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Heat-resistant alloys are used extensively in
coal-fired power plants, as gas turbine materials for gas-fired power
plants, or as reactor internals for nuclear power plants. With
increasing demands imposed on structural materials operating at high
temperatures, there is a growing need to predict the service life and
reliability of components experiencing creep deformation and fracture.
In this presentation two numerical models for the creep deformation and
fracture of Cr-Mo heat resistant steels are introduced.
The first approach is strip-yield modeling, which is a numerical method
to simulate crack growth under constant or variable amplitude loading.
A numerical strip-yield model was developed to simulate creep crack
incubation in heat-resistant steels. The time evolution of the plastic
deformation ahead of a crack loaded in tension is modeled using a
stress power law for the steady state creep stage. The evolution with
time of the crack-tip plastic zone, crack-tip opening displacement and
yield strength in the plastic zone are computed at constant temperature
for center-crack panels.
The second approach is a micromechanical model developed for the
evaluation of creep deformation and rupture times of modified Cr-Mo
steels. Creep deformation in metallic materials is generally induced by
the dislocation generation, motion, and annihilation. The evolution of
dislocation density in the crystalline lattice was modeled by
considering the generation and annihilation of single and dipole
dislocations. In addition to the dislocation motion as a basis for
creep deformation, there are other factors which are involved in the
creep resistance of this type of steel. Among these, the most
significant are precipitate coarsening, changes in the concentration of
the solid solutions, void nucleation and crack formation. The evolution
of these mechanisms during creep deformation was modeled by introducing
specific continuum damage equations. Creep tests were also performed at
several stress and temperature levels. The comparison of the numerical
model results with the experimental data shows satisfactory agreement.
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