1 When PWHT is Generally Not Required
For most corrosion-resistant applications in chemical processing, petrochemical, and general industry, Inconel 600 welds are often used in the as-welded condition.
This is because:
Inconel 600 is a solid-solution strengthened alloy and does not rely on precipitation hardening for strength, so there is no need for a solution heat treatment or aging treatment after welding.
The alloy has good ductility and toughness in the as-welded condition, and the HAZ is generally stable.
PWHT may not significantly improve corrosion resistance in many environments and can sometimes be detrimental if not properly performed.
2 When PWHT May Be Beneficial or Required
PWHT may be considered or specified in the following cases:
2.2.1 Stress Relief for Thick Sections or High Restraint Joints
Welding of thick plates or highly restrained structures can result in high residual stresses.
A stress-relief heat treatment may be performed to reduce these stresses and minimize the risk of:
Stress corrosion cracking (SCC) in certain corrosive environments (e.g., caustic solutions, especially at elevated temperatures).
Distortion during subsequent machining or service.
Typical stress-relief temperatures for Inconel 600 are in the range of 700–900°C (1290–1650°F), followed by air or furnace cooling. The exact temperature and time depend on material thickness and code requirements.
2.2.2 Improvement of Mechanical Properties
While Inconel 600 does not undergo precipitation hardening, PWHT can:
Reduce hardness in the weld and HAZ.
Improve ductility and toughness, especially in heavily welded structures.
This may be important for applications subject to cyclic loading or low-temperature service.
2.2.3 Compliance with Codes and Standards
Some industry codes and standards (e.g., ASME Boiler and Pressure Vessel Code) may mandate or recommend PWHT for certain types of joints, thicknesses, or service conditions.
For example, in pressure vessel applications, PWHT may be required to meet code requirements for stress relief and quality assurance.
2.2.4 Specific Corrosive Environments
In strong caustic (NaOH) environments, especially at temperatures above approximately 300°C (570°F), stress relief is often recommended to reduce the susceptibility to caustic stress corrosion cracking (CSCC).
Inconel 600 is widely used in caustic handling, and PWHT is frequently specified for such applications to enhance reliability.
2.3 When PWHT Should Be Avoided or Used with Caution
Overheating (e.g., above 1000°C) can cause excessive grain growth, reducing toughness and fatigue resistance.
In some cases, prolonged exposure to high temperatures may slightly reduce high-temperature strength, although the effect is less pronounced than in precipitation-hardened alloys.
Therefore, PWHT should be performed within the recommended temperature range and for a controlled duration.
2.4 Summary
Inconel 600 does not require post-weld heat treatment for most general applications and is often used in the as-welded condition.
PWHT may be beneficial or required for:
Thick sections or highly restrained joints to relieve residual stresses.
Applications in aggressive environments (e.g., high-temperature caustic) to reduce SCC risk.
Compliance with specific codes and standards.
The decision to perform PWHT should be based on:
Service temperature and environment.
Material thickness and joint restraint.
Design codes and customer specifications.
Potential benefits in terms of stress relief, mechanical properties, and corrosion resistance.
