Influence of Formation and Coarsening of the Laves Phase on the Mechanical Properties of Heat-Resistant Ferritic Steels

• Published in: Steel research international 2014-05, Vol.85 (5), p.851-862
• Main Authors: Klein, Simon, Nabiran, Nilofar, Weber, Sebastian, Theisen, Werner
• Format: Article
• Language: English
• Subjects: Alloys; Annealing; Coarsening; ductility; Ferritic stainless steels; heat-resistant ferritic steels; Laves phase; long-term annealing; Mechanical properties; Microstructure; Precipitation; Steel alloys; Steels; strength; Tensile strength
• ISSN: 1611-3683; 1869-344X
• DOI: 10.1002/srin.201300257

Three newly designed heat‐resistant ferritic alloys containing the intermetallic Laves phase were investigated with respect to an annealing dwell time of up to 1440 h at 900°C and were compared with commercially available steels. A detailed characterization of the microstructure evolution in dependence of the annealing dwell time was performed. In order to estimate the influence of Laves phase formation and coarsening on the strength, ductility and toughness, the results of the microstructural analysis were correlated with tensile tests at room temperature and with Charpy‐V impact tests. Precipitates of the Laves phase were observed in the recrystallized state with a mean particle diameter about 0.25 μm. The Laves phase in all investigated alloys showed rapid growth and coarsening with increasing annealing time. In spite of this behavior, the strength and ductility of the newly designed alloys were conserved, even after annealing for 1440 h. However, the toughness decreased with coarsening of the Laves phase, which is expressed by a shift of the ductile‐to‐brittle transition temperature to a higher temperature. Overall, it was shown that the influence of grain growth on the mechanical properties is more significant than the presence of the Laves phase. Precipitation of Laves phase lowers the mobility of the grain boundaries so that grain growth can be avoided. To improve mechanical properties of ferritic heat‐resistant steels at high temperature, steels were developed that contain a certain amount of Laves phase. In the present work, the effect of its precipitation and coarsening on mechanical properties at room temperature was investigated. It was found, that the Laves phase rises DBTT, but retards grain coarsening and therefore stabilizes mechanical properties during annealing.