Hydrogen Manufacturing Units which have a Pre-reformer often use large diameter, thick-wall austenitic stainless steel (SS) piping operating at 540–650°C (1000–1200°F). In numerous units, circumferential welds in this piping have experienced failures due to stress relaxation cracking (SRC). Similarly, SRC has been reported in SS piping operating in this temperature range in hydroprocessing, olefin and coking units and in steam superheater coils in various units. This paper describes the in-service SRC mechanism and reports on some cracking case histories. It will also define how the risk varies depending on the process conditions, pipe size and thickness, fabrication steps and more. Additionally, this paper briefly lists the pros and cons of different material choices for this piping based on operating conditions, and ranks the materials for SRC resistance. Various other damage mechanisms also need to be considered and affect the optimum materials choice. The paper recommends steps for minimizing the risk of SRC of SS piping when SS is determined to be the preferred material choice. One of the key steps for providing resistance to in-service SRC is postweld heat treatment of the welds, but this has led to cases of reheat cracking during the fabrication stage. Guidelines for predicting and minimizing this risk are also reported.

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