Cast duplex stainless steels are popularly used in the petroleum industry due to their good corrosion resistance and mechanical properties. One of the applications of cast duplex stainless steels in the petroleum industry is in the manufacture of valves in corrosive service. The good properties of duplex stainless steels are attributed to their balanced microstructure containing austenite and ferrite phases. The balanced microstructure in the duplex stainless-steel components is achieved by the optimum heat treatment and manufacturing process. Improper cooling during manufacturing or accidental exposure of the component to high temperatures can induce the precipitation of secondary phases in the microstructure of duplex stainless steel, which can seriously influence the corrosion resistance and mechanical properties. The paper deals with the failure investigation of the first-generation cast duplex stainless steel valves that failed in a short duration of service. The microstructure of the failed valve was characterized by optical and scanning electron microscopy. The effect of the microstructure on the mechanical properties was assessed by hardness measurements, tensile testing, and impact toughness testing. The study indicated that the properties of the duplex stainless-steel valves were affected due to the precipitation of secondary phases and the root cause of the failure was embrittlement caused by the precipitated secondary phases.
Duplex stainless steels are a family of stainless steels possessing excellent corrosion resistance with good mechanical properties.1-5 The duplex stainless steels possess austenite-ferrite microstructure typically containing about 30-60% ferrite phase. The good properties of duplex stainless steels are due to their balanced microstructure and controlled addition of alloying elements. The corrosion resistance of duplex stainless steels is a function of chromium, molybdenum, tungsten, and nitrogen content in the alloy. These alloying elements, in addition to improving corrosion resistance also help to maintain the phase balance of the alloy. Duplex stainless steels are available commercially in various product forms processed through wrought and cast manufacturing routes. Cast duplex stainless steel components are being used in the petroleum industry for applications, such as valves and pumps in corrosive services, such as seawater, effluent water, etc. The components typically perform well in such services, when they are optimally manufactured with microstructure containing only austenite and ferrite phases. When the duplex stainless-steel components are heat treated inadequately or exposed to elevated temperatures, secondary phases can precipitate in the microstructure.6-12 The secondary phases that precipitate in duplex stainless steels are rich in chromium and molybdenum and are also hard and brittle.13,14 The precipitation of such secondary phases can impact the good properties of duplex stainless-steel components. The precipitated secondary phases can result in inferior corrosion resistance and embrittlement, thus affecting the service life of the components.6,7,15-20 Due to the same reason, duplex stainless steel components are typically specified in industrial applications of up to about 315°C.21,22 There are many incidents of failure of duplex stainless steel components due to improper microstructure.23-25 The failures of cast duplex stainless steel components in the petroleum industry affect the economics of operation in terms of lost production and replacement costs, in addition to unsafe conditions and poor reliability. Considering the importance of maintaining reliability and better economics of operation, efforts are made to mitigate component failures in the petroleum industry.
