Abstract
Low thermal expansion precipitation strengthening Ni-base superalloy, Ni-20Cr-10Mo-1.2Al-1.6Ti alloy (USC141TM), was developed for 700°C class A-USC steam turbine material by Hitachi, Ltd and Hitachi Metals, Ltd. USC141 is usually solution treated and then aged to increase high temperature strength for turbine blades and bolts. As the estimated 105h creep rupture strength at 700°C is about 180MPa, USC141 could also be expected to apply for boiler tubes. On the other hand, this alloy seems to be only solution treated to apply for boiler tubes because tubes are usually jointed by welding and bended by cold working and thus tube alloys should have low hardness before welding and bending and should be used as solution treated. In this study, the creep properties of USC141 as solution treated was evaluated, and the results and microstructures after creep tests were compared with those as aged. As a result, USC141 as solution treated exhibited almost as same creep rupture properties as that as aged because precipitation at grain boundaries and in grains gradually increased at testing temperatures around 700°C. Furthermore seamless tubes of USC141 were produced and various properties including creep properties are now being evaluated.
