A Study on the Paste Boronizing Treatment of 12 percent Cr Steel for Steam Turbine Nozzle and High Temperature Surface Properties

摘要

12 percent Cr steel used for a nozzle in steam turbine was paste-boronized and its surface properties at high temperature were evaluated. Typical tooth structure, which consisted of outer layer of FeB and inner layer of Fe_2B, was observed at the boronized layer of the specimen and its activation energy was 286.7 kJ/mole. Cracks due to the difference of thermal expansion coefficient and brittleness of FeB layer were observed. Voids were also observed at tip area of tooth structure. Weight of the specimen exposed at 740 deg C increased rapidly from the initial stage of oxidation. Iron oxide was mixed in chromium oxide at outer layer of the oxidized specimen, but chromium oxide and molybdenum were mixed in iron oxide at inner layer. Borenized specimen has shown excellent erosion resistance up to 20 times higher than that of non-treated specimen.