Influence of Texture and Anisotropy on Microstresses and Flow Behavior in a Duplex Stainless Steel during Loading

The load partitioning between the two phases in a cold rolled duplex stainless steel sheet have been experimentally studied in situ during loading, via X-ray diffraction, for different loading directions. The microstresses in the two phases were found to decrease when loading in the transverse direction, while they increase during loading in the rolling and 45°-direction. Due to strong crystallographic texture in the ferritic phase the material is anisotropic with a higher stiffness and yield strength in the transverse direction compared to the rolling direction. The texture have been measured and used as input to theoretical predictions of both elastic and plastic anisotropy. The predicted anisotropic material properties have then been used in finite element simulations to study the flow behaviour of the material in different directions. The predicted flow behaviour was found to be in good agreement with the experimentally observed load partitioning between the phases for loading in the rolling and transverse direction. However, the yield strength of the ferritic phase during loading in the 45°-direction was found to be lower than what can be predicted by the crystallographic texture. INTRODUCTION Duplex stainless steels possess microstructures containing comparable volume fractions of austenite and ferrite. Due to the difference in coefficient of thermal expansion, residual tensile stresses are formed in the austenitic phase during cooling from the solution annealing temperature. These stresses are balanced by compressive stresses in the ferritic phase. Since the two phases also have different elastic and plastic properties, the microstresses between the two phases may change during deformation. The interactions between the two phases thus give rise to a complex flow behaviour. In many of the reported studies that include modelling work on the deformation behaviour of duplex stainless steels, the ferritic phase has been considered as the strong phase and austenite the softer and more ductile phase [1-4]. However, experimental studies on duplex stainless steels show the opposite behaviour, i.e. the austenitic phase is the stronger phase [5-8]. The reason for this is that modern duplex stainless steels are alloyed with nitrogen in order to increase the strength of the material, and to balance the phase contents. Nitrogen is mainly increasing the strength of the austenitic phase. Other factors contributing to a complex load partitioning are that, as a result of hot or cold rolling, duplex stainless steel exhibit anisotropic material properties, with values of yield and tensile strength that are higher in the transverse direction (TD) than in the rolling direction (RD) [7]. This study emphasises determination of the plastic properties of the constituent phases in a duplex stainless steel and the influence of texture and anisotropy on load partitioning, flow behaviour and evolution of microstresses during uniaxial loading in different directions. Copyright(c)JCPDS-International Centre for Diffraction Data 2001,Advances in X-ray Analysis,Vol.44 1 229