Views: 6 Author: Monica Publish Time: 2026-07-20 Origin: Site
Table of Contents
Inconel 625 (UNS N06625) is a highly weldable nickel-chromium-molybdenum alloy that can be joined using GTAW (TIG), GMAW (MIG), and SMAW (stick) processes with ERNiCrMo-3 (AWS A5.14) or ENiCrMo-3 (AWS A5.11) filler metals. No preheat is required, post-weld heat treatment is generally unnecessary, and interpass temperature should be controlled below 175°C (350°F).
The primary welding challenges are solidification cracking from niobium segregation, porosity from contamination, and Laves phase formation from excessive heat input. The table below provides a rapid-reference summary of the key welding parameters.
Parameter | Value | Notes |
ASME P-Number | P-No. 43 | Nickel and nickel alloys |
Matching Filler Metal (GTAW/GMAW) | ERNiCrMo-3 (AWS A5.14) | Also: Inconel Filler Metal 625, AMS 5837 |
Matching Filler Metal (SMAW) | ENiCrMo-3 (AWS A5.11) | Also: Inconel Welding Electrode 112 |
Preheat Required | No | Room temperature welding acceptable |
Maximum Interpass Temperature | 175°C (350°F) | General fabrication; 100°C for 9% Ni steel |
PWHT Required | No (generally) | Solution anneal optional for critical service |
Shielding Gas (GTAW) | 100% Argon | 10–20 L/min flow rate |
Shielding Gas (GMAW) | 100% Ar or Ar + 20–30% He | CO2 strictly prohibited |
Back Purging | Mandatory | For pipe and tube root passes |
Hydrogen Cracking Risk | None | FCC microstructure immune to HIC |
PREN (Weld Metal) | ≥45 | Excellent pitting resistance |
Its weldment retains strength and corrosion resistance without post-weld heat treatment. Success depends on three controls: correct filler metal (ERNiCrMo-3), scrupulous cleanliness, and moderate heat input with stringer beads.
Inconel 625 is highly weldable because its solid-solution strengthening mechanism does not depend on heat treatment. Unlike Inconel 718, the weldment retains full mechanical strength and corrosion resistance in the as-welded condition, eliminating the need for complex post-weld thermal processing.
The alloy's face-centered cubic (FCC) austenitic microstructure is inherently ductile and resistant to hydrogen-induced cracking (HIC). Inconel 625 forgiving of minor welding imperfections that would cause catastrophic failure in carbon steel.
Niobium tends to segregate to interdendritic regions during weld solidification, forming low-melting-point constituents that can cause solidification (hot) cracking under high restraint or excessive heat input. Proper welding procedure design manages this through controlled heat input, stringer bead techniques, and appropriate joint preparation.
Three welding processes dominate Inconel 625 fabrication: GTAW (TIG) for precision and root passes, GMAW (MIG) for production welding and overlay cladding, and SMAW (stick) for field repair and restricted-access work.
Process | Best For | Typical Thickness | Heat Input Control | Deposition Rate | Key Advantage |
GTAW (TIG) | Root passes, thin sheet, precision joints, pipe welding | 1–12 mm | Excellent (lowest HI) | Low | Highest quality, minimal spatter |
GMAW (MIG) | Production welding, thick plate, overlay cladding | 3–25+ mm | Good (pulsed mode) | Medium-High | Higher productivity than GTAW |
SMAW (Stick) | Field repair, restricted access, maintenance | 3–20 mm | Fair (manual control) | Medium | No shielding gas required |
SAW (Submerged) | Thick section overlay cladding, heavy fabrication | 10–50+ mm | Moderate | High | Highest deposition rate for cladding |
PAW (Plasma) | Keyhole welding of thin sections, precision | 1–6 mm | Excellent | Low-Medium | Deep penetration, narrow HAZ |
LBW (Laser) | Thin sheet, dissimilar joints, AM post-processing | 0.5–5 mm | Excellent (very low HI) | Medium | Minimal distortion, precise HAZ |
GTAW produces the highest-quality welds with the cleanest microstructure and smallest heat-affected zone (HAZ), making it the preferred choice for root passes in pipe welding, aerospace components, and any application where weld integrity is critical. GMAW offers significantly higher deposition rates and is the workhorse for thick-section fabrication and weld overlay cladding of carbon steel components. SMAW remains indispensable for field repairs where shielding gas delivery is impractical.
The standard filler metal for Inconel 625 welding is ERNiCrMo-3 for GTAW and GMAW processes, and ENiCrMo-3 for SMAW. These filler metals are composition-matched to the base metal, producing weld deposits with equivalent corrosion resistance and mechanical properties. The deposited weld metal contains approximately 60% Ni, 22% Cr, 9% Mo, 3.5% Nb+Ta, and 5% Fe maximum.
Filler Metal Forms and Specifications
Form | AWS Classification | Common Diameters | Typical Application | AMS Spec |
TIG Wire (bare) | ERNiCrMo-3 | 1.6, 2.0, 2.4, 3.2 mm | Root passes, precision welding, thin sheet | AMS 5837 |
MIG Wire (spooled) | ERNiCrMo-3 | 0.9, 1.0, 1.2, 1.6 mm | Production welding, overlay cladding | AMS 5837 |
Stick Electrode (covered) | ENiCrMo-3 | 2.5, 3.2, 4.0 mm | Field repair, restricted access | — |
SAW Wire + Flux | ERNiCrMo-3 | 2.4, 3.2 mm | Thick-section overlay cladding | — |
Filler Metal Selection for Dissimilar Metal Welding
When welding Inconel 625 to other metals, filler metal selection depends on the dissimilar combination. ERNiCrMo-3 is acceptable for most dissimilar joints because its high nickel content is compatible with both nickel alloys and steels.
Base Metal Combination | Recommended Filler (GTAW/GMAW) | Recommended Filler (SMAW) | Critical Note |
Inconel 625 + Inconel 625 | ERNiCrMo-3 | ENiCrMo-3 | Matching composition; standard procedure |
Inconel 625 + 316L / 904L SS | ERNiCrMo-3 | ENiCrMo-3 | Never use stainless filler; corrosion loss |
Inconel 625 + Carbon Steel | ERNiCrMo-3 or ERNiCr-3 | ENiCrMo-3 or ENiCrFe-3 | Control Fe dilution; consider buttering layer |
Inconel 625 + Duplex 2205/2507 | ERNiCrMo-3 | ENiCrMo-3 | Low heat input; preserve duplex balance |
Inconel 625 + 9% Ni Steel | ERNiCrMo-3 | ENiCrMo-3 | Interpass temperature < 100°C |
Inconel 625 + Inconel 825 | ERNiCrMo-3 | ENiCrMo-3 | ERNiCrMo-3 overmatches 825; acceptable |
Inconel 625 + Monel 400 | ERNiCu-7 | ENiCu-7 | Use Monel filler for Cu compatibility |
Why Never Use Stainless Steel Filler? Using 316L or other stainless steel filler metal to join Inconel 625 dilutes the nickel and molybdenum content in the weld deposit, dropping the PREN (Pitting Resistance Equivalent Number) from 45+ to below 30. This makes the weld seam vulnerable to pitting and crevice corrosion in chloride environments, defeating the entire purpose of specifying Inconel 625. Always use nickel-based filler (ERNiCrMo-3) even when welding to stainless steel. | |||
GTAW welding of Inconel 625 uses direct current electrode negative (DCEN) polarity with 100% argon shielding gas and ERNiCrMo-3 filler wire. Current ranges from 80–220 A depending on wire diameter and material thickness, with voltages of 12–18 V. Stringer beads are preferred over weave techniques because the molten pool in nickel alloys is more viscous (sluggish) than in steel, meaning wider weaves trap inclusions and increase cracking risk.
Wire Diameter | Current Range (DCEN) | Voltage Range | Suitable Thickness | Travel Speed |
1.6 mm (1/16") | 80–130 A | 12–15 V | 1–3 mm | 90–150 mm/min |
2.4 mm (3/32") | 120–175 A | 13–17 V | 3–6 mm | 100–180 mm/min |
3.2 mm (1/8") | 150–220 A | 14–18 V | 6–12 mm | 120–200 mm/min |
GTAW Technique Essentials
• Tungsten electrode: 2% thoriated (EWTh-2) or 2% lanthanated (EWLa-2), ground to a truncated cone tip with a small flat face
• Gas flow rate: 10–15 L/min through a gas lens nozzle for laminar flow; increase to 15–20 L/min for outdoor or drafty conditions
• Back purging: mandatory for pipe and tube root passes; use 100% argon at 5–10 L/min until root pass is complete
• Arc length: keep short (1–3 mm) to ensure stable arc and minimize atmospheric contamination
• Travel technique: use stringer beads; avoid weaving wider than 2.5–3 times the wire diameter
• Torch angle: 10–15° from vertical in the direction of travel; filler wire fed into the leading edge of the pool
• Trailing shield: recommended for titanium-sensitive applications and extended welds runs to protect the solidifying bead
GMAW welding of Inconel 625 uses ERNiCrMo-3 wire in either short-circuit transfer (90–170 A, 100% Ar) or spray transfer (200–260 A, Ar + 20–30% He).
Pulsed GMAW is strongly recommended because it reduces average heat input while maintaining stable spray transfer, producing cleaner welds with less spatter and lower dilution in overlay applications.
Transfer Mode | Wire Diameter | Current Range | Voltage Range | Shielding Gas | Best For |
Short-Circuit | 0.9–1.2 mm | 90–170 A | 19–23 V | 100% Ar | Thin plate, positional welding, root passes |
Spray Transfer | 1.2–1.6 mm | 200–260 A | 28–32 V | Ar + 20–30% He | Flat position, thick plate, high deposition |
Pulsed Spray | 1.0–1.2 mm | Peak 300–400 A / Background 50–100 A | Variable | 100% Ar or Ar+He | All-position welding, overlay cladding |
CO2 Is Strictly Prohibited Never use CO2 or any CO2-containing gas mixture for welding Inconel 625. Carbon dioxide reacts with the molten nickel alloy to cause severe porosity, carbon pickup, and oxidation of chromium and niobium. Use only 100% argon or argon-helium mixtures. This rule has zero exceptions. | |||||
SMAW welding of Inconel 625 uses ENiCrMo-3 covered electrodes with DC reverse polarity (DCRP, electrode positive). Current ranges from 95–240 A depending on electrode diameter.
Electrode Diameter | Current Range (DCRP) | Voltage Range | Typical Thickness | Notes |
2.5 mm (3/32") | 95–140 A | 15–18 V | 3–6 mm | All-position; root and fill passes |
3.2 mm (1/8") | 120–190 A | 18–20 V | 6–12 mm | Flat and horizontal; general fabrication |
4.0 mm (5/32") | 160–240 A | 20–22 V | 12–20 mm | Flat position only; high deposition fills |
SMAW Technique Essentials
• Electrode storage: keep in sealed original packaging until use; once opened, store in a holding oven at 110–150°C to prevent moisture pickup
• Moisture is the enemy: damp electrodes almost certainly produce porosity; if electrodes absorb moisture, re-bake according to manufacturer instructions before use
• Slag removal: clean thoroughly between passes using chipping hammer and wire brush; residual slag causes slag inclusions in subsequent passes
• Travel speed: maintain a steady pace to produce a convex bead; avoid concave beads which are crack-prone
• Arc length: keep short, approximately equal to the electrode core wire diameter
• Bead overlap: when cladding, overlap each bead by approximately one-third of the bead width for uniform layer thickness
The standard shielding gas for Inconel 625 GTAW is 100% argon at 10–20 L/min, delivered through a gas lens nozzle for optimal laminar flow.
For GMAW, 100% argon is used in short-circuit mode, while argon-helium mixtures (Ar + 20–30% He) improve wetting and penetration in spray transfer and thick-section welding. Back purging with 100% argon is mandatory for all pipe and tube root passes to prevent internal oxidation.
Application | Gas Composition | Flow Rate | Purpose |
GTAW shielding (standard) | 100% Argon | 10–15 L/min | Protect weld pool from oxidation |
GTAW shielding (thick section) | 75% Ar + 25% He | 12–18 L/min | Increase heat input and penetration |
GMAW short-circuit | 100% Argon | 15–20 L/min | Stable arc, low spatter |
GMAW spray transfer | 70–80% Ar + 20–30% He | 18–25 L/min | Improved wetting, higher deposition |
Back purge (root protection) | 100% Argon | 5–10 L/min | Prevent internal oxidation and sugaring |
Trailing shield (optional) | 100% Argon | 10–15 L/min | Protect solidifying bead behind torch |
Formier gas (alternative purge) | 90% N2 + 10% H2 | 5–10 L/min | Cost-effective alternative for purging |
Back purging deserves special emphasis. When welding Inconel 625 pipe or tube, the inside surface of the root pass reaches temperatures above 500°C and will oxidize rapidly if exposed to atmospheric oxygen. This oxidation, called sugaring, produces a rough, blackened surface with reduced corrosion resistance and increased susceptibility to pitting. Purging the inside of the pipe with argon displaces oxygen and prevents this damage. Continue purging until the root pass has cooled below 200°C.
Inconel 625 does not require preheating or post-weld heat treatment (PWHT) for standard fabrication. The alloy's solid-solution strengthening mechanism means the weldment retains full strength and corrosion resistance in the as-welded condition.
Preheating is not only unnecessary but can be counterproductive: excessive preheat raises the interpass temperature, increases heat accumulation, and promotes Laves phase formation and distortion.
Temperature Control Guidelines
Parameter | Requirement | Rationale |
Preheat | Not required (room temperature) | FCC microstructure; no martensitic transformation risk |
Interpass Temperature (general) | ≤ 175°C (350°F) | Prevent cumulative heat buildup and Laves phase |
Interpass Temperature (9% Ni steel) | ≤ 100°C (212°F) | Protect 9% Ni steel toughness |
Interpass Temperature (overlay cladding) | 150–200°C per WPS | Balance deposition rate and dilution control |
PWHT (standard) | Not required | Solid-solution strengthened; no aging needed |
Solution Anneal (optional) | 1060–1150°C, 1–2 h, rapid cool | Dissolve Laves/delta phases for critical service |
Stress Relief (optional) | 871°C, 1 h/inch, air cool | Reduce residual stresses in non-corrosive service |
The only scenario where post-weld heat treatment may be considered is for the most critical applications: nuclear components, aerospace flight hardware, or sour gas service where maximum corrosion resistance and microstructural uniformity are mandatory. In these cases, a solution anneal at 1060–1150°C dissolves any Laves phase or carbide precipitates that formed during welding, restoring the weldment to a fully homogeneous condition.
For dissimilar joints to carbon steel, a buttering technique is used: ERNiCrMo-3 is deposited on the steel side first, the steel is then given its required PWHT, and finally the Inconel 625 is joined to the buttered layer, keeping the 625 weldment away from the high PWHT temperatures.
The three most common Inconel 625 welding defects are solidification (hot) cracking from niobium segregation, porosity from contamination, and Laves phase embrittlement from excessive heat input. Understanding the root cause of each defect is essential for designing welding procedures that produce defect-free welds consistently.
Defect | Root Cause | Prevention Strategy | Detection Method |
Solidification Cracking (Hot Crack) | Nb and Mo segregate to interdendritic regions; low-melting-point films form under high restraint | Low heat input; stringer beads; minimize joint restraint; ERNiCrMo-3 filler; proper groove angle | VT, PT, RT |
Porosity | Surface contamination (oil, grease, oxides, moisture); inadequate shielding; damp electrodes | Acetone cleaning; dry filler metal; correct gas flow; back purge; store electrodes properly | RT, VT |
Laves Phase Formation | Excessive heat input or slow cooling; Nb-rich intermetallic Ni2Mo/Ni2Nb forms | Control heat input; increase travel speed; avoid wide weaves; solution anneal if detected | Metallography, SEM-EDS |
Lack of Fusion | Insufficient heat input, excessive travel speed, or poor joint preparation | Match current to thickness; proper bevel angle and root face; maintain consistent arc length | RT, UT |
Tungsten Inclusion (GTAW) | Tungsten electrode tip contacts weld pool or filler wire | Maintain correct arc length; use correct tungsten grind angle; avoid touching pool | RT |
Distortion | Cumulative heat buildup from multiple passes; poor sequencing | Balanced welding sequence; skip welding; clamping fixtures; interpass temperature control | Dimensional inspection |
Oxidation (Sugaring) | Insufficient back purge on pipe/tube root passes | Mandatory argon back purge; continue until root cools below 200°C; verify oxygen content <0.1% | VT (internal inspection) |
Solidification Cracking: The Primary Challenge
Solidification cracking, also called hot cracking, is the most significant welding defect in Inconel 625. During weld solidification, niobium and molybdenum atoms do not distribute evenly: they concentrate in the last liquid to freeze, which occupies the interdendritic spaces between solidifying crystal grains. If the joint is under high restraint (thick sections, rigid fixturing, or multi-pass welds), the shrinkage stresses can tear these still-liquid films apart before they fully solidify, producing centerline cracks.
Think of it like ice forming on a pond: the ice crystals grow from many directions simultaneously, and the last water to freeze contains all the impurities that the growing crystals rejected. If something stretches the ice while the last bit of water is still liquid, a crack opens along that impure centerline. The solution is to freeze the weld faster (lower heat input, faster travel) so there is less time for impurities to segregate, and to reduce the stretching forces (lower restraint, proper joint design).
Contamination Control: The Golden Rule Nickel alloys are extremely sensitive to sulfur, phosphorus, lead, zinc, and low-melting-point metals. Even trace amounts of these elements from fingerprints, cutting fluids, marker pens, or grinding debris can cause catastrophic hot cracking or porosity. The golden rule: clean every joint surface with acetone immediately before welding, use dedicated stainless steel wire brushes (never used on carbon steel), and never use cutting tools that have touched sulfur-containing materials. This is not optional. |
Inconel 625 can be successfully welded to carbon steel, stainless steel, duplex stainless steel, and other nickel alloys using ERNiCrMo-3 filler metal in most cases. The key considerations for dissimilar welding are controlling iron dilution from the steel side, managing differences in thermal expansion, and selecting filler metals that are compatible with both parent metals.
Dissimilar Combination | Filler Metal | Dilution Concern | Special Requirement |
625 to Carbon Steel (A106, A516) | ERNiCrMo-3 | High Fe dilution in first layer | Control Fe < 10% in weld deposit; use buttering layer |
625 to 316L/304L Stainless | ERNiCrMo-3 | Moderate Fe dilution | Never use SS filler; PREN drops below threshold |
625 to Duplex 2205/2507 | ERNiCrMo-3 | Low dilution acceptable | Low heat input to preserve duplex phase balance |
625 to 9% Ni Steel (LNG) | ERNiCrMo-3 | Moderate Fe and Ni dilution | Interpass < 100°C; protect Ni steel toughness |
625 to Inconel 825 | ERNiCrMo-3 | Low; both nickel-based | ERNiCrMo-3 overmatches; standard procedure |
625 to Cu-Ni 90/10 | ERNiCu-7 or ERNiCrMo-3 | Cu migration concern | Use Monel filler for best Cu compatibility |
625 to Titanium | Not directly weldable | Intermetallic formation | Use solid-state bonding or transition joint |
Buttering Technique for Dissimilar Joints
When welding Inconel 625 to carbon steel that requires PWHT (such as pressure vessel applications), the buttering technique protects the 625 weld from thermal damage. The process: (1) deposit a layer of ERNiCrMo-3 on the carbon steel side, (2) perform the required PWHT on the steel component, (3) machine or grind the buttering layer to sound metal, (4) weld the Inconel 625 to the buttered surface. This keeps the 625 weldment away from the high PWHT temperatures that could cause carbide precipitation and sensitization.
Weld overlay cladding with Inconel 625 deposits a corrosion-resistant nickel alloy layer onto a cheaper carbon steel substrate, combining the strength and cost advantages of steel with the corrosion resistance of Inconel 625. The critical challenge is controlling iron dilution: the first layer of weld metal picks up iron from the melting steel substrate, which can reduce the corrosion resistance of the overlay if iron content exceeds 10–15%.
Layer | Process | Typical Fe Content | Corrosion Resistance | Purpose |
First Layer (bond) | GMAW or GTAW | 15–25% Fe | Reduced (Fe dilution) | Metallurgical bond to substrate |
Second Layer (fill) | GMAW or GTAW | 5–10% Fe | Good | Restore chemistry; barrier layer |
Third Layer (surface) | GMAW or GTAW | < 5% Fe | Excellent (meets spec) | Final corrosion-resistant surface |
Buttering Layer (optional) | GTAW | 10–15% Fe | Moderate | Buffer between steel and 625 |
Industry best practice for critical service overlays (subsea, sour gas, chemical processing) is a two-layer minimum approach: the first layer provides the metallurgical bond but has elevated iron content, and the second layer restores the surface chemistry to within Inconel 625 specification limits. For the most demanding applications, a three-layer approach is used with each layer's chemistry verified by positive material identification (PMI) before proceeding to the next.
Iron Dilution Control Strategies
• Use low heat input: reduces the depth of substrate melting and therefore iron pickup
• Pulsed GMAW: the pulsed waveform reduces average heat input while maintaining stable arc characteristics
• Stringer bead technique: narrow beads with minimal overlap reduce dilution compared to wide weaves
• Multi-layer deposition: each subsequent layer sees less dilution as the previous layer acts as a buffer
• Chemical verification: use PMI (XRF or OES) to verify Fe content in each layer before proceeding
Joint preparation for Inconel 625 welding requires scrupulous cleanliness, appropriate bevel geometry, and careful control of root gap and land dimensions. The alloy's sluggish molten pool means that joint design must provide adequate access for the torch and filler wire, and contamination levels that would be acceptable for carbon steel welding will cause defects in nickel alloy welding.
Joint Type | Bevel Angle | Root Face | Root Gap | Best For |
Square Butt | — | — | 0–2 mm | Thin sheet (< 3 mm); single-pass GTAW |
V-Groove | 60–70° | 1–2 mm | 2–3 mm | General purpose; 3–12 mm thickness |
U-Groove | 15–25° + radius | 1–3 mm | 2–4 mm | Thick sections (12–25 mm); less weld volume |
Double V-Groove | 60° each side | 1–2 mm | 2–3 mm | Thick plate (> 12 mm); balanced distortion |
J-Groove | 10–20° + radius | 1–3 mm | 2–4 mm | Thick sections; minimal weld metal |
Cleanliness Protocol
Cleanliness is not just a recommendation for Inconel 625 welding; it is a mandatory quality control requirement. The joint surfaces and surrounding 50 mm on each side must be free of all contaminants before welding begins. The protocol: (1) remove all oxide scale by grinding with aluminum oxide wheels (never carbon steel wheels), (2) degrease with acetone or a suitable solvent using lint-free wipes, (3) verify no residual marker pen, oil, or cutting fluid remains, (4) use dedicated stainless steel wire brushes that have never touched carbon steel, (5) keep filler metal in sealed packaging until the moment of use.
Dedicated Tools Rule Never use grinding wheels, wire brushes, or files that have been used on carbon steel for Inconel 625 preparation. Iron particles embedded in the tool surface will transfer to the nickel alloy and cause localized corrosion and weld defects. Maintain a complete set of dedicated tools marked clearly as Nickel Alloy Only. This is a non-negotiable best practice in any fabrication shop working with corrosion-resistant alloys. |
Inconel 625 welding is governed by a hierarchy of standards covering filler metal chemistry, welding procedure qualification, and inspection requirements. ASME Section IX assigns Inconel 625 to P-Number 43 (nickel and nickel alloys), which defines the essential variables for procedure qualification. AWS A5.14 and A5.11 specify filler metal composition and mechanical properties. NACE MR0175/ISO 15156 governs welding for sour oil and gas service.
Standard | Scope | Relevance to Inconel 625 Welding |
ASME Section IX | Welding procedure and welder qualification | P-No. 43; WPS/PQR required for pressure equipment |
AWS A5.14 / ASME SFA-5.14 | Nickel filler metal specifications | ERNiCrMo-3 chemistry and mechanical properties |
AWS A5.11 / ASME SFA-5.11 | Nickel covered electrodes | ENiCrMo-3 electrode chemistry and usability |
AMS 5837 | Aerospace welding wire | ERNiCrMo-3 wire for aerospace GTAW/GMAW |
NACE MR0175 / ISO 15156 | Sour service materials | Hardness limits; welding requirements for H2S service |
API RP 582 | Welding of corrosion-resistant alloys | Recommended practices for CRA welding |
ASTM B443/B446 | Base metal specifications | Plate/sheet (B443) and bar (B446) delivery conditions |
ISO 15614-1 | Welding procedure qualification (European) | Alternative to ASME IX for international projects |
For pressure equipment fabrication (vessels, piping, heat exchangers), ASME Section IX qualification is mandatory. The WPS (Welding Procedure Specification) documents all essential variables: base metal thickness range, filler metal classification, welding position, preheat/interpass limits, shielding gas composition, and PWHT. The PQR (Procedure Qualification Record) provides the test data (tensile, bend, and if required, impact tests) that validate the WPS.
Inconel 625 weld quality is verified through a combination of non-destructive testing (NDT), mechanical testing, and chemical verification. The inspection plan depends on the service severity: general industrial applications typically require visual inspection (VT) and dye penetrant testing (PT), while critical service (pressure equipment, subsea, sour gas) demands radiographic (RT) or ultrasonic (UT) examination plus hardness testing.
Inspection Method | What It Detects | When to Apply | Acceptance Standard |
Visual Testing (VT) | Surface defects, bead profile, undercut | Every pass (interpass) and final weld | ASME Section VIII or project spec |
Dye Penetrant Testing (PT) | Surface-breaking cracks, porosity | Root pass, final weld, repaired areas | ASME Section VIII, Appendix 8 |
Radiographic Testing (RT) | Internal defects: porosity, slag, lack of fusion | Critical welds: pressure boundary, subsea | ASME Section VIII, UW-51 or UW-52 |
Ultrasonic Testing (UT) | Internal planar defects, lack of sidewall fusion | Thick section welds (> 12 mm) | ASME Section V, Article 4 |
Hardness Testing | Hardness exceeds specification (sour service) | Sour service welds per NACE MR0175 | NACE MR0175: typically ≤ 22 HRC |
Positive Material Identification (PMI) | Chemical composition verification | Overlay layers, dissimilar joints | Project specification (Fe, Cr, Mo, Nb) |
Ferrite Number (FN) | Ferrite content (for dissimilar to SS) | Dissimilar welds to stainless steel | 3–10 FN for SS joints |
Heat input control is the single most important parameter for producing defect-free Inconel 625 welds. Excessive heat input promotes Laves phase formation, increases niobium segregation, widens the heat-affected zone, and increases distortion. The recommended heat input range is 0.5–1.5 kJ/mm for GTAW and 1.0–2.0 kJ/mm for GMAW, with the exact limits specified in the qualified WPS.
Heat input is calculated using the formula: HI = (V × I × 60) / (S × 1000), where HI is heat input in kJ/mm, V is voltage, I is current in amps, and S is travel speed in mm/min. For pulsed processes, the average current is used. Maintaining this value within the qualified range ensures consistent microstructure and mechanical properties across the weldment.
Process | Recommended Heat Input | Maximum Heat Input | Consequence of Exceeding Limit |
GTAW (TIG) | 0.5–1.2 kJ/mm | 1.5 kJ/mm | Laves phase; excessive HAZ; distortion |
GMAW (MIG) Short-Circuit | 0.8–1.5 kJ/mm | 2.0 kJ/mm | Dilution increase; interpass temp rise |
GMAW (MIG) Spray | 1.2–2.0 kJ/mm | 2.5 kJ/mm | High dilution; Laves phase; cracking risk |
SMAW (Stick) | 1.0–1.8 kJ/mm | 2.0 kJ/mm | Slag inclusions; high interpass; distortion |
SAW (Submerged Arc) | 1.5–3.0 kJ/mm | 3.5 kJ/mm | High dilution; coarse microstructure |
Q: Is Inconel 625 hard to weld?
A: No, Inconel 625 is one of the most weldable nickel alloys. Its solid-solution strengthening means it does not require post-weld heat treatment, and its FCC microstructure is immune to hydrogen-induced cracking. The main challenges are managing niobium segregation (which can cause solidification cracking) and maintaining scrupulous cleanliness. With correct filler metal (ERNiCrMo-3), moderate heat input, and proper joint preparation, Inconel 625 welds are consistently defect-free.
Q: What filler wire do you use for Inconel 625?
A: ERNiCrMo-3 (AWS A5.14) is the standard filler wire for GTAW and GMAW welding of Inconel 625. For SMAW (stick welding), the equivalent covered electrode is ENiCrMo-3 (AWS A5.11). These filler metals are composition-matched to the base metal, producing weld deposits with Ni ≥58%, Cr 20–23%, Mo 8–10%, and Nb+Ta 3.15–4.15%. For aerospace applications, AMS 5837 specifies the same wire chemistry with additional quality control requirements.
Q: Does Inconel 625 need post-weld heat treatment?
A: No, Inconel 625 does not require post-weld heat treatment (PWHT) for standard applications. The alloy is solid-solution strengthened, so it retains full strength and corrosion resistance in the as-welded condition. PWHT (specifically solution annealing at 1060–1150°C) is optional and is reserved for the most critical service conditions where dissolving any Laves phase or carbide precipitates is necessary, such as nuclear, aerospace, or sour gas applications.
Q: Can you weld Inconel 625 to carbon steel?
A: Yes, Inconel 625 can be welded to carbon steel using ERNiCrMo-3 filler metal. The key consideration is controlling iron dilution from the steel substrate, which can reduce the corrosion resistance of the weld deposit. For pressure vessel applications where the carbon steel requires PWHT, use the buttering technique: deposit ERNiCrMo-3 on the steel side first, perform PWHT on the steel, then weld the Inconel 625 to the buttered surface.
Q: What is the maximum interpass temperature for Inconel 625 welding?
A: The maximum interpass temperature for Inconel 625 welding is 175°C (350°F) for general fabrication. For welding to 9% nickel steel (LNG applications), the interpass temperature should be limited to 100°C (212°F) to protect the nickel steel's cryogenic toughness. Exceeding these limits increases heat accumulation, promotes Laves phase formation, and raises the risk of solidification cracking.
Q: Why does Inconel 625 crack during welding and how to prevent it?
A: Inconel 625 cracks during welding primarily due to solidification (hot) cracking caused by niobium and molybdenum segregating to interdendritic regions during solidification. Prevention requires four controls: (1) use ERNiCrMo-3 filler metal with correct chemistry, (2) maintain low to moderate heat input (0.5–1.5 kJ/mm for GTAW), (3) use stringer beads instead of wide weaves, and (4) ensure scrupulous joint cleanliness to eliminate sulfur, phosphorus, and lead contamination that lowers the solidification temperature range.
