DEVELOPMENT OF HIGH STRENGTH 9CR STEEL BY COMBINATION OF FINE MX-TYPE NITRIDES AND NO CARBIDE

摘要

The improvement of creep strength has been achieved for 9Cr steel by optimized distributions of fine MX-type nitrides. The creep test was carried out for 9Cr-3W-VNb-N steels containing 0.002 and 0.08 mass%C at 923K and 160MPa for up to about 10{sup}4h. The creep rupture strength is much larger for the extremely low carbon steel (0.002mass%C) than for the steels containing carbon of 0.08 mass%. The creep life of the extremely low carbon steel at 923K and 160MPa is about ten times as large as that of the steel containing 0.08 mass%C. The microstructures of the steels examined are observed to be tempered martensite. A large number of fine MX identified as nitrides having a size of 2-20nm are distributed on grain boundaries as well as on lath boundaries in the extremely low carbon 9Cr steel after tempering. A combination of the removal of any carbide whose coarsening rate is much larger than that of nitrides and the fine distributions of MX-type nitrides is significantly effective for the stabilization of a high density of dislocations as well as a variety of boundaries in the martensitic 9Cr steel for up to long times at 923K.