Effect of delta ferrites on the anisotropy of impact toughness in martensitic heat-resistant steel

To avoid brittle failure of the final-stage turbine blades, it is necessary to understand the anisotropy mechanism in martensitic heat-resistant steel 10Cr12Ni3Mo2VN. This study focused on the effect of delta ferrites on the anisotropy of impact toughness by investigating the relationship between the microstructures and the anisotropy of impact toughness. It was mainly interested in the effect of banded delta ferrites on the transverse impact toughness of experimental steel after quenching and tempering. It was found that banded delta ferrites cause banded brittle cracks and induce tempered martensite matrix to fracture in quasi-cleavage mode, which leads to the embrittlement in transverse specimens and the anisotropy of impact toughness. The embrittlement caused by banded delta ferrites is unrelated to other precipitates such as M23C6-type carbides or MX-type precipitates. In addition, by removing the banded delta ferrites, it can eliminate the anisotropy of impact toughness in the experimental steel. It was also found that conventional heat treatment could not improve the isotropy of impact toughness since it has little influence on the content of banded delta ferrites. As suggested, the use of upsetting-stretching forging strategy promises a great of improvement of isotropy of impact toughness as it can remarkably reduce the amount of banded delta ferrites. (C) 2019 Brazilian Metallurgical, Materials and Mining Association. Published by Elsevier Editora Ltda.