Effect of creep-fatigue loading condition on crack tip fields of grade 91 steel at crack initiation and growth

The Sodium Fast-cooled Reactor (SFR), are generation IV nuclear power plants, have a target operating temperature of 550°C which makes creep-fatigue behavior more critical than a generation III nuclear power plants. So it is important to understand the nature of creep-fatigue behavior of the piping material, Grade 91 steel. The creep-fatigue damage diagram of Grade 91 steel used in ASME-NH was derived using a conventional time-fraction testing method which was originally developed for type 300 stainless steels. Multiple studies indicate that the creep-fatigue damage diagram of Grade 91 steel developed using this testing method has excessive conservatism in it. Therefore, an alternative testing method was suggested by separating creep and fatigue using interrupted creep tests. The suggested method makes it possible to control creep life consumption freely which was difficult with the previous method. It also makes it easier to observe the interaction between creep and fatigue mechanisms and microstructural evolution. In conclusion, an alternative creep-fatigue damage diagram for Grade 91 steel at 550°C was developed using an interrupt creep fatigue testing method and FE model simulation.

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Impact of Thermomechanical Fatigue on Microstructure Evolution of a Ferritic-Martensitic 9 Cr and a Ferritic, Stainless 22 Cr Steel

Applied Sciences ◽

10.3390/app10186338 ◽

2020 ◽

Vol 10 (18) ◽

pp. 6338 ◽

Cited By ~ 1

Author(s):

Bernd Kuhn ◽

Jennifer Lopez Barrilao ◽

Torsten Fischer

Keyword(s):

Material Selection ◽

Fatigue Loading ◽

Thermomechanical Fatigue ◽

Power Engineering ◽

Microstructural Stability ◽

Damage And Failure ◽

Trade Name ◽

Energy Conversion Systems ◽

Grade 91 ◽

Grade 91 Steel

The highly flexible operation schemes of future thermal energy conversion systems (concentrating solar power, heat storage and backup plants, power-2-X technologies) necessitate increased damage tolerance and durability of the applied structural materials under cyclic loading. Resistance to fatigue, especially thermomechanical fatigue and the associated implications for material selection, lifetime and its assessment, are issues not considered adequately by the power engineering materials community yet. This paper investigates the principal microstructural evolution, damage and failure of two steels in thermomechanical fatigue loading: Ferritic-martensitic grade 91 steel, a state of the art 9 wt % Cr power engineering grade and the 22 wt % Cr, ferritic, stainless Crofer® 22 H (trade name of VDM Metals GmbH, Germany; under license of Forschungszentrum Juelich GmbH) steel. While the ferritic-martensitic grade 91 steel suffers pronounced microstructural instability, the ferritic Crofer® 22 H provides superior microstructural stability and offers increased fatigue lifetime and more forgiving failure characteristics, because of innovative stabilization by (thermomechanically triggered) precipitation of fine Laves phase particles. The potential for further development of this mechanism of strengthening against fatigue is addressed.

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Unified Viscoplastic Model Coupled With Damage for Evaluation of Creep-Fatigue of Grade 91 Steel

Volume 6A: Materials and Fabrication ◽

10.1115/pvp2017-65849 ◽

2017 ◽

Author(s):

Nazrul Islam ◽

David J. Dewees ◽

Tasnim Hassan

Keyword(s):

Damage Mechanics ◽

Creep Damage ◽

Continuum Damage Mechanics ◽

Creep Model ◽

Damage Evaluation ◽

Creep Fatigue ◽

Grade 91 ◽

Term Creep ◽

Grade 91 Steel

A continuum damage mechanics (CDM) coupled unified viscoplasticity model has been developed to predict the creep-fatigue life of modified Grade 91 steel. A tertiary creep model termed MPC-Omega codified in Part 10 of API (and also implemented in the ASME BP&V Code for Grade 22V and more recently Grade 91 Steel) is also employed for creep damage evaluation. As MPC-Omega has a direct relationship with Larson-Miller parameter (LMP) coefficients, creep damage coefficients in the unified constitutive model (UCM) are tied with MPC-Omega coefficients in order to utilize WRC and API 579-1 Grade 91 creep rupture database. The model is validated against long-term creep, LCF, creep-fatigue and TMF experimental responses at T = 20–600°C.

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