In order to conserve the energy resources and to reduce the dependence to oil, the large coal fired power plant which was improved heat efficiency has been currently developed. A very effective method of improving the heat efficiency of steam power plant is the raise of steam conditions. It is important to develop the high temperature turbine valve forging which is used as the major structural material of this steam power plant. Modified 9Cr-1Mo (Grade 91) steel has been incorporated to the parts of boiler in thermal plants due to its excellent properties at high temperatures. In recent years, this steel has also been applied to the parts of turbine in ultra super critical plants. Former experience with a process for manufacturing forged valves of low alloy steel in thermal power plants have enabled the application of F91 steel to forge as valves. A 25-ton ingot was made by the basic electric furnace and was molded to valve bodies by combination of forging and shaping with Gas. The manufacture of trial valve with F91 steel have successfully been performed. They exhibited high quality, good creep rupture properties and excellent toughness.
