49346
2019
eng
lecture
0
--
--
--
Cyclic Operation Performance of 9-12% Cr Ferritic-Martensitic Steels. Part 2: Microstructural Evolution during Cyclic Loading and its Representation in a Physically-based Micromechanical Model
The current trend towards cyclic, “flexible” operation of fossil-fueled power plants constitutes a major issue regarding lifetime and operational safety of the respective installations and their components, as was outlined in our complementary contribution (Part 1). The present contribution reports on the investigation of the microstructure evolution in cyclically loaded ferritic-martensitic steels and its representation in a physically-based micromechanical model.
For this purpose, specimens of P92 steel grade from the mechanical test programme outlined in our companion contribution (Part 1) were analyzed by scanning electron microscopy (SEM), including backscattered diffraction (EBSD) mapping, and transmission electron microscopy (TEM). A novel method was implemented to improve angular resolution of EBSD scans. Additionally, a correlative microscopy approach was developed and used to correlate EBSD and TEM measurements on the same locations of thick regions of electron transparent specimens. By applying these techniques, a detailed quantitative microstructure description of the as-received material condition, namely in terms of subgrain morphology and dislocation density/distributions, was established. Comparisons of as-received and cyclically loaded conditions from tests interrupted at different stages of lifetime indicate a rapid redistribution of in-grain dislocations with a strong interaction between mobile dislocations and low angle grain boundaries (LABs).
The proposed micromechanical model is formulated in a viscoplastic self-consistent (VPSC) scheme, which is a mean-field approach that allows us to include the crystal details at the level of slip systems while avoiding the considerable computational costs of full-field approaches (such as the classical crystal plasticity finite element analysis). Being physically-based, the model uses dislocation densities and includes the interaction between dislocations, e.g. annihilation of mobile dislocations, and evolution of microstructure, e.g. the grain coarsening. Particularly, the constitutive laws for dislocation evolution and interaction between dislocations and low angle boundaries are calibrated based on two-dimensional discrete dislocation dynamic (2D DDD) simulations, which are performed at a micro-/meso-scale. The results of the beforementioned EBSD experiments are considered as a direct input, involving e.g. the amount of geometrically necessary dislocations, average misorientations and grain characteristics.
45th MPA-Seminar 2019
Leinfelden-Echterdingen, Germany
01.10.2019
02.10.2019
false
true
0
Leonardo Agudo Jácome
Farshad Roohbakhshan
eng
uncontrolled
Tempered martensite ferritic steel
eng
uncontrolled
Dislocation
eng
uncontrolled
Electron backscattered diffraction (EBSD)
eng
uncontrolled
Transmission electron microscopy (TEM)
eng
uncontrolled
Microstructure
eng
uncontrolled
Physically based material model
Angewandte Physik
5 Werkstofftechnik
5.1 Materialographie, Fraktographie und Alterung technischer Werkstoffe
5.5 Materialmodellierung
Energie
Datei im Netzwerk der BAM verfügbar ("Closed Access")
Präsentation
50052
2019
eng
80
85
conferenceobject
MPA (Materialprüfungsanstalt Universität Stuttgart)
Stuttgart
0
--
--
--
Cyclic operation performance of 9-12% Cr ferritic-martensitic steels part 2: Microstructural evolution during cyclic loading and its representation in a physically-based micromechanical model
The current competitive situation on electricity markets forces conventional power plants into cyclic operation regimes with frequent load shifts and starts/shutdowns. In the present work, the cyclic mechanical behavior of ferritic-martensitic 9-12 % Cr steels under isothermal and thermomechanical loading was investigated for the example of grade P92 material. A continuous softening was observed under all loading conditions. The introduction of hold periods to the applied cycles reduced material lifetime, with most prominent effects at technologically relevant small strain levels. The microstructural characterization reveals a coarsening of the original “martensitic” lath-type microstructure to a structure with polygonal subgrains and reduced dislocation density. The microstructural data forms the input for a physically-based modelling approach.
Fit for Future - Advanced Manufacturing Technologies, Materials and Lifetime - Proceedings of the 45th MPA-Seminar
45. MPA-Seminar
Leinfelden-Echterdingen, Germany
01.10.2019
02.10.2019
false
true
Leonardo Agudo Jácome
Gert Nolze
Farshad Roohbakhshan
Bernard Fedelich
Jürgen Olbricht
Birgit Skrotzki
eng
uncontrolled
Tempered Martensite Ferritic Steels
eng
uncontrolled
P92
eng
uncontrolled
TEM
eng
uncontrolled
EBSD
eng
uncontrolled
Micromechanical model
Angewandte Physik
5 Werkstofftechnik
5.1 Materialographie, Fraktographie und Alterung technischer Werkstoffe
5.2 Metallische Hochtemperaturwerkstoffe
5.5 Materialmodellierung
Energie
Datei im Netzwerk der BAM verfügbar ("Closed Access")
Graue Literatur
47101
2018
eng
lecture
0
--
--
--
Dislocation-based modeling of high temperature deformation and fatigue in P92 ferritic-martensitic steels
The employment of renewable energy resources, which are naturally intermittent, for electricity generation has altered the working conditions of conventional power plants from continuous (baseload) to cyclic or flexible operation. For a long time, 9-12% Cr ferritic-martensitic stainless steels have been widely used in power plants due to their favorable characteristics such as high creep strength at high temperatures and oxidation and corrosion resistance. The components of power plants are subjected to long term cyclic loadings including fatigue and creep-fatigue at high temperatures. As ferritic-martensitic steels are known to exhibit cyclic softening when subjected to such loading scenarios, it is crucial to study the material response in such conditions. Since it is impossible to test the material behavior exactly as the operation conditions of power plants, due to technical difficulties and cost issues, it is necessary to develop physically-based material models that can predict the material behavior in more realistic situations.
In recent years, many material models have been proposed to describe the behavior of 9-12% Cr ferritic-martensitic stainless steels, which follow phenomenological or physically-based approaches. Phenomenological models provide a stress-strain relation based on empirical observations although they usually lack physical background. To alleviate this drawback and to allow for more flexibility and wider ranges of strain-rate and temperature, physically-based models are suggested. In this approach, microstructural evolution, dislocation movement and/or kinetics of plastic deformation processes are included. The physically-based models allow for a better extrapolation from the experiment results to other operation conditions and their material constants can be interpreted physically. Compared to the former approach, the material behavior can be described more accurately and flexibly and the number of material constants is less in general. In the presented work, the existing micromechanical models developed for P92 steel are compared and extended to allow for new dislocation-based strengthening/cyclic mechanisms. Their performance is assessed in the light of mechanical test data from creep-fatigue and thermo-mechanical fatigue experiments and detailed characterizations of the microstructure evolution in the fatigued material.
12th International Fatigue Congress
Poitiers, France
27.05.2018
01.06.2018
false
true
0
Farshad Roohbakhshan
eng
uncontrolled
9-12% Cr ferritic-martensitic steels
eng
uncontrolled
Cyclic softening
eng
uncontrolled
Micromechanical model
eng
uncontrolled
Microstructural evolution
eng
uncontrolled
P92 steels
Angewandte Physik
5 Werkstofftechnik
5.2 Metallische Hochtemperaturwerkstoffe
5.5 Materialmodellierung
Energie
Datei im Netzwerk der BAM verfügbar ("Closed Access")
Präsentation
47118
2018
eng
259
265
conferenceobject
MPA/University of Stuttgart
0
--
--
--
Cyclic loading performance and related microstructure evolution of ferritic-martensitic 9-12% Cr steels
The current competitive situation on electricity markets forces power plants into cyclic operation regimes with frequent load shifts and starts/shutdowns. In the present work, the cyclic mechanical behavior of ferritic-martensitic 9-12 % Cr steels under isothermal and thermomechanical loading was investigated for the example of grade P92 material. A continuous softening was observed under all loading conditions. The introduction of hold periods to the applied cycles reduced material lifetime, with most prominent effects at technologically relevant small strain levels. The microstructural characterization reveals a coarsening of the original “martensitic” lath-type microstructure to a structure with polygonal subgrains and reduced dislocation density. The microstructural data forms the input for a physically-based modelling approach.
Fit for Future - Advances in Materials, Manufacturing and Assessment. Proceedings of the 44th MPA-Seminar
44th MPA-Seminar
Leinfelden/Stuttgart, Germany
17.10.2018
18.10.2018
false
true
Jürgen Olbricht
Leonardo Agudo Jácome
Maria Jürgens
Farshad Roohbakhshan
Bernard Fedelich
Birgit Skrotzki
eng
uncontrolled
Ferritic-martensitic steels
eng
uncontrolled
Cyclic loading
eng
uncontrolled
Microstructure evolution
Ingenieurwissenschaften und zugeordnete Tätigkeiten
Angewandte Physik
5 Werkstofftechnik
5.1 Materialographie, Fraktographie und Alterung technischer Werkstoffe
5.2 Metallische Hochtemperaturwerkstoffe
5.5 Materialmodellierung
Energie
Material
Degradation von Werkstoffen
Datei im Netzwerk der BAM verfügbar ("Closed Access")
Graue Literatur