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.