For stainless steel pipes using liquid ammonia as the medium, is post-weld pretreatment necessary?

The answers you’re looking for can be found in 7.8.1 of SH 3075 and 6.8.5 of HG 20581:2020.

Liquid ammonia is highly corrosive to stainless steel; therefore, post-weld preprocessing is necessary. It is generally recommended to perform weld inspection and cleaning to remove any harmful substances that may be generated during the welding process. Specific specifications can be found in ASME or relevant industry standards, such as the “Safety Technical Regulations for Pressure Piping”. It is recommended to consult a professional to ensure compliance with all safety requirements. .

Stainless steel should be avoided from undergoing heat treatment; otherwise, it means the material was selected incorrectly!

For stainless steel pipes using liquid ammonia as the medium, whether post-weld pre-treatment is required must be determined through a comprehensive evaluation based on specific operating conditions, material type, and regulatory requirements. The following are the key analysis points: 1. The peculiarities of a liquid ammonia environment. Risk of stress corrosion cracking (SCC): Liquid ammonia—especially when containing trace amounts of moisture or air—can induce stress corrosion cracking (SCC) in austenitic stainless steels. Residual stress after welding is one of the main contributing factors; therefore, post-weld stress relief treatments (such as heat treatment) are generally recommended. Material sensitivity: Austenitic stainless steels (such as 304 and 316) are relatively sensitive to SCC in liquid ammonia environments, while duplex stainless steels or high-nickel alloys exhibit better resistance. 2. Code and standard requirements ASME B31.3: If the pipeline complies with ASME B31.3 (Process Piping Code), the determination shall be based on the material category and design conditions. Stainless steel pipes used in liquid ammonia services often require post-weld heat treatment (PWHT) to reduce residual stresses. NACE MR0175/ISO 15156: Standards for wet H₂S or acidic environments may not be directly applicable, but their stress-relief concepts can be referenced. Industry experience: The chemical industry generally applies PWHT to liquid ammonia stainless steel pipelines, especially in high-pressure or low-temperature conditions. 3. Key measures for preprocessing: Post-weld heat treatment (PWHT): Temperature range: usually 850–900°C (for austenitic stainless steels, the sensitization range must be avoided). Purpose: To eliminate welding residual stresses and reduce the risk of SCC. Pickling and passivation: Removes the oxide layer from the welding area, restores the passivation film, and improves corrosion resistance (but it cannot replace PWHT in eliminating residual stresses). Non-Destructive Testing (NDT): After welding, RT (Radiographic Testing) or PT (Penetrant Testing) is required to ensure there are no defects. 4. Exceptions: Low-pressure or room-temperature liquid ammonia: If it is at low pressure, at room temperature, and without cyclic loading, PWHT may be exempted, but an engineering assessment is required. Alternative measures: Use vibration stress relief (VSR) or mechanical stretching to relieve stress (efficacy needs to be verified). Conclusion and recommendations: Post-weld heat treatment (PWHT) is recommended, especially for austenitic stainless steel pipes, to comply with standards and reduce the risk of SCC. Pickling and passivation are used as auxiliary measures to ensure surface corrosion resistance. Refer to the design documents (such as PID, material specifications) and adhere to project standards (such as API or EN standards specified by the client). In the event of a dispute, it is recommended to conduct a specialized risk assessment by a materials engineer or corrosion expert.