Analysis of the cyclic hardening of fully ferritic high Chromium Steels at elevated temperatures

1)P. Lehner, 2)T. Fischer, 1)B. Blinn, 2)B. Kuhn, 1)T. Beck

1)Lehrstuhl für Werkstoffkunde (WKK), TU Kaiserslautern

2) Institut für Energie- und Klimaforschung, Werkstoffstruktur und -eigenschaften (IEK-2), Forschungszentrum Jülich

The ongoing transformation in energy conversion systems is leading to more pronounced fluctuations in the operating levels of power plants, which results in an accelerated damage of the materials used. Consequently, power plant materials with optimized thermomechanical and especially cyclic properties are required. In this context fully ferritic high chromium steels developed at FZ Jülich, IEK2 show a higher resistance to fatigue crack initiation and growth in relation to conventional advanced ferritic martensitic (AFM) steels. This is predominantly caused by thermomechanically induced precipitation of intermetallic Laves phase, which finely disperse in the volume of the fully ferritic stainless steels [1]. Thereby, the precipitation of the Laves phase depends on the operation temperature and time as well as the amount of plastic deformation. To exploit the whole potential of this new class of power plant materials, a sound knowledge of the mechanical, and especially cyclic properties is a prerequisite.

In the present work, interrupted out of phase TMF (thermomechanical fatigue) test were conducted at a fully ferritic high Chromium steel and, as a reference, the well-established AFM steel P91. In the test interruptions, instrumented cyclic indentation tests (CIT), which enable an analysis of the cyclic hardening potential [2], were performed. During the fatigue lifetime an increase in cyclic hardening potential was observed for the fully ferritic steel, which correlates with the evolution of the Laves phase particles induced by a combination of cyclic plastic deformation and elevated temperatures up to 650°C [3]. In contrast to that, the P91 did not show significant changes in cyclic hardening potential during TMF, which finally results in a shorter fatigue strength and lifetime [3]. Consequently, in this work a high correlation of the cyclic hardening potential and the evolution in microstructure, i.e., precipitation of Laves phase, was shown.

Based on this, the influence of the temperature, strain rate as well as stress level on the cyclic deformation behavior of another batch of fully ferritic stainless steels is analyzed in ongoing work. Therefore, fatigue tests are performed at temperatures of 600°C and 650°C, whereby different frequencies (0.005 to 5 Hz) and stress amplitudes are used. Moreover, CIT are performed at loaded and as-received specimens to determine the interrelation of the test condition, the cyclic deformation behavior, and the resulting precipitation of the Laves phase.

[1] Kuhn et al.: Impact of Thermomechanical Fatigue on Microstructure Evolution of a Ferritic Martensitic 9 Cr and a Ferritic, Stainless 22 Cr Steel. 2020. Appl. Sci. 10, 6338

[2] Görzen et al.: Influence of Cu precipitates and C content on the defect tolerance of steels. 2021 International Journal of Fatigue 144, 106042.

[3] Blinn et al.: Analysis of the Thermomechanical Fatigue Behavior of Fully Ferritic High Chromium Steel Crofer22H with Cyclic Indentation Testing. 2020. Appl. Sci. 10, 6461