Effects of welding and long-term service on the microstructural evolution of superheater tubes of TP347H stainless steel used in power plants were investigated by optical microscope (OM), scanning electron microscope (SEM), electron back-scattered diffraction (EBSD), and transmission electron microscope (TEM). Analyses after welding or long-term service, showed fine NbCs in grains, which will precipitaion strengthen the matrix. When TP347 was long-term serviced in power plants, M 23 C 6 formed preferentially on the grain boundaries and on twin boundaries, which was attributed to the embrittlement and the intergranular corrosion and fracture. The steam side had less recrystallization rate and more oxide compared to the fire side, which is part of the reason for the cracking from steam side to the fire side. And HAZ is more brittle than the matrix, because of α-Fe phase and coarse grains, due to which, cracks tend to initiate in the steam side of HAZ and propagate to the fire side.
The Nb-stabilized and Cu-strengthened austenitic stainless steel Super304H was aged at 600~700°C up to 20,000 hrs. Scanning electron microscope (SEM), and transmission electron microscope (TEM) were used to study the effect of aging on the microstructural evolution of the specimen, with focus on the precipitation behavior in relation to the temperature and time. Only NbCs in size range 2 μm ~ 50 nm were observed to be scarcely distributed in matrix in the as-received specimen. Upon aging, precipitation of σ-phase (~5 μm) and Cr-rich M 23 C 6 (~1 μm) along grain boundary, and nanosized Cu precipitates (~65 nm) in the grain interior were formed. The size and fraction of the σ-phase and M 23 C 6 increased with the increase of aging temperature and/or aging time, with higher sensitivity to the temperature. The size of Cu precipitates was relatively stable, while the fraction and number density increased with the aging temperature/time. The microhardness upon aging increased with increase of aging temperature/time, due to the precipitation of the nanosized Cu precipitates.
Low alloy Cr–Mo–V steels are often used in steam power generation units. The evolution of carbide often leads to embrittlement during elongated service. Therefore, the determination of carbide evolution mechanism during long time service is important to understand and prevent premature failures such as temper embrittlement. In this study, low alloy Cr–Mo–V steels used as main steam pipes in a thermal power plant were studied after various service times as well as in the as fabricated condition. Electron microscopic analyses were carried out on extraction replicas to observe and analyse the morphology and composition of the carbides. Predominant plate-like vanadium rich carbides were observed in the as fabricated condition. When exposed to high temperature as a result of the use, the V rich carbides transformed to Mo rich carbides, which have a typical H type morphology. The change in morphology and composition of the carbide is mainly due to the gradual depletion of Mo from the solid solution, and a non-destructive carbide extraction method was established for the examination of the precipitates in the working turbine rotor.
Journal Article Microstructural Evolution and Oxidation Behavior of T91/T92 Steel upon Long-Term Steam Test Get access K Shin, K Shin School of Nano and Advanced Material Engineering, Changwon National University, Changwon 51140, Korea Search for other works by this author on: Oxford Academic Google Scholar HY Ma, HY Ma School of Nano and Advanced Material Engineering, Changwon National University, Changwon 51140, Korea Search for other works by this author on: Oxford Academic Google Scholar YS He, YS He School of Nano and Advanced Material Engineering, Changwon National University, Changwon 51140, Korea Search for other works by this author on: Oxford Academic Google Scholar SY Bae SY Bae Advanced Materials Group, Korea Electric Power Company, Daejeon 305308, Korea Search for other works by this author on: Oxford Academic Google Scholar Microscopy and Microanalysis, Volume 23, Issue S1, 1 July 2017, Pages 2072–2073, https://doi.org/10.1017/S1431927617011023 Published: 04 August 2017
13Cr martensitic stainless steels are widely used in gas industry, which are usually manufactured by quenching-tempering treatment. Microstructural study of 13Cr steel through various heat treatments was carried out for determining the optimum parameters for industry manufacture. After quenching treatment at 975 °C for 20 min, precipitation-free martensitic structures were formed. During tempering, recovery of martensite through grain boundaries migration and dislocations annihilation was found to soften the steel. In addition, transformation of needle-like Cr 7 C 3 carbides to the irregular shaped Cr 23 C 6 carbides was observed when tempering temperature is above 710 °C. The phase transformation induced precipitation strengthening is discussed. The optimum heat treatment parameters of 13Cr steel for avoiding over tempering and the precipitation hardening are found.
The P92 steels were aged at 632°C for 500hrs and 1,000hrs, and creep ruptured at 650°C~625°C with stress of 120~110 MPa. The creep rupture life (CRL) of the aged samples was decreased with the aged time. The microstructure of the P92 steel was observed as fine tempered lath with dispersion of Cr-rich M 23 C 6 along various grain boundaries. Upon aging and creep, recovery of lath and precipitation of coarse W-rich Laves phase were characterized as the main microstructural change. The M 23 C 6 is relatively stable upon the short-term aging and creep. Decrease of the CRL of the aged specimens is considered as the degradations of microstructure such as the recovery of lath due to the dislocation annihilation and precipitation of coarse Laves phase.
In this study, a Cu-Sn sintered bronze, used largely for con-rod bushing and automotive transmission, was treated by ultrasonic nanocrystalline surface modification (UNSM). Then, Vickers hardness and microstructural evolution of the treated region were investigated by using scanning electron microscope (SEM), X-ray diffraction (XRD) and transmission electron microscope (TEM). The hardness of the treated surface doubled, which is attributed to the developed of nanoscale grains, deformation twins, and high density of dislocations induced by the UNSM. Microstructural modification beneath the UNSM treated surface was typically characterized with increase of the depth: (i) nanoscale grains (top surface), (ii) intersection of deformation twins (~30 μm), (iii) high density nanoscale twin/matrix lamellae (~50 μm), (iv) interception of micro band and deformation twins (~100 μm), (v) dislocation arrays (~200 μm), (vi) low density dislocations (~300 μm) and (vii) pre-existing coarse grains and annealing twins in unaffected region (400 μm ~deeper).
The microstructures of a series of ultralow carbon steels with various carbon contents (50–200 ppm) and Ti addition (<500 ppm) were studied by optical microscope, scanning electron microscope and transmission electron microscope. The specimens were fabricated under the process of hot rolling, cold rolling and two-step continuous annealing. The experimental results showed that the increase in carbon content decreased the grain size, whereas the addition of Ti resulted in a larger grain size compared with those without Ti addition. Dislocation (cell) structures were introduced by cold rolling, but it was removed by two-step annealing. Only MnS precipitated from the Ti free specimens. However, a larger amount of fine TiN, a few coarse TiS and an extremely low number of much coarser Ti3AlC were observed in the Ti added specimens. In addition, the reduction in MnS inclusion was obtained by the addition of Ti.
Super304H is austenitic steel used predominantly for boiler and turbine components in thermal power plants due to the high strength, excellent creep and oxidation resistance in high-temperature steam environments. Microstructural degradation is inevitable upon long-term high-temperature exposure. Therefore, understanding the processes of degradation is critical for the determination of residual lifetime of the parts. In this study, the hardness, tensile strength and creep strength of a Super304H steel that was in service as a reheater for ~8 years at about 600°C was investigated with corresponding microstructural analysis. Grain growth is evident, but microhardness and tensile strength did not decrease, due to the precipitation of nanosized NbX and Cu-rich particles in grain interior. However, the creep rupture life of the aged S304H steel is ~90% lower at 650°C than that of the virgin S304H, due to the coarsening of σ phase and formation of M 23 C 6 on the grain boundary. Our measurements and analysis of the specimens in this study indicates the remaining service life longer than those in the NIMS database.
'%3e%3cg id='b'%3e%3cpath id='a' stroke='%23000' stroke-width='2' d='M-6-25H6m-12 3H6m-12 3H6'/%3e%3cuse xlink:href='%23a' y='44'/%3e%3c/g%3e%3cpath stroke='%23fff' d='M0 17v10'/%3e%3ccircle r='12' fill='%23cd2e3a'/%3e%3cpath fill='%230047a0' d='M0-12A6 6 0 0 0 0 0a6 6 0 0 1 0 12 12 12 0 0 1 0-24z'/%3e%3c/g%3e%3cg transform='rotate(-123.69)'%3e%3cuse xlink:href='%23b'/%3e%3cpath stroke='%23fff' d='M0-23.5v3M0 17v3.5m0 3v3'/%3e%3c/g%3e%3c/svg%3e)
'/%3e%3cuse xlink:href='%23a' transform='rotate(23.036 .093 25.536)'/%3e%3cuse xlink:href='%23a' transform='rotate(45.87 1.273 16.18)'/%3e%3cuse xlink:href='%23a' transform='rotate(69.945 .996 12.078)'/%3e%3cuse xlink:href='%23a' transform='rotate(20.66 -19.689 31.932)'/%3e%3c/svg%3e)
