Magnetic evaluation of microstructure changes in 9Cr-1Mo and 2.25Cr-1Mo steels using electromagnetic sensors

This paper presents results from a multi-frequency electromagnetic sensor used to evaluate the microstructural changes in 9Cr-1Mo and 2.25Cr-1Mo power generation steels after tempering and elevated temperature service exposure. Electromagnetic sensors detect microstructural changes in steels due to changes in the relative permeability and resistivity. It was found that the low frequency inductance value is particularly sensitive to the different relative permeability values of both steels in the different microstructural conditions. The changes in relative permeability have been quantitatively correlated with the microstructural changes due to tempering and long-term thermal exposure, in particular to changes in martensitic/bainitic lath size and number density of carbide precipitates that determine the mean free path to reversible domain wall motion. The role of these microstructural features on pinning of magnetic domain wall motion is discussed

Creep-fatigue interactions in a 9 Pct Cr-1 Pct Mo martensitic steel, part II. Microstructural evolutions

International audienceA multiscale characterization of the microstructural evolutions taking place in 9 to 12 pct Cr martensitic steels subjected to fatigue and creep-fatigue (CF) loadings is presented. Specimens of a P91 steel subjected to high-temperature cyclic loadings are examined using several experimental techniques. Bright-field transmission electron microscopy (TEM), electron backscattered diffraction (EBSD), and TEM orientation mapping are used to characterize and quantify the microstructural evolutions. A recovery phenomenon consisting of the coarsening of the subgrains and a decrease of the dislocation density is observed. This coarsening is heterogeneous and depends on the strain amplitude and on the applied hold time. The size distribution of subgrains and the dislocation density are measured from bright-field TEM observations. Orientation mapping on scanning electron microscopy (SEM) and TEM show that, even though a correlation between the crystallographic orientation and the recovery phenomenon is highlighted, a complex dependency related to the orientation of neighboring blocks exists. These microstructural observations are consistent with the very fast deterioration of creep properties due to cyclic loadings (reported in the first part of this study)

Effect of Precipitation Control on Strength and Impact Toughness in Advanced Pressure Vessel Steel

This paper reports the effect of increase in vanadium (V) content on changes in precipitation reaction and on mechanical properties in SA1017 grade 23 steel (2.25Cr-1.5W-VNbTi) for application to pressure vessels in advanced nuclear reactors. Increase in V content prohibits formation of coarse M23C6 carbide and allows only fine MX carbo-nitrides precipitation in this steel during tempering. Calculation based on Ashby-Orowan equation suggests that increase in yield strength is mainly attributed to precipitation hardening of fine MX carbo-nitrides and it shows a good agreement with experimental results. Meanwhile, step-cooling heat treatment causing temper embrittlement decreases Charpy impact toughness of steels at sub-zero temperatures, but higher V-alloyed steel shows higher toughness. Because interfaces between MX carbo-nitrides and the matrix act as strong trap sites of segregating P, its segregation toward prior austenite grain boundaries is decreased. Therefore, alloy design of V content for SA1017 grade 23 improves strength without degradation of impact toughness and temper embrittlement. (C) The Minerals, Metals & Materials Society and ASM International 201911Nsciescopu