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The deformation process of the microstructure in 2205 duplex stainless steel (DSS) under thermo-mechanical coupling at 250 °C was investigated using digital image correlation (DIC). A thermal tension test of duplex stainless steel (2205DSS) with a banded structure was carried out to observe the initial deformation of the microstructure. It was found that inhomogeneous strain fields occurred primarily in austenite. The maximum normal strain in austenite was almost positive, while that in ferrite was almost negative. In addition, a thermal cyclic-loading test was conducted, and the strain field was characterized by e11. Strain heterogeneities were induced after 400 cycles, which spread within the austenite and at the phase boundaries with the load increasing. The high tensile-strain regions were always located adjacent to regions of intense compressive strain. Based on the strain matrix sum vs. cycle number, we found that hardening occurred in the early cycles followed by softening.
Duplex stainless steel (DSS), which consists of about the same amount of ferrite and austenite, combines the characteristics of ferritic and austenitic stainless steels. Because of its high strength, good corrosion resistance, and other improved mechanical properties, DSS has become a good alternative to the single-phase (austenitic or ferritic) steel[1–3] in many industrial applications, such as nuclear power plants, chemical industry, pipes for heat exchangers, offshore applications, and other general engineering applications.[4–6]
Body-centered-cubic (bcc) ferrite and face-centered-cubic (fcc) austenite have different mechanical properties, which results in different deformation behaviors of DSS on the micro scale. The deformation properties of the two phases affect not only each other but also the load transfer characteristics, which can cause strain and stress partitioning.[7] Because of the different thermal expansion coefficients of ferrite and austenite, the residual micro-stress is always present in DSS. Micro-stress in duplex microstructures changes during deformation because of the different elasto-plastic properties of the two phases, which likely affects both plastic strain and strain localization.[8] In recent years, the micro-behavior of duplex stainless steel has been studied widely and intensely.[9–17] Duplex stainless steel is generally used below 316 °C according to the ASME boiler and pressure vessel code[18] because of the risk of precipitation of embrittling phases. However, the embrittlement phenomenon of DSS can already occur at 250 °C.[19] Quantitative micro-deformation measurements and analysis of 2205DSS at about 250 °C are required for many applications. Few studies focus on the micro-deformation behavior of 2205DSS in this temperature range.
In this paper, we investigated both the initial micro-deformation characteristics and the cyclic loading response of 2205DSS under thermo-mechanical coupling at 250 °C. The microstructure of the surface was observed in real-time during quasi in-situ loading using a scanning electron microscope (SEM). Furthermore, the surface micro-deformation was calculated via digital image correlation (DIC),[19–23] which used real-time images taken during thermal tension and thermal cyclic-loading tests. The micro-deformation mechanism in this temperature range can be better understood through surface observation and surface deformation measurements. The observation and analysis of the micro-deformation enables the optimization of both the material performance and the microstructure for the application of DSS in challenging environments.
In DIC, the displacement/strain information is obtained by comparing the deformed image with the reference image using a correlation algorithm.[24] Artificial speckles/textures of samples can be used to provide the deformation information. These can be obtained by tracking/matching the same pixel points between reference and deformed images. As shown in Fig.
Steel 2205DSS with a banded microstructure was provided by Zewee industrial co., LTD (Shanghai, China). The chemical composition is shown in Table
A GATAN MTEST 5000W in-situ micro-tension stage with a heating function and cooling system attached to a ZEISS EVO MA15 scanning electron microscope was used for loading and orientation imaging microscopy, as shown in Fig.
Mechanical testing was conducted when the sample was heated to and kept at 250 °C. The tests were carried out at a speed of 1.0 mm/min, and SEM images were captured in succession. One of the images was used as the reference image, while the other images were treated as the deformed images. Pristine texture images of the sample surface were used for the correlation calculation of DIC.
A series of zero-deformation tests were performed to observe and analyze the system errors. The system errors primarily resulting from drift and any spatial distortion in the SEM images should be determined first.[25,26] Hence, the zero-deformation tests were performed. Nine images were captured in succession and with different magnifications at 250 °C. The first image was used as the reference image. Subsequently, the mean and standard deviation of the displacements were determined.
Next, a 2205DSS sample with the banded structure was used to study the initial deformation response of the microstructure. The (ferrite and austenite) deformation of the microstructure in this experiment was characterized using maximum normal strain, which is denoted by
Subsequently, a cyclic-loading test was performed to study the cycling response of the microstructure of 2205DSS with banded features. The strain fields were characterized by e11. The deformed images were captured at 400, 1000, 4000, 5000, 9000, 13000, and 14000 cycles, respectively. The image captured before the cyclic loading test at 250 °C was used as the reference image. The loading route for the cyclic-loading test is shown in Fig.
The main parameters for DIC are described below. The calculation step was 5 pixels, and the size of the subset was 61 × 61 pixels, which were kept unchanged during all experiments. The size of the region of interest (ROI) was 451 × 401 pixels during the thermal tension test but 851 × 556 pixels during the thermal cyclic-loading test. Furthermore, the stress-controlled cyclic loading test (R = 0.5) with nonzero mean-stress was performed using a GATAN MTEST 5000W in-situ micro-tension stage with a maximum load of 450 MPa.
The mean and standard deviation of the displacements are shown in Fig.
The maximum normal strain maps of the microstructure obtained using DIC are shown in Fig.
The data along the red line L shown in Fig.
As shown in Fig.
Overall, during the thermal tension tests, the deformation of ferrite was delayed and the deformation of austenite accelerated. This was due to different micro-stresses generated by the different thermal expansion coefficients between the two phases. This caused a sharp contrast in the strain fields between the two phases, which generated high strain gradients, as shown in Fig.
Figure
Figure
It is well known that micro-stresses must balance between phases, while residual stresses are self-balanced.[27] The strain field obtained by DIC is a type of accumulated residual micro-deformation. In other words, the strain fields are also balanced. To validate the strain fields obtained by DIC, the mean values (M11) of the strain matrices were calculated, as shown in Fig.
These phenomena in the thermal cyclic-loading test are related to the difference between the elasto-plastic properties and thermal expansion coefficients. However, they are also related to the crystallographic orientation relationships and slip system difference between neighboring ferrite and austenite. The reason that the area and size of strain localization in zones 1–3 increased with the number of cycles (see Fig.
The sums of the strain matrices were studied, as shown in Fig.
The behavior of 2205 duplex stainless steel under thermo-mechanical coupling was studied using digital image correlation. The results can be summarized as follows.
To observe the initial deformation response of the microstructure of 2205DSS under thermo-mechanical coupling, as-received 2205DSS with a banded structure was studied in a thermal tension test. The inhomogeneous strain fields occurred predominantly in austenite and occasionally at the phase boundaries. This is similar to the results observed in the tension test at room temperature. However, the decisive difference is that the maximum normal strain was almost positive in austenite and negative in ferrite, which is mainly due to the different thermal expansion coefficients. After comparing the true stress–strain curve at 250 °C with that at room temperature, we found that the plasticity, yield, and ultimate strength significantly decreased at 250 °C.
The development of strain distribution of the microstructure in as-received 2205DSS with a banded structure was studied during the thermal cyclic-loading test. It was found that both magnitude and size of the strain localization increased with increasing cycle numbers in austenite and at the phase boundaries. This is attributed to micro-strain accumulation due to strain arrested at the grain or phase boundaries. For these boundaries, the Kurdjumov–Sachs relationship was not satisfied. Furthermore, an intense tensile strain region was always found to be adjacent to an intense compressive strain region of similar magnitude. In addition, hardening occurred within 4000 cycles followed by softening. The hardening stage is due to work-hardening in austenite.
In summary, it was shown that DIC in combination with SEM can be used to analyze the micro-deformation of 2205DSS at 250 °C. The results with respect to the micro-deformation behavior of 2205DSS under thermo-mechanical coupling improve the understanding of many properties of DSS.
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