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

Abstract

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.

Analysis of the Thermomechanical Fatigue Behavior of Fully Ferritic High Chromium Steel Crofer®22 H with Cyclic Indentation Testing

“…As shown in Reference [5], the differences in fatigue behavior of both steel types are caused by different microstructural evolution. To quantify the microstructural evolution caused by cyclic loading, several experimental methods can be applied.…”

“…While strengthening of P91 is mainly achieved by M 23 C 6 (M: Mo, Cr)-and MX (M: V, Nb; X: N)-particles, as well as solid solution and sub-grain hardening [3], the fully ferritic Crofer ® 22 H, can be strengthened by a combination of solid solution hardening and intermetallic Laves phase (Fe,Cr,Si) 2 (Nb,W) precipitates [4], whereby Crofer ® 22 H has a limited solubility of C and N in the ferritic matrix. For Crofer ® 22 H, the decisive factor for hardening in (thermomechanical) fatigue loading is the thermomechanically induced precipitation of the Laves phase, which takes place during application in the temperature range from 600 to 700 • C [5].…”

“…Moreover, Crofer ® 22 H exhibits a stable grain structure under thermomechanical fatigue (TMF)-loading, which is in contrast to the AFM steels, showing unstable martensite lath structure and "polygonization" at TMF-loading conditions [5]. Additionally, in Reference [5], a higher damage tolerance of Crofer ® 22 H compared to P91 is indicated, leading to a higher TMF-lifetime for Crofer ® 22 H. Investigations at another fully ferritic, Laves phase strengthened steel [6] show a higher amount of hardening and additionally a sub-grain formation in front of the crack tip, resulting in crack branching or deflection at sub-grain boundaries and crack deflection at Laves phase particles [6]. Furthermore, it is assumed, that plastic deformation or high dislocation densities in the vicinity of a crack tip, as well as sub-grain boundaries, can boost the Laves phase precipitation, because both can act as nucleation sites [6].…”

“…As the TMF-tests performed in Reference [5] indicate a higher damage tolerance for Crofer ® 22 H in relation to P91, CITs were performed at various states of TMF-loading to elaborate the difference in damage tolerance, as well as its TMF-induced evolution. Additionally, local investigations of the damage tolerance in the vicinity of a fatigue crack were performed using CITs with reduced indentation force, thereby quantifying local differences in mechanical properties.…”

“…Additionally, local investigations of the damage tolerance in the vicinity of a fatigue crack were performed using CITs with reduced indentation force, thereby quantifying local differences in mechanical properties. Moreover, the underlying microstructural changes reported in Reference [5] are compared to the results obtained in CITs. Consequently, the results presented here are an extension of the investigations from Reference [5].…”