In the research project EPRI WO9000-38 the microstructure stability during creep exposure of the newly developed 9Cr/12Cr-W steels P92 and P122 was studied. Microstructure measurements included: subgrain sizes, phase isolation, lattice parameters, and particle sizes by energy filtered TEM and field emission gun SEM. The investigations focussed on the behaviour of tungsten, which is the major new alloying element in the steels. Major part of the 1,8% Tungsten in both steels precipitate as intermetallic Laves phase during isothermal ageing or creep exposure at 600°C or 650°C, but the loss of tungsten from solid solution does not seem to affect the long-term creep stability. Laves phase particles precipitate and grow during 1.000 hours at 600°C in steel P122 and during 10.000 hours at 600°C in steel P92. After this the Laves phase particles coarsen at a rate similar to the M_(23)C_6 carbides. Calculations of Orowan back stresses and carbide coarsening rates indicate a positive effect of tungsten on microstructure stability and creep strength caused by Laves phase precipitation hardening and stabilisation of M_(23)C_6 carbides.
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