Why Is Inconel So Hard to Weld

When it comes to high-performance alloys used in extreme environments, Inconel stands out as one of the toughest and most reliable materials available. Known for its outstanding strength, heat resistance, and corrosion protection, Inconel is widely used in industries such as aerospace, chemical processing, oil and gas, and power generation.

However, anyone who has worked with this nickel-based superalloy knows that it is notoriously difficult to weld. The same properties that make it ideal for extreme environments also create significant challenges during welding and fabrication.

In this article, SASAALLOY explores the reasons why Inconel is hard to weld, the metallurgical challenges behind it, and the best practices that engineers use to overcome these difficulties.

1. What Is Inconel?

Inconel is a family of nickel-chromium-based superalloys designed to withstand extreme heat, pressure, and corrosion. These alloys retain their strength even when exposed to temperatures exceeding 1000°C, where many stainless steels and other materials begin to weaken.

Some of the most common Inconel grades include:

• INCONEL 600 – For corrosion resistance in chemical industries

• INCONEL 625 – For seawater and acid corrosion resistance

• INCONEL 718 – For high strength and fatigue resistance in aerospace

• INCONEL 617 – For oxidation resistance in power and energy systems

Each of these grades offers a unique balance of mechanical and chemical properties, but they all share one characteristic — they are challenging to weld due to their complex metallurgical behavior.

2. Why Is Inconel Difficult to Weld?

The difficulty in welding Inconel arises from a combination of factors, including its high nickel content, thermal expansion rate, oxide layer formation, and tendency to crack under heat stress. Let’s explore these factors in detail.

1. High Nickel Content Causes Work Hardening

Inconel alloys contain over 50 percent nickel, which provides corrosion resistance and high-temperature strength. However, nickel also causes the metal to work-harden rapidly when subjected to heat or mechanical stress.

During welding, the area surrounding the weld pool (known as the Heat Affected Zone or HAZ) can become extremely hard and brittle. This makes it difficult to control the weld bead and increases the risk of cracking.

Key Issue:
Once hardened, the material becomes difficult to machine or rework without cracking, requiring precise heat control during the entire welding process.

2. High Thermal Expansion and Low Thermal Conductivity

Inconel expands significantly when heated but does not dissipate heat efficiently. This mismatch of properties creates thermal stress during welding.

The weld area cools unevenly, which can result in:

• Warping and distortion

• Residual stress buildup

• Cracking in the heat-affected zone

These thermal effects make welding Inconel far more complex than welding materials with stable heat dissipation, such as carbon steel or stainless steel.

3. Formation of Oxide Layers

Inconel forms a tough oxide film when exposed to air at high temperatures. This oxide protects the alloy from corrosion during operation but becomes problematic during welding.

The oxide layer must be removed before welding; otherwise, it can lead to porosity, lack of fusion, or inclusions in the weld. Even with surface cleaning, the rapid reformation of oxides during high-temperature welding can interfere with weld pool stability.

4. Hot Cracking Tendency

Inconel is prone to hot cracking (also known as solidification cracking) because of its high nickel and low sulfur content.

As the weld pool solidifies, impurities and segregated elements such as sulfur or phosphorus can concentrate at grain boundaries. This weakens the solidified structure, leading to micro-cracks that propagate under stress.

INCONEL 718 is particularly susceptible if improper welding parameters are used or if the base material has residual impurities.

5. High Resistance to Melting and Fusion

The melting temperature of Inconel alloys typically ranges from 1290°C to 1380°C, much higher than common steels. This requires intense heat input during welding, which increases the size of the heat-affected zone and makes controlling the weld pool more difficult.

Improper heat control may lead to incomplete fusion, undercuts, or surface contamination.

6. Complex Alloy Chemistry

Inconel’s alloying elements — nickel, chromium, molybdenum, cobalt, niobium, and aluminum — make it extremely resistant to corrosion and oxidation but also metallurgically complex.

When welded, these elements can interact and form unwanted carbides, intermetallic compounds, or gamma prime precipitates, which reduce ductility and promote cracking.

3. Metallurgical Challenges in Inconel Welding

To understand the welding challenges fully, it’s important to examine the metallurgical reactions that occur during the welding process.

1. Segregation of Alloying Elements

During solidification, molybdenum and niobium tend to segregate to grain boundaries. This creates weak zones susceptible to cracking, especially under thermal or mechanical stress.

2. Formation of Precipitates

In precipitation-hardened grades like INCONEL 718, excessive heat during welding can cause unwanted γ′ (gamma prime) and γ″ (gamma double prime) phase formation in the heat-affected zone. These precipitates can embrittle the material if not properly controlled.

3. Carbide Precipitation

At intermediate temperatures, chromium and carbon combine to form carbides along grain boundaries, leading to sensitization and reduced corrosion resistance.

4. Common Welding Methods for Inconel

Despite these challenges, Inconel can be successfully welded with the right procedures and expertise. Common welding methods include:

1. Gas Tungsten Arc Welding (GTAW / TIG)

This is the most widely used process for Inconel. It provides:

• Excellent control of the weld pool

• High-quality and clean welds

• Minimal contamination

However, it requires skilled operators and controlled shielding gas (argon or helium) to prevent oxidation.

2. Gas Metal Arc Welding (GMAW / MIG)

MIG welding is used for thicker Inconel sections. It offers faster deposition rates but requires precise control to avoid overheating and distortion.

3. Electron Beam Welding (EBW) and Laser Welding

Used for aerospace and nuclear components, these methods offer deep penetration with narrow heat-affected zones, reducing cracking risk.

5. Best Practices for Welding Inconel

To overcome its welding difficulties, engineers and fabricators follow several critical practices:

1. Pre-Weld Preparation

• Remove oxides and surface contaminants by grinding or chemical cleaning.

• Ensure joint edges are smooth and properly aligned.

• Use filler metals compatible with the base alloy (e.g., INCONEL 625 filler for 625 base).

2. Controlled Heat Input

• Use low to moderate heat input to minimize distortion and cracking.

• Avoid prolonged high-temperature exposure.

• Preheat is generally unnecessary, but interpass temperature should be controlled below 175°C.

3. Inert Gas Shielding

Use high-purity argon or helium gas to prevent oxidation of the molten pool. In multi-pass welding, back-purging is also necessary to protect the root side from oxidation.

4. Post-Weld Heat Treatment (PWHT)

Certain Inconel grades (especially 718) require aging heat treatment after welding to restore ductility and strength by reprecipitating the γ′ and γ″ phases uniformly.

5. Avoid Rapid Cooling

Rapid cooling can cause residual stress and cracking. Controlled cooling ensures a more stable weld microstructure.

6. Differences Between Welding INCONEL 617 and 718