Introduction to Superalloy Welding
Superalloys, also known as high-performance alloys or high-temperature alloys, are engineered to operate in extreme environments involving high temperatures, corrosive media, and mechanical stress. The most commonly welded superalloys belong to the nickel-based family, including Inconel, Hastelloy, Monel, and Incoloy grades. Welding these materials requires specialized knowledge of their metallurgical behavior, proper filler metal selection, precise heat input control, and rigorous procedural compliance to avoid solidification cracking, liquation cracking, and heat-affected zone (HAZ) degradation.
This guide provides comprehensive welding procedures for the most widely used superalloys, covering process selection, filler metal matching, parameters, and quality control requirements.
Superalloy Classification and Weldability
Nickel-Based Superalloys
| Alloy | UNS | Primary Application | Weldability |
|---|---|---|---|
| Inconel 625 | N06625 | Marine, chemical processing, aerospace | Excellent |
| Inconel 718 | N07718 | Aerospace turbine components, high-strength | Good (requires post-weld heat treatment) |
| Inconel 825 | N08825 | Oil & gas, sour service, chemical | Good |
| Inconel 600 | N06600 | Heat treatment, chemical processing | Good |
| Hastelloy C-276 | N10276 | Chemical processing, flue gas desulfurization | Good (low heat input required) |
| Hastelloy C-22 | N06022 | Chemical, pharmaceutical, pollution control | Excellent |
| Hastelloy B-3 | N10675 | Hydrochloric acid service | Fair (high cracking susceptibility) |
| Monel 400 | N04400 | Marine, hydrocarbon processing | Good |
| Incoloy 800H/HT | N08810/N08811 | Petroleum cracking, heat treatment | Good |
Welding Processes for Superalloys
GTAW (Gas Tungsten Arc Welding / TIG)
GTAW is the preferred process for superalloy welding due to its precise heat input control, superior weld quality, and excellent shielding gas coverage.
- Current: DCEN (Direct Current Electrode Negative) for all nickel alloys
- Shielding gas: 100% Argon for thickness up to 6mm; Argon + 2-5% Hydrogen for thicker sections (increases heat input)
- Back purging: Mandatory for all superalloy welds; argon back purge to <50 ppm oxygen
- Tungsten: 2% ceriated or 1.5% lanthanated; pure tungsten for AC only (not used for Ni alloys)
GMAW (Gas Metal Arc Welding / MIG)
GMAW is used for higher deposition rates on thicker sections:
- Transfer mode: Pulsed spray or short-circuit for positions; spray transfer for flat/horizontal
- Shielding gas: Argon + 1-3% O₂ or Ar + 5-25% He for improved wetting
- Wire diameter: 0.8–1.2mm typical
SAW (Submerged Arc Welding)
SAW is used for heavy-wall longitudinal and circumferential seams in pipe and vessel fabrication:
- Matched wire and flux combinations per alloy manufacturer recommendations
- Basic fluxes preferred for better mechanical properties and lower diffusible hydrogen
Filler Metal Selection
| Base Metal | GTAW Filler (AWS) | GMAW Filler (AWS) | SAW Filler |
|---|---|---|---|
| Inconel 625 | ERNiCrMo-3 | ERNiCrMo-3 | ENiCrMo-3 wire + basic flux |
| Inconel 718 | ERNiCrMo-3 or ERNiFeCr-2 | ERNiCrMo-3 | Per alloy manufacturer |
| Inconel 825 | ERNiCrMo-3 | ERNiCrMo-3 | ENiCrMo-3 wire + basic flux |
| Hastelloy C-276 | ERNiCrMo-4 | ERNiCrMo-4 | ENiCrMo-4 wire + basic flux |
| Hastelloy C-22 | ERNiCrMo-10 | ERNiCrMo-10 | ENiCrMo-10 wire + basic flux |
| Monel 400 | ERNiCu-7 | ERNiCu-7 | ENiCu-7 wire + basic flux |
| Dissimilar: Ni alloy to carbon steel | ERNiCr-3 (INCONEL 82 filler) | ERNiCr-3 | Per procedure qualification |
Pre-Weld Preparation
Cleaning Requirements
- Remove all oil, grease, paint, marking compounds, and surface contaminants within 25mm of the weld joint
- Use acetone or alcohol-based solvent for degreasing; avoid chlorinated solvents
- Mechanical cleaning with stainless steel wire brush (dedicated to nickel alloys only)
- For critical applications, vapor degreasing followed by acetone wipe
Joint Design
- Root opening: 1.6–3.2mm for GTAW; 2.0–4.0mm for GMAW
- Land thickness: 0–1.6mm; thinner lands reduce fusion issues
- Bevel angle: 60–70° included angle for GTAW; 60–75° for GMAW
- Avoid sharp corners and abrupt transitions that concentrate stress
Welding Parameters
GTAW Parameters for Inconel 625
| Thickness (mm) | Current (A) | Wire Diameter (mm) | Travel Speed (mm/min) | Heat Input (kJ/mm) |
|---|---|---|---|---|
| 2–3 | 80–120 | 1.6–2.0 | 80–120 | 0.5–0.8 |
| 4–6 | 120–180 | 2.0–2.4 | 60–100 | 0.6–1.0 |
| 8–12 | 150–220 | 2.4–3.2 | 50–80 | 0.8–1.2 |
| >12 | 180–280 | 3.2–4.0 | 40–70 | 1.0–1.5 |
Crack Prevention
Solidification Cracking
Nickel alloys are susceptible to solidification (hot) cracking due to their wide freezing range and high sulfur/phosphorus sensitivity:
- Control heat input: Keep within the qualified range; excessive heat input increases cracking risk
- Maintain low dilution: Use buttering layers when welding to steel to reduce dilution of the nickel weld metal
- Avoid excessive restraint: Design joints to minimize shrinkage restraint
- Use appropriate filler: Over-alloyed fillers (higher Mo, Nb) improve crack resistance
Liquation Cracking (HAZ Cracking)
Liquation cracking occurs in the HAZ of precipitation-hardened alloys (718, X-750) where grain boundary constituents melt during welding:
- Solution anneal before welding when possible
- Use low heat input and multiple passes to minimize HAZ peak temperature
- Post-weld solution anneal for 718: 980°C ± 15°C for 1 hour, air cool + aging treatment
Post-Weld Heat Treatment (PWHT)
| Alloy | PWHT Requirement | Temperature | Duration |
|---|---|---|---|
| Inconel 625 | Generally not required | Stress relieve: 870–900°C if specified | 1–2 hours |
| Inconel 718 | Mandatory for full properties | Solution: 980°C → Age: 720°C (8h FC to 620°C, 8h) | Per AMS 5663 |
| Hastelloy C-276 | Not required | N/A | N/A |
| Monel 400 | Optional stress relief | 550–620°C | 1–4 hours |
| Incoloy 800H | Solution anneal for creep properties | 1120–1175°C | Per specification |
Summary
Welding superalloys such as Inconel, Hastelloy, and Monel demands strict adherence to qualified procedures, proper filler metal selection, controlled heat input, and comprehensive cleaning protocols. The primary challenges are solidification cracking, HAZ liquation cracking, and maintaining corrosion resistance in the weld zone.
CoreMetal Steel supplies nickel alloy pipe, tube, plate, and fittings in Inconel 625, 718, 825, Hastelloy C-276, C-22, and Monel 400 with full material traceability and mill test certificates. Contact our technical team for alloy selection and welding consumable guidance.