5. Conclusions
Study of the effect of the dwells at maximum temperature in
thermomechanical out-of-phase fatigue and the comparison with in-phase
fatigue and thermomechanical fatigue without dwells of superaustenitic
Sanicro 25 steel led to the following conclusions:
1. Cyclic stress-strain response is substantially affected by dwells
leading to more rapid initial hardening both in OPD-TMF and in IPD-TMF
cycling. In OPF-TMF cycling due to the extension of the fatigue life
long period of stress amplitude saturation was found. Introduction of
dwells results in the shift of cyclic stress-strain curves to lower
stress amplitudes.
2. Internal dislocation structure after cycling with dwells changes
markedly in comparison with cycling without dwells. Random distribution
of dislocation segments without any arrangement in bands parallel to the
crystallographic directions was found. Dislocations are often pinned by
nanoparticles. Dislocation density is lower in the case of OPD-TMF
cycling than in the case of IPD-TMF cycling.
3. The introduction of dwells led to the increase of the nanoparticle
density. In addition to Cu-rich and Nb-rich particles larger W-rich
particles were identified. Systematic formation of Cr rich
M23C6 carbide on the grain boundaries
was found.
4. Surface fatigue crack in IPD-TMF cycling starts in the oxidized grain
boundaries and continues intergranularly. Its growth is accelerated by
the formation of internal cracks which join the principal crack during
its growth.
5. In OPD-TMF cycling the crack initiation on the grain boundaries is
suppressed. The thick oxide layer is formed and when its thickness
reaches critical depth, parallel open cracks perpendicular to the stress
axis arise. The oxygen has locally access to the metal and localized
oxidation and oxide cracking leads to early crack growth. Fatigue cracks
grow transgranularly.