Inconel 718 (UNS N07718) is the most widely used precipitation-hardened nickel alloy, found in turbine discs, aerospace fasteners, rocket engines, and nuclear components. Unlike solid-solution alloys (625, 600), its strength comes from heat treatment. That creates a fundamentally different welding challenge: you must weld 718 in the solution-annealed (soft) condition, then age-harden it after welding. Welding the aged material and re-aging creates strain-age cracking, the most dangerous failure mode in nickel alloy welding.
The filler is ERNiFeCr-2, which matches the base metal’s chemistry and allows the weld deposit to respond to the same aging heat treatment as the base metal.
Why 718 Is Different from 625
Inconel 625 is solid-solution strengthened. Its properties come from the alloy chemistry itself, and welding doesn’t change them. Inconel 718 is precipitation-hardened. Its strength comes from gamma-double-prime (Ni3Nb) and gamma-prime (Ni3(Al,Ti)) precipitates formed during a specific aging heat treatment.
| Property | Inconel 625 | Inconel 718 (Solution Annealed) | Inconel 718 (Aged) |
|---|---|---|---|
| Tensile strength (ksi) | 120-145 | 120-140 | 180-210 |
| Yield strength (ksi) | 60-75 | 60-80 | 150-175 |
| Hardness (HRC) | ~20 HRC | ~25 HRC | 36-44 HRC |
| Strengthening mechanism | Solid solution | Precipitation hardening (gamma-double-prime + gamma-prime) | |
| PWHT required? | No | Yes (for full properties) | |
| Weldability | Good | Good (annealed) | Poor (aged) |
The 718’s advantage over other precipitation-hardened nickel alloys (like Waspaloy or Rene 41) is its relatively slow aging response. The gamma-double-prime precipitate forms more slowly than gamma-prime, giving a wider window between welding and heat treatment without premature hardening in the HAZ. This is what makes 718 the most weldable of the precipitation-hardened superalloys.
Pre-Weld Conditioning: Solution Annealing
Before welding Inconel 718, the material should be in the solution-annealed condition. Solution annealing dissolves the hardening precipitates back into solid solution, maximizing ductility and minimizing residual stress.
| Condition | Temperature | Time | Cooling |
|---|---|---|---|
| Solution anneal (standard) | 1750-1800F | 1 hour per inch of thickness | Air cool or faster |
| Solution anneal (high temp) | 1900-1950F | 1 hour per inch | Air cool or faster |
The higher temperature anneal (1900-1950F) dissolves delta-phase (Ni3Nb) in addition to the gamma-double-prime, producing a fully recrystallized structure with maximum ductility. It’s preferred before welding when the component will go through a complete PWHT cycle after.
The standard anneal (1750-1800F) retains some delta-phase at grain boundaries, which pins grain boundaries and prevents excessive grain growth. This option is used when grain size control matters.
Filler Metal: ERNiFeCr-2
| Element | ERNiFeCr-2 (typical %) | Inconel 718 base (typical %) |
|---|---|---|
| Nickel | 53 | 53 |
| Chromium | 19 | 19 |
| Iron | 18 | 18.5 |
| Niobium | 5.0 | 5.0 |
| Molybdenum | 3.0 | 3.0 |
| Titanium | 0.9 | 0.9 |
| Aluminum | 0.5 | 0.5 |
ERNiFeCr-2 matches the base metal and responds to the same aging treatment. After PWHT, the weld deposit develops similar strength and hardness to the base metal.
Alternative filler: ERNiCrMo-3 (Inconel 625 filler) can be used as an alternate. It won’t respond to aging heat treatment (it’s a solid-solution alloy filler), so the weld deposit stays softer than the aged base metal. This is sometimes acceptable for non-critical joints or when the weld doesn’t need to match base metal strength.
TIG Welding Procedure
DCEN TIG with 100% argon, 18-25 CFH through a gas lens. Same basic approach as other nickel alloys: stringer beads, low heat input, clean everything.
| Thickness | Tungsten | Filler | Amps | Notes |
|---|---|---|---|---|
| 0.063 in | 1/16 in | 1/16 in ERNiFeCr-2 | 30-55 | Thin sheet; fast travel |
| 0.125 in | 3/32 in | 3/32 in ERNiFeCr-2 | 55-100 | Standard procedure |
| 0.250 in | 1/8 in | 1/8 in ERNiFeCr-2 | 100-160 | Multi-pass V-groove |
| 0.375 in | 1/8 in | 1/8 in ERNiFeCr-2 | 140-200 | Multi-pass; monitor interpass temp |
Heat Input Rules
Keep heat input below 35 kJ/inch on all passes. Calculate heat input as:
Heat Input (kJ/in) = (Amps x Volts x 60) / (Travel Speed in ipm x 1000)
Lower heat input produces a finer grain structure in the weld and HAZ, which improves both mechanical properties and resistance to strain-age cracking during PWHT.
Interpass Temperature
Maximum 300F. This is stricter than many other nickel alloys. High interpass temperatures promote precipitation of hardening phases in the HAZ during welding, which increases susceptibility to strain-age cracking later during PWHT.
Measure with a contact pyrometer, not a temp stick (temp sticks leave contamination). If the joint exceeds 300F, stop and wait.
Strain-Age Cracking (SAC)
Strain-age cracking is the defining risk when welding Inconel 718. It occurs during PWHT, not during welding. Here’s the mechanism:
- Welding creates residual tensile stress in the HAZ and weld metal.
- During the heating phase of PWHT (specifically in the 1100-1500F range), hardening precipitates form rapidly.
- The precipitates strengthen the alloy and reduce its ductility before the residual stresses have relaxed.
- The combination of high stress and low ductility causes intergranular cracking.
Why 718 is better than other superalloys: The gamma-double-prime precipitate in 718 forms more slowly than the gamma-prime in alloys like Waspaloy, Rene 41, or IN-100. This gives 718 a window where stress relaxation can occur before significant hardening. It’s the reason 718 can be welded at all, while some other gamma-prime-strengthened alloys are essentially unweldable.
SAC Prevention
| Strategy | How It Helps |
|---|---|
| Weld in solution-annealed condition | Minimizes residual stress; maximizes ductility |
| Low heat input welding | Reduces residual stress and HAZ grain growth |
| Low interpass temperature (<300F) | Prevents premature aging in HAZ during welding |
| Rapid heating through 1100-1500F during PWHT | Minimizes time in the cracking-susceptible temperature range |
| Intermediate stress relief before aging | Relaxes residual stress before hardening occurs |
| Stringer beads (no weaving) | Reduces heat input and residual stress per pass |
Some specifications call for an intermediate stress relief at 1325-1375F for 4-6 hours between the solution anneal and the aging treatment. This step relaxes welding residual stresses before the alloy hardens during aging.
Post-Weld Heat Treatment (PWHT)
The standard PWHT for welded Inconel 718 restores full mechanical properties to both the weld and the HAZ.
Full PWHT Cycle
- Solution anneal (if not done before welding): Heat to 1750-1800F or 1900-1950F. Hold 1-2 hours. Air cool or faster.
- First aging step: Heat to 1325F. Hold 8 hours. Furnace cool to 1150F at a rate of about 100F/hour.
- Second aging step: Hold at 1150F for a total combined aging time of 18 hours. Air cool.
Modified PWHT (For Stress-Rupture Applications)
- Solution anneal: 1900-1950F, 1 hour, air cool.
- First age: 1400F, 10 hours, furnace cool to 1200F.
- Second age: 1200F, hold for a total aging time of 20 hours, air cool.
This modified cycle produces slightly lower room-temperature tensile properties but better stress-rupture life at elevated temperatures.
Expected Properties After PWHT
| Property | Base Metal (Aged) | Weld Metal (ERNiFeCr-2, Aged) |
|---|---|---|
| Tensile strength (ksi) | 185-210 | 170-195 |
| Yield strength (ksi) | 150-175 | 130-160 |
| Elongation (%) | 12-20 | 10-18 |
| Hardness (HRC) | 36-44 | 33-42 |
The weld deposit typically comes in slightly below the base metal’s properties because the as-cast weld structure has some microsegregation (niobium-rich regions) that reduces the uniformity of the aging response.
Joint Preparation
Same cleanliness standards as other nickel alloys, plus the additional requirement of pre-weld condition verification:
- Verify condition: The material must be in solution-annealed condition. If the incoming stock is aged (hardness above 30 HRC), it must be solution annealed before welding.
- Mechanical prep: Machine or grind bevels. Use carbide tooling or silicon carbide/aluminum oxide wheels. Don’t use tools previously used on carbon steel.
- Degrease: Acetone on lint-free wipes.
- Mechanical clean: Stainless steel brush dedicated to nickel alloy work.
- Fit-up: Standard V-groove or U-groove per WPS. 60-75 degree included angle for V-groove.
NDE After Welding
Inconel 718 welds typically require full NDE (non-destructive examination) before PWHT:
- Visual inspection: Check for cracks, porosity, undercut, incomplete fusion.
- Fluorescent penetrant inspection (FPI): Standard for aerospace 718 welds. Detects surface-breaking cracks that visual inspection might miss.
- Radiographic testing (RT): Required for most pressure-containing joints and aerospace applications.
Additional FPI after PWHT detects any strain-age cracking that occurred during the heat treatment cycle. This post-PWHT inspection is critical because SAC cracks form during heat treatment, not during welding.
Applications
Inconel 718 accounts for roughly 50% of all nickel alloy production by weight. Major applications:
- Gas turbine discs and blades (jet engines, power generation)
- Aerospace fasteners (bolts, studs rated to 1200F)
- Rocket engine components (combustion chambers, turbopumps)
- Nuclear reactor internals (springs, fasteners)
- Downhole tools (oil and gas drilling, high-pressure/high-temp)
- Cryogenic equipment (retains ductility to -423F)
For comparison with Inconel 625 (solid-solution alloy), see the welding Inconel 625 guide. For Monel procedures, see the Monel welding guide. For the complete filler chart, see the nickel alloy filler selection guide.
Back to the nickel alloys welding category.