Nickel　is　an　important　raw　material　for　fabrication　of　high-temperature　alloys　and　corrosion-resistant　alloys　due　to　its　high　melting　point,corrosion　resistance,oxidation　resistance,ductility　and　other　excellent　properties.　As　one　kind　of　key　strategic　resources,　nickel　has　been　widely　used　in　aerospace,defense,nuclear　energy　and　other　important　fields.　However,China　is　a　typical　nickel-deficit　country,and　the　dependence　on　foreign　nickel　ore　reaches　as　high　as　95%.　Recycling　of　nickel-contained　scrap　can　effectively　alleviate　the　shortage　of　nickel　resources,and　is　of　great　significance　to　ensure　the　safe　and　sustainable　supply　of　nickel　resources　in　China.　Compared　with　other　types　of　nickel-contained　scraps,nickel-based　high-temperature　alloy　scrap　exhibits　higher　recovery　value　because　of　its　advanced　preparation　process　and　containing　various　kinds　of　strategic　metal　elements.　Currently,different　methods　including　pyrometallurgical,hydrometallurgical,and　pyro-hydrometallurgical　processing　are　applied　for　recycling　scrap　nickel-based　high-temperature　alloys.　However,these　methods　have　disadvantages　such　as　multi-step　processing　and　relatively　low　utilization　rates.　What＇s　more,the　achieved　products　by　the　above　methods　usually　are　metal　elements　or　corresponded　salts.　To　achieve　same-level　utilization　of　scrap　nickel-based　alloys,a　direct　re-melting　method　through　vacuum　induction　melting　is　used　to　recycle　scrap　690　alloy.　However,　re-melted　690　alloy　shows　poor　performance,especially　deterioration　of　ductility,which　is　resulted　from　a　large　number　of　Cr2O3　inclusions　distrusted　in　re-melted　690　alloy.　Consequently,Cr2O3　inclusions　cannot　be　removed　effectively　through　the　direct　re-melting　method.　Therefore,carbon　thermal　reduction　was　proposed　to　solve　this　problem　in　this　study.　Scrap　690　alloy　was　obtained　through　aging　treatment　at　850　℃　for　360　h　by　considering　that　oxidation　as　well　as　microstructure　coarsening　were　the　main　factors　for　serving　failure　of　690　alloys.　The　above　scrap　690　alloy　were　re-melted　using　carbon　thermal　reduction　at　1550　℃,and　the　carbon　reduced　690　alloy　was　obtained.　Fe　content　of　the　alloy　was　analyzed　using　inductively　coupled　plasma　optical　emission　spectrometer　(ICP-OES),while　titration　was　used　to　determine　Cr　content.　Oxygen-nitrogen-hydrogen　analyzer　was　utilized　to　analyze　O　and　N　content,and　carbon-sulfur　analyzer　was　used　to　determine　C　and　S　content.　The　microstructure　and　fracture　morphology　of　three　types　of　690　alloys　were　observed　using　optical　microscopy(OM),scanning　electron　microscopy(SEM)equipped　with　energy　dispersive　spectroscopy(EDS),and　X-ray　diffraction(XRD).　Microhardness　was　performed　using　Vickers　microhardness　tester　under　conditions　of　Load　retention　for　10　s　at　test　force　of　1960　N.　The　tensile　properties　of　three　types　of　690　alloys　were　tested　by　electronic　universal　testing　machine　at　room　temperature　and　650　℃.　SEM　images　indicated　that　a　large　amount　of　Cr23C6　precipitates　were　uniformly　distributed　in　carbon　reduced　690　alloy,while　there　were　almost　no　inclusions　of　Cr2O3.　Moreover,the　oxygen　content　of　carbon　reduced　690　alloy　decreased　to　88×10-6.　Therefore,it　could　be　reasonably　included　that　the　inclusions　of　Cr2O3　were　successfully　transformed　into　precipitates　of　Cr23C6　through　the　carbon　thermal　reduction　method.　As　a　result,the　mechanical　properties　of　carbon　reduced　690　alloy　significantly　improved.　The　hardness　of　carbon　reduced　690　alloy　was　HV　278.　The　tensile　strength　of　carbon　reduced　690　alloy　at　room　temperature　and　650　℃　was　672　and　395　MPa,respectively,which　was　10.5%　and　44.2%　higher　than　those　of　standard　690　alloy.　More　importantly,the　elongation　at　room　temperature　and　650　℃　of　carbon　reduced　690　alloy　was　increased　18.8%　and　96.0%,respectively,compared　with　direct　re-melted　690　alloy.　The　difference　in　mechanical　properties　between　the　two　types　of　remelted　690　alloys　was　mainly　related　to　the　type　and　distribution　of　second-phase　particles.　The　inclusions　of　Cr2O3　were　brittle　particles,which　were　prone　to　stress　concentration　at　the　interface　between　inclusions　and　matrix.　Their　distribution　would　not　get　changed　accordingly　during　plastic　deformation,where　voids　and　cracks　were　easily　formed,leading　to　an　obvious　decrease　in　the　tensile　ductility　of　direct　re-melted　690　alloy.　For　carbon　reduced　690　alloy,the　unfavorable　inclusions　of　Cr2O3　were　transformed　into　beneficial　precipitates　of　Cr23C6　benefited　from　the　carbon　thermal　reduction　method.　During　the　tensile　test,Cr23C6　precipitates　could　slow　down　the　aggregation　and　growth　of　micropores,hinder　crack　generation,and　improve　the　ductility　of　alloy.　Furthermore,a　large　amount　of　uniformly　distributed　Cr23C6　precipitates　contributed　to　the　higher　strength　of　carbon　reduced　690　alloy　due　to　dispersion　strengthening　effect.　Also,these　precipitates　of　Cr23C6　contributed　to　the　fine　grain　microstructure,where　grain　boundary　strengthening　was　achieved.　These　results　indicated　that　recycling　of　scrap　690　alloy　using　carbon　thermal　reduction　could　effectively　remove　the　oxide　inclusions,and　improve　both　the　strength　and　ductility　of　re-melted　690　alloy.　©　2024　Editorial　Office　of　Chinese　Journal　of　Rare　Metals.　All　rights　reserved.