Heat treatment issues after welding of hydrogen sulfide-resistant pipelines
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The supervisor told me that pipelines exposed to hydrogen sulfide require post-weld heat treatment, and asked me to specify the relevant requirements in the design instructions. The material chosen is 20ABTI-H2S as specified in GB9948, but I noticed that GB50235 does not specify any requirements regarding heat treatment temperature or holding time for wall thicknesses less than 19 mm
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A thickness greater than 19 is just one of the situations that require post-weld heat treatment; there is also a case where stress corrosion exists, and in that case as well post-weld heat treatment is needed. Media containing hydrogen sulfide will suffer from stress corrosion
To resist stress corrosion caused by hydrogen sulfide, concentrated alkalis, liquid ammonia, etc., heat treatment is required, following the methods used for carbon steel.
Unless specified otherwise in the design documents, heat treatment is required for carbon steel and carbon-manganese steel with a thickness greater than 19 mm, and requirements are set for the heat treatment temperature and time. Regarding the carbon steel pipes you mentioned for use in hydrogen sulfide-resistant applications, heat treatment is required to prevent stress corrosion; the specific time and temperature can be the same as those used for pipes with a diameter of 19 mm or more. The mechanism of common stress corrosion is as follows: under the combined action of stress and a corrosive medium, the oxide film on the surface of a part or component is corroded and damaged. The damaged surface and the undamaged surface act as the anode and cathode respectively; the metal at the anode is dissolved into ions, generating an electric current that flows toward the cathode. Since the anode area is much smaller than that of the cathode, the current density at the anode is high, further corroding the already damaged surface. With the effect of tensile stress, cracks gradually form at the failure site, and these cracks expand over time until fracture occurs. Such cracks can develop not only along the grain boundaries of the metal but also through the grains.