Welding Inconel: Techniques And Filler Metal Selection

Introduction

Inconel alloys are among the most widely used nickel-based superalloys in industries requiring high-temperature strength, oxidation resistance, and superior corrosion performance. From aerospace engine components to chemical processing equipment and power generation systems, Inconel materials operate reliably in environments where most stainless steels fail. However, achieving sound welded joints in Inconel requires a thorough understanding of the alloy's metallurgical behavior, appropriate welding process selection, and correct filler metal matching. This guide provides engineers and procurement professionals with practical information on welding techniques, filler metal selection, and process parameters for the most commonly welded Inconel grades.

Inconel Alloys Overview

Inconel 600 (UNS N06600)

Inconel 600 is a nickel-chromium-iron alloy with a minimum nickel content of 72%. It offers excellent resistance to oxidation at temperatures up to 1093°C and maintains good mechanical properties across a wide temperature range. Its high nickel content provides outstanding resistance to chloride stress corrosion cracking and alkaline environments. Inconel 600 is commonly used in furnace components, heat-treating equipment, and nuclear reactor parts.

Inconel 625 (UNS N06625)

Inconel 625 is a nickel-chromium-molybdenum-niobium alloy strengthened by solid-solution strengthening rather than precipitation hardening. The combination of 8-10% molybdenum and 3.15-4.15% niobium provides exceptional tensile and creep strength from cryogenic temperatures up to 980°C. Its superior pitting and crevice corrosion resistance makes it a preferred choice for marine, chemical processing, and oil and gas applications.

Inconel 718 (UNS N07718)

Inconel 718 is a precipitation-hardening nickel-chromium alloy containing significant amounts of iron, niobium, molybdenum, and aluminum. It delivers the highest yield strength among wrought superalloys at temperatures below 650°C, making it the material of choice for gas turbine components, aerospace fasteners, and oil field tooling. Its excellent weldability, including resistance to strain-age cracking, sets it apart from other precipitation-hardening superalloys.

Chemical Composition Comparison

Filler Metal Selection Guide

Selecting the correct filler metal is critical for achieving weld joints that match the base metal's corrosion resistance and mechanical properties. The general principle is to use a filler metal whose composition matches or slightly overmatches the base alloy.

ERNiCr-3 is one of the most versatile nickel-base filler metals. It is used not only for welding Inconel 600 and 601 but also for dissimilar joints between nickel alloys, stainless steels, and carbon steels. ERNiCrMo-3 is specifically designed for Inconel 625, providing matching chemistry with niobium stabilization to prevent sensitization and intergranular corrosion.

Welding Process Parameters

GTAW (Gas Tungsten Arc Welding)

GTAW is the preferred process for Inconel welding due to its precise heat input control and excellent arc stability. Argon shielding gas with a purity of 99.99% is standard.

GMAW (Gas Metal Arc Welding)

GMAW offers higher deposition rates for thicker sections. A shielding gas mixture of 75% Ar / 25% He is commonly used to improve wetting and penetration.

Welding Techniques and Best Practices

Surface Preparation

Thorough cleaning is essential for Inconel welding. Surface contaminants such as oxides, oils, grease, and sulfur compounds can cause porosity and cracking. Clean the weld zone and adjacent areas with acetone or a suitable solvent, followed by mechanical removal of oxide layers using stainless steel brushes or carbide tools. Never use carbon steel tools, as iron contamination can initiate corrosion.

Heat Input Control

Inconel alloys have lower thermal conductivity and higher thermal expansion than carbon steel, which means heat concentrates in a smaller area and distortion is more likely. Maintain low to moderate heat input, typically below 10 kJ/cm for Inconel 625. Stringer bead techniques are preferred over wide weave patterns to minimize heat input and control the weld pool.

Interpass Temperature Management

Controlling interpass temperature is critical, especially for Inconel 625 and 718. Excessive interpass temperatures can promote carbide precipitation and microsegregation in the heat-affected zone, reducing corrosion resistance. For Inconel 625, the interpass temperature should not exceed 100°C. For Inconel 718, a maximum of 95°C is recommended to avoid microcracking.

Shielding Gas and Back Purging

Use high-purity argon (99.99% minimum) for both shielding and back purging. For GMAW, adding 25% helium improves arc energy and weld pool fluidity. Back purging is mandatory for full-penetration welds on pipe and tubing to prevent oxidation of the root pass. Maintain back purge gas flow until the root pass is at least 3 mm thick.

Post-Weld Heat Treatment

• Inconel 600: Generally no PWHT required. Solution annealing at 980-1150°C may be specified for critical service.
• Inconel 625: No PWHT required for most applications. For severe corrosive service, a solution anneal at 1093°C followed by rapid cooling restores optimal corrosion resistance.
• Inconel 718: Requires a two-stage precipitation hardening heat treatment after welding: solution anneal at 968°C for 1 hour, air cool, then age at 720°C for 8 hours, furnace cool to 620°C, hold for 8 hours, and air cool. This develops the full strength of the weldment.

Common Welding Issues and Solutions

1. Hot Cracking

Hot cracking or solidification cracking occurs when low-melting-point impurities segregate to grain boundaries during solidification. Inconel alloys with high sulfur and phosphorus content are particularly susceptible. Mitigation strategies include:

• Using filler metals with controlled low impurity levels (S ≤ 0.015%, P ≤ 0.02%)
• Maintaining low heat input and smaller weld beads
• Ensuring proper joint design to avoid excessive constraint

2. Porosity

Gas porosity is typically caused by inadequate shielding, surface contamination, or moisture in the filler metal. Prevention requires:

• Maintaining adequate gas flow (10-15 L/min for GTAW)
• Cleaning all surfaces thoroughly before welding
• Storing filler metals in sealed, dry containers
• Using ERNi-1 filler with added titanium for pure nickel welds, as Ti acts as a deoxidizer

3. Lack of Fusion

Due to the sluggish nature of nickel alloy weld pools, lack of fusion at sidewalls and root can occur. Address this by:

• Using slightly wider joint angles (70-80° for V-grooves)
• Maintaining proper arc length and travel speed
• Using stringer beads rather than weave techniques for thick sections

4. Microcracking in Inconel 718

Inconel 718 is susceptible to liquation cracking in the heat-affected zone due to niobium segregation. This is mitigated by:

• Keeping heat input low and interpass temperatures below 95°C
• Using ERNiFeCr-2 filler metal that matches the base alloy composition
• Avoiding unnecessary weld repairs, which accumulate heat cycles

Application Guide

Aerospace

Inconel 718 is the dominant alloy for aerospace welding applications, including engine casings, turbine discs, and structural components. GTAW with ERNiFeCr-2 filler is the standard process, often performed with automated orbital welding equipment for consistent quality. Post-weld precipitation hardening is always applied to achieve design strength.

Oil and Gas

Inconel 625 cladding and weld overlays protect carbon steel pipelines and pressure vessels from sour gas and chloride corrosion. GMAW with ERNiCrMo-3 is widely used for cladding applications, with controlled heat input to dilute the weld deposit and maintain corrosion resistance. Submerged arc welding (SAW) with INCOFLUX 7 flux is also used for heavy-section cladding.

Chemical Processing

Inconel 600 and 625 are used for reactor vessels, heat exchangers, and piping in chemical plants. GTAW with matching filler metals is standard for pressure-retaining welds. Solution annealing after welding may be specified for vessels handling strongly oxidizing media.

Power Generation

Inconel 600 is used in nuclear reactor components and steam generator tubing, while Inconel 625 and 718 find applications in gas turbine hot-section components. Welding procedures must comply with ASME Section IX qualification requirements, with strict controls on heat input and interpass temperature.

FAQ

What is the best welding process for Inconel?

GTAW (TIG welding) is generally the best process for Inconel alloys due to its precise heat input control, clean weld environment, and ability to produce high-quality welds in all positions. For thicker sections or higher productivity requirements, GMAW (MIG welding) with pulsed transfer is an effective alternative.

Do Inconel welds require preheating?

Preheating is generally not required for Inconel alloys. In fact, preheating should be avoided for Inconel 625 and 718, as elevated temperatures before welding can increase the risk of carbide precipitation and microcracking. Room temperature welding with controlled interpass temperature is the standard practice.

Can you weld Inconel to carbon steel?

Yes, Inconel can be welded to carbon steel using dissimilar filler metals such as ERNiCr-3 (for Inconel 600) or ERNiCrMo-3 (for Inconel 625). These nickel-base fillers provide ductile weld metal that accommodates the thermal expansion difference between the two materials. Proper joint preparation and low heat input are essential to minimize dilution.

Why does my Inconel weld have porosity?

Porosity in Inconel welds is most commonly caused by inadequate gas shielding, surface contamination (oils, oxides, moisture), or contaminated filler metal. Ensure shielding gas purity is at least 99.99% argon, clean all surfaces with acetone before welding, store filler metals properly, and verify that gas flow rates are sufficient (10-15 L/min for GTAW).

What is the difference between ERNiCr-3 and ERNiCrMo-3 filler metals?

ERNiCr-3 is a nickel-chromium filler (approximately 72% Ni, 20% Cr) used for welding Inconel 600, 601, and Incoloy 800, as well as dissimilar joints. ERNiCrMo-3 is a nickel-chromium-molybdenum-niobium filler (58% Ni, 22% Cr, 9% Mo, 3.6% Nb) designed for welding Inconel 625. The molybdenum and niobium in ERNiCrMo-3 provide superior pitting resistance and higher creep-rupture strength for demanding applications.

Conclusion

Welding Inconel alloys successfully depends on three pillars: matching the filler metal to the base alloy, controlling heat input and interpass temperature, and maintaining meticulous cleanliness throughout the process. By following the process parameters and best practices outlined in this guide, fabricators can produce reliable, high-performance welds for aerospace, oil and gas, chemical processing, and power generation applications. For specific welding procedure specifications (WPS), always consult ASME Section IX or the relevant industry code applicable to your service environment.