Influence of thermal cycles on properties of duplex stainless steels (Duprop) : final report

Duplex stainless steels become more and more attractive, both from technical and economical points of view. However, use of duplex stainless steels is limited to 300 °C. Metallurgical transformation and phase precipitations can affect the mechanical and corrosion properties over long-term exposure at elevated temperatures. In some cases, however, it can be interesting to submit a duplex stainless steel to a thermal cycle. In this work the influence of short thermal cycles on the properties of duplex stainless steel has been studied. Four duplex stainless steel grades have been selected: two superduplex grades, one standard grade and one lean duplex. The influence of thermal cycles has been studied on base materials and welds. The influence of plastic deformation has also been investigated. The influences of thermal cycles have been investigated on toughness and corrosion properties. The microstructural evolutions have also been studied. Results show that thermal cycles can affect toughness properties and corrosion properties in different ways. For example, at the lower temperature, toughness can be strongly reduced while localised corrosion resistance is not affected. At the higher temperature, both the corrosion resistance and the toughness can be significantly reduced. This is because of the microstructural evolution in the steels. The possible influence of thermal cycles on stress relaxation of welded structures has been studied experimentally and with the help of thermal simulation. It has been demonstrated that rate of stress relaxation up to 70 % can be achieved. The results of this work can be used to determine whether thermal cycles are suitable for a particular application. The thermal cycles acceptable depend on the base material, on the weld technique used and on the material specification.