With the increasing requirements of environmental protection, energy conservation and emission reduction, efficient, clean and low emission ultra supercritical power generation technology has become the main development direction of thermal power units in the world. China began to vigorously develop 620 ℃ high-efficiency ultra supercritical power generation technology in 2013. By the end of 2018, China has put into operation more than 160 620 ℃ ultra supercritical units, including more than 80 million KW units, and the number of units and installed capacity are among the best in the world. As the steam temperature of thermal power units continues to increase and the operating conditions are more stringent, the traditional 12%cr martensitic heat-resistant steel can no longer meet the service requirements of 620 ℃ steam turbine cylinders, valves and other large castings of thermal power units. Therefore, European and Japanese research institutions have developed new high-temperature materials for steam turbines, such as tos110 (Toshiba), hr1200 (Hitachi), mtr10a (Mitsubishi) and cost-fb2/cb2 (Europe), based on the improved 12%cr steel (containing Mo and W elements) and by adding CO and Trace B, for the manufacture of high-temperature components of steam turbines of 620 ℃ grade thermal power units.
(1) CB2 large steel castings have many kinds of alloy elements, large casting sizes and complex shapes, so it is difficult to smelt, complex manufacturing process and long production cycle, which puts forward higher requirements for the manufacturer’s process and technology level. Among them, smelting, pouring and heat treatment are the key processes that determine the quality of CB2 steel castings.
(2) On the basis of 9% Cr steel, CB2 steel has higher creep strength by adding W, CO and a small amount of B, and the maximum service temperature can reach 625 ℃. The main strengthening phases are M23C6 carbides and a small amount of MX phase distributed at the grain boundary and lath boundary. Because the addition of B greatly delays the size coarsening of M23C6 carbides, CB2 cast steel has good high temperature stability.
(3) Domestic and imported CB2 steel castings are at the same level in terms of alloy element content and conventional mechanical properties. Compared with imported products, domestic steel castings are at the same level in terms of non-metallic inclusion content δ There are differences in ferrite content and grain size, which are closely related to the steelmaking process level of the manufacturer, the purity control of molten steel, and the heat treatment process parameters.
(4) CB2 cast steel is a new material, and there is no relevant technical standard at home and abroad at present. In order to strengthen product quality inspection and acceptance and ensure the long-term operation safety of CB2 steel castings in power stations, it is suggested to formulate a unified CB2 steel castings manufacturing acceptance specification as soon as possible.
CB2 steel is a new 9%cr martensitic heat-resistant material developed in the European cost (co operation in science and Technology) joint research project cost-522 (1998-2003). Compared with the high and medium pressure inner cylinder materials used in 600 ℃ units, the high temperature endurance strength of CB2 steel at 620 ℃ /105h reaches 85mpa, and its maximum service temperature can reach 625 ℃. At present, China’s 620 ℃ high-efficiency ultra supercritical units use CB2 martensitic steel to make large steel castings such as high and medium pressure inner casing of steam turbine, main steam / reheat steam valve, etc. Because CB2 cast steel is a new material used in 620 ℃ high-efficiency thermal power units, there are still few reports on the performance of this material, which has brought great trouble to the quality acceptance and metal supervision of key components of steam turbines in thermal power units. The material characteristics and manufacturing quality status of CB2 steel castings will be systematically described in order to provide technical support and theoretical basis for the order acceptance, quality monitoring and operation supervision of CB2 steel castings for thermal power units.