Inconel 625 (UNS N06625) is a nickel-chromium-molybdenum alloy built for high-temperature strength and corrosion resistance in the toughest chemical, marine, and power generation environments. It welds with ERNiCrMo-3 filler, runs on DCEN TIG with 100% argon, and demands tight heat input control to prevent hot cracking. The weld puddle is noticeably sluggish compared to steel or stainless, and that slow-flowing puddle is normal. Don’t fight it by cranking up the amps.
Inconel 625 is solid-solution strengthened, meaning it doesn’t need heat treatment after welding to develop its properties. The as-welded deposit has excellent corrosion resistance and strength to about 1800F. This makes it the go-to overlay material for carbon steel vessels that need corrosion resistance without building the entire vessel from nickel alloy.
Alloy Characteristics
| Property | Inconel 625 |
|---|---|
| UNS | N06625 |
| Nominal composition | 62Ni-22Cr-9Mo-3.5Nb |
| Tensile strength (ksi) | 120-145 (annealed) |
| Yield strength (ksi) | 60-75 (annealed) |
| Melting range (F) | 2350-2460 |
| Density (lb/in3) | 0.305 |
| Thermal conductivity (BTU/hr-ft-F) | 5.7 (at room temp) |
| Max service temperature | 1800F (continuous) |
Notice the low thermal conductivity: about 1/5th of carbon steel. Heat stays concentrated in the weld zone rather than spreading out. This means you need less amperage than you’d expect for the material thickness, and the HAZ is narrow. But it also means the weld zone stays hot longer, increasing hot cracking risk.
Filler Metal: ERNiCrMo-3
ERNiCrMo-3 per AWS A5.14 is the matching filler for Inconel 625. Its composition closely matches the base metal, preserving corrosion resistance and high-temperature strength in the weld deposit.
| Element | ERNiCrMo-3 (typical %) | Inconel 625 base (typical %) |
|---|---|---|
| Nickel | 62 (min 58) | 62 (min 58) |
| Chromium | 22 | 22 |
| Molybdenum | 9 | 9 |
| Niobium (Columbium) | 3.5 | 3.5 |
| Iron | 1.0 max | 5.0 max |
ERNiCrMo-3 is also the workhorse filler for:
- Dissimilar joints between 625 and stainless steel (304, 316, 321, 347)
- Dissimilar joints between 625 and carbon steel
- Corrosion-resistant overlay (weld cladding) on carbon steel vessels
- Joining other nickel alloys (600, 800, 825) where corrosion resistance is critical
For the full nickel alloy filler selection matrix, see the nickel alloy filler metal guide.
TIG Welding Procedure
Shielding Gas
100% argon at 18-25 CFH through a gas lens. Argon/helium blends (up to 25% He) can increase arc energy for thicker sections without raising amperage. Back purge with argon on pipe and enclosed sections until oxygen drops below 50 ppm.
Parameters
| Material Thickness | Tungsten Dia. | Filler Dia. | Amps (DCEN) | Travel Speed |
|---|---|---|---|---|
| 0.063 in (1/16) | 1/16 in | 1/16 in | 30-60 | 4-6 ipm |
| 0.125 in (1/8) | 3/32 in | 3/32 in | 60-100 | 3-5 ipm |
| 0.188 in (3/16) | 3/32 in | 3/32 in | 90-130 | 3-5 ipm |
| 0.250 in (1/4) | 1/8 in | 1/8 in | 120-170 | 2-4 ipm |
| 0.375 in (3/8) | 1/8 in | 1/8 in | 150-200 | 2-4 ipm |
These amperages are lower than what you’d run on stainless steel of the same thickness. That’s intentional. Inconel 625’s low thermal conductivity means the heat stays in the joint, so you need less input to maintain the puddle.
Technique for the Sluggish Puddle
The Inconel 625 puddle is viscous and slow-moving compared to steel. It doesn’t flow ahead of the arc or self-level the way stainless does. This is normal behavior for high-nickel alloys. Techniques to work with it:
- Stringer beads only. No weaving, no oscillation wider than 2.5 times the filler rod diameter. Wide weaves concentrate sulfur and phosphorus at the bead centerline, triggering hot cracking.
- Add filler continuously. The puddle doesn’t wet out on its own; it needs the filler rod to keep it moving. Dip at a steady, consistent rate.
- Short arc length. Keep the tungsten close, about 1/16 inch. A long arc spreads the heat column and reduces penetration.
- Push angle. A 5-10 degree push angle helps the gas coverage and lets you see the leading edge of the puddle.
Hot Cracking Prevention
Hot cracking is the primary weld defect on Inconel 625. It occurs during solidification when trace elements (sulfur, phosphorus, silicon, boron) form low-melting-point films at grain boundaries. As the weld contracts, these films tear apart.
Causes and Countermeasures
| Factor | Why It Causes Cracking | Countermeasure |
|---|---|---|
| Sulfur contamination | Sulfur forms NiS films (melting point 1135F) at grain boundaries | Clean obsessively; no markers, tape, anti-seize with sulfur |
| Wide weave beads | Centerline segregation of impurities | Stringer beads only; max 2.5x rod diameter width |
| High heat input | Increases grain size and segregation | Low amps, fast travel, thin beads |
| High interpass temp | Extends time at cracking-susceptible temperature | Keep below 300F; measure with pyrometer |
| High restraint | Concentrates shrinkage stress at solidifying grain boundaries | Balanced weld sequence, reduce clamping, preheat |
| Concave bead profile | Thin center section can't resist shrinkage stress | Add filler to build slightly convex profile |
| Remelting previous passes | Dissolves impurities back into the puddle, concentrates them | Place beads side by side, not on top; don't re-melt the toes |
The most common shop contamination source is marking materials. Soapstone, paint markers, and grease pencils all deposit sulfur, silicon, or organic compounds on the joint surface. Mark parts with a vibrating engraver, laser, or light scribe line outside the weld zone. If you must mark near the joint, use markers specifically tested for nickel alloy welding.
Joint Preparation
- Bevel with machining, grinding (silicon carbide or aluminum oxide wheels), or plasma. If plasma cutting, remove the recast layer by grinding 1/16 inch past the cut edge.
- Clean with acetone on lint-free wipes. Follow with a solvent wipe to remove grinding residue.
- Mechanical cleaning with a stainless steel brush or Scotch-Brite (aluminum oxide type). Dedicate tools to nickel alloy work only.
- Fit-up per the WPS. Standard V-groove: 60-75 degree included angle, 1/32 to 1/16 inch root face, 3/32 inch gap for open root.
Handle cleaned material with clean gloves only. No bare hands, no leather gloves contaminated with oil or sweat. Weld within 4 hours of final cleaning, or re-clean if there’s a delay.
Weld Overlay (Cladding) Applications
Inconel 625 overlay on carbon steel is one of the most common applications for this alloy. The 625 provides corrosion resistance while the carbon steel provides structural strength.
Overlay is typically done with GMAW (MIG) using ERNiCrMo-3 wire in spray or pulse transfer mode, or with strip cladding (submerged arc with strip electrodes) for large vessel interiors.
Key overlay considerations:
- Minimum of two layers to ensure the surface composition meets the 625 chemistry specification. A single layer has too much iron dilution from the carbon steel base.
- First layer heat input should be low to minimize dilution. Use a slower wire speed and faster travel.
- Second layer restores the full 625 chemistry at the surface.
- Dilution control: Target less than 10% iron in the final surface layer. Higher iron content reduces corrosion resistance.
MIG Welding Inconel 625
GMAW with ERNiCrMo-3 wire works on sections over 3/16 inch where production speed matters.
| Thickness | Wire Dia. | Voltage | WFS (ipm) | Gas | Mode |
|---|---|---|---|---|---|
| 3/16 in | 0.035 in | 22-25 | 200-280 | 100% Ar | Pulse |
| 1/4 in | 0.035 in | 24-27 | 250-350 | 100% Ar or Ar/He | Pulse/Spray |
| 3/8 in | 0.045 in | 26-29 | 200-300 | 100% Ar or Ar/He | Spray |
Pulse MIG provides better heat input control than spray transfer and is preferred for most 625 work. 100% argon is standard; argon-helium blends (up to 25% He) increase arc energy for thicker material.
Do not use CO2-containing shielding gas blends on nickel alloys. Carbon pickup from CO2 reduces corrosion resistance and can cause intergranular attack in service.
Post-Weld Heat Treatment
Inconel 625 is a solid-solution alloy and doesn’t require PWHT to develop its mechanical properties. The as-welded deposit has the same corrosion resistance and roughly 90% of the base metal’s tensile strength. PWHT is specified only when:
- The code or owner specification requires stress relief for pressure-containing components (typically 1600-1700F for 1-2 hours)
- The service involves stress-corrosion cracking environments where residual welding stress might initiate failure
- Dimensional stability is critical and residual stress could cause distortion during machining
If PWHT is performed, avoid the 1200-1600F range where long-term exposure can precipitate carbides and topologically close-packed (TCP) phases that reduce ductility. Either stay below 1200F or go above 1600F for stress relief.
Common Applications
Inconel 625 shows up in aggressive environments across multiple industries:
- Chemical processing: Reactor vessels, heat exchangers, piping for acids and chlorides
- Marine: Seawater piping, submarine components, offshore platform hardware
- Power generation: Superheater tubing, flue gas desulfurization systems, nuclear plant components
- Aerospace: Combustion liners, exhaust systems, turbine seals
- Oil and gas: Downhole production tubing, flowlines, subsea equipment
- Pollution control: Flue gas desulfurization (FGD) systems, scrubbers, and ducts exposed to sulfuric and hydrochloric acid
For each application, the welding procedure must address the specific service conditions (temperature, corrosive media, fatigue loading) that determined the alloy selection in the first place. Weld procedures qualified per ASME Section IX, AWS D10.11, or applicable owner specifications govern most code-regulated 625 welding.
For Inconel 718 (precipitation-hardened variant requiring different procedures), see the welding Inconel 718 guide. For the complete nickel alloy filler chart, see the filler selection guide. For Monel welding, see the Monel 400 welding guide.
Back to the nickel alloys welding category.