Microstructural stability of ODS steels in cyclic loading

Abstract

A B S T R A C T The remarkable microstructural stability of high chromium steels prepared by powder metallurgy and strengthened by dispersion of nanometric yttrium oxides in cyclic loading at high temperatures is reported. Contrary to the continuous cyclic softening and profound changes in the microstructure during fatigue of common high chromium steels, the addition of 0.3 wt% Y 2 O 3 stabilizes the microstructure and significantly reduces cyclic softening of investigated steels. The evolution of microstructure as a result of fatigue loading at room temperature, 650 and 750°C, was examined by means of transmission electron microscopy. Only minor changes in the microstructure were detected. The stability of oxide particles after high-temperature exposure was confirmed by energy dispersion spectroscopy chemical analysis. The microstructural features are discussed in relation to the cyclic behaviour of the oxide dispersion strengthened steels. The analysis of the hysteresis loop indicates that oxide nanoclusters are intersected and dissolved in slip bands of ODS Eurofer steel. This process contributes to cyclic softening.Keywords steel; microstructure; fatigue.N O M E N C L A T U R E EBSD = electron backscattered diffraction EDS = energy dispersion spectroscopy LAB = low angle boundary ODS = oxide dispersion strengthened RAF = reduced activation ferritic RAFM = reduced activation ferritic-martensitic SEM = scanning electron microscopy STEM = scanning transmission electron microscopy TEM = transmission electron microscopy E eff = effective elastic modulus (MPa) N = number of cycles (À) ε r = relative strain (À) σ a = stress amplitude (MPa) σ Ε = effective stress (MPa) σ I = internal stress (MPa) σ r = relative stress (MPa)

Towards improved ODS steels: A comparative high‐temperature low‐cycle fatigue study

“…The first assumption was confirmed by Kuběna et al with a cyclic stress level of almost double magnitude (also see Figure where the studied ODS steels as well as ODS F/M P91 show higher cyclic strength than the conventional non‐ODS version of P91; the tests on P91 versions (for more details about these materials see Straßberger et al and Führer and Aktaa 32 ) were performed on the same type of sample, using the same test set‐up). In contrast, the second presumption of cyclic softening as shown still persists in the present 9YWT‐MATISSE, ODS P91 and was previously also reported for ODS EUROFER, see Kuběna et al However, even though the studied ODS steels exhibit cyclic softening, it is significantly lower than that of the non‐ODS P91 (Figure ).…”

Cyclic Creep Behavior and Modified Life Prediction of Bainite 2.25Cr-1Mo Steel at 455 °C

“…Many engineering structures and components are frequently subjected to cyclic loadings during normal operation, which generally leads to a complicated failure mechanism caused by a combined damage mode of creep, fatigue, and their interactions [1][2][3][4][5]. Especially, stress loading is more common in practical engineering applications than strain loading [6].…”

Cyclic creep behaviour and strain classification of a bainite 2.25Cr-1Mo steel at 455oC

“…High temperature equipment is generally subjected to cyclic loadings during operation. [1][2][3][4][5] Hence, the failure mechanism of these kinds of equipment is complicated and often with mixed damages modes of creep, fatigue, as well as their interactions. Under realistic loading conditions, such as nuclear reactor components, stress loading is considered to be more common than strain loading.…”