Precipitation behavior of type 347H heat-resistant austenitic steel during long-term high-temperature aging
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The microstructural evolution of type 347H heat-resistant austenitic steel during long-term aging at 700–900 °C was investigated by using a transmission microscope, a scanning electron microscope, and electron energy spectrum technology. The microstructural examination showed the typical micrographs of MX carbonitrides and M23C6 carbides after aging. The existence of the Z phase (NbCrN) at the grain boundaries during aging was identified. Meanwhile, the possible precipitation sequence of these particles was also confirmed. In the beginning of aging, fine Nb(C,N) precipitates first, then, M23C6 carbides precipitate along the grain boundaries. Finally, the Z phase is also observed at the grain boundaries. Moreover, the influence of isothermal holding temperature on the precipitation of MX carbonitrides and M23C6 carbides was discussed. The various microstructural characterizations showed that the M23C6 carbides and MX carbonitrides precipitate more easily with the increase of aging temperature. Furthermore, the number and the size of MX particles and M23C6 carbides increase when the isothermal holding time is prolonged.
I. INTRODUCTION
Contributing Editor: Eric A. Stach a) Address all correspondence to this author. e-mail: [email protected] DOI: 10.1557/jmr.2015.343
former elements compared with Cr, the purpose of additions of niobium is the formation of Nb(C,N) carbonitrides during aging.6 The Nb(C,N) precipitates stabilize the austenite steel against the precipitation of M23C6 carbides, which would lead to intergranular corrosion. Meanwhile, the dissolved Nb(C,N) carbonitrides during solution heat treatment are capable of providing excellent creep resistance in the steel.8 It is well known that the precipitation behavior of the steel during long-term service at elevated temperature leads to the change of mechanical properties of the materials. Therefore, it is necessary to clarify the evolution of secondary phases in the steels. Previous investigations show that MX carbonitrides, M23C6 carbides, Z-phase, laves phase, and sigma phase are the probable phases that existed in type 347H austenitic steel during long-time isothermal aging. As discussed above, MX carbonitrides provide superior thermal strength because of pinning of dislocation during aging.9 The M23C6 carbide is commonly found after isothermal treatment in austenite steel. When type 347H austenitic steel is exposed at the temperature range of 700–900 °C, the laves phase (Fe2Nb) precipitates at the grain boundaries in the matrix. However, the precipitation of the Laves phase results in the reduction of ductility, toughness, and corrosion resistance.10 The Z phase (NbCrN) can strengthen austenite steel due to its stability and is usually observed in nitrogen and niobium containing heat resistant steels during their service at high temperatures.
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Ó Materials Research Society 2015
Concerns over the protection of the environment and the improvement of thermal efficiency prompted the development of ultra-supercritical (USC) power plants that can operate at much
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