Creep Rupture Properties of KA-STPA28 Steel Aged Material Welded Joints and Ultra Miniature Specimen

Creep rupture properties of weld joints and ultra miniature specimen of KA-STPA28 steel, Japanese standard of Gr. 91 steel for high temperature piping, aged material was investigated. Time dependence of creep rate of each test were compared and the results were discussed with scanning electron microscopy (SEM) observation of fracture surfaces and cross section of specimens. The welded joint creep specimens tested at high stress fractured in weld metal, and the specimens tested at low stress fractured in fine grain HAZ. It was suggested that the fine grain HAZ scattered in weld metal has inferior high temperature strength to those of base metal. Creep rate of UMC specimens at the initial stage of tests were about 5 times higher than those of round bar specimens. By comparing relationship between modified minimum creep rate and rupture time of each tests, in case of UMC specimen, excessive creep strain corresponded to those at 3% of rupture time was considered to be accumulated during initial stage of tests. Duration time between when specimen showed corresponded to minimum creep rate and rupture was relatively longer in case of UMC specimen because of higher ductility showed at fracture. Specimen surface oxidation was well suppressed by employing appropriate Ar gas flow rate.