Inconel 625 is one of the most widely welded nickel alloys in heavy industry. It appears in offshore platform seawater systems, aerospace combustion hardware, chemical processing vessels, and nuclear reactor components. Its reputation for good weldability — relative to other precipitation-hardened nickel alloys — is well deserved. But “good weldability” does not mean “easy to weld.” The alloy’s sensitivity to cleanliness, its tendency toward heat-affected zone (HAZ) sensitization in the 650–875°C range, and its susceptibility to cracking from excessive heat input or restrained joint design all require deliberate technique. This article provides the specific parameters, filler metal selection logic, and procedural guidance needed to produce sound Inconel 625 welds reliably.
Inconel 625: Why Weldability Is Good — But Not Simple
Inconel 625 is classified as a solid-solution strengthened alloy, not a precipitation-hardened alloy. This distinction matters for welding. Alloys strengthened by coherent precipitates (Inconel 718 with γ″, Waspaloy with γ′) are susceptible to strain-age cracking — a weldability problem caused by precipitation during the weld thermal cycle in the HAZ. Inconel 625’s strengthening comes primarily from molybdenum and niobium in solid solution, with secondary γ′ (Ni₃Al, Ni₃Ti) and γ″ (Ni₃Nb) precipitation contributing to its high strength. This means Inconel 625 does not experience the same severe precipitation-related cracking risk as its precipitation-hardened cousins.
However, the alloy’s 3.5% niobium content introduces a different constraint: niobium has a strong tendency to segregate during solidification. In high heat input welds or slow freezing conditions, niobium enriches the last-solidifying interdendritic liquid, creating regions susceptible to microfissuring — a form of hot cracking.
The other important metallurgical constraint is the HAZ sensitization window. Between 650°C and 875°C (923K–1148K), niobium and molybdenum precipitate as γ″ (Ni₃Nb, approximately 700°C) and δ phase (Ni₃Nb, orthorhombic, approximately 725°C). These precipitates can reduce both creep performance and corrosion resistance in the HAZ. The practical engineering implication: keep interpass temperatures low, and avoid multiple high-heat passes that allow the HAZ to linger in this temperature band.
Filler Metal Selection: ERNiCrMo-3 vs ERNiCrMo-4 vs ERNiCrMo-10
The choice of filler metal is the single most consequential decision in Inconel 625 welding. Three AWS A5.14 electrodes are commonly considered.
ERNiCrMo-3: The standard choice for Inconel 625
ERNiCrMo-3 (AWS A5.14, UNS N06625) is the designated filler metal for welding Inconel 625 to itself. Its composition mirrors the base metal closely — including niobium at 3.15–4.15% — producing weld metal that matches the base metal’s corrosion resistance and mechanical properties.
AWS A5.14 ERNiCrMo-3 composition (typical):
| Element | Range | Typical |
|---|---|---|
| Carbon | ≤0.10% | 0.03% |
| Manganese | ≤0.50% | 0.20% |
| Iron | ≤5.0% | 1.5% |
| Phosphorus | ≤0.030% | 0.005% |
| Sulfur | ≤0.015% | 0.003% |
| Silicon | ≤0.50% | 0.15% |
| Chromium | 20.0–23.0% | 21.5% |
| Molybdenum | 8.0–10.0% | 9.0% |
| Niobium | 3.15–4.15% | 3.6% |
| Titanium | ≤0.40% | 0.20% |
| Aluminum | ≤0.40% | 0.15% |
| Nickel | Balance | ~61% |
Mechanical properties of as-deposited ERNiCrMo-3 weld metal:
- Tensile strength: 760–900 MPa (typically ~830 MPa)
- Yield strength (0.2% offset): 380–460 MPa (typically ~415 MPa)
- Elongation: 30–40%
ERNiCrMo-3 weld metal delivers excellent resistance to pitting (PREN ~53 for weld metal), stress corrosion cracking, and oxidizing/reducing acid environments. It is suitable for service temperatures from cryogenic to approximately 550°C.
ERNiCrMo-4: For Hastelloy C-276 and matching chemistries
ERNiCrMo-4 (UNS N10276) contains no niobium and a higher molybdenum content (15.0–17.0%). It is the correct choice for welding Hastelloy C-276 to itself. Using ERNiCrMo-4 on Inconel 625 is technically acceptable but suboptimal: the absence of niobium means the weld metal’s mechanical strength will be lower than base metal, and the corrosion resistance profile is different (optimized for C-276’s environments rather than 625’s).
ERNiCrMo-4 weld metal has higher molybdenum than ERNiCrMo-3 (15–17% vs. 8–10%), making it more resistant in reducing acid environments (hydrochloric acid, sulfuric acid). However, its lower chromium (14.5–16.5% vs. 20–23%) makes it less resistant in oxidizing environments.
When to use ERNiCrMo-4 on Inconel 625: When welding a component that will be internally clad with Hastelloy C-276 overlay, or when fabricating a transition joint where Inconel 625 will eventually be overlaid with C-276.
ERNiCrMo-10: For maximum corrosion resistance
ERNiCrMo-10 (UNS N06210) is the highest-alloy filler metal in this group: 19–21% Cr, 15–17% Mo, plus 2.5–3.5% tungsten. Like ERNiCrMo-4, it contains no niobium. Its corrosion resistance in oxidizing acid environments (nitric acid, mixed acid) is superior to ERNiCrMo-3, and its resistance to chloride-induced pitting is among the highest available.
When to use ERNiCrMo-10 instead of ERNiCrMo-3: In highly corrosive process environments — mixed acid streams, oxidizing chloride solutions, or any service where the specific corrosion profile of ERNiCrMo-10 is known to be superior. For matching Inconel 625 service, ERNiCrMo-3 remains the first choice.
Filler metal selection summary
| Application | Recommended Filler | Notes |
|---|---|---|
| Inconel 625 to Inconel 625 | ERNiCrMo-3 | Standard choice; matches base metal |
| Hastelloy C-276 to C-276 | ERNiCrMo-4 | Matches C-276 chemistry |
| Inconel 625 in aggressive oxidizing acids | ERNiCrMo-10 | Higher Cr, no Nb |
| Inconel 625 to carbon steel | ERNiCrMo-3 | Buttering with 309L also common |
| Inconel 625 to 316L stainless | ERNiCrMo-3 or ERNiCrMo-3/ER309L | Match strength; 309L for dissimilar |
| Transition to Hastelloy C-276 overlay | ERNiCrMo-4 | For compatibility with overlay |
GTAW (TIG) Welding Inconel 625: Parameters by Thickness
Gas Tungsten Arc Welding (GTAW) — also called TIG welding — is the preferred process for Inconel 625 when weld quality is paramount: nuclear components, aerospace hardware, instrumentation piping, and thin-walled process vessels. The process delivers the best control over bead profile, penetration, and porosity.
Equipment setup
Tungsten electrode: EWCe-2 (2% ceriated tungsten) in 2.4 mm diameter for most applications. Use DCEN (straight polarity). Grind the electrode to a 30–45° included angle; a blunt end increases arc wander and reduces arc stability.
Shielding gas: 100% Argon. Argon-helium mixtures (75% Ar / 25% He) can be used for deeper penetration on thicker sections but are not necessary for most Inconel 625 GTAW work. Helium increases heat input and can exacerbate hot cracking in niobium-bearing alloys if misused.
Back purge: Essential for root pass quality. Use 100% Argon at 5–10 CFH (14–28 L/min) through a purge dam or backing gas fixture. Inconel 625 weld roots are highly susceptible to oxidation, which appears as gray discoloration and causes porosity.
Welding parameters by joint configuration
| Thickness | Joint Type | Current (A) | Voltage (V) | Travel Speed | Heat Input |
|---|---|---|---|---|---|
| 1.5–3 mm sheet | Butt | 50–80 | 10–12 | 100–150 mm/min | ~2–4 kJ/in |
| 3–6 mm plate | Butt / Vee | 80–150 | 12–14 | 80–120 mm/min | ~3–5 kJ/in |
| 6–12 mm plate | Vee (60°) | 150–220 | 14–17 | 60–100 mm/min | ~5–7 kJ/in |
| SCH 10–40 pipe | Butt Vee | 100–160 | 12–15 | 80–120 mm/min | ~3–5 kJ/in |
| SCH 80 pipe | U-groove | 150–200 | 14–16 | 60–90 mm/min | ~5–7 kJ/in |
Key rule: Keep heat input in the 2–5 kJ/in range. Below 2 kJ/in: cold lap and lack of fusion. Above 7 kJ/in: increased hot cracking risk and HAZ sensitization from niobium segregation.
Shielding gas flow rates
| Torch size | Flow rate |
|---|---|
| 4–5 mm collet body | 10–15 CFH (280–425 L/min) |
| 6–8 mm collet body | 15–20 CFH (425–565 L/min) |
| Back purge | 5–10 CFH (140–280 L/min) |
GMAW (MIG) and FCAW for Inconel 625
GMAW (MIG) with ERNiCrMo-3 filler wire is used for thicker sections where deposition rate matters more than the precise bead control of GTAW. The spray transfer mode is preferred for Inconel 625 GMAW, using 100% Argon or Ar+20–30% He.
GMAW parameters (typical, 1.2 mm wire):
| Thickness | Current | Voltage | Wire Feed Speed | Gas |
|---|---|---|---|---|
| 3–6 mm | 180–220 A | 26–30 V | 6–8 m/min | 100% Ar |
| 6–12 mm | 220–280 A | 28–32 V | 8–10 m/min | Ar+25% He |
| 12+ mm | 280–350 A | 30–35 V | 10–12 m/min | Ar+25% He |
FCAW (flux-cored arc welding) with nickel alloy FCAW electrodes is possible but less commonly used for Inconel 625. FCAW introduces slag, which must be completely removed between passes, and the resulting weld metal chemistry is less precise. When FCAW is used, gas-shielded FCAW (FCAW-G) with an argon-CO₂ or argon-O₂ shielding gas is preferred over self-shielded FCAW, which produces nitrogen porosity in nickel welds.
Preheat, Interpass Temperature, and PWHT
Preheat
Inconel 625 does not require preheat. In fact, preheating is counterproductive: it raises the interpass temperature, widens the HAZ sensitization band, and increases the risk of hot cracking. Keep the base metal dry and at room temperature.
The only exception is for thick-section welds in a shop environment where ambient temperature is below 10°C and surface condensation is present — in which case light局部预热 to 15–20°C may be used to drive off moisture, but never above 50°C.
Interpass temperature: The most critical process control parameter
Maximum interpass temperature: 150°F (65°C). This is not a suggestion. It is the controlling limit for maintaining HAZ corrosion resistance in Inconel 625.
Between 650°C and 875°C, niobium-rich precipitates form in the HAZ. Each pass reheats the previous pass’s HAZ into this temperature band. At temperatures below 150°F (65°C), the time in this sensitization window is short enough that precipitate formation is limited. Above 150°F, longer time at temperature allows γ″ and δ phase to accumulate, degrading corrosion resistance.
For multi-pass welds on thick sections, monitor interpass temperature with a contact pyrometer or thermocouple probe. Do not begin the next pass until the temperature has fallen below 150°F.
Post-weld heat treatment (PWHT)
For most Inconel 625 corrosion applications, PWHT is not required. Inconel 625 is used precisely because it provides adequate strength in the solution-annealed condition. Applying PWHT unnecessarily introduces risk: the stress relief temperatures typically used for carbon steel (550–650°C) fall directly within the sensitization window.
When PWHT is required (pressure vessel code compliance):
- Stress relief at 600–650°C for code-stamped vessels, but understand the trade-off in corrosion performance.
- Solution anneal at 1,100–1,150°C for maximum corrosion resistance after fabrication, followed by rapid air cooling. This dissolves all HAZ precipitates and restores the base metal chemistry. However, solution annealing of weldments requires furnace access and careful temperature control.
The metallurgical recovery shortcut: If solution annealing is not feasible but HAZ sensitization is a concern, a stabilization hold at 875°C (1,148K) for 5 hours will dissolve γ″ and δ phases. This is not equivalent to full solution annealing but significantly reduces HAZ carbide precipitation.
Dissimilar Metal Welding: Inconel 625 to Stainless Steel and Carbon Steel
Inconel 625 to 316L stainless steel
This is the most common dissimilar combination involving Inconel 625, typically encountered when a 316L stainless steel piping system transitions to an Inconel 625 section in a seawater or acidic service.
Two valid approaches:
Option 1 — Direct weld with ERNiCrMo-3: Acceptable when the 316L is the thinner member and the service environment favors the Inconel 625 weld metal. The weld dilution zone will contain some iron but retains sufficient nickel and chromium to maintain reasonable corrosion resistance.
Option 2 — Buttering with ER309L, then weld with ERNiCrMo-3: Preferred for production welds where repeatability matters. Buttering the stainless steel surface with ER309L (~3 mm layer) creates a transition zone that reduces dilution concerns. Then complete with ERNiCrMo-3 over the butter layer.
Inconel 625 to carbon steel
Carbon steel to Inconel 625 dissimilar welds are challenging because of the large difference in thermal expansion coefficients and the tendency for carbon migration across the weld interface.
Recommended approach:
- Butter the carbon steel surface with ER309L (minimum 3 mm build-up).
- Post-weld heat treat the butter layer (normalizing or PWHT as required by code).
- Machine or grind the butter surface to clean metal.
- Complete the joint with ERNiCrMo-3.
ERNiCrMo-3 directly applied to carbon steel will produce a fusion boundary with carbon migration into the weld metal, forming a brittle heat-affected zone on the carbon steel side. This approach is acceptable only for non-critical applications with low stress and benign environment.
Common Defects and Prevention
Hot cracking (microfissuring): Inconel 625 welds are susceptible to microfissures in the weld metal and partially fused zone due to niobium segregation in the last-solidifying interdendritic liquid. Prevention: keep heat input moderate (3–5 kJ/in), use small electrode/wire diameters for better control, and ensure adequate filler metal addition to reduce restraint at the solidification front.
Porosity: Caused by moisture, oil, grease, sulfur, or inadequate gas coverage. Prevention: strict surface cleaning before welding (solvent wipe + acetone; wire brush dedicated to nickel alloys only — never use a brush previously used on stainless steel), adequate shielding gas flow rate and coverage, and proper back purge on pipe/root passes.
Lack of fusion: More common in GTAW of Inconel 625 than in steel welding because the weld pool “lies flat” rather than flowing forward, making it easy to miss the fusion line. Prevention: use a tighter arc length, angle the tungsten to pre-heat the joint wall ahead of the pool, and reduce travel speed slightly to allow the pool to wet the sidewalls.
HAZ corrosion degradation: If interpass temperatures exceed 150°F (65°C) repeatedly, the HAZ accumulates γ″ and δ phase precipitates that reduce corrosion resistance. The HAZ may appear visually acceptable but fail in service by preferential attack along the sensitization band. Prevention: interpass temperature control; if sensitization is suspected, evaluate with ASTM G28 Method A (ferric sulfate-sulfuric acid test) or ASTM A262 Practice E (Huey test).
Summary: The Five Non-Negotiable Rules for Inconel 625 Welding
- ERNiCrMo-3 is the default filler metal. Do not substitute ERNiCrMo-4 or ERNiCrMo-10 unless the specific service environment justifies it. Changing filler metals changes the corrosion profile of the weldment.
- Keep interpass temperature below 150°F (65°C). This is the single most important process control parameter. It directly governs HAZ sensitization and weld metal mechanical properties.
- Cleanliness is non-negotiable. Dedicated stainless steel brushes only. No oil, grease, sulfur compounds, or moisture. Solvent wipe followed by acetone wipe. Any contamination produces porosity that is difficult to detect and catastrophic in service.
- Control heat input. 3–5 kJ/in for GTAW. Too low: lack of fusion. Too high: hot cracking from niobium segregation.
- Match the filler metal to the service environment, not just the base metal. For seawater: ERNiCrMo-3 weld metal with ~53 PREN. For high-temperature oxidation: consider ERNiCrMo-10. For mixed acid: evaluate ERNiCrMo-10 vs. ERNiCrMo-4 against the specific acid composition.
Related articles: Welding Nickel Alloys: Common Challenges and Solutions, Corrosion Resistance Comparison: Which Nickel Alloy Lasts Longest?, How to Inspect Nickel Alloy Materials: PMI and Beyond
