Microstructure Evolution of Fe–Ni-Based Alloy HR6W during Isothermal Aging

Abstract

The age-hardening behavior of the FeNi-based alloy HR6W was investigated in the temperature range between 973 K and 1073 K. A twostep increase of hardness was detected for the alloy at every aging temperature; the first increase of hardness results from the precipitation of M 23 C 6 carbides, and the second increase corresponds to precipitation of the C14Fe 2 W Laves phase. The timetemperatureprecipitation diagram for the alloy was established on the basis of the results of hardness measurements and microstructure observations, where the precipitation of the C14Fe 2 W Laves phase was slower than that of the M 23 C 6 carbides by three orders of magnitude and the nose temperature of the Laves phase was greater than 1073 K. The M 23 C 6 carbides precipitated with a plate-like morphology along grain boundaries at the early stage of aging, followed by the precipitation of the C14Fe 2 W Laves phase with a granular morphology with increasing aging time. The M 23 C 6 carbides and C14Fe 2 W Laves phase are aligned under the stress condition because of their precipitation on the dislocations introduced during creep deformation. [

Microstructural Stability of Secondary Phases in an Ods Ferritic Steel after Thermal Aging at 873 K

“…Furthermore, the streaked diffraction spots observed in Fig. 9(d) are typically mentioned in the literature as characteristic of Laves phase presence [46][47][48][49]. These results confirm that, although some Cr-W carbides remain after the aging process, Laves phase precipitated.…”

“…7 do not contain any C. Cr and W generally exhibit a high C affinity; therefore, the formation of the carbides can be seen as a process concurrent to a possible precipitation of the Laves phase. Laves phase formation at grain boundaries has been widely observed in long-term aged steels and Fe-based alloys [27,[46][47][48][49]56,57], as W is likely to segregate to grain boundaries and this effect is further favored by the long-term aging treatment. When W concentration is high and beyond the accommodation of the grain boundaries, the Laves-phase will begin to precipitate.…”

Impact of Temperature on Low-Cycle Fatigue Characteristics of the HR6W Alloy

“…It can also be seen that the maximum stress in the first load cycle at room temperature is higher than the maximum stress in the first cycle at 650 • C. This behavior of the material which differs in the saturated state in relation to the first load cycle results from the simultaneous impact of the temperature and the number of load cycles on the strengthening process. On the one hand, an increased temperature affects the weakening of the material; on the other hand, its long-term effect favors precipitation processes that lead to strengthening [12].…”

“…What is required from materials under high temperatures and pressures, which occur in boilers under increased operating parameters, is high creep resistance and thermalmechanical and low-cycle fatigue resistance as well as high corrosion resistance . Unfortunately, in contrast to the large number of generally available data in the field of research on basic mechanical properties at room temperature and elevated temperature, creep, and corrosion resistance of the HR6W alloy [1][2][3][4][5][6][7][8][9][10][11][12][13][15][16][17][18][19], there is less data on thermomechanical and low-cycle fatigue testing of this alloy [2,6,14,20]. The benefits of using data obtained in low-cycle fatigue tests at elevated temperature for prediction of durability under thermo-mechanical fatigue loading for the HR6W alloy were demonstrated by Noguchi et al [20].…”

Precipitation Behavior of Ni-Based Superalloy Alloy 625 for A-USC Power Plants

“…2) In advanced ultra-supercritical (A-USC) thermal power plants, where the operating steam temperature is set above 973 K, the ferritic and austenitic heat-resistant steels are not well suited for boiler piping from the viewpoints of high-temperature creep and oxidation characteristics. 3) Accordingly, Ni-based superalloys 4,5) and Fe-Ni-based alloys, [6][7][8] which are applicable to severe vapor pressure conditions at elevated temperatures, have attracted attention in recent years.…”