Thispaperdescribesthealloydesignphilosophyfortheimprovementoflong-termcreepstrengthoftemperedmartensitic9Crsteel,includingweldedjoints.Thecreeplifetrisinverselyproportionaltotheminimumcreeprateεmintimestheincreaseincreepratebystraindlnε/dεintheaccelerationregionastr=1.5/[(εmin)（dlnε/dε）].Theparametersεminanddlnε/dεarecloselycorrelatedwiththetimetominimumcreepratetmandthestraintominimumcreeprateεm,whichcharacterizethecreepdeformationbehaviorinthetransientregion.Theboundaryandsub-boundaryhardeningisshowntobethemostimportantstrengtheningmechanismincreepof9Crsteelandisenhancedbyfinedispersionsofprecipitatesalongboundaries.TheadditionofboronreducesthecoarseningrateofM23C6carbidesalongboundariesnearprioraustenitegrainboundariesduringcreep.Theenhancementofboundaryandsub-boundaryhardeningincreasesthetmanddecreasestheεmin,whichimprovesthecreeplife.Theboundaryandsub-boundaryhardeningissignificantlyreducedinfine-grainedregionofheataffected-zone（HAZ）ofconventionalsteelP92weldedjoints,promotingTypeⅣfracture.InNIMS9Crboronsteelweldedjoints,thedistributionofcarbonitridesalongboundariesaresubstantiallythesamebetweentheHAZandbasemetal,suppressingtheTypeⅣfracture.
