Evolution of the residual stress state in a duplex stainless steel during loading

The evolution of micro- and macrostresses in a duplex stainless steel durin g loading has been investigated in situ by X-ray diffraction. A 1.5 mm cold -rolled sheet of alloy SAF 2304 solution treated at 1050 degrees C was stud ied. Owing to differences in the coefficient of thermal expansion between t he two phases, compressive residual microstresses were found in the ferriti c phase and balancing tensile microstresses in the austenitic phase. The in itial microstresses were almost two times higher in the transverse directio n compared to the rolling direction. During loading the microstresses incre ase in the macroscopic elastic regime but start to decrease slightly with i ncreasing load in the macroscopic plastic regime. For instance, the microst resses along the rolling direction in the austenite increase from 60 MPa, a t zero applied load, to 110 MPa, at an applied load of 530 MPa. At the appl ied load of 620 MPa a decrease of the microstress to 90 MPa was observed. D uring unloading from the plastic regime the microstresses increase by appro ximately 35 MPa in the direction of applied load but remain constant in the other directions. The initial stress state influences the stress evolution and even after 2.5% plastic strain the main contribution to the microstres ses originates from the initial thermal stresses. Finite element simulation s show stress variations within one phase and a strong influence of both th e elastic and plastic anisotropy of the individual phases on the simulated stress state. (C) 1999 Acta Metallurgica Inc. Published by Elsevier Science Ltd. All rights reserved.