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Incoloy 825 (UNS N08825 / W.Nr. 2.4858 / GB NS1402) is a titanium-stabilized, solid-solution strengthened Ni-Fe-Cr-Mo-Cu alloy. Unlike precipitation-hardenable superalloys, it is not age-hardenable, and its corrosion resistance—particularly resistance to intergranular attack after welding—relies on correct Ti/C ratio (Ti/C ≥ 8–12) and proper solution annealing of the parent material. Welding does not require post-weld heat treatment (PWHT) for corrosion resistance, but it does require careful control of heat input, interpass temperature, and filler metal selection to avoid microfissuring, carbide precipitation, or dilution issues. This guide provides specification-based welding procedures, filler metal recommendations, and troubleshooting for common defects.
1. Weldability Overview of Incoloy 825
Good general weldability using GTAW (TIG), GMAW (MIG), SMAW (stick), and SAW (submerged arc, with caution).
Hot cracking / microfissuring susceptibility: Higher than 300-series stainless steels due to low melting point constituents (S, P, residual low-m.P. phases); minimized by:
Using over-alloyed filler metals (higher Mo, Cr, Ni than base metal)
Controlling heat input and interpass temperature
Maintaining proper joint geometry and bead profile
No PWHT required for corrosion (Ti-stabilized base + proper filler = HAZ remains immune to intergranular corrosion). Stress-relief annealing < 400°C may be done for dimensional stability only.
Base metal must be in solution-annealed condition (per ASTM B424/B423/B425: ≥ 980°C + rapid quench) prior to welding.
2. Recommended Filler Metals (AWS A5.14 / A5.11)
|
Welding Process |
Recommended Filler |
AWS Classification |
Rationale |
|---|---|---|---|
|
GTAW (TIG) / GMAW (MIG) |
ERNiCrMo-3 (Inconel 625 type) — Preferred |
AWS A5.14 ERNiCrMo-3 |
Over-alloyed (22% Cr, 9% Mo, ≥58% Ni); highest resistance to microfissuring & pitting; commonly used even though chemistry differs from base |
|
GTAW / GMAW |
ERNiFeCr-1 |
AWS A5.14 ERNiFeCr-1 |
Closer match to base Ni/Fe ratio; acceptable for less aggressive media, but slightly higher microfissuring tendency than ERNiCrMo-3 |
|
SMAW (Stick) |
ENiCrMo-3 (Inco 112 type coated electrode) |
AWS A5.11 ENiCrMo-3 |
Matching over-alloyed composition to ERNiCrMo-3 wire |
|
SMAW |
ENiFeCr-1 |
AWS A5.11 ENiFeCr-1 |
Fe-matched version; use where dilution/color match prioritized over max fissure resistance |
|
SAW (Submerged Arc) |
ERNiCrMo-3 / ERNiFeCr-1 + matching flux (basic, low Si/Mn) |
— |
Flux must be dry; watch for Si pickup which can promote hot cracking |
Best Practice: For critical corrosive service (seawater, sour gas, sulfuric acid), ERNiCrMo-3 / ENiCrMo-3 is the industry default despite being "over-matched" — it gives maximum tolerance to heat-affected-zone (HAZ) micro-cracking and superior pitting resistance (PREN ~ 48+ vs. base ~ 34).
3. Typical Welding Parameters & Technique
|
Parameter |
Recommendation |
|---|---|
|
Joint Prep |
Machine or fine-grind (stainless wire brush only for 825). Remove oxides, oil, chalk. Bevel angle 60–70°, root gap 1.5–2.5 mm for GTAW root. |
|
Shielding Gas (GTAW/GMAW) |
Argon (pure) for GTAW root & fill; Ar + up to 25–30% He for better penetration/bead shape on thicker sections. Back purge: Pure Ar on root side to prevent oxidation (color: silver-white = good; blue/dark = inadequate purge). |
|
Heat Input |
Low-to-moderate: ~ 0.8–1.6 kJ/mm (adjust per thickness). Avoid > 2.0 kJ/mm on thin sections — excessive heat input enlarges HAZ and can promote hot cracking. |
|
Interpass Temperature |
≤ 150°C (recommended ≤ 100°C). Allow cooling between passes. High interpass promotes grain boundary liquation in HAZ. |
|
Travel Speed |
Moderate; too slow → wide bead, higher heat input; too fast → lack of fusion. Stringer beads preferred over weaving (> 3× electrode diameter weave width discouraged). |
|
Polarity (GTAW) |
DCEN (Direct Current Electrode Negative) |
|
Cleaning Between Passes |
Stainless steel wire brush (dedicated to Ni-alloys), or rotary SS brush; remove oxide tints especially before next pass. |
|
Root Run |
Ensure full penetration; back purge essential. Tack welds must be re-melted or removed. |
4. Common Welding Defects & Solutions
|
Defect |
Probable Cause |
Corrective Action |
|---|---|---|
|
Microfissuring / Hot Cracking (in weld or HAZ) |
• Excessive heat input / interpass temp too high |
✓ Use ERNiCrMo-3 / ENiCrMo-3 over-alloyed filler |
|
Lack of Fusion / Incomplete Penetration |
• Too fast travel speed |
✓ Proper joint prep (gap, land) |
|
Discolored Root (Blue / Black Oxide inside pipe) |
• Inadequate back purging or purge gas contaminated |
✓ Extend purge time; verify O₂ < 50 ppm in purge zone |
|
Porosity |
• Moisture in filler / flux (SMAW/SAW) |
✓ Bake electrodes per mfr (typically 200–300°C × 1 h before use) |
|
Carbide Precipitate / Sensitization Appearance in HAZ (rare for Ti-stabilized 825) |
• Base metal not properly Ti-stabilized or C/Ti ratio off-spec |
✓ Procure certified solution-annealed 825 with Ti/C ≥ 8 documented |
|
Distortion / Warp |
• Uneven weld sequence |
✓ Balanced weld sequencing (back-step, symmetrical passes) |
5. Post-Weld Considerations
No PWHT required for corrosion resistance. If stress relieving for dimensional reasons, hold ≤ 375–400°C; do not soak in 400–815°C range which could sensitize improperly stabilized material or promote σ-phase.
Pickling / Passivation: Remove discoloration (especially on cap pass exterior) by:
HNO₃ + HF pickle (15–20% HNO₃ + 2–4% HF @ 50–60°C), or
Mechanical abrasion + acid dip.
Note:Ti-stabilized 825 naturally passivates well; pickling mainly removes weld tint which may harbor crevice sites in severe service.
Inspection: Visual (VT), Dye Penetrant (PT) or MT for surface cracks; Radiography (RT) or UT (where applicable) for volumetric checks per project spec (often ASME BPVC Section IX + Section VIII for pressure equip).
6. Applicable Standards
|
Standard |
Scope |
|---|---|
|
AWS A5.14 |
ERNiCrMo-3, ERNiFeCr-1 filler metal specs |
|
AWS A5.11 |
ENiCrMo-3, ENiFeCr-1 covered electrodes |
|
ASME BPVC Section IX |
Welding Procedure Specification (WPS) / Procedure Qualification Record (PQR) guidelines |
|
ASTM B424 / B423 / B425 |
Parent material must be solution-annealed; welding shall not degrade corrosion rating below spec |
|
NACE MR0175 / ISO 15156 |
Hardness ≤ 35 HRC in sour service; proper solution-annealed + welded 825 with over-alloyed filler meets requirement |
7. Summary
Use ERNiCrMo-3 (Inconel 625-type) filler as first choice for GTAW/GMAW and ENiCrMo-3 for SMAW.
Keep interpass ≤ 150°C, use stringer beads, and back purge with Ar.
No PWHT for corrosion; Ti-stabilized base + over-alloyed filler = HAZ remains intergranular-corrosion resistant.
Control heat input, cleanliness, and filler storage to avoid microfissuring and porosity.
Procure only solution-annealed (≥ 980°C + rapid quench) 825 with Ti/C ≥ 8 documented.
