, beam delivery and fixturing, and its own procedure qualification.
Side by side
| Question | GMAW | Hot-wire GTAW | Strip (SAW / ESW) | Laser |
|---|---|---|---|---|
| Typical surfaces | Shells, components, repairs | Bores, internals, valves | Large shells, heads, tube sheets | Local areas, edges, heat-sensitive parts |
| Tendency to dilute | Higher on the first layer | Controlled; arc energy decoupled from filler | Lower; energy spread over a wide bead | Low; small fused zone |
| Area per pass | Medium | Small to medium | Large | Small |
| Main coverage variable | Bead overlap, travel speed | Bead pitch, wire/arc balance | Strip-to-strip overlap, run ends | Track overlap, powder/wire feed |
| Equipment | Common | Dedicated heads | Dedicated cladding station | Laser cell |
The table gives tendencies, not values. The dilution, deposition rate and number of layers that apply to your job come from the qualified procedure for that base metal and filler, and from chemical analysis at the specified depth.
Qualification comes with the choice
Whatever the process, the overlay procedure is qualified on a coupon that represents the job: ASME Section IX for work built to ASME, or ISO 15614-7, the overlay part of the ISO 15614 series, for work built to an ISO or EN route. For corrosion-resistant overlay the qualification includes chemical analysis at the specified depth. Changing the process, or an essential variable within it, generally means a new qualification. That is why the process should be settled before the procedure is written, not while the first coupons are being cut.
The weld dilution calculator helps estimate dilution from bead cross-section areas on a macro section. It is a planning aid, not a substitute for the chemical analysis the qualification requires.
What to monitor once the process is chosen
Each process drifts in its own way, and the drift is easier to catch while the layer is being deposited than after the next layer has covered it:
- GMAW: heat build-up along the run, bead overlap and travel speed, arc instability.
- Hot-wire GTAW: wire current and feed against the arc, bead pitch.
- Strip cladding: strip-to-strip overlap, heat at run starts and stops, edges.
- Laser cladding: track overlap, heat field around the small pool, feed interruptions.
A thermal record per pass shows where the heat field changed, where a bead or strip drifted and where coverage may be thin. Weld overlay cladding monitoring with thermal imaging describes that setup in more detail.
Where Therness fits
Therness does not sell overlay welding equipment and does not recommend a process for a specific job: that decision belongs to the fabricator’s welding coordination and the qualified procedure. Therness works on the monitoring side. HeatCam IR-S is a compact thermal camera built for permanent installation on robot heads and machine frames, and the HeatCore platform records the thermal signature of each pass so it can be compared against passes from a qualified reference. A pilot on your process and alloy pair shows which drift modes the thermal record catches before the layer is covered.
Picked the overlay process? Plan what to monitor
Therness can help list the drift modes of your overlay process and check in a pilot which of them a per-pass thermal record catches.
Discuss your overlay jobTherness solution: Welding camera — Thermal and high-speed imaging that watches the arc, the pool and the cooling seam while the weld is made.
Frequently Asked Questions
Which welding processes are used for corrosion-resistant weld overlay?
The common choices are GMAW, including pulsed variants, GTAW with a hot-wire feed, strip cladding with submerged arc or electroslag, and laser cladding with wire or powder. They differ in how much base metal they melt into the deposit, how much area they cover per pass, and which part geometries they can reach.
Which overlay process gives the lowest dilution?
There is no single answer that holds for every alloy pair, part and parameter set. Processes that spread the energy over a wide, shallow pool or keep the melt small tend to dilute less than a concentrated, high-energy arc. The dilution that counts is the one measured by chemical analysis at the specified depth during procedure qualification.
How many overlay layers are needed?
As many as it takes to reach the required chemistry at the depth stated in the specification. A second layer dilutes against the first layer rather than the base metal, which is why two layers are common. Whether one layer is enough is proven on the qualification coupon, not assumed from the process name.
Is strip cladding always better for large areas?
Strip cladding suits large, fairly flat or large-diameter surfaces where a wide bead can be laid in long runs. It is less suited to small bores, nozzles, tight radii and short or interrupted runs, where the strip width and the flux or slag handling get in the way.
When does laser cladding make sense for an overlay?
When the part needs a thin, low-heat deposit with tight control of the fused zone, for example on heat-sensitive or distortion-prone parts, or on local areas and edges. It needs a laser cell, beam delivery and fixturing, and its own procedure qualification.
What should be monitored during overlay deposition?
The variables that change dilution and coverage for the process in use: heat input and travel speed, bead or strip overlap, wire or strip feed, and the heat field of the deposit layer by layer. A thermal record per pass shows where heat built up or a bead drifted, before the layer is covered by the next one.
