Behavior of Carbon in Low Temperature Plasma Nitriding Layer of Austenitic Stainless Steel (2)
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Behavior of carbon in low temperature plasma nitriding layer of austenitic stainless steel M. Tsujikawa a, * , N. Yamauchi b , N. Ueda b , T. Sone b , Y. Hirose c a Department of Materials Science, Graduate School of Engineering, Osaka Prefecture University, 1-1 Gakuen-cho, Sakai-shi, 599-8531, Japan b Technology Research Institute of Osaka Prefecture, 7-1, Ayumino 2, Izumi-shi, Osaka 594-1157, Japan c Department of Materials Science and Engineering, Kanazawa University, Kakuma-machi, Kanazawa, 920-1192, Japan Available online 28 October 2004 Abstract Expanded austenite by low temperature plasma nitriding of austenitic stainless steels pushes carbon ahead of the nitrided layer. Apparently, this phenomenon accompanies uphill carbon diffusion. Clarifying the mechanism of this carbon push-ahead effect is important for controlling treated-layer microstructure. This investigation is intended to clear the push-ahead effect of specimens treated with a combination of carburizing and nitriding. Plasma treatments were carried out for 18Cr–8Ni steel in 667 Pa at 673 K. We used the following gas-flow ratios: 80% N 2 –20% H 2 for nitriding; Ar gas mixed with 5% CH 4 for carburizing; and 80% N 2 mixed with from 5% CH 4 and balancing H 2 for simultaneous carburizing and nitriding. Glow-discharge optical emission spectroscopy (GDOES) indicated an accumulated carbon layer beyond the nitrogen plateau on the nitrided specimen of pre-carburized sample. Such a carbon accumulation profile also resulted from simultaneous treatment. Furthermore, carbon accumulated similarly in cases of carburizing after nitriding treatment. Nitrided specimens of pre-carburized samples have a small carbon peak just under the surface on GDOES profile. This small carbon peak weakened and disappeared with increasing nitriding time. X-ray diffraction analysis revealed formation of e (Fe 2–3 N) phase just under the surface of the nitrided pre-carburized sample. D 2004 Elsevier B.V. All rights reserved. Keywords: Diffusion; Carburizing; Microstructure control; Austenitic stainless steel; Surface improvement 1. Introduction Low temperature plasma nitriding hardens the surface of austenitic stainless steel without deterioration of corrosion resistance [1–5]. The distribution of dissolved carbon in austenitic stainless steels changes by low temperature nitriding. Expanded austenite by low temper- ature plasma nitriding of austenitic stainless steels pushes carbon ahead of the nitrided layer. Thereby, the changed carbon profile shows the carbon accumulation beyond the front of the nitrided layer. This phenomenon has been observed widely with a variety of profiling techniques [5–8]. This phenomenon has been described with a trapping model, in which nitrogen is trapped in the treated layer, presumably by chemical binding with chromium [9]. The carbon is pushed ahead of the incoming nitrogen. This is qualitatively consistent with trapping and detrapping, assuming the trap binding energy for carbon to be lower than that of nitrogen [7,10]. On the other hand, low temperature carburizing similarly make the expanded austenite phase as hard and corrosion- resistant case [11]. Combination of carburizing and nitriding introduces new structural control of surface modification layer [12]. It is important for development of the controlled structure to clarify the behavior of carbon in connection with nitriding. This study examines elemental profiles of carbon and nitrogen of 304 stainless steel after plasma treatment in various combinations of carburizing and nitriding. In 0257-8972/$ - see front matter D 2004 Elsevier B.V. All rights reserved. doi:10.1016/j.surfcoat.2004.08.179 * Corresponding author. Tel.: +81 72 254 9317; fax: +81 72 254 9912. E-mail address: [email protected] (M. Tsujikawa). Surface & Coatings Technology 193 (2005) 309– 313 www.elsevier.com/locate/surfcoat
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plasma nitriding of austenitic stainless steel
Transcript of Behavior of Carbon in Low Temperature Plasma Nitriding Layer of Austenitic Stainless Steel (2)
BehaviorofcarboninlowtemperatureplasmanitridinglayerofausteniticstainlesssteelM. Tsujikawaa,*, N. Yamauchib, N. Uedab, T. Soneb, Y. HirosecaDepartmentofMaterialsScience,GraduateSchoolofEngineering, OsakaPrefectureUniversity, 1-1Gakuen-cho,Sakai-shi,599-8531, JapanbTechnologyResearchInstituteofOsakaPrefecture, 7-1, Ayumino2, Izumi-shi, Osaka594-1157, JapancDepartmentofMaterialsScienceandEngineering, KanazawaUniversity, Kakuma-machi, Kanazawa, 920-1192, JapanAvailableonline28October2004AbstractExpanded austenite by lowtemperature plasma nitriding of austenitic stainless steels pushes carbon ahead of the nitrided layer.Apparently,thisphenomenonaccompaniesuphillcarbondiffusion.Clarifyingthemechanismofthiscarbonpush-aheadeffectisimportantfor controlling treated-layer microstructure. This investigation is intended toclear the push-aheadeffect of specimens treated with acombination of carburizing andnitriding. Plasma treatments were carried out for 18Cr8Ni steel in 667 Pa at 673 K. We used thefollowinggas-flowratios: 80%N220%H2fornitriding; Argasmixedwith5%CH4forcarburizing; and80%N2mixedwithfrom5%CH4andbalancingH2for simultaneouscarburizingandnitriding. Glow-dischargeoptical emissionspectroscopy(GDOES) indicatedanaccumulatedcarbonlayer beyondthenitrogenplateauonthenitridedspecimenof pre-carburizedsample. Suchacarbonaccumulationprofile also resulted fromsimultaneous treatment. Furthermore, carbon accumulated similarly in cases of carburizing after nitridingtreatment. Nitridedspecimensofpre-carburizedsampleshaveasmall carbonpeakjust underthesurfaceonGDOESprofile. Thissmallcarbonpeakweakenedanddisappearedwithincreasingnitridingtime. X-raydiffractionanalysisrevealedformationofe(Fe23N)phasejust under thesurfaceof thenitridedpre-carburizedsample.D2004ElsevierB.V. Allrightsreserved.Keywords:Diffusion;Carburizing;Microstructurecontrol;Austeniticstainlesssteel;Surfaceimprovement1. IntroductionLowtemperature plasma nitridinghardens the surfaceof austenitic stainless steel without deterioration ofcorrosion resistance [15]. The distribution of dissolvedcarbon in austenitic stainless steels changes by lowtemperaturenitriding. Expandedaustenitebylowtemper-atureplasmanitridingof austeniticstainless steels pushescarbonaheadof the nitridedlayer. Thereby, the changedcarbonprofileshowsthecarbonaccumulationbeyondthefront of the nitrided layer. This phenomenon has beenobserved widely with a variety of profiling techniques[58].This phenomenon has been described with a trappingmodel, inwhich nitrogen is trappedinthe treated layer,presumablybychemical bindingwithchromium[9]. Thecarbonis pushedaheadof theincomingnitrogen. This isqualitatively consistent with trapping and detrapping,assumingthe trapbindingenergyfor carbontobelowerthanthatofnitrogen[7,10].On the other hand, low temperature carburizing similarlymaketheexpandedaustenitephaseashardandcorrosion-resistant case [11]. Combination of carburizing and nitridingintroduces newstructural control of surface modificationlayer[12].Itisimportantfordevelopmentofthecontrolledstructure to clarify the behavior of carbon in connection withnitriding.This study examines elemental profiles of carbon andnitrogen of 304 stainless steel after plasma treatment invarious combinations of carburizing and nitriding. In0257-8972/$-seefrontmatterD2004ElsevierB.V. Allrightsreserved.doi:10.1016/j.surfcoat.2004.08.179* Corresponding author. Tel.: +81 72 254 9317; fax: +81 72 254 9912.E-mailaddress: [email protected](M.Tsujikawa).Surface&CoatingsTechnology193(2005)309313www.elsevier.com/locate/surfcoataddition, it elucidates the formation and change of phases intreated layers.2. ExperimentalproceduresThe material used in this work is austenitic stainless,which satisfies the grade of AISI 304. The chemicalcomposition of the specimen is Fe18.8 Cr8.3 Ni0.94Mn0.21Mo0.31Cu0.4Si0.037P0.003S0.06Cinmass%. Roughlycut AISI304platesweresolutiontreatedat1303 Kfor2.7 ks.Thentheyweredrygroundslightlytothespecimenshapeof25mmwidth, 50mmlengthand5mmthickness. These preparations gave the specimens afullyaustenitic structure, whichwas confirmedbyX-raydiffractionanalysis[13].Plasma thermo-chemical treatment was performed with alaboratorytypeapparatus withad.c. power source. Eachspecimen attached with a thermo couple was set in the furnaceas cathode. After evacuation up to 1.33101Pa, thepressureofthemixedgasforeachspecimenwasadjustedto6.67102Pa. All thermo-chemical treatment was per-formed in this study at 673 K, with the exception of the 4N4C450 specimen. Its second process was treatment at 723 K.Table 1 shows details of each plasma treatment withspecimen symbols. For example, the 8Nspecimen wasplasma treated at 673 K with a mixture of 80% nitrogen gasand20%hydrogengasfor8h. The8(C+N)specimenwastreatedbyplasmacarbo-nitridingprocessfor8hat 673Kwithamixtureof nitrogen, methane, andhydrogengases.Similarly, the4C4Nspecimenwasfirsttreatedbyplasmacarburizing for 4 h at 673 K, then followed by plasmanitridingfor4hat673K.Insuchsequentialprocesses,gasmixturechangesweremadeatwithin10swithouttemper-aturechange. Aftertreatment, thespecimenswereallowedtocoolintheevacuatedfurnace.X-raydiffractionanalysis(XRD) usingCu-Ka(40kV,150mA)radiationwithamonochrometer, wasundertakentoidentifythecrystalstructureatthesurfaceoftheplasmatreated surface. Glow discharge emission spectrometry(GDOES) was used to analyze the elemental depth profilingoftreatedsurfaces.3. Resultsanddiscussion3.1. PushaheadofcarburizedcarbonbynitridingElemental depth profiles of carburized only specimendesignated for 4C is shown in Fig. 1(a) and that ofTable1PlasmatreatmentandspecimendesignationProcess Symbol Firstprocess Secondprocess Totalprocesstime(h)Temperature(K)Gasflowratio(%) Processtime(h)Temperature(K)Gasflowratio(%) Processtime(h)CH4 N2 H2 Ar CH4 N2 H2 ArNitriding 8N 673 80 20 8 8Carburizing 4C 673 5 15 80 4 4Simultaneousprocess8(C+N) 673 5 80 15 8 8Sequentialprocess4C4N 673 5 15 80 4 673 80 20 4 88C4N 673 5 15 80 8 673 80 20 4 1216C4N 673 5 15 80 16 673 80 20 4 2024C4N 673 5 15 80 24 673 80 20 4 284C8N 673 5 15 80 4 673 80 20 8 124C16N 673 5 15 80 4 673 80 20 16 204C24N 673 5 15 80 4 673 80 20 24 284N4C 673 80 20 4 673 5 15 80 4 84N4C450 673 80 20 4 723 5 15 80 4 8Fig. 1. GDSdepthprofiles of carburizedspecimen(a) andnitridedpre-carburizedspecimen(b).M. Tsujikawaetal. /Surface&CoatingsTechnology193(2005)309313 310carburizedandpost-nitridedspecimendesignatedfor 4C4N in Fig. 1(b). The plasma treatment condition forspecimen 4C worked successfully to carburize the 304austeniticstainless steel. Thecarbonpush-aheadeffect ofnitrogen is evident when comparing Fig. 1(a) and (b).However, somecarbonremains inthenitridedlayer as asmall carbonpeakremainsinthesub-surfacelayer. Fig. 2shows the microstructure of surface layer of the sequen-tially processed 4C4N specimen. There are obviousduplex layers as reported by Blawert at simultaneousprocessing[12].The XRDanalysis of Fig. 3 indicates that the u2udiffractionpatternof 4C4Nis fundamentallythesumofthe nitrided specimens and the carburized specimenspatterns. Expandedaustenitephasebynitrogen(S(N)) andbycarbon(S(C)) appearedseparately, but somechromiumcarbides of the 4Cspecimen disappeared. These resultsindicate that movement of the S(C)phase is possible byapplyingnitridingprocessafter carburizing.3.2. Competitive diffusion of carbon and nitrogen, andcarbondiffusionthroughnitridedlayerFig. 4(a) shows the GDOESdepthprofile of simulta-neous carburizing and nitriding processed specimen8(C+N). Carbondiffusedbeyondthe nitridedlayer; also,carbonaccumulatedat thefront ofthenitridedlayer. Highconcentrationofcarbonatthesurfacedecreasedsharplyinthe nitridedlayer. This carbonconcentrationresults fromcompetition to get trap sites in the austenitic phase. BecauseFig. 2. Microstructure of duplex surface layer formed at surface ofcarburizedandpost-nitridedspecimen(4C4N).Fig. 3. Comparisonof XRDpatternsof nitridedpre-carburizedspecimen(4C4N),nitridedspecimen(8N)andcarburizedspecimen(4C).Fig. 4. GDS depth profiles of simultaneous nitriding and carburizingprocessed specimen (a: 8(C+N)), nitrided and post-carburized specimen (b:4N4C)andnitridedandpost-carburizedat723specimen(c:4N4C450).M. Tsujikawaetal. /Surface&CoatingsTechnology193(2005)309313 311oftheweakaffinityofcarbontotrapsite[9], carbonmustremain in the diffusion site [10]. Thereby carbon at diffusionsitesdiffusedtoinnerspacesaccordingtotheconcentrationgradientofinterstitialatoms.Similardiffusionofcarbononnitridedlayeroccurredatnitrided and post-carburized specimen 4N4C. Fig. 4(b)shows diffusion of carbon through a pre-nitrided layer.Slight accumulation of carbon occurs at the front of nitridedlayer.Fig. 5 shows that these two specimens have similarXRDpeaks. Separationof S(N)peaks andS(C)peaks arevague in comparison with the pattern of 4C4N thatappears in Fig. 3 and the CrNpeak is shown in the8(C+N) specimen[12].Althoughthe4N4C450specimenhasahugeaccumu-lationof carbonbeyondthe nitridedlayer showninFig.4(c), the circumstance of the carbon diffusion was quitedifferent fromthe4N4C. S(N)phasewasdecomposedtoCrNandferriticiron(a-Fe)bysecondaryprocessat723K[14] asshowninFig. 5. Theconcentrationof nitrogeninFig. 4(c) displays not the interstitial nitrogen but thenitrogeninCrN.3.3. FormationofE-nitrideat sequential processingFig. 6showsGDOESprofilesfor carburizedandpost-nitrided specimens with various carburizing time beforenitridingfor4 h.Intensitiesofelementalconcentrationvaryin each measurement. In Fig. 6, there are iron (Fe)concentration profiles of each specimen as an intensityindex. TheFecontent of substrates showedgoodconsis-tency for carbon and nitrogen concentration comparison.The concentration and the depth of accumulated carbonincreasewiththepre-carburizingtime.Thepeakatthesub-surfacealsoincreased.Theheightofthepeaksincreaseswithcarburizingtime.However,thepositionsofthepeaksarealmostatthesamedepth. Equally, the concentrationof nitrogendecreases withcarburizingtime.Fig.7showsthesetofGDOESprofilesof4CxN(x=4,8, 16, 24) specimens. The accumulationof carbonat thefront of nitrided layer moves with the nitriding front.However, the small sub-surface peak of carbon did notmove into a deeper position. Furthermore, weakening of thesub-surfacepeakisapparentwithincreasingnitridingtime.On the 4C24Nspecimen, the carbon sub-surface peakalmostdisappeared.Fig. 8shows XRDpatterns of 4CxNspecimens. Onthe 4C4Nspecimen, as mentionedbefore, theS(C)peakcanberecognized. However, S(C)peaksnolonger appearon 4C8N specimen because the carbon accumulationmoves to a deeper position. On the other hand, the E-Fe23NpeaksandS(N)peaksareclarifiedwithincreasingnitridingtime. Especially, the XRD peaks of e phase seemtoFig.5. XRDanalysisofcombinedprocessedspecimens.Fig. 6. InfluenceofcarburizedtimeonGDSdepthprofilesafternitriding(nitridingtime:4h).Fig. 7. Effect of post nitridingtimeonGDSdepthprofilesof carburizedspecimens(carburizingtime:4h).M. Tsujikawaetal. /Surface&CoatingsTechnology193(2005)309313 312correspondtothe sub-surface peakof carboninGDOESanalysis. Theyareshownclearlywithincreasingnitridingtime.Theseresults suggest that formationof E-Fe23Nphaseoccursinprecarburizedspecimensatthesub-surfacewhennitridingstarts becauseof thepresenceof super saturatedcarbonreleasedfromtrapsites byincomingnitrogen. Atthis position, trapping of nitrogen by affinity withchromium[7]renderstheformationofchromiumcarbidesor chromiumnitridesdifficult. Throughthehelpof super-saturated carbon, E-Fe23Nmay be formed at the sub-surfacelayer. FurthernitridingdilutesthedissolvedcarboninEphase.4. ConclusionsAseriesofnitridingandcarburizingbyplasmaprocess-ingat relativelylowtemperaturequalitativelyclarifiedthebehaviorofcarboninnitridingasfollowings:(1) MovementofS(C)phaseispossiblebypost nitriding.Carburized layer was pushed ahead by incomingnitrogen. Withsuchsequential carburizingandnitrid-ingtheduplexsurfacelayer wasformedat austenitic304stainlesssteel.(2) Accumulation of carbon at the front of nitridedlayer alsooccurredinbothsimultaneous carburizingand nitriding process and in the carburizing prenitrided process. The carbon accumulation at thefront of nitrided layer also occurred in bothsimultaneous carburizing and nitriding process andin the nitriding and post-carburizing process. How-ever, chromiumcarbide was detected by XRDinthese specimens.(3) E-Fe23N phase is formed at the sub surface ofsequentially carburized and post-nitrided specimens,but no chromium compounds were detected byXRD.(4) Carbon concentration of the sub surface E-Fe23Nphasedecreasedwithincreasingpost-nitridingtime.References[1] K. Ichii, K.Fujimura,T. Takase,NetsuShori25(1985)191.[2] Z.I. Zhang, T. Bell, Surf.Eng. 1(2)(1985)131.[3] K.-T. Rie, E.Broszeit,Surf. Coat. Technol.7677(1995)425.[4] C. Blawert,Surf. Eng.15(2)(1999)154.[5] N. Yamauchi, N. Ueda, K. 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Perina,Surf. Coat. Technol.128129(2000)219.[13] K. Hamaishi, H. Sueyoshi, J. Kiyofuji, Y. Nakamura, J. Jpn. Inst.Metal59(2)(1995)133.[14] Y. Sun, T. Bell, Z. Kolosvary, J. Flis, Proc. Int. Current Status SeminaronThermochemicalSurf. Eng. Osaka,2001,p. 65.Fig. 8. XRDpatterns for nitridedpre-carburizedspecimens withvariousnitridingtimes.M. Tsujikawaetal. /Surface&CoatingsTechnology193(2005)309313 313
