Abstract ▼ AI-Helper

The mod. (9-12)Cr-1Mo steels have been the most powerful materials for boiler tube and turbine materials in the steam turbine engine due to their excellent creep strength with oxidation resistance.
In this study, the influence of solidification rates and Cr equivalents on the formation of the δ-...

The mod. (9-12)Cr-1Mo steels have been the most powerful materials for boiler tube and turbine materials in the steam turbine engine due to their excellent creep strength with oxidation resistance.
In this study, the influence of solidification rates and Cr equivalents on the formation of the δ-ferrite was studied by directional solidification in mod. (9-12)Cr-1Mo steels. It was found that the volume fraction of δ-ferrite increased as increasing Cr equivalent and solidification rate. The volume fraction of the δ-ferrite showed much higher at low solidification rates with the planar or cellular interface than that at high solidification rates with the dendritic interface. The volume fraction of δ-ferrite in solidification microstructure showed much higher than that in extruded and heat-treated alloys. It depends on not only solidification rate and Cr equivalent but also the solidification fraction. At low solidification rates, there occurs segregation because the solid/liquid interface was planar or cellular, and it makes composition change with solidification fractions. The formation of δ-ferrite has been discussed with Cr equivalent and solidification rate.
And then, the behavior of boride formation has been examined with solidification rates and boron contents by observing the solid/liquid interface using directional solidification in mod. (9-12)Cr-1Mo steels. The phase transformation temperatures of liquidus, solidus, eutectic formation, and final solidification were analyzed by DTA. In order to analyze the crystallographic structure of the boride, extractive method, extracting the boride from the matrix, was used. The boride was proved as M3B2, and this boride phase expected to be formed by eutectic reaction. It has been also found that the boride formation depends on contents of born as well as the other alloying elements, such as C and W. As increasing boron contents, the amount of boride eutectic increased. Also, the alloying element W was shown to enhance the formation of the boride.

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