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The high-temperature temper embrittlement of martensitic heat-resistant 10Cr12Ni steel was studied. The results demonstrate that there is some irreversible temper embrittlement when the steel is tempered at 625 °C. The irreversible temper embrittlement can be overcome by re-tempering at higher temperature. The tempered martensite embrittlement at 625 °C is attributed to the precipitation of M 23 C 6 -type carbides along the martensite lath and prior austenite grain boundaries. The tempering process can be divided into three stages: In the first stage, the martensite laths recover and M 7 C 3 -type carbides precipitate inside the martensite laths, leading to the improvement of the toughness. In the second stage, M 7 C 3 -type carbides dissolve, and M 23 C 6 -type carbides precipitate along the martensite laths and prior austenite grain boundaries. The M 23 C 6 -type carbides play a nucleating role in the development of cracks, leading to tempered martensite embrittlement. In the third stage, the toughness gradually recovers with the further recovery of the martensite laths. [ABSTRACT FROM AUTHOR]

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. • Large-size carbides cause serious anisotropy of toughness and plasticity. • Big carbides promote the formation and propagation of microcracks. • Big carbides cause long brittle cracks in transverse impact specimens. • Big carbides cause shallow chain-distributed dimples in transverse specimens. [ABSTRACT FROM AUTHOR]

The hot ductility of as-cast 11Cr–3Co–3W martensitic heat resistant steel was studied by tensile tests in the temperature range from 900 °C to 1200 °C. A serious hot embrittlement was observed at 1150–1200 °C, appearing as a whole intergranular fracture. And the ductility fully recovered with deformation temperature decreasing to 1100 °C and below, appearing as a dimple fracture. The fracture appearance, microstructure and precipitates were analyzed to investigate the hot embrittlement mechanism. The phase transformation of precipitates and their effect on hot embrittlement were mainly discussed. Results show that the hot embrittlement is caused by the precipitation of (Cr, Fe, W)-rich sigma phase, which precipitates and forms flaky structure at grain boundaries of austenite and delta ferrite at 1150 °C and above in the experimental steel. The flaky structure composed by brittle sigma phase seriously destroys the bonding of grain boundaries and causes intergranular cracks. The recovery of ductility with deformation temperature decreasing to 1100 °C is attributed to the phase transformation of precipitates from sigma phase to M6C-type carbides. Flaky-distributed sigma phase is unstable at 1100 °C and below and tends to transform to isolated-distributed M6C-type carbides, which have less harmful effect on ductility. [ABSTRACT FROM AUTHOR]