The Effects of Carbon and Austenite Stabilizing Elements (Co, Cu, Ni and Mn) on the Microstructural Changes and the Creep Rupture Strength in 9-12 Cr Ferritic Heat Resistant Steels

摘要

The effects of carbon element and austenite stabilizing elements, such as Ni, Mn, Co and Cu, on the precipitation of Laves phase, Fe_2W, and the creep rupture strength (CRS) at 600 °C to 700 °C have been investigated by the creep test and microstructural observation in 9-12 % Cr steels. Both Ni and Mn had nearly no effect on the formation of Laves phase and coarse carbide like M_6C up to about 1 %. As the Co content increases from 0 % to 3 %, the fraction of Laves phase at 650 °C of Co alloy was increased by about 17 %. The addition of Co element did not contribute to the improvement of long-term CRS at 650 °C but caused the rapid decrease of CRS in a short time. The addition of Cu caused the precipitation of fine Cu-rich precipitates. Cu particle acts on a nucleation site for Laves phase and M_(23)C_6 carbide. In both Cu and CuC alloys, needle type Laves phase different from globular type Laves phase in Ni and Co alloys occurred and it contributed to the improvement of CRS in Cu alloy. As the carbon content increases from 0.1 % to 0.2 %, the fraction of Laves phase at 650 °C was reduced by about 20 % in the calculation by Thermo-Calc. It was found that the increment of carbon content is the most effective method to suppress the formation of Laves phase. In Co alloy with higher W content, the increase of carbon content from 0.09 % to 0.19 % had a good influence on the CRS at 650 °C, while the increment of carbon content in Ni and NiMn alloys lowered CRSs at 600 °C to 700 °C. The CRSs of Cu alloy at 600 °C to 700 °C showed an improving trend more or less due to the reduction of Laves precipitation caused by carbon addition. Cu alloy, therefore, shows the most superior CRS among the alloys with 0.2 % carbon.