A Breakdown of Post-Weld Heat Treatment (PWHT) Requirements for Carbon Steel to Nickel Alloy Junctions

A Breakdown of Post-Weld Heat Treatment (PWHT) Requirements for Carbon Steel to Nickel Alloy Junctions

Welding dissimilar metals, particularly carbon steel to nickel alloys, is a common necessity in power generation, petrochemical, and oil & gas industries. These joints are critical in components like reactor transition zones, boiler tubing, and high-temperature piping. However, the inherent differences in these materials’ physical and metallurgical properties make the post-weld heat treatment (PWHT) process complex and fraught with potential pitfalls. Incorrect PWHT can severely degrade the joint’s mechanical properties and corrosion resistance.

This guide breaks down the purpose, challenges, and precise requirements for successfully performing PWHT on carbon steel to nickel alloy welds.

The Purpose of PWHT

PWHT is a controlled heating and cooling process applied to a weldment to achieve specific goals:

• Stress Relief: To reduce residual stresses induced by welding, minimizing the risk of stress corrosion cracking (SCC) and catastrophic brittle fracture.

• Hardness Control: To temper hard, brittle microstructures in the Heat-Affected Zone (HAZ) of the carbon steel, restoring toughness and ductility.

• Microstructural Stability: To improve the microstructure of the weld metal and HAZ.

The Core Challenge: Carbon Migration

The primary metallurgical challenge when PWHT’ing carbon steel to nickel alloy joints is carbon migration.

• The Mechanism: Carbon atoms are highly mobile at elevated PWHT temperatures. They tend to diffuse from the region of higher carbon activity (the carbon steel, e.g., A516 Gr. 70) to the region of lower carbon activity (the high-nickel alloy weld metal, e.g., ERNiCr-3).

• The Result:

  1. Decarburized Zone: A soft, weak layer forms on the carbon steel side of the fusion line.

  2. Carburized Zone: A hard, brittle layer forms on the nickel alloy side of the fusion line.

• The Consequence: This creates a weak, brittle interface that is highly susceptible to failure under mechanical stress or thermal cycling, completely undermining the purpose of the PWHT.

PWHT Strategy: The “Buffer Layer” Technique

To combat carbon migration, the standard and code-approved practice is to use a buttering layer or transition weld.

Procedure:

1. The carbon steel surface is prepared for welding.

2. A buffer layer is deposited onto the carbon steel using a nickel alloy filler metal (e.g., AWS ERNiCr-3 / INCONEL Filler Metal 82). This layer is typically ~3-5 mm thick.

3. Crucially, this buttered assembly undergoes a full PWHT cycle before the final weld is made. This allows any carbon in the steel to migrate into the buttering layer before the final, service-critical joint is completed. The brittle layer is safely contained within the buttering layer.

4. After this initial PWHT, the buttered carbon steel is welded to the nickel alloy component using the same nickel alloy filler metal.

5. The final weldment typically does not require another PWHT cycle. This avoids a second episode of carbon migration, which would damage the final joint.

This process is illustrated in the following flowchart:

Key Considerations and Best Practices

1. Filler Metal Selection: Nickel-based filler metals are always used to join carbon steel to nickel alloys. Common choices include:

   • ERNiCr-3 (INCONEL 82): Excellent strength and corrosion resistance. Most common for buttering.

   • ERNiCrFe-3 (INCONEL 182): Nickel-chromium-iron electrode for SMAW.

   • ERNiMo-3 (HASTELLOY W): For environments containing hot acids.
     The nickel content acts as a barrier to carbon diffusion.

2. PWHT Parameters: When PWHT is required (for the buttering layer), the temperature and hold time are dictated by the carbon steel base metal, not the nickel alloy.

   • Temperature: Typically in the range of 595°C – 675°C (1100°F – 1250°F). This is below the lower transformation temperature (Ac1) of the carbon steel to avoid forming new austenite.

   • Time: A soak time of 1 hour per inch (25 mm) of thickness is standard, with a minimum of 1 hour.

   • Heating and Cooling Rates: Must be controlled to prevent excessive thermal stresses. ASME BPVC Sec. VIII typically mandates rates above 315°C (600°F) not to exceed 400°C/hr (720°F/hr) divided by the maximum metal thickness in inches, but not more than 205°C/hr (370°F/hr) nor less than 55°C/hr (100°F/hr).

3. Code Compliance: Always adhere to the relevant construction code:

   • ASME Boiler and Pressure Vessel Code (BPVC), Section VIII, Div. 1: Mandatory rules for PWHT of pressure vessels.

   • ASME B31.3 Process Piping: Provides rules and guidance for piping systems.

   • These codes often provide specific exemption curves and guidelines for when PWHT can be avoided based on material grade and thickness.

4. When to Avoid PWHT: In some cases, especially with thin sections or specific service conditions, codes may allow an exemption from PWHT to entirely avoid the issue of carbon migration. The decision is based on the material P-Number and thickness. Consulting the applicable code is essential.

Summary: To PWHT or Not to PWHT?

Conclusion: Successfully managing PWHT for carbon steel to nickel alloy junctions hinges on understanding and mitigating carbon migration. The buttering technique is the industry-standard solution, allowing the carbon steel to be properly heat-treated without compromising the integrity of the final dissimilar metal weld. Always defer to qualified welding procedures (WPS/PQR) and the specific requirements of the governing construction code to ensure a safe, reliable, and long-lasting joint.