Proper Design and Fabrication of Socket Welds for Use in Sour Service

摘要

Socket welds are commonly used to assemble small-bore carbon steel process piping systems because they can be fabricated fairly quickly and are somewhat tolerant of field fit-up issues. In the most simplistic terms, these welds are made by inserting the pipe into a socket and then seal welding using gas tungsten arc welding around the gap between the outside pipe wall and the end of the fitting. This leaves a notch in the root of the seal weld that is open to the process fluid. This notch can act both as a stress riser and a crevice capable of concentrating chemical species. In many process applications, these socket weld notches do not cause any in-service problems. However, in the case of sour service, these notches can become problematic because of the potential for sulphide stress corrosion cracking (SSCC). For SSCC to occur, the right combination of stress, environment, and material susceptibility must be present. Therefore, for socket welds to be used in sour service without the risk of failure, these parameters need to be controlled by proper design and fabrication of the welded joint. Since a crevice is created as a natural by-product of a socket weld geometry, where the SSCC environment will be present, the parameter most readily controlled to minimize susceptibility to SSCC is the microstructure and residual stress of the weld filler metal. As will be shown, this resistance to SSCC can be accomplished by designing a welding procedure specification to include the combination of both a second pass seal weld and by post-weld heat treatment (PWHT). It will also be shown that a complete second pass is crucial to proper tempering and refining the microstructure of the root pass. Without this microstructural tempering and refining by the second pass, the root pass can still undergo SSCC even with subsequent application of PWHT. Examples of SSCC initiating from crack-like weld root defects caused by poor welding techniques and propagating into single-pass socket welds that had been subjected to PWHT will be presented.