Effectofagingat700°ConprecipitationandtoughnessofAISI321andAISI347austeniticstainlesssteelweldsOriginalResearchArticle

AdetailedknowledgeofchangesinmicrostructuresandmechanicalbehaviourthatoccurinausteniticstainlesssteelswithorwithoutNb/Ti-stabilizedweldduringheattreatmentisofgreatinterest,sincetheductilityandtoughnessofthematerialmaychangedrasticallyafterlongagingtimes.Twokindsofmaterials,i.e.AISI321baseandwithoutTi-stabilizedweldsteelandAISI347basewithNb-stabilizedweldsteel,werecomparedduringagingat700°Cupto6000h.Bothmaterialspresentincreasedamountofprecipitateanddecreasedimpactenergyastheagingtimeincreases.Thedecreasedextentofimpactenergywithagingisalmostthesameforbothbasematerials.However,itpresentsdifferencesfor347and321weldsamples.Thelattershowsamoredrasticdecreaseofimpactenergythantheformerduetothedifferentamountofprecipitates.321weldsampleprecipitatesmorenumerouslythan347weldsampleduetotheabsenceofstabilizedTi/Nbontheformer.Largeamountofcarbidesisformedon321weldsampleimmediatelyafterwelding.Thecarbidesaretransformedtosigmaphase,whichismainlyresponsibleforthemuchmoresigmaphaseprecipitationcomparedwithothersamples,afterhigh-temperatureaging.Thefractographsshowed,ingeneral,brittlefracturemodein321weldimpact-fracturedspecimensafteragingat700°Cfor6000h.However,othersamplesshowductilefracturemodeingeneral.Severalapproachesshouldbeemployedtocontrolsigmaphaseprecipitationinweldmaterial.Theseapproachesinclude:decreasingcontentofferriteandM23C6carbideinweldandselectingNbaddedweldwireduringwelding.