Inconel TIG Welding: A Complete Practical Guide

The first time many fabricators weld Inconel, the puddle looks wrong before it even looks hot. It sits there, thick and reluctant, refusing to flow into the root the way stainless steel does, and when they finish the bead they find a crack running straight down the centerline. Nothing about the procedure was unreasonable — they simply applied stainless-steel habits to a nickel-chromium superalloy. This guide walks through what actually changes when you TIG weld Inconel: the fillers, the gas, the joint geometry, the heat input math, and the specific metallurgical traps that cause cracking, porosity, and loss of corrosion resistance.

Why Inconel TIG Welding Behaves Nothing Like Stainless Steel

Here is a job that lands in nearly every nickel fabrication shop. A 3/8-inch Inconel 625 header section comes back from the field with a crack along the weld. An Inconel 718 flange in the customer’s skid fails a hydrostatic test the day it arrives. The welder on both jobs is experienced, the machine is set up properly, and the filler came from the supplier the customer named. And yet.

On the 625, the crack runs straight down the centerline of the bead and stops dead at the stop weld, which is the signature of solidification cracking. On the 718, the bead is narrow and almost vertical in profile, and the flange cracks again the moment someone heats it for stress relief. Neither job failed from lack of skill. Both failed because stainless-steel habits were applied to a nickel-chromium superalloy.

So the question most fabricators actually have: can Inconel be TIG welded? Yes. Nickel alloys are readily welded, and most can be fusion welded using gas-shielded processes such as TIG or MIG, according to TWI’s weldability guidance on nickel and nickel alloys. The conditions are simply non-negotiable: correct filler, inert shielding, and heat input you control rather than drift. Get those three right and the metal cooperates. Get them wrong and it fails in ways that take a failure analyst to unwind.

Inconel is a family, not an alloy

“Inconel” is a trademark, not a material designation. It covers a family of nickel-chromium superalloys from Special Metals, and the members differ enormously in chemistry and in welding behavior. Alloy 600 and 601 are solid solution grades. 625 is the corrosion workhorse. 718 is precipitation hardened. 690 is a high-chromium grade built for nuclear steam generators. Special Metals’ data sheet for Inconel Filler Metal 625 shows how tightly each family member is matched to its own consumables.

That distinction matters commercially and technically. The filler you buy, the condition you weld the base metal in, and the crack tendency you inherit all depend on which grade is actually on your table.

Five reasons the weld behaves differently

1. Very high alloy content. Nickel alloys carry chromium, molybdenum, niobium, titanium, and more in solid solution. The weld pool is chemically complex, and the microstructures that form on solidification are unforgiving.
2. A sluggish, low-fluidity pool. Most nickel alloys have lower fluidity than carbon manganese steel or stainless, producing a pool that does not wet out easily or travel as you expect.
3. No self-cleaning oxide flux. Stainless relies on fluxing action to sweep the weld clean. Nickel alloys do not. Oxide must be physically removed before welding, and nickel oxides have very high melting points that let them survive into the bead.
4. A wider freezing range. The alloy spends a long time in a mushy state, which is exactly where low-melting grain boundary films form and where contraction stress does its damage.
5. Precipitation-hardened grades. Alloys like 718 must be welded in the solution-treated condition. Weld aged material and you invite strain-age cracking during post-weld heat treatment or high-temperature service.

One consequence surprises people who are used to stainless: you cannot fix penetration by turning up the amperage. The pool’s fluidity, not its heat, is the limiting factor. More current just overheats the tungsten, widens the arc, and raises the risk of porosity and tungsten inclusion.

The four failure modes that dominate nickel welding

TWI lists the recurring imperfections in nickel alloy welds, and they map directly onto the four problems you will actually fight:

• Porosity, oxide inclusions, and lack of inter-run fusion
• Weld metal solidification cracking
• Microfissuring

Nickel alloys are naturally susceptible to hot cracking to some extent, and the severity depends on weld restraint, bead profile, alloy type, and the level of impurities in the material. That is why bead shape matters so much in this guide: a convex bead can absorb contraction strain that a narrow, deep, undercut profile will tear open.

Cleanliness is the other constant. Nickel alloys are readily welded only if the surface is cleaned immediately before welding: degrease, remove all surface oxide by machining, grinding, or scratch brushing, then degrease again. The window between cleaning and welding is short, and it is measured in hours, not days.

This guide is written for job-shop welders, engineers specifying nickel work, and hobbyists moving up from stainless and mild steel. You will find filler selection, gas and tungsten setup, heat input math, joint geometry, and troubleshooting broken down in the sections that follow. If you are new to the material, the aerospace TIG welding standards and techniques overview is a good parallel read, because 718 work is where nickel welding discipline is most strictly policed.

What Counts as “Inconel”: Alloy Families and Their Welding Personalities

Before you can weld Inconel well, you have to know which Inconel you have. “Inconel” is not an alloy but a trademark family of nickel-chromium superalloys owned by Special Metals, and the members behave very differently under the arc. One of them is genuinely difficult and will punish an impatient welder every time.

Here is the mental model for the rest of the guide. Nickel alloys split into two camps, and the split predicts nearly everything about how they weld. Solid-solution (SSS) alloys get their strength from the base metal itself, nothing precipitates after welding, and the joint stays about as ductile as the parent metal. Precipitation-strengthened (PS) alloys get strength from a second phase deliberately aged in after solution treatment, and that phase is also why they crack. As TWI notes, PS nickel alloys are prone to strain-age cracking, while solid solution alloys generally show superior weldability.

The alloys you will actually meet

Alloy	UNS	Primary service environment	Typical TIG filler	Single most important welding caution
Inconel 600	N06600	Oxidation, caustic service, furnace parts, mufflers	ERNiCr-3 (FM-82, N06082)	Low heat input; sensitization-type intergranular attack if held at 600–800 °C
Inconel 601	N06601	High-temperature oxidation, sulfur-bearing service, radiant tubes	ERNiCr-3 or ERNiCrMo-3	Stubborn Cr-rich oxide skin; grind or blast the surface before welding
Inconel 625	N06625	Offshore, chemical process, seawater, sour gas, chloride SCC	ERNiCrMo-3 (FM-625, N06625)	Weld as clean as the parent metal; Mo-bearing welds punish contamination
Inconel 718	N07718	Aerospace, rocket engines, gas turbine casings and discs, to about 650 °C	ERNiFeCr-2 (FM-718, AMS 5832) or ERNiCr-3 for non-aged joints	Weld solution-treated only, then re-solution anneal and double age
Inconel 690	N06690	Nuclear steam generators, high-chromium caustic service	ERNiFeCr-8 (N06690)	Creep and IGSCC sensitive; control heat input and cool slowly
Inconel 704	N06604	High-temperature oxidation with good thermal stability	ERNiCrMo-3 (N06625)	Leaner in Mo than 625; not a drop-in substitute, filler matching matters
Inconel 725	N07725	High-strength, sour service, springs and fasteners	ERNiFeCr-2 (N07718-class)	Like 718, but a much narrower heat input window
Monel 400	N04400	Hydrofluoric acid, seawater, alkalis, marine hardware	ERNiCu-7 (N04080)	Runs hot and fluid; conservative heat, plenty of purge on thin sections
Hastelloy C-276	N10276	Extreme chemical corrosion, scrubbers, waste incineration	ERNiCrMo-10 (N10312)	Low heat input only; high heat promotes segregation and cracking

Each of these alloys fails in its own characteristic way, so the same filler and the same amperage can be excellent on one and a liability on another.

600 and 601: the solid-solution workhorses

600 and 601 carry roughly 15 to 17 percent chromium for oxidation resistance, plus 8 percent molybdenum in 601 for sulfur-bearing service. With no precipitation reaction they can usually be used as-welded, which makes them the right training metal for a shop moving up from stainless.

625: the corrosion workhorse

Inconel 625 is nominally 62 percent nickel, 21.5 percent chromium, 9.0 percent molybdenum and 3.7 percent niobium. Molybdenum plus niobium gives resistance to pitting and crevice corrosion, and the niobium stabilizes the alloy against sensitization. Special Metals documents ERNiCrMo-3 for GTAW and GMAW of INCONEL 625, INCOLOY 825, INCOLOY 25-6MO and austenitic and super austenitic stainless steels, which is why it is the default filler on stainless-to-nickel work. The problem is contamination: Mo-rich weld metal picks up sulfur, lead and zinc, and those seed cracking. Section 8 covers the hygiene protocol.

718: the hard one

Inconel 718 built the reputation of nickel welding as difficult. It is a nickel-chromium-iron alloy strengthened by gamma prime, Ni₃(Al,Ti) and gamma double prime, Ni₃Nb, serving rocket combustors, gas turbine casings, turbine discs and pressure vessels to about 650 °C. A research review of IN718 notes its cryogenic rocket engine use and points to sluggish gamma double prime precipitation, which gives 718 excellent resistance to strain-age cracking and its aerospace and pressure-vessel dominance. That resistance is also why 718 welds to aged condition with a deliberate post-weld heat treatment sequence (Section 9). 725 is the newer, higher-strength member of the same family: same rules, faster strengthening, narrower heat input window.

690 and the nuclear service alloys

Inconel 690 carries about 30 percent chromium, far more than 600 or 625, which drives its use in nuclear steam generators, where the concerns are caustic cracking and intergranular stress-corrosion cracking in the heat-affected zone. A coarse HAZ grain size makes the joint more susceptible, so low heat input, controlled interpass temperature and careful filler selection all matter more than on 625.

The Inconel / Incoloy confusion

Inconel and Incoloy are two separate trademark families from the same company. INCONEL alloys are nickel-chromium, with nickel as the dominant balance element; INCOLOY alloys are nickel-iron-chromium, with iron as the significant balance element. INCOLOY 800, 825 and 25-6MO are common, and 825 is routinely welded with ERNiCrMo-3 filler alongside 625. Short Note: A purchase order saying “Inconel” without a number is not a specification. Insist on the UNS number and the mill condition, especially for 718.

When it is not even a nickel alloy family member

Monel 400 and the Hastelloy alloys share Inconel’s fundamental rules because they share its metallurgy: inert gas shielding, a clean joint, low and controlled heat, no self-cleaning flux. Monel 400 handles hydrofluoric acid and seawater and runs more fluid than Inconel, so it wants less heat and more shielding. C-22 is commonly welded with ERNiCrMo-10, B-2 with ERNiMo-7, and all of them demand low heat input because high heat input drives segregation and cracking in the weld metal.

The joint-prep and cleanliness discipline that separates a good aluminum MIG welding job from a bad one also separates a good Inconel TIG weld from a cracked one, and the penalty for skipping it is much higher.

Choosing Filler Metal: AWS A5.14 Classifications Explained

Choosing the TIG rod for Inconel is the quiet decision that decides whether the job passes inspection. The base metal sets the strength the joint must carry; the filler decides how the puddle behaves as it freezes, how the bead fills, and whether the centerline survives. Pick by trade name alone and you can burn a shift learning why.

How to read an AWS A5.14 classification

AWS A5.14 covers nickel and nickel-alloy welding electrodes and filler metals. Every classification is assembled from a few pieces, and once you can read them you know exactly what you are buying.

• ER means a rod or wire consumed by the arc, which is why GTAW nickel filler always carries the prefix.
• Ni, NiFe, NiCr, NiCrMo, NiCrCoMo give the base family: nickel, nickel-iron, nickel-chromium, nickel-chromium-molybdenum, nickel-chromium-cobalt-molybdenum, and so on.
• The number is the nominal alloying content above the nickel balance. ERNiCr-3 means nickel-chromium carrying roughly 3 percent total alloying additions (chromium plus niobium); ERNiCrMo-3 means nickel-chromium-molybdenum with about 3 percent molybdenum plus niobium.
• The trailing dash number separates variants inside that family.

Trade names tell you nothing about chemistry. “Inconel 82,” “FM-82,” and “Ni6082” all describe the same rod. Learn the classification and you can cross-shop suppliers, verify a mill certificate, and check a WPS without guessing.

The filler choices you will actually use

Base alloy	AWS A5.14 classification	Trade name	UNS number	Primary use
600 / 601	ERNiCr-3	INCONEL Filler Metal 82	N06082	Solid-solution, oxidation and caustic service
625	ERNiCrMo-3	INCONEL Filler Metal 625	N06625	625 joints and general dissimilar work
718	ERNiFeCr-2	INCONEL Filler Metal 718 (AMS 5832)	N07718	718 joints that will be post-weld aged
C-22	ERNiCrMo-10	Alloy 22 filler	N06622	High-chromium corrosion service
690	ERNiFeCr-8	690 filler	N06690	Nuclear steam generator tubing
617	ERNiCrCoMo-1	INCONEL Filler Metal 617	N06617	High-temperature, creep-resistant service

That mapping matches TWI’s guidance on welding nickel and nickel alloys: alloy 600 pairs with Ni6082 / ERNiCr-3, alloy 625 with Ni6625 / ERNiCrMo-3, and alloy 718 with Ni7718 / ERNiFeCr-2. TWI also lists alloy 625 filler as an accepted alternative for 718 when no post-weld heat treatment will be applied.

Nominal chemistry matters when you are chasing strength. FM 82 runs about 72 Ni, 20 Cr, 3.0 Mn and 2.5 Nb, balance iron. FM 625 is 62 Ni, 21.5 Cr, 9.0 Mo and 3.7 Nb, with a minimum tensile strength of 105,000 psi (724 MPa) and 30 percent elongation per the Special Metals data sheet. FM 718 carries 52.5 Ni, 19 Cr, 3.0 Mo, 5.1 Nb, 0.90 Ti and 0.50 Al, balance iron, because it must reproduce the gamma-prime response of the parent metal.

ERNiCrMo-3 is also a common choice for dissimilar joints between nickel alloys, stainless steel and carbon steel, because its chromium and molybdenum content tolerates mixed dilution better than most alternatives.

When to dilute on purpose

Matching filler is not automatically the best answer. In aged 718 the base metal is precipitation hardened, and a weld metal matching it in strength can crack during a later solution anneal or aging cycle. Many shops deliberately run a softer, more ductile ERNiCr-3 weld in 718, trading joint strength for crack resistance and forgiving ductility. If the part never sees heat treatment, ERNiFeCr-2 is unnecessary and ERNiCrMo-3 becomes a sensible choice instead.

Rod, spool and diameter

Shop supply comes as cut lengths, typically 36 inch (1 m) sticks, or as spooled TIG wire on 1 kg, 11 lb (5 kg) or 33 lb (15 kg) spools. Cut rods suit hand welding and give a clean, uncontaminated feed; spooled wire suits automated cells and repeat orders but needs sealed storage and clean handling. Rod forms and handling are covered in our all-position welding rod guide.

Diameter follows joint thickness and heat control. A starting point for GTAW nickel work:

• 1/16 in (1.6 mm) for material up to about 3 mm and joints needing fine puddle control
• 3/32 in (2.4 mm) for roughly 3 to 6 mm
• 1/8 in (3.2 mm) for roughly 6 to 10 mm
• 3/16 in (4.8 mm) and larger for heavy section or fillet work at high travel speed

When in doubt, step down one size. Smaller rod dumps less heat into the puddle per stroke, and on a sluggish nickel pool that is often the difference between a sound bead and a centerline crack. On aerospace-grade 718, qualified procedures and welder qualification come first; our aerospace TIG welding guide covers that side.

Verify before you buy

Never buy filler on the label alone. Check the AWS A5.14 classification, the UNS number, the applicable AMS specification (AMS 5832 for 718, for example), the lot or heat number and the manufacturer. If a reseller cannot produce those numbers, the rod is a guess. And remember the trademark: Inconel is a Special Metals trademark, so anything genuinely carrying that name should trace back to a real mill certificate.

Shielding Gas, Tungsten Choice, and Machine Setup

Two things happen on an Inconel job that rarely happen on mild steel: the gas you pick becomes a metallurgical decision, and the consumables you load into the torch decide whether you get a clean fusion zone or a contaminated one. Neither is optional. Get the shielding, tungsten, and machine setup wrong and no amount of technique will save the weld.

Argon Is the Default, and Here Is Why

Argon is the default shielding gas for GTAW on nickel alloys, and it is the standard choice for back purging as well. Three reasons drive that decision. Argon is chemically inert, so it will not react with the molten pool. It is denser than air, roughly 1.65 times the weight of air, so it settles and displaces the surrounding atmosphere effectively rather than getting mixed with it. And it costs a fraction of what helium costs while producing a similar arc for most practical work on nickel.

Helium does have a place: it produces a hotter, more focused arc that improves penetration and heat concentration in heavy sections, and some codes permit or require it for specific applications. But the arc is less stable on very thin material, it pushes the pool harder, and it is generally a cost decision rather than a metallurgical one for typical Inconel work.

TWI’s guidance on welding nickel alloys notes that when using argon in TIG and MIG welding, attention must be paid to the shielding efficiency of the weld pool, including the use of a gas backing system. The same source observes that argon-hydrogen gas mixtures tend to produce cleaner welds in TIG. The reason is understandable: a small hydrogen addition stabilizes the arc and can improve the wetting and cleanliness of the bead on some nickel alloys.

The tradeoffs are real, though. Hydrogen is a known contributor to porosity, and hydrogen pickup is precisely what you are trying to avoid in a nickel weld. Mixtures also cost more and require additional regulator hardware, and hydrogen handling brings regulatory and safety considerations beyond ordinary cylinder management. If you are considering a mixture, treat it as a qualified procedure choice, not a shop-floor experiment.

Gas handling itself deserves a moment. If you are buying cylinders or arranging a supply contract, it helps to understand how the different gases are packaged and delivered. This guide to gas services and safety and this overview of cylinders, regulators and safety practices cover the practical side, including the regulators, hose, and flash-back arrestor setup that keeps a nickel weld running.

Back Purging: Where It Matters Most

Back purging means flooding the inside of a joint with inert gas before welding so the root of the weld is shielded from air. It is required for tube work, for any full-penetration closed section, and for joints where the root surface will later be in a corrosive service. Nickel alloys do not self-clean, so an oxidized root is not a cosmetic problem. It is a fusion and corrosion-resistance problem.

A working purge setup needs several things:

• Purge dams at both ends of the joint, usually inflatable expanding dams for tube or purpose-made sealing dams for flat work.
• Purge before striking the arc. Start the gas flow, allow the trapped air to displace, and confirm the oxygen level is low before you strike. An arc struck into an unpurged cavity will oxidize the root and defeat the entire operation.
• A controlled vent path. The displaced gas has to go somewhere. Without a controlled outlet, purge gas creates turbulence that draws air back into the joint at exactly the wrong moment.
• Restricted outlet flow on small diameters. On tube under about 50 mm, an open vent can suck in atmosphere. Bleed the purge gas through a restricted outlet or a side vent so flow stays low and inward-directed.
• Verification with an oxygen meter. Read the oxygen level at the outlet and confirm it is at the level your procedure requires before welding.

Choosing the Right Tungsten

The electrode has to stay hot enough to sustain the arc without contaminating the pool. Wrong geometry or wrong dopant and you will get tungsten inclusions, wandering arcs, or a contaminated weld. For nickel alloys the usual choices are:

Electrode	Polarity	Best Use
Pure tungsten	DCEN	AC only; rarely used for nickel, since the arc is not stable on AC
Ceriated (2% Ce)	DCEN	Excellent DCEN arc start and stability, good for most Inconel TIG work
Lanthanated (1.8% La)	DCEN, also usable on AC	Popular all-round choice, holds a good point, forgiving on mixed polarity
Thoriated (2% Th)	AC or DC	Superb DCEN stability and heat capacity, but carries a radiation handling note
Zirconiated	DCEN	Used for AC where arc stability on aluminum-type work is needed

On thoriated electrodes, note the handling guidance: they contain radioactive thorium, so avoid grinding them without respiratory protection and follow supplier disposal rules. In shops that already grind tungsten constantly, ceriated or lanthanated is the safer everyday answer for nickel.

Diameter has to match current. A 1.6 mm electrode handles roughly 70 to 150 A DCEN, 2.4 mm about 150 to 250 A, and 3.2 mm about 250 to 400 A. Using an electrode that is too small for your current is the classic cause of tungsten contamination in the bead. Staying below the rated range also gives you a sharper arc.

Geometry matters just as much. A pointed ground tip concentrates the arc and lets you work into a tight root gap or a narrow land without the arc wandering onto the sidewall. A blunt tip spreads the arc, which softens penetration and raises heat input. For Inconel root work, use a fine pointed or 2% lanthanated ground with a small included angle, and grind it on dedicated wheels kept away from other metals.

Machine Setup

Run GTAW on DCEN, electrode negative. It concentrates the majority of the heat in the workpiece, which is exactly what you want on an alloy where heat input has to be controlled rather than dumped into the joint.

A few more setup details that separate a good Inconel weld from a bad one:

• Foot pedal control. Pedal control lets you modulate current and, just as importantly, manage the pool and the crater. A cold crater is where centerline cracks start.
• HF start. High-frequency start gives a clean, low-contamination arc ignition without a scratch starter. Some codes prohibit lift-arc start on nickel work for exactly that reason.
• Water-cooled copper leads and a properly seated ground clamp. Direct the heat into the workpiece and away from the power supply. For code work on pressure equipment, remember that 29 CFR 1926.353 requires that cylinders be secured upright and that hoses be protected from damage.
• Constant current versus pulsed or synergic control. A plain constant-current machine gives you manual control of everything through the pedal and it will do the job. Pulsed and synergic TIG add value on Inconel because they let you set peak and base current, frequency and duty cycle, which controls heat input, limits the pool size and influences the grain structure of the fusion zone. On 718, that control is a real metallurgical lever, not a convenience feature.

Finally, do not forget the air around you. AWS guidance on welding fume is simple and worth repeating: keep your head out of the fumes, use enough ventilation or exhaust at the arc to keep fume out of your breathing zone, and use air sampling if ventilation is questionable. If your shop does aerospace or pressure work, this is also the section where your aerospace TIG welding procedures and equipment expectations will be most stringent.

Joint Preparation and Fit-Up for a Sluggish Nickel Pool

Carbon steel forgives bad geometry; Inconel does not. The nickel pool is thin, dense and reluctant: it will not climb a steep wall or wash into a narrow root, and penetration with nickel base alloys is significantly lower than with carbon manganese steel. Joint prep is a metallurgical control.

Bevel Angle and Root Geometry

Guidance calls for a wider bevel, about 80° included angle on a single V, versus tighter angles on carbon manganese and stainless. A wider angle opens the root for the sluggish pool; a narrow bevel leaves no bottom opening, so the arc wanders to one side and the root is fused on one edge only: lack of fusion, a crack starter.

Geometry Feature	Nickel Alloy Guidance	Why It Matters
Included angle, single V	About 80°	Opens the root for the sluggish pool
Root gap	1.6 to 3.2 mm, by thickness and process	Too tight and the root will not fill
Root face / land	Thin or zero on most GTAW	A thick land traps the torch

A thin land of 0.5 to 1.0 mm controls the root and keeps the arc from blasting through, but a heavy land fights penetration. Most Inconel GTAW runs no land at all, the bevel faces meeting at a sharp root, gap held by spot tacks or clamps.

Backing: Strip or Tabs

With a backing strip, copper or stainless, the pool rests on a chill bar that soaks up heat and supports the root, so reinforcement builds on the front face, ground flush later. Copper is preferred: it does not melt into the weld and pulls heat faster than steel, cutting burn-through on thin wall.

Run-on and run-off tabs do the same job at the ends of the joint, giving the arc somewhere to start and finish without freezing the crack-prone centerline. Welding without backing is legitimate, but full penetration in one pass and deliberate root reinforcement are your responsibility. A flat or dished root invites cracking.

Fit-Up, Misalignment and Bead Profile

Fit-up tolerances on nickel should be tighter than most fabricators expect. Misalignment of even 1 to 2 mm on thin-gauge stock shifts heat sideways, so one side of the groove gets more penetration, concentrating contraction strain down the centerline of a hot-crack-prone material.

Bead shape follows the same logic. Solidification cracking is typically associated with a flat or concave weld profile and can normally be prevented by producing a larger, more convex bead, which lets reinforcement grow as the joint shrinks and cuts centerline tensile strain.

Cleaning the Base Metal

Heavy oxides: nickel oxides have very high melting points and survive into the pool as inclusions or porosity nuclei. Unlike stainless steel, nickel alloys do not self-clean, so oxide must be removed by grinding, abrasive blasting or pickling before it reaches the arc. Contaminants: sulfur, phosphorus, lead, zinc and tin are low-melting and promote cracking, arriving through machining lubricant, crayons, oil, grease and paint.

Guard against recontamination from grinding wheels and wire brushes. Dedicated stainless-only tools are recommended, and a marked container protects nickel filler rods.

Weave Width and Filler Placement

The nickel pool will not self-flow, so the welder has to move it. Weaving works molten metal into the groove sides. Keep the weave within three times the wire diameter: a 1.0 mm filler weaves across roughly 3 mm, a 2.4 mm filler within 7 mm. Beyond that the weave edges freeze before the filler reaches them, causing cold laps and undercut, while the wide, slow motion traps heat mid-joint.

Pause at each side, holding the torch still long enough for the filler to wet the sidewall. Skip the pause and the sidewall sees only a quick glance of the arc, leaving undercut along the bead toe. Add filler at the leading edge of the puddle, not on top of it; let the arc melt the rod end, then the base metal.

Tacking Without Setting Up a Crack

Tacks are where nickel welds quietly fail. A centerline crack found at inspection often started in a tack made with the wrong filler or too much heat. Use the same filler as the final weld, tack with low heat and short beads, and grind out tack nuggets: a partially remelted nugget is a porosity source mid-root.

On heavy or highly restrained sections, consider sequential fill and weld: it shortens the weld length, reducing shrinkage and lowering centerline restraint while the joint is hot. Where geometry permits, electromagnetic stirring refines grain structure and reduces columnar centerline grains.

Get the joint right first; for heat input, see the parameter section next.

Setting TIG Parameters: Amps, Volts, Travel Speed and Heat Input Math

With Inconel’s sluggish puddle, guessing at your machine settings is one of the quickest ways to invite cracking. Rather than chasing a magic number for every job, the goal should be to calculate and control heat input to fit the specific alloy that you’re working with.

Calculating Heat Input

There are two industry standard formulas that you’ll see on procedure qualification records (PQRs). Knowing both is crucial, as codes can call for either one.

Standard	Formula	Notes
ASME/AWS	Heat Input = (Arc Voltage × Arc Current) ÷ Travel Speed	Gives the theoretical arc energy delivered to the joint. No process efficiency factor is applied.
EN (European)	Heat Input = (Arc Voltage × Arc Current × η) ÷ Travel Speed	Accounts for the actual heat transferred to the workpiece. For GTAW/TIG, the thermal efficiency factor (η) is typically taken as 0.6.

Worked Example: Let’s say you’re running 110 A at 16 V with a travel speed of 100 mm/min on Inconel 625.

• Convert travel speed: 100 mm/min ÷ 60 = 1.6667 mm/s
• ASME/AWS: (16 × 110) ÷ 1.6667 = 1760 ÷ 1.6667 ≈ 1.06 kJ/mm (roughly 26.9 kJ/inch)
• EN (η = 0.6): (16 × 110 × 0.6) ÷ 1.6667 = 1056 ÷ 1.6667 ≈ 0.634 kJ/mm of effective heat input

Published Heat Input and Interpass Limits

When it comes to nickel alloys, restraint and grain growth are the enemy. According to TWI, heat input should be kept moderate, around 2 kJ/mm maximum to limit grain growth and HAZ size. However, more sensitive precipitation strengthened alloys, such as 718, C-22 and C-276, warrant a much tighter ceiling of 1 kJ/mm maximum.

For high Cr + Mo alloys that are destined for corrosive service, a widely used rule of thumb is a maximum interpass temperature of 150 °C and a maximum heat input of 1.5 kJ/mm. In regards to interpass temperature as a whole, TWI states that it should not be allowed to rise above 250 °C, though some alloy suppliers recommend an interpass as low as 100 °C for alloys such as Alloy C-276. For Inconel 718 specifically, shop practice often aims for around 120 °C (250 °F).

Most importantly, do not use temperature indicating thermal crayons to check your interpass temperature. These can introduce surface contamination to an already oxide sensitive nickel alloy. Instead, utilize a calibrated thermocouple or an approved contact pyrometer.

Amperage, Tungsten and Travel Speed Control

The vast majority of Inconel TIG welds are performed in DCEN. As with any process, cleanliness is paramount before striking an arc. This is a shared principle seen throughout preparation practices, such as those found within aluminum cleaner for welding, just with the added necessity of removing tenacious nickel oxides.

Use the table below as a realistic starting point for flat, well fitted joints. These align closely with published GTAW parameters for Inconel 625 of 80–125 A, 14–18 V, 90–120 mm/min travel speed, DCEN, a 175 °C maximum interpass and 10–20 L/min of argon flow.

Material Thickness	Suggested Amperage (DCEN)	Tungsten Diameter (Ceriated/Lanthanated)	Tip
1.0 mm / 0.040″	40–65 A	1.0 mm / 0.040″	Sharp, pointed tip for a tight, controllable arc.
2.0 mm / 0.080″	70–95 A	1.6 mm / 1/16″	Utilize foot pedal control for puddle management.
3.0 mm / 0.120″	95–115 A	1.6 mm / 1/16″	Stay vigilant with heat input between passes.
6.0 mm / 0.236″	110–125 A	2.4 mm / 3/32″	Multi-pass with strict interpass temperature control.

The Crack Problem: Solidification, Liquation, and Strain-Age Cracking

For most fabricators, the biggest hurdle with inconel tig welding isn’t porosity or spatter, it’s cracking. Nickel alloys simply don’t behave like stainless steel when they contract. Their grain boundaries become vulnerable at different stages of the weld cycle, which is why cracking can appear in the bead, beside it in the heat-affected zone, or even days after welding. As laid out in TWI’s guide to the weldability of nickel and nickel alloys, these failures fall into three distinct families.

Solidification Cracking: The Centerline and Crater Split

Solidification cracking occurs as the weld metal freezes. As the puddle shrinks, it pulls into tension while alloying elements such as chromium, niobium, molybdenum, iron, and carbon segregate to the very last liquid. This remaining liquid forms a low-melting eutectic film along the grain boundaries. With nowhere to bond back together, the boundaries split, creating a crack along the centerline where that film collected. The same principle causes crater cracks, making crater fill non-negotiable for Inconel.

To keep this under control, focus on these practical measures:

• Bead shape: A slightly convex bead is far more forgiving than a concave one. A deep, concave root is practically a pre-crack.
• Arc-off technique: Ramp current down, hold filler in the puddle until contraction slows, then break the arc. Avoid tapping the crater to cover a shrinkage pocket.
• Restraint: Minimise rigid fixturing where possible. Back-stepping, skip welding, or welding from the center out can relieve contraction stress.
• Heat input: Going too low is counterintuitive. A narrow, fast-frozen bead concentrates strain. Pushing heat input towards the upper end of your allowed range will widen the pool and create coarser, more crack-resistant grains.
• Cleanliness: Sulfur, phosphorus, lead, zinc, and tin all widen the freezing range and drop the eutectic temperature. No parameter can compensate for contaminated base metal.

HAZ Grain Boundary Liquation Cracking

Unlike solidification cracking, liquation cracking forms beside the weld, in the heat-affected zone (HAZ). Nickel alloys are packed with strengthening precipitates, such as MC carbides (NbC, TiC), M6C, Laves, and sigma phases. Under the rapid heating of TIG, these particles don’t fully dissolve. As they partially break down, they form a low-melting eutectic film along the parent metal’s grain boundaries. With weld contraction pulling on those boundaries, the liquid film opens them up and fissures the HAZ.

According to TWI’s second article on welding nickel alloys, susceptibility is driven by alloy composition, grain size, boundary character, and impurities. Coarse grains (above roughly ASTM 6), excess carbon or boron, and elements such as sulfur and phosphorus are all known to increase this risk.

The most reliable defenses are metallurgical:

• Material condition: For alloys like Inconel 718, welding in the solution-annealed condition between 955 and 980 °C (1750 to 1800 °F) is the standard for weldability. Overaged material is often the most crack-resistant if stress relief is the goal.
• Control alloying: Keep aluminium, titanium, and niobium within specification. Excess niobium is a well known driver for NbC and Laves phase formation.
• Thermal control: Keep heat input low and travel speed reasonably fast. This limits peak HAZ temperatures and reduces the time for low-melting films to accumulate.
• Cleanliness: Any contamination at the edge of preparation can drop the local eutectic temperature and make liquation far more likely.

Strain-Age and Reheat Cracking

The final family is the sneakiest. Strain-age, or reheat, cracking won’t show up in the puddle at all. It can appear hours, days, or during hot service after welding or post-weld heat treatment (PWHT). Gamma-prime strengthened alloys, such as 718, 625, and 690, precipitate these hardening phases when exposed to elevated temperatures. This precipitation makes the HAZ grain boundaries brittle, whilst residual stresses from welding try to pull them apart.

Because of this, the cure must come before striking the arc. Weld in the solution-treated or overaged condition, and for precipitation-hardened base metals that will see PWHT, consider diluting the joint with a more ductile filler. Fillers such as Inconel Filler Metal 625 are widely used for this exact reason. The trade-off may be some strength, however the gain in ductility and resistance to reheat cracking is often worth it for these applications.

The Narrow Window for IN718

Nowhere is this balancing act tighter than with Inconel 718. In published TIG welding work on Inconel 718, a microstructure free of a straight centerline grain boundary was achieved below 135 J/mm of heat input. However, full penetration could not be achieved below 112 J/mm, and longitudinal grain formation became present at welding speeds below 3.39 mm/s. These numbers don’t form a universal production set, but they perfectly illustrate the race between penetration, solidification structure, and HAZ integrity.

This isn’t universal across all superalloys either. A study of IN-939 superalloy TIG welds found that HAZ cracks grew perpendicular to the melting zone along grain boundaries, and that pre-weld heat treatment had a greater effect on crack susceptibility than heat input alone. For projects with aerospace or pressure-vessel requirements, qualification takes precedence, with resources such as aerospace TIG welding standards and training guiding the path.

Porosity, Oxides, Contamination and Other Defects

With Inconel TIG welding, porosity will reject a part far more often than cracking will. Gas becomes trapped in the weld metal, and on a radiograph it can appear as round pores, scattered specks, or worm-like indications. Knowing the shape is often enough to trace it back to its source.

Where Porosity Comes From

According to TWI’s weldability guidance for nickel and nickel alloys, there are three distinct causes to look out for:

1. Air entrainment. Leaks, failed purge, or a torch angle that allows atmosphere to shear into the arc can introduce oxygen and nitrogen into the molten pool.
2. Breakdown of surface oxides. Even with perfect shielding, oxides already on the joint can release oxygen and nitrogen as they break down.
3. Hydrogen from contamination. Oil, grease, cutting fluids, marking compounds, water, or moisture on a wire spool can all release hydrogen, which is extremely soluble in liquid nickel.

This is why most Inconel TIG consumables are over-deoxidized. For example, an ERNiCr-3 rod contains deliberate additions of aluminium and titanium to combine with oxygen in the pool, rather than letting it trap as gas.

Oxides That Will Not Melt

Unlike stainless steel, nickel alloys have no self-cleaning mechanism. The oxides of nickel and chromium have very high melting points and will not melt into the weld pool. If left on the joint, they become entrained as oxide inclusions and show up on radiography.

To prevent this, removal must be done before striking the arc by one of the following:

• Grinding with a dedicated stainless or aluminum wheel down to bright metal.
• Abrasive blasting with clean, oxide-free media such as glass bead or garnet.
• Pickling in an appropriate acid for the alloy, followed by thorough rinsing and drying.

For full-penetration or closed section joints, the root side must be back purged with argon. Once welded, it cannot be cleaned, so protection from the start is the only option.

Low Melting Point Contaminants

Not every contaminant forms porosity. Elements such as sulfur, lead, phosphorus, zinc, and tin form low melting point films and eutectics at grain boundaries, widening the freezing range and drastically increasing hot cracking susceptibility. Sulfur and lead are especially severe, with lead able to cause cracking at trace levels. Zinc, often from galvanizing or brass fixtures, and tin from solder or bronze tooling are equally damaging.

Housekeeping For Nickel Alloys

Nickel alloys punish cross-contamination more than mild steel ever could. A strict

Housekeeping For Nickel Alloys

Nickel alloys punish cross-contamination far more than mild steel does. A workable protocol covers the whole job, not just the joint:

• Dedicated stainless brushes and wire wheels for nickel work. Never the brush that touched carbon steel. A single carbon steel particle can seed a crack in a 718 joint.
• New or dedicated grinding wheels. Treat a nickel wheel the way you treat one for aluminum or stainless.
• No shared files, wire wheels, or dirty benches. If sharing is unavoidable, clean the tool first.
• Sealed filler rod storage, off the floor and away from rust, dust, and moisture.
• Clean handling of finished parts with gloves so sweat and skin oils never reach the weld zone.
• Removal of all marking and thermal crayons from the joint area. Approved temperature-indicating crayons still leave a film, and solvent-based markers can be worse.

Lack of Fusion, Cold Lap, Tungsten Inclusion, Spatter, Undercut and Arc Strikes

Lack of fusion and cold lap both come from the sluggish nickel pool. The molten metal will not wet out and flow into the joint the way stainless does, so too much travel speed, too little amperage, or a narrow bevel lets the filler run over the base metal without bonding to it. You see a dark line along the bead toe, and radiography shows a sharp unfused boundary. Fix it with more heat, slower travel within your heat input limit, a wider bevel, and a bead that overlaps both sides of the joint.

Tungsten inclusion happens when the tungsten dips into the puddle, the arc wanders during a dip, or the electrode is ground into the work. It reads as a hard, dense defect on radiography. Keep the tungsten 10 to 15 degrees off the work surface, and if inclusions become frequent, your arc length is too long or your amperage exceeds the tungsten rating.

Spatter on nickel alloys is uncommon but appears with excessive amperage for the joint thickness, an overly long arc, or contaminated surfaces. High amperage on a dirty joint produces the worst spatter, and spatter on nickel is stubborn to remove.

Undercut comes from too fast a travel speed, an excessive weave, or too much amperage. The narrow, sluggish pool lags behind a wandering torch, so the arc digs into the side of the joint instead of melting it. Narrow the weave, pause at each side, and check heat input against the limits before reducing anything else.

Arc strikes on parent metal outside the joint are a serious problem, not a cosmetic one. A strike on Inconel can leave a hard, cracked spot that fails NDE. Grind them out completely and re-inspect, or relocate your start and stop points and use run-on tabs.

Defect to Cause to Fix

Defect	Likely cause	Corrective action
Round scattered porosity	Air entrainment, inadequate shielding, failed purge, worn torch cup, arc too long	Check gas flow and leaks, inspect cup and gas ring, shorten arc, verify purge with oxygen meter
Fine clustered porosity	Hydrogen from oil, grease, moisture, or dirty wire spool	Degrease and dry the joint, check spool and rod storage, increase deoxidized filler usage
Worm-like or linear indications	Trapped oxide film, slag, or unmelted contamination rolled into the bead	Grind or blast the joint back to bright metal, clean between passes, check for copper or tool contamination
Oxide inclusions	Nickel and chromium oxides with very high melting points that will not melt into the pool	Mechanical or chemical oxide removal, maintain back purge on the root side
Lack of fusion, cold lap	Sluggish pool, insufficient heat, too fast travel, too narrow a bevel	Open the bevel, widen the bead, lower travel speed, confirm heat input is within limits
Tungsten inclusion	Tungsten dipped or touched the puddle, arc too long, amperage above tungsten rating	Correct torch angle and arc length, replace the tungsten, re-inspect the area
Undercut	Excessive travel speed, wide weave, too much amperage	Narrow the weave, pause at each side, reduce amperage or increase travel speed within limits
Spatter	Excessive amperage for thickness, long arc, contaminated surface	Correct amperage range, clean the joint, shorten the arc
Cracks at grain boundaries	Sulfur, lead, zinc, tin, or phosphorus contamination; aged base metal	Restore cleaning discipline, use certified low-impurity filler, verify the stock is in the correct condition
Arc strike crack	Arc started or stopped on the parent metal	Grind out fully, re-inspect, use run-on and run-off tabs

One safety point closes this section. Nickel and chromium in welding fume can cause dermatitis or a skin rash in sensitive individuals, and nickel alloy fume also carries elements such as manganese and copper. Check the safety data sheet for the specific alloy and filler, respect the local exhaust requirements in 29 CFR 1926.353 for ventilation and respiratory protection, and read the OSHA welding safety fact sheet alongside AWS Safety and Health Fact Sheet No. 4. Good fume control protects the welder and keeps the work area clean enough that contamination stays out of the joint.

For more on shielding and gas handling before moving to inspection and qualification, our AllGas welding supply and gas services guide covers cylinder selection, regulators, and flowmeter setup in detail.

Inconel 718 in Detail: Heat Treatment, Laves Phase and Aging

Most nickel alloys in this guide can be welded and used roughly as-welded. Inconel 718 cannot. It is a precipitation-hardened superalloy whose strength comes from gamma-prime (Ni₃(Al,Ti)) particles that only form in a controlled ageing cycle after welding. Weld it aged and you get a hard, crack-sensitive heat-affected zone beside an un-tempered weld.

The procedure is not a mystery. It is a defined sequence used on jet engine parts, pressure vessels and high-temperature piping for decades, and most 718 failures trace back to two shortcuts: welding material that was already aged, or skipping the post-weld solution anneal.

Why 718 Must Be Welded in the Solution-Treated Condition

TWI’s review of nickel alloy weldability confirms the rule: precipitation-hardened alloys must be welded in the solution-treated or annealed condition. Precipitates on grain boundaries in the heat-affected zone are brittle, and when the weld contracts they crack. The fix is simple: remove them before you light the torch.

The Standard Production Sequence

1. Solution anneal the base material to 955–980 °C (1750–1800 °F). Strengthening precipitates dissolve, leaving a soft, single-phase, ductile austenitic structure. Plate for welding is normally supplied in this condition.
2. Weld in the solution-annealed condition, with a filler matched to the service requirement and the heat input discipline covered earlier in this guide.
3. Post-weld re-solution anneal at the same 955–980 °C (1750–1800 °F) range. This relieves residual stress and homogenises the as-welded microstructure, and it is done promptly after welding.
4. Double age at 720 °C (1325 °F) for 8 hours, furnace cool to 620 °C (1150 °F), then hold to a total ageing time of 18 hours.

Run correctly, that sequence delivers a joint with strength at 90 percent or better of the base metal, including the weld zone that failed under test before heat treatment.

What Goes Wrong When the Sequence Is Broken

Weld aged material, then re-age it without a solution anneal in between, and you get an overaged heat-affected zone that ends up softer and more crack-prone than the base metal. Strain-age cracking risk rises as well, because the HAZ carries a hardened, non-ductile grain boundary film while the weld is still contracting. TWI’s guide to welding nickel alloys is blunt: any weldment that will be post-weld heat treated must be welded from solution-treated material and given a full solution anneal before ageing.

Niobium Segregation and Brittle Laves Phase

The other failure path is metallurgical. Niobium has a strong affinity for the liquid phase, so it concentrates in the last liquid to freeze along the weld centreline. When that enriched liquid solidifies it forms brittle, Nb-rich Laves phase alongside niobium carbide, often as a semi-continuous film along grain boundaries with almost no ductility. That is where centreline cracks start.

Research on GTAW of Inconel 718 attributes the narrow weldability window to this mechanism: high heat input and slow cooling give segregation and Laves phase time to proceed, while a refined fusion zone delivers improved tensile properties. One study on IN718 weldability and fusion-zone refinement makes the case directly, and testing of welded 718 sheet reports weld strength and elongation below base metal values with fracture initiating in the fusion zone.

• Low heat input, so the weld spends less time in the vulnerable temperature range.
• Small beads, which shorten the time the pool sits at peak temperature.
• Fast cooling, so niobium stays fine and dispersed rather than forming a continuous brittle film.
• Fine grain stock, ASTM 5 or finer on the certificate, which reduces heat-affected-zone microfissuring susceptibility, a requirement spelled out in AWS D17.1 for aerospace fusion welding of nickel alloys.

The Deliberate Dilution Trade-off

Not every 718 joint gets aged. Field repairs, dissimilar joints and structures that stay at ambient temperature are common cases, and diluting deliberately with a softer, more ductile filler is often smarter. ERNiCr-3 (Inconel Filler Metal 82, UNS N06082), classified under AWS A5.14, gives far better ductility and solidification cracking resistance than a matching-age filler.

The price is strength. An ERNiCr-3 weld in 718 comes out at roughly half the aged strength of the base metal: fine for a seal weld that will never be heat treated, hopeless for a hot-section joint that must match parent metal. For corrosion or dissimilar work, Inconel Filler Metal 625 (ERNiCrMo-3) is another common choice.

Refining the Fusion Zone on Critical Work

Where segregation cannot be tolerated, fabricators reach for process control rather than chemistry alone, and this is where advanced inconel TIG welding setups earn their keep. Pulsed GTAW separates heat into a fast melting cycle and a shorter holding cycle, flattening the bead profile and giving the pool less time to segregate. Magnetic arc oscillation stirs the pool and refines fusion-zone grains, while a constricted arc concentrates heat and narrows the fusion line. On aerospace 718 work these are written into the procedure rather than left to operator judgment. Our aerospace TIG welding guide covers the standards and training side.

Put simply: 718 gives you strength only if you sequence it correctly, and a sound weld only if you keep the pool small and fast. Check the certificate for condition and grain size before the plate reaches the shop floor.

Inspection, Qualification, Codes and a Practical Troubleshooting Workflow

The weld is finished, the bead looks clean, everyone walks away. Two months later a customer finds a centerline crack in a 625 header. That is the scenario that keeps fabricators up at night, and the reason inspection and qualification are not paperwork bolted onto the end of a job. For nickel alloys they are part of the welding operation itself. Specialist fabricators note that nickel alloy welds in critical service often call for penetrant or radiographic inspection, and the smart move is to put inspection requirements on the drawing before the job is quoted, because NDT on Inconel is a real line item and surprises show up as change orders.

Which Standard Applies to Your Job

Three frameworks cover most Inconel TIG work, and they answer different questions.

• AWS D17.1 is the aerospace fusion welding standard, and it explicitly covers nickel-based alloys along with titanium and steel. If your Inconel work goes into an engine, a nacelle, a structural frame or anything else aerospace-adjacent, this is your governing document. See the broader requirements in our aerospace TIG welding standards guide.
• ASME Section IX governs qualification of welders and welding procedures for pressure vessels, boilers and piping. It is the paperwork that lets you legally weld an Inconel pressure part. Filler metal classification comes in through ASME Section II Part C, which references the AWS A5.14 filler metal specifications, so the rod classification on your WPS and the certificate in your rod box have to agree.
• AWS B4.0 is the standard for weld testing and inspection, covering the mechanical test methods and acceptance criteria used to qualify welds. It is also the test standard used by researchers who publish interpass temperature studies on nickel alloys.

The key paperwork concept is the WPS (Welding Procedure Specification) and its PQR (Procedure Qualification Record). The WPS states the variables: joint design, filler classification and diameter, gas and flow rates, amperage, voltage, travel speed, interpass temperature limits, heat input ceiling, cleaning method and inspection method. The PQR documents the coupon that proved the procedure works, with the actual results and the qualified ranges. Compare that with how all-position filler selection forces you to think in qualified ranges rather than a single lucky setting.

Inspection Methods and What Each One Actually Finds

Method	What it detects	Limitations
Visual (VT)	Profile, reinforcement, undercut, bead shape, spatter, arc strikes, misalignment	Surface only; depends on lighting and access
Dye penetrant (PT)	Fine surface-breaking cracks and porosity	Misses subsurface defects; needs clean, uncoated surface
Radiography (RT)	Internal porosity, lack of fusion, inclusions, some cracks	Costly, access both sides, radiation controls
Ultrasonic (UT)	Volumetric and deep flaws, laminations	Troublesome on rough nickel surfaces; needs a skilled technician
Macro / microetch	Grain structure, fusion boundary, filler dilution, Laves phase and Nb segregation	Destructive, usually on cut-up coupons

Visual inspection catches more than most people expect. A dull, lumpy bead on Inconel usually means the filler was cold, the arc was too long, or the pool was contaminated. The same bead might pass a casual look but show up as a reject under magnification.

For general structural work, AWS D1.1 gives the reference points most shops already know: maximum profile reinforcement and a limit on undercut depth, with criteria tightening as required strength rises. Exact numbers depend on the specification, joint orientation and code, so read the clause rather than working from memory. The same discipline applies to heat input. Verifying interpass temperature with a calibrated thermocouple or contact pyrometer is standard practice in critical work, whereas thermal crayons are a rough visual aid. Crayons leave residue on nickel alloy, and that residue must be removed before welding or it ends up in the weld.

Troubleshooting Workflow: Symptom to Cause to Fix

Symptom	Likely cause	Corrective action
Centerline crack in the bead	Solidification cracking, narrow bead, high restraint, low heat input	Widen the bead, more convex profile, add crater fill, lower restraint, check heat input is not below minimum
Cracking in the HAZ after PWHT	Strain-age cracking in aged or precipitation-hardened material	Weld in solution-treated condition, use ductile diluting filler such as 625 or C-263, control heating and cooling
Porosity, scattered and round	Oxygen or nitrogen entrainment, wet filler, leaking gas	Purge before striking the arc, verify purge with an oxygen meter, check gas lines and moisture, dry filler storage
Black specks or slag-like particles	Oxide that did not melt (nickel oxides melt at very high temperature)	Grind, abrasive blast or pickle the joint; nickel alloys do not self-clean like stainless
Lack of fusion	Sluggish pool, insufficient prep, too little heat, oxide in the joint	Open the bevel angle, increase root gap and heat, clean the joint immediately before welding
Undercut	Travel speed too fast, weave too wide, no pause at bead edges	Slow down, narrow the weave, pause at each side
Tungsten inclusion	Broken or dipped tungsten, contaminated filler	Stop the arc before touching the pool, replace the tungsten, check for loose connections

The theme running through every row is the one behind this entire guide. Nickel alloys behave differently from stainless, so stainless habits fail. TWI’s guidance on welding nickel alloys is consistent: keep the joint clean, keep the shielding inert, control the heat, and use stringer beads with deliberate filler placement. Research on IN718 hot cracking adds a caveat: process parameter identification remains largely empirical, but controlling composition, grain size, traverse speed and heat input is exactly what makes fine-grained wrought alloys such as IN718 and Waspaloy weldable in the first place. Inspection cannot rescue a weld made outside that window, and no paperwork substitutes for it.

The Economics of Doing It Right

Inconel welds cost more, and it is worth being honest about why. Base material is expensive. Filler metal is expensive, and nickel filler carries the alloy content plus machining and certification costs. Argon adds up by volume, especially with back purging on long runs. Labor runs slower because the pool will not self-flow, the joint must be prepped wider, and interpass limits mean you wait. NDT adds a significant line item.

Cost control comes from doing the boring things right. Reuse purge dams instead of taping fresh ones every joint. Buy filler in spooled form for production volume and cut lengths for short runs to avoid shelf life loss. Use a smaller diameter rod on thin material to reduce heat input and rework. Set up fit-up tooling so joints land square the first time. Use pre-qualified procedures so you are not burning hours developing settings a WPS already covers. Firms that publish their equipment and consumable sourcing openly tend to win the bids that matter.

Conclusion and Field Checklist

Inconel TIG welding rewards discipline. Pick the right filler for the alloy and service condition, keep the joint clean, shield it properly, control heat input, use narrow stringer beads, and inspect to a standard you can name. The cracks that ruin Inconel jobs trace to four things: porosity, lack of fusion, solidification cracking, or microfissuring.

• Identify the alloy by UNS number, not trade name. Know if it is solid solution or precipitation hardened.
• Confirm filler classification against AWS A5.14, ASME Section II Part C, or the AMS spec on the certificate.
• Verify the gas: argon, correct flow, purge started before the arc strikes.
• Set the procedure within a documented heat input and interpass range, and measure temperature with a thermocouple.
• Keep the joint clean with dedicated stainless tools. Never let a carbon steel wheel touch the metal.
• Use stringer beads, keep the weave narrow, fill the crater, let the pool cool between passes.
• Choose the inspection method from the drawing and the code: VT, PT, RT, UT, macroetch.
• Keep the WPS, PQR and inspection records. They are what makes the job defensible later.

Weld it clean, weld it cool, weld it slow, and weld it once. That habit will do more for your Inconel work than any piece of equipment on the bench.

Frequently Asked Questions

What filler metal should I use for TIG welding Inconel 625?

The standard matching filler is ERNiCrMo-3 (AWS A5.14), sold as INCONEL Filler Metal 625, UNS N06625, typically with roughly 62% Ni, 21.5% Cr, 9% Mo and 3.7% Nb. It is also the most common choice for dissimilar joints involving Inconel 625, stainless steel and carbon steel. ERNiCr-3 (Inconel 82 type) is a softer, more ductile alternative when higher crack resistance matters more than matching strength.

Do I need to preheat Inconel before welding?

Usually not. Preheating is generally unnecessary unless the risk of porosity from condensation exists, for example when the base metal is colder than the workshop air temperature. Below roughly 5 °C, a moderate preheat of 40–50 °C is recommended to prevent condensation and the porosity it causes. Preheating is not a substitute for keeping the joint clean and dry.

What is the maximum interpass temperature for Inconel?

TWI advises that interpass temperature should generally not exceed 250 °C, though some suppliers specify as low as 100 °C for particular alloys such as C-276. For alloys with high chromium and molybdenum content that will see corrosive service, guidance suggests a maximum interpass of 150 °C with maximum heat input of 1.5 kJ/mm. For Inconel 718 practice, an interpass limit around 120 °C (250 °F) is common.

Why does my Inconel TIG weld crack along the centerline?

Centerline solidification cracking usually points to a flat or concave bead profile, a wide freezing range, low-melting films at the grain boundaries, or excessive restraint. Produce a fuller, more convex bead, fill the crater completely, clean the joint of sulfur and other contaminants, and check heat input and travel speed. Remember that going too fast to hold heat input down can narrow the bead and make centerline cracking worse rather than better.

Can I weld Inconel with stainless steel TIG wire?

No. Stainless fillers contain no niobium, molybdenum or the other alloying elements Inconel depends on for corrosion resistance and strength, and they introduce unwanted dilution into the weld. Always use an AWS A5.14 nickel-alloy filler matched to the base metal or a deliberately chosen dissimilar filler such as ERNiCrMo-3.

Is argon the right shielding gas for Inconel TIG welding?

Yes, argon is the default shielding gas for GTAW on nickel alloys and for back purging because it is inert and heavier than air. TWI notes that argon-hydrogen mixtures tend to produce cleaner TIG welds, but hydrogen increases porosity risk from contamination and argon is the standard, widely approved choice. Never use a shielding gas that is not certified for the application, and never weld without adequate ventilation.

How hot should I run the arc on Inconel 625?

Published starting points for GTAW on 625 are around 80–125 amps, 14–18 volts and90–120 mm/min travel speed with DCEN, argon flow of 10–20 L/min and interpass held at 175 °C or below. Treat these as a starting range for procedure development only; qualify your own WPS and confirm the heat input stays inside the limit specified for your service condition and alloy.

Does Inconel need to be heat treated after welding?

Post-weld heat treatment is generally not needed to restore corrosion resistance in solid solution alloys. It may be required for precipitation-hardening alloys such as 718, which is normally welded in the solution-annealed condition, re-solution annealed after welding, and then double aged. Certain alloys, such as UNS N02200, may need PWHT for optimum stress corrosion resistance in specific environments.

What causes porosity when TIG welding Inconel?

Three main causes: oxygen and nitrogen entrained from the air, surface oxides that will not melt and are carried into the pool, and hydrogen from surface contamination. Remove oxides mechanically or by pickling immediately before welding, degrease again afterwards, and use filler metals containing deoxidizers such as aluminium and titanium. Check shielding gas flow, purge effectiveness and argon purity.

Is TIG the right process for Inconel?

For thin to moderate thickness, repair work, and precise or highly cleanable joints, TIG is usually the best balance of control and quality, and it is the standard for many718 job-shop and aerospace repairs. For thicker sections or production volume, GTAW can be slower and less economical than GMAW or submerged arc, and electron beam or laser welding offer deeper, narrower, lower-heat-input fusion. The process choice should follow the governing code and the joint design.