Page - 80 - in 3D Printing of Metals

Metals 2016,6, 218 acquiredfromsamples treatedaccordingtodifferentagingconditions,whilequantitativeevaluation of theγ-Fephase fraction is reported inTable2. Amarked increase inaustenite content is induced byaging. Furtherslightdecompositionofmartensitecouldbeachievedbyalso increasingtheaging temperatureandbyextensivelyover-agingthesamples (e.g., aging8hat540 ◦C). Figure8.XRDspectraof the18-Ni300steelasa functionofagingcondition. Table2.Austenite (γ-Fe) fractionandcorrespondingtensilepropertiesasa functionofagingtemper. Standarddeviationofdata isalsogiven inbrackets. Condition γ-FeFraction(%) YS(MPa) UTS(MPa) Elongation(%) asbuilt 11.38 (1.02) 914.9 (12.5) 1187.6 (10.4) 6.14 (1.33) 460 ◦C8h 15.55 (0.73) 1956.8 (43.3) 2017.1 (57.7) 1.51 (0.20) 490 ◦C4h 16.12 (0.57) 1793.3 (96.5) 1814.6 (95.1) 1.20 (0.13) 540 ◦C1h 17.17 (0.68) 1870.3 (53.8) 1956.9 (54.2) 2.07 (0.32) 540 ◦C8h 21.30 (0.77) 1545.9 (103.1) 1655.5 (87.5) 1.77 (0.05) 600 ◦C10min 17.15 (0.68) 1557.2 (140.4) 1659.1 (119.4) 1.60 (0.09) Themeasuredmodificationsalsoreflectedintochangesoftheobservedmicrostructure. InFigure9, acollectionofrepresentativeSEMmicrographs isgiven.Onaging,an increasedamountofaustenite (brightconstituent inFigure9)decoratingcellboundariesbecamevisiblewhileover-agingadditionally promotedprecipitationofausteniteat intragranularsites (Figure9d,f). Table 2 also summarizes themain average data collected by room temperature tensile tests. Low-temperatureagingpromotesadramatic increase instrengthover theasbuilt condition. Theyield strength improvedfrom915MPaof theasbuilt steel to1957and1793MPafor samplespeak-aged at460and490 ◦C,respectively.Conversely, fractureelongationunderwentadropfrom6.1%to1.5% and1.2%for the samematerials. Theultimate tensile strength (UTS) and0.2%yield strength (YS) given inTable2 forstandardpeakagingat490 ◦Caresubstantiallyconsistentwithspecifications for wroughtproducts (e.g.,ASTMA538,AMS6514,andMIL-S-46850standards)andingoodagreement with literaturedataonSLMed type300maraging steel [10]. Tensileductilityvaluesbetween1.2% and2.1%wererecordedfromtests, irrespectiveofagingtemperatureandresultingaustenite fraction. ThepoorfractureelongationofSLMedspecimensafterpeakagingisalsoconsistentwithliterature[10]. Figure10reports the tensilecurvesofsamplespeakagedat460 ◦Candofasbuilt samples tohighlight thedramaticdifferences in tensilebehaviourbetweenthese twoconditions. 80