Page - 79 - in 3D Printing of Metals

Metals 2016,6, 218 (a)ȱ (b)ȱ Figure6. DSCscansat aheating rateof 20 ◦C/mincomparing theagingbehaviourof as-built and solution-treatedsamples (a).Hardnessevolutionof the twocorrespondingsamplesduring isothermal holdingat490 ◦C(b). Isothermal aging curves collected starting from as built samples are depicted in Figure 7. Theexpectedstrengthening trendasa functionofaging time is revealed forall of the investigated temperatures. Peakagingtimescanbe identifiedas: 10min,1h,4h,and8hfor the temperaturesof 600,540,490,and460 ◦C,respectively.Markedover-agingeffects (i.e., sharphardnessdroponce the peakhardness timeisexceeded)wereobservedfor thehighest temperature levels (540and600 ◦C), whileover-agingat460and490 ◦Conly ledtomoderate loss inhardness. ȱ Figure7.Vickershardnessvs. agingtimeofas-built samplescollectedatdifferentagingtemperatures. Samplesselectedfor furtheranalysesaremarkedwitharrows. Basedontheaboveresults, furtheranalysesonmechanicalbehaviourwere focussedonsamples agedto theirpeakhardnessconditionsat thedifferentagingtemperatures.Anadditionalcondition wasselectedbyover-agingfor8hat540 ◦C.This temperallowscomparisons tobemadewitha600 ◦C peak-agedsample (comparablehardnessbutdifferentagingtimes)anda460 ◦Cpeak-agedsample (sameagingtimebutdifferent resultinghardness). Sinceholdingat temperaturesexceeding500 ◦Cisexpected topromoteextensive reversionof martensite intoaustenite inwroughtmaragingsteels [1,3,5–9],XRDandmicrostructuralanalyseswere carriedout toassesphasebalance in the investigatedsamples. Figure8summarizes theXRDspectra 79