| The effects of W and B on the stress-rupture and impact properties of modified 9∼12 Cr-ferritic steel for high temperature use were studied. Stress-Rupture tests were carried out using a lever arm creep tester at 550℃, 600℃ and 650℃ in air, and impact tests were performed at -196℃, 0℃ and 25℃. The stress-rupture strength of 0.1C-9.8Cr-0.6Mo-2W-0.01B steel was lower than that of 0.18C-10.5Cr-1.0Mo-0.2V-0.07Nb steel. The high strength in the latter steel was due to the high C content, larger grain size of prior austenite and carbide precipitation. The deterioration of stress-rupture strength with time of 0.1C-9.8Cr-0.6Mo-2W-0.01B steel was, however, less than that of 0.18C-10.5Cr-1.0Mo-0. 2V-0.07Nb steel. It is, thus, expected that the former steel has better stress-rupture properties in long term tests. In either steel, by improving the cleaness, impart energy was increased and ductile to brittle fracture transition temperature(DBTT) was lowered below 0℃. |
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