Duplex stainless steels, consisting of approximately equal amounts of austenite and ferrite, often combine the best features of austenitic and ferritic stainless steels. They generally have good mechanical properties, including high strength and ductility, and the corrosion resistance is often better than conventional austenitic grades. This has lead to a growing use of duplex stainless steels as a material in mechanically loaded constructions. However, detailed knowledge regarding its mechanical properties and deformation mechanisms are still lacking. In this thesis special emphasis has been placed on the residual stresses and their influence on mechanical behaviour of duplex stainless steels. Due to the difference in coefficient of therrnal expansion between the two phases, tensile rnicrostresses are found in the austenitic phase and balancing compressive rnicrostresses in the ferritic phase.
The first part of this thesis is a literature survey, which will give an introduction to duplex stainless steels and review the fatigue properties of duplex stainless steels and the influence of residual stresses in two-phase material.
The second part concems the evolution of the residual stress state during uniaxial loading. Initial residual stresses were found to be almost two times higher in the transverse direction compared to the rolling direction. During loading the absolute value of the rnicrostresses increased in the macroscopic elastic regime but started to decrease with increasing load in the macroscopic plastic regime. A significant increase of the rnicrostresses was also found to occur during unloading. Finite element simulations also show stress variation within one phase and a strong influence of both the elastic and plastic anisotropy of the individual phases on the simulated stress state.
In the third part, the load sharing between the phases during cyclic loading is studied. X-ray diffraction stress analysis and transmission electron rnicroscopy show that even if the hardness and yield strength are higher in the austenitic phase, more plastic deformation will occur in this phase due to the residual rnicrostresses present in the material. These rnicrostresses were also found to increase from 50 MPa to 160 MPa in the austenitic phase during the first 100 cycles when cycled in tension fatigue with a maximum load of 500 MPa. A sharpening of the texture was found in the ferritic phase, while a small decrease was found in the austenitic phase. The changes in texture lead to a reduction of the stiffness in the loading direction.
Linköping: Linköpings universitet , 1999. , 15 + 3 papers p.