Wolfgang Bleck

High-chromium ferritic stainless HiperFer steels were developed for high-temperature applications in power conversion equipment. The presented research describes the precipitation behavior of the Laves phase after the thermomechanical treatment of Fe-17Cr-0.6Nb-2.4W HiperFer alloys with and without the addition of 55 ppm boron. The boron-alloyed variant was produced with the aim of enhancing grain boundary strengthening and consequently increasing creep resistance. The focus is set on the effect of boron on the thermomechanically induced precipitation of (Fe,Cr,Si)2(Nb,W) Laves phase at grain boundaries. The addition of boron modifies the diffusion conditions in the area of grain boundaries. Consequently, the formation of Laves phase is promoted and the particle growth and coarsening process are suppressed. The impact of boron addition was validated by performing creep and thermomechanical fatigue testing in the standard processing state of HiperFer steel. In the B-alloyed variant, increased creep ductility through the modification of the particle-free zone widths at high-angle grain boundaries was encountered. Nevertheless, an optimized thermomechanical treatment is necessary to fully utilize the increased ductility effect for the creep strength optimization of the B-alloyed grade.

Development of the descriptors of the multiphase microstructure, which will allow comparing material's local fracture resistance, is the objective of the article. A hypothesis is made that local gradients of properties can realistically represent the tendency to the local fracture and can be an effective indicator for the stretch‐flangeability properties. A novel gradient evaluation algorithm was developed within the study. The algorithm calculates the distribution of gradients for the data given in the form of a cloud of points, independently of the source of these data. Two industrial grade dual‐phase (DP) and complex‐phase (CP) steels are produced from the same cast chemical composition and with the same processing route; however, different parameter setups are studied using a correlative microstructure characterization approach to supply data for gradients evaluation. Obtained electron backscattered diffraction maps are used to generate the representative volume elements and statistically similar representative volume elements, which are subjected to deformation and used to calculate the properties gradients theoretically. The high‐resolution hardness maps are used to directly obtain the hardness gradients. Both methods of gradients calculations are applied to the DP and CP microstructures and prove to be an effective and reliable method for the comparison of materials’ stretch‐flangeability.