Creep-Fatigue Behavior and Analysis of the Load- and Grain Structure-Dependent Laves Phase Formation in Ferritic High Temperature Steel with High Chromium Content

• Applicant: Professor Dr.-Ing. Tilmann Beck
• Subject Area: Mechanical Properties of Metallic Materials and their Microstructural Origins
• Term: since 2020
• Project identifier: Deutsche Forschungsgemeinschaft (DFG) - Project number 450763904

Fully ferritic high temperature steels with Cr-contents of around 17 wt.-% (High Performance Ferrites – HiPerFer), which are precipitation strengthened by the formation of finely dispersed Laves phase particles are, due to their high creep and fatigue strength, and superior corrosion resistance, a promising alternative to established martensitic steels with 9 wt.-% Cr content. While at the beginning of the recently completed 1st funding period (FP) of this project, a sound state-of-knowledge existed regarding the creep behavior of both material classes, in this FP a comprehensive and detailed comparison of the fatigue performance of a HiPerFer material with 17 wt.-% Cr content, and P91 as representative of 9% Cr martensitic steels, was established for the first time. In this context, the cyclic deformation behavior of both materials as well as fatigue-induced changes in the microstructure, and in the micromechanical properties determined by instrumented cyclic microhardness testing (CHT), were considered in detail. In summary, it was concluded that the fatigue behavior of the P91 is characterized by continuous cyclic softening, which is accelerated by increasing testing temperatures, while dynamic strengthening processes due to the continuous precipitation of finely dispersed Laves phase enables the HiPerFer-material to counteract the temperature- and fatigue-induced degradation. Albeit the limits of these unique strengthening effects of HiPerFer are not quantitatively clarified yet, the results obtained in the 1st FP rise the assumption that the HiPerFer material could outperform the P91 regarding its fatigue strength, especially at temperatures of around 650°C.In this context, it’s currently unclear how creep strains applied before a cyclic loading as well creep processes induced by superimposed mean stresses during fatigue loadings do affect the microstructure, and the fatigue performance of HiPerFer materials. This shall be clarified by a comparative study on both materials in the 2nd FP, for which a duration of 24 months is envisaged. For this, experiments with sequential application of creep and subsequent High Cycle Fatigue- (HCF-) loadings as well as HCF tests with superimposed mean stress are planned, each at a temperature of 650°C. Additionally, high-temperature fatigue tests with stress-free dwell times after each cycle will be performed to separate the influence of time / test duration from the influence of cyclic loading on the strengthening effects induced by Laves phase precipitation. To establish an in-depth understanding of the results achieved in the fatigue tests outlined above, comprehensive microstructural analyses as well as CHT on the micro and nano scale are planed which will also contribute to the clarification of the influence of the local grain orientation on the Laves phase precipitation observed in the 1st FP.

• DFG Programme: Research Grants
• Co-Investigators: Dr.-Ing. Bastian Blinn; Dr.-Ing. Bernd Kuhn