Duplex stainless steels (DSSs) have microstructures of α and γ-phases of iron and exhibit changing of their mechanical properties by aging above 300°C, so that strain behavior is expected to be complicated under stress loading. In order to estimate lattice strains and to determine their behavior, lattice strain measurements of aged DSSs have been performed using synchrotron radiation white X-rays. As a result, it was found that the lattice strain was not always correspond with applied macro strain, and the lattice strains of γ-phase showed no elongation in comparison with those of α-phase in the aged specimen. These results mean that complex strain distributions between (hkl) planes already exist in the early stage of elastic deformation, and suggest that fracturing of aged DSSs under tensile loading occurs preferentially on γ-phase rather than α-phase.
The fire damage FFS assessment is based on the material degradation associated with heat exposure. The proposed identification of heat exposure zone will be provided in the API 579-1/ASME FFS-1 Standard. As the useful judgments, temper colors and mechanical properties such as tensile strength, hardness, impact energy, ductility and so on for the damaged materials. However, the reference temper colors are shown only by the names of colors without photographic examples and the details of metallurgical analysis are not available in the Standard. This paper presents the temper color reference shows by presenting several photographic examples of different exposed temperature, elapsed time and cooling rates for carbon steel and stainless steel. The metallurgical analysis guide describes typical degradations in mechanical properties and appearances in microstructure observed in the materials exposed to fire, evaluation criteria, procedure for replica preparation, personnel qualification and so on.
In-situ diffraction measurements were carried out on a tensile specimen of SUS304 stainless steel using a small autoclave at BL22XU at SPring-8. The temperature of circulating water in the autoclave was changed from room temperature to 561 K under ambient pressure and also under a pressure of 8.8 MPa. Tensile stress of 260 MPa was applied to the specimen at 561 K under 8.8 MPa, and a two dimensional strain distribution was obtained by calculation based on measured lattice spacings of the γ-Fe311 plane under various conditions. The interior region of the specimen showed higher strain compared with that on the surface region. After the tensile deformation in the autoclave, CT images of cross sections of the specimen were taken. Void-like images with about 50 μm in size were observed.
The mechanical properties of duplex stainless steel (DSS) are known to display embrittlement due to ageing effects. Knowledge of the micro behaviour of aged DSS is important for understanding the mechanisms of deformation and failure, but it has proved difficult to obtain due to the complex effects created by the duplex structure. Lattice strain measurements of DSS aged under various conditions of temperature and time have been performed in order to determine the strain behaviour of individual (hkl) planes in the α- and γ-phases using white synchrotron radiation X-rays at SPring-8. It was found that the lattice strain of α-Fe(200) depends strongly on the ageing conditions and that the lattice strain behaviour in the γ-phase is more stable than that in the α-phase. These results suggest that the changes in macro mechanical properties, including yield strength and ultimate strength, which are revealed by standard tensile tests, are solely due to ageing effects in the α-phase. The results further suggest that the lattice strain of α-Fe(200) can be used as an index to evaluate 475 °C ageing effects in DSS.
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