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.