Effect of different loading conditions on the accumulation of residual strain in a creep resistant 1%CrMoV steel:a neutron and X-ray diffraction study
Michael Andreas Weisser
Plastic deformation of multi-phase materials can generate significant amount of stresses between the microstructural constituents due to their different mechanical properties. In ferritic carbon steels, the main microstructural constituents are the polycrystalline ferritic matrix and the cementite particles. Several X-ray and neutron diffraction studies report on the interplay between the cementite and the ferrite. These studies, however, have been limited to ambient temperatures, where
... e is known to be a hard and brittle phase. Under that condition, large stresses between these two phases are created due to the load-redistribution from the plastifying ferrite to the cementite. The strengthening mechanisms at both ambient and elevated temperatures in creep resistant bainitic 1% CrMoV steels are governed by the interplay between the ductile ferrite matrix and the carbides, among which vanadium carbide and cementite are the main constituents. In this dissertation, the residual stress (actually strain) accumulated during RT tensile deformation is studied and compared to the residual strain accumulated during high temperature (565°C) tensile and creep deformation. A temperature of 565°C was chosen because it is the maximum operating temperature for this material when used as a rotor steel in steam turbines. The complementary use of Time-of-flight (ToF) neutron and synchrotron X-ray diffraction accounts for the inhomogeneous microstructure and the low volume fraction of the second phase particles (3%), respectively. ToF neutron diffraction on pre-deformed samples shows that the accumulation of residual strains strongly depends on the deformation condition: Large interphase strains are created during RT tensile deformation whereas very little strains are created after creep deformation. On the other hand, large intergranular strains are introduced for every deformation sequence but the loadredistribution between the ferrite grain families appears to be different at ambient and elevated temperatures. In situ neutron and X-ray diffraction elucidates that the cementite contributes to the build-up of interphase strain during during deformation at both RT and HT, but only until creep mechanisms become dominating. The intergranular load-redistribution is discussed in terms of the elastic anisotropy of the ferrite grain families, which appears to be more pronounced at elevated temperatures. The small volume fraction of cementite in the 1%CrMoV is responsible for a significant accumulation of residual interphase strain, comparable to the amount in some high-carbon steels. It appears that the microstructure and the morphology of the cementite particles can significantly influence the amount of residual strain. In addition, the cementite characteristics during tensile deformation in the 1%CrMoV have been studied and compared to that of a pearlitic and a spheroidized microstructure with spherical cementite particles. Cementite shows an elastic anisotropy and an extensive diffraction peak broadening during plastic deformation. This broadening is discussed in terms of the range of local stress states, the individual particles experience.