Effect of large-size carbides on the anisotropy of mechanical properties in 11Cr-3Co-3W martensitic heat-resistant steel for turbine high temperature blades in ultra-supercritical power plants

Abstract

9–12%Cr martensitic heat-resistant steels (MHRS) are the key materials for making ultra supercritical (USC) unites turbine blades. In this study, the anisotropy of mechanical properties in Cr-Co-W-type MHRS 1Cr11Co3W3NiMoVNbNB is discussed. The samples were prepared for impact and tensile tests. By utilizing characterization techniques, such as OM, FESEM, TEM, and EDS, steel sample fracture surfaces and the microvoids as well as the microstructure-anisotropy mechanical properties relationship were thoroughly investigated. It was pointed out that a strong anisotropy of mechanical properties was observed as much lower impact toughness and tensile plasticity in transverse direction in the steel sample after a heat treatment process. The anisotropy was mainly due to the large-size directionally distributed M23C6-type and M6C-type carbides. Large-size directionally distributed carbides lead to anisotropy of mechanical properties mainly by promoting the formation of microcracks and the propagation of cracks in transverse specimens. For transverse impact specimens, the propagation of microvoids is promoted to banded brittle cracks by large-size directionally distributed carbides. These banded brittle cracks become the pre-cracks of matrix and markedly decrease the toughness. For transverse tensile specimens, the propagation of microvoids is promoted to shallow chain-distributed dimples by large-size directionally distributed carbides, which markedly decrease the plasticity.