Duplex stainless steel (DSS) materials are extensively used for various critical applications in the Oil & Gas industry. During the past 30 years, Petroleum Development Oman (PDO) an upstream operator is one of the major end-user of DSS materials. A variety of applications such as wellhead piping, flow lines, headers, pipelines, vessels, coolers, tubing, pump and piping components, etc. are installed with DSS materials. Fabrication, quality assurance and quality control measures of these DSS components are critical elements for achieving long-term plant design integrity with a minimal life cycle cost. The objectives of this paper are to share selected case studies of premature failures (less than two years) experienced with DSS components during operation and to discuss recent quality assurance (QA) & quality control (QC) challenges encountered during procurement, forming/ forging, welding, heat treatment, and inspection & testing of DSS components. Even with thirty years of user experience, the well-known DSS material issues continue to challenge the material, corrosion & quality engineers, as the quality failures recurrence is alarming.
In recent decade, the applications of DSS have significantly increased in oil & gas industry, due to their attractive properties compared to austenitic grades with similar corrosion resistance1. The DSS products exhibit a better resistance to pitting, stress corrosion cracking and higher mechanical properties compared to other austenitic stainless steel grades. The microstructure of these materials consists of approximately 50% austenite (γ) and 50% ferrite (α) phases, obtained by means of a solution heat treatment.2-4 The temperature of solution annealing is chosen to obtain the equal distribution of ferrite and austenite. However, if the DSS is incorrectly heat-treated, these materials suffer from precipitation of intermetallic phases such as sigma-phase (σ-phase) and chi-phase (χ-phase), which effect the corrosion and mechanical properties of the materials5-9. Therefore, the quality of fabrication, heat treatment and lab testing/evaluation of DSS is critical to ensure integrity of the product. PDO has been one of the major end-users of DSS materials, for wide range of applications as shown in Figure 1. The expanding use of DSS material means it has been increasingly being treated as a commodity alloy and the manufacturers do not always pay sufficient attention to the QA/QC of these materials. The impact of DSS quality failures during various stages of manufacturing, commissioning and operations was not always recognized5-9. Various standards and specifications considered DSS as a traditional routine material under the family of stainless steels.
