Rodrigo Medeiros

Residual stresses and fatigue in a duplex stainless steel

Duplex stainless steels, consisting of approximately equal amounts of austenite and ferrite, ofte... more 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 thermal expansion between the two phases, tensile microstresses are found in the austenitic phase and balancing compressive microstresses in the ferritic phase.

$Figure 2: Coordinate system used in stress measurements with diffraction.$

$The load sharing between phases and the evolution of microstresses and macrostresses during cyclic loading has been investigated in a 1.5 mm cold rolled sheet of the duplex stainless steel SAF 2304. X-ray diffraction stress analysis and transmission electron microscopy show that even if the hardness and yield strength is higher in the austenitic phase more plastic deformation will occur in this phase due to the residual microstresses present in the material. Figure 5 clearly show that the hysteresis during one fatigue cycle is higher in the austenite compared to the ferrite. The origin of the microstresses is the differences in coefficient of thermal expansion between the two phases, which will lead to tensile microstresses in the austenite and compressive in the ferrite. These microstresses was also found to increase from 50 MPa to 160 MPa in the austenite during the first 100 cycles when cycled in tension fatigue with a maximum load of 500 MPa. The cyclic loading response of the material was thus mainly controlled by the plastic properties of the austenitic phase. It was also found that initial compressive macrostresses on the surface increased from -40 to 50 MPa in tension during the first 10° cycles, see Figure 6(a). The microstresses at$

$Table 5: Residual stresses in a duplex stainless steel after 12% plastic strain measured on several diffraction planes for sir’ yw $0.8 [54] From the measurements on the 12% plastically strained samples, which covered several different reflection lines, dg were calculated under the assumption o,, =0 for each phase. Again the distribution are nearly linear. The relatively large scatter of the data is assumed to be at least partly due to the low signal-to-noise ratio of the X-ray intensities. From the slope of the straight lines, obtained by regression analysis, residual stresses were calculated for each phase. The results are listed in Table 5.$

$Table I: Chemical composition of duplex stainless steel SAF 2304 The volume fraction, determined from point counting of each phase, is 45+5% for the ferritic phase and 55+5% for the austenitic phase. The micrograph in Figure 1 show, as a result of the hot rolling, a heavily banded microstructure in all three directions with austenitic islands in a ferritic matrix. The size of the austenitic islands scatters a lot with typical value in the range of 0.5-3.0 um in the normal direction, 5-25 tim in the transverse direction and 15-200 im in the rolling direction. The distance between the austenitic islands is typically 3 um in the normal, 25 [im in the transverse, 150 fim in the rolling direction, respectively. The grain size is similar for both phases and about 10 um in the rolling and transverse direction and 1.5 jm in the normal direction. No secondary phases were observed in the optical micrographs or in the X-ray diffractograms.$

$The evolution of the stresses during loading is plotted in Figure 6 and Figure 7. Figure 6 show the total stress and the macrostress measured by X-ray diffraction as a function of the applied stress measured by the load cell. In Figure 7 the microstresses has been separated from the total stress. In this figure the total plastic strain applied to the specimen has also been included. One can see that the microstress is always lower in the rolling direction than in the transverse direction. The microstresses are increasing with increasing load in the macroscopic elastic regime, but start to decrease when the macroscopic load reaches the plastic stage.$

$Table 1: Chemical composition of duplex stainless steel SAF 2304 The micrograph in Figure 1 shows as a result of hot rolling a heavily banded microstructure with austenitic islands in a ferritic matrix. The volume fraction, determined from point counting of each phase, is 45+5% for the ferritic phase and 55+5% for the austenitic phase. The size of the austenitic islands scatters a lot with typical values in the range of 0.5-3.0 um in the normal direction, 5-25 lum in the transverse direction, and 15-200 um in the rolling direction. The distance between the austenitic islands is typically 3 um in the normal, 25 um in the transverse, and 150 um in the rolling direction. The grain size is similar for both phases and about 10 ym in the rolling and transverse directions and 1.5 um in the normal direction. No secondary phases were observed in the optical micrographs or in the X-ray diffractograms.$

Uploads

Papers by Rodrigo Medeiros