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metals Article AgingBehaviourandMechanicalPerformanceof 18-Ni300SteelProcessedbySelectiveLaserMelting RiccardoCasati 1, JannisN.Lemke1,AusonioTuissi 2 andMaurizioVedani1,* 1 DipartimentodiMeccanica,PolitecnicodiMilano,MilanoI-20156, Italy; riccardo.casati@polimi.it (R.C.); jannisnicolas.lemke@polimi.it (J.N.L.) 2 Institute forCondensedMatterChemistryandTechnologies forEnergy,CNR,LeccoI-23900, Italy; ausonio.tuissi@cnr.it * Correspondence:maurizio.vedani@polimi.it;Tel.:+39-02-2399-8230 AcademicEditor:ManojGupta Received: 2August2016;Accepted: 5September2016;Published: 8September2016 Abstract: An 18-Ni 300 grade maraging steel was processed by selective laser melting and an investigationwas carried out onmicrostructural andmechanical behaviour as a function of aging condition. Owing to the rapid cooling rate, the as-built alloy featured a full potential for precipitate strengthening, without the need of a solution treatment prior to aging. The amount of reversed austenite found in themicrostructure increased after aging and revealed to depend onaging temperature and time. Similarly to the correspondingwrought counterpart, also in the selective laser-melted18-Ni300alloy,agingpromotedadramatic increase instrengthwithrespect to theas-built conditionandadrop in tensileductility.Nosystematic changeswere foundin tensile propertiesasa functionofmeasuredamountofaustenite. It isproposed that thesubmicrometric structure and the phase distribution inherited by the rapid solidification condition brought by selective lasermeltingaresuchthatchanges in tensile strengthandductilityaremainlygovernedby theeffectsbroughtbythestrengtheningprecipitates,whereas theconcurrent reversionof theγ-Fe phase indifferentamountsseemstoplayaminorrole. Keywords: selective lasermelting;maragingsteel; agingbehaviour; reversedaustenite 1. Introduction Maraging steels areultra-high strengthFe-Nialloysdevelopedmainly for aircraft, aerospace, andtoolingapplications. Theiroutstandingcombinationofstrengthandtoughnessresults fromthe agingofarelativelysoftmartensite formedbycoolingatmoderaterates fromtheNi-containingγ-Fe solidsolution. Aging leads to the formationof strengtheningprecipitates. It is reported that in the 18-Ni 300 alloyand in similar grades, theNi3Tiphase (ormoregenerally,Ni3XwhereX=Ti,Mo, V, andW)readily formonshort-termagingat lowtemperatures (400–450 ◦C), followedbyFe2Mo orFe7Mo6precipitation [1–6]. Agingat temperaturesexceeding500 ◦Cconcurrentlypromotes the formationofaustenitebyadiffusion-controlledreaction,which isalso favouredbythereleaseofNi into thematrixduetodecompositionofNi3Tiphase [1,3,5–9]. Due to their good ductility andweldability, maraging steels recently attracted the attention as candidate alloys for additive manufacturing processes, such as selective laser melting (SLM). DuringSLM, layersofpowderareselectivelymoltenbyascanning laserbeamandconsolidatedon topofeachother. Fullydensepartscan thusbegeneratedfeaturingmechanicalpropertiescomparable to thoseofcomponentsprocessedbystandardroutes [10–12]. The mechanical behaviour of SLM parts is strongly affected by the refined microstructure inheritedfromfastcoolingandbygenerationofsmallsolidificationfaults [13–16]. Indeed, thelocalized meltingprocessexperiencedduringSLMisnotyet fullyunderstood. It isgovernedby laserbeam Metals 2016,6, 218 74 www.mdpi.com/journal/metals