On precipitates in Fe–Ni base alloys used for USC boilers

Abstract

This paper overviews the effects of some precipitates in Fe–Ni base boilers materials. The volume fraction of these precipitates is usually below 10 for maintaining the microstructure stability and manufacture ability. They appear in a number of morphologies, but all of them improve the creep strength when they exist in small size. For the alloys serviced below 650°C, the strengthening precipitates disperse within the matrix and maintain their diameter below 100 nm during long time exposure. For the alloys serviced above 650°C, the strengthening precipitates are normally larger than 100 nm in equivalent diameter. They may experience long growth duration and thus keep their size below 500 nm. Meanwhile, the precipitates may improve creep strength when they are dispersed over the grain boundaries.

Analysis of Phase Transformation in Nickel-Base Alloy after Ageing

“…As the energy of the incoherent twin boundary represents ap-proximately 0.7 and the energy of the coherent boundary represents 0.2-0.3 of the energy of the large-angle boundary [11], at the next stage the precipitation took place at the twin boundaries and inside the grains. In creepresistant alloys, precipitates at the grain boundaries hamper sliding at the boundaries, thus increasing the creep resistance [4,5,12]. However, this effect disappears in case of formation of the so-called continuous network of precipitates by particles, which results in the increase in brittleness and contributes to a change in cracking mechanism from ductile to brittle, frequently intercrystalline [3].…”

“…M 23 C 6 carbides ( Fig. 4a) are characterised by low stability at elevated temperature and a tendency to form the so-called continuous network of precipitates at the grain boundaries, which results in the decrease in mechanical properties, and also in the reduction in corrosion resistance [3][4][5]13]. According to [5,11,12], not only the amount of M 23 C 6 particles precipitated at the grain boundaries, but also their shape determine their impact on the properties.…”

“…4b) in an alloy of this type are considered favourable provided that the phase is relatively fine. The growth of this phase leads to the reduction in creep strength and crack resistance [2,4,10]. Inside the grains, in addition to primary MX precipitates, numerous relatively fine secondary precipitates of M 23 C 6 carbides are observed.…”

“…The suitability of creep-resistant steels and alloys for use in components and installations of power units is determined by labour-and time-consuming tests of microstructure stability and mechanical properties under ageing conditions at a temperature similar to that of the expected service. For creep-resistant steels and alloys with austenitic matrix the main processes affecting the microstructure degradation and reduction in functional properties, including corrosion resistance, are precipitation and changes in the morphology of secondary phases [3][4][5][6][7][8]. Knowledge of the sequence of secondary phase precipitations and linking them with changes in mechanical properties will allow building a database and characteristics of specific alloy in terms of its applicability for long-term service under creep conditions [2,3].…”

Microstructural study in a Fe–Ni-base superalloy during creep–rupture at intermediate temperature

“…It impose a requirement of the long term rupture strength (100 MPa for 10 5 h) at the service temperature [4][5][6][7]. Unfortunately, it exceeds the capacity of the traditional commercial boiler materials, such as ferrite and austenitic steel [8,9]. Recently, Fe-Ni-base superalloy has attracted much attention as a candidate selection for use as boiler material in A-USC power plants [10][11][12][13][14].…”