Thermodynamics, Kinetics and Microstructural ...

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of the rate-determining mechanisms governing nucleation and growth of the compound layer during gaseous nitriding and nitrocarburising of pure iron.

HEATTREATMENT OF METALS 2000.4p.92-102

Thermodynamics, Kinetics and Microstructural Evolution of the Compound Layer; a Comparison of the States of Knowledge of Nitriding and Nitrocarburising* M.A.J. SOMERS

Technical University of Denmark

The author provides an overview of current understanding of the rate-determining mechanisms governing nucleation and growth of the compound layer during gaseous nitriding and nitrocarburising of pure iron. INTRODUCTION Ferritic nitrocarburising comprises a thermochemical surface treatment in which nitrogen and carbon are suppliedsimultaneouslyto a steelsurfaceat a temperature between 823 and 853K (550-580"C).Provided that the nitrogen and carbon activities imposed by the nitrocarburisingagenton the steelsurfaceare sufficientlyhigh,a compound layer is formed at the sudace of a ferritic steel, which consistspredominantlyof e and/ory' phases.In the region underneath the compound layer, the so-called diffusion zone develops.The compound layer has an interestingcombinationof wear and atmosphericcorrosion performance.Thediffusionzone brings about an improvement of the endurancelimit as comparedwith an untreated component. Hence, nitrocarburisingis perhaps the most versatile surface treatment for ferritic steel and has a potentialfor wide application. From the literature on nitrocarburising,an incomplete fragmentarypicture emerges of the relationshipbetween nitrocarburisingconditions, microstructuraldevelopment and the properties/performanceof nitrocarburised workpieces. This hinders modelling of the process-microstructure-propefi relationshipsand, thereby,optimisation of the processconditionsfor tailoringa certaincombination of properties.The present paper describes aspects of the thermodynamics,kineticsand microstructuralevolutionof the compound layer on pure iron during nitrocarburising, by comparingthe currentstatusof qualitativeunderstanding with that for nitriding,which is largelyquantitative.lron was taken as a model case in order to assessthe actual effect of the interstitial elements, nitrogen and carbon, providedby the nitridingand nitrocarburising gases. THERMODYNAMICS OF NITROCARBURISING The nitriding and carburising potentials imposed on a gas mixture can be specimensurfaceby a nitrocarburising representedby the nitrogen and carbon activities'that would occur in a solid solution if (local) chemical equilibriumprevailedbetweenthe gas mixtureand the solid solution(at the surface).Below the thermodynamicsin the xManuscriptbased on an invited lecture given at the 5th ASM Heat Treatment and Surface Engineering Conference in Europe incorporating the 3rd lnternational Conferenceon Heat Treatment with Atmospheres,7-9June 2000,Gothenburg,Sweden. 92

gas phaseand in the solid state are treated separately. Thermodynamics of the gas phase A definitionof the nitriding and carburisingpotentials,as well as the correspondingnitrogenand carbonactivities,as imposedby gaseousnitridingand carburisingatmospheres was given in Refs.1 and2. Nitriding reaction For a gas mixture containingammonia, the dissolutionof nitrogeninto the solid phase t ) o

! r t

(r-

NH, content (vol. %) Fig.4.Boundariesbetween Fe-N-Cphasesas a function of the contents of ammonia and carbonmonoxide in a flowing NH,-CO-H2gas mixture at 773K(500'C)16. Theammonia and carbonmonoxide contentscannot be directlyconveftedto nitrogen and carbon activities. O/1

Fig.5.Phasediagram of the Fe-Nsystem. HeatTreatmentof Metals2000.4

M.A.J. Somers Phasediagrams layers occurredfor combinationsof nitrogen and carbon By combining the model description of the nitrogen contentsbeyond those indicatedin the Fe-N-Cdiagram in absorptionisothermsfor the various Fe-N phasesand the Ref.18. With respectto these discrepancies, it is noted that p h a s eb o u n d a r i e si n t h e L e h r e rd i a g r a m t, h e F e - Nd i a g r a m the Fe-N-Cdiagram at 848K (575"C)in Ref.18was obtained can be calculated.The Fe-N diagram thus obtainedlT by interpolationbetweenexperimentaldataat 823K(550.C) is in excellent correspondence with (independent) and 873K (600'C),and should thereforebe consideredas experimentally-determined phase boundaries.Hence, a approximate*. consistentquantitativedescriptionof the thermodynamics The Fe-N-Cphase diagram has been a topic of debatefor of the Fe-Nsystem is obtainedfor the temperatureregion the last 15 years.Severalresearchgroups have attempted where the absorption isotherms were experimentally to solve the discrepancybetween the Fe-N-Cdiagram determined.TheFe-Nphasediagram (not the one resulting proposedin Ref.18and the (apparent)phaseequilibriaand from the calculationsmentioned above) is presented in compositionsin as-growncompound layersby calculating Fig.5.ltis notedthat both ferriteand y'-nitridehavea narrow the ternary Fe-N-Cdiagram10-13'23,24. Thesecalculationswere homogeneityrange as comparedwith the e-nitridephase. based on the sub-regularsolution descriptionsfor binary Note that these phases have a rather broad chemical Fe-N and Fe-Cdiagrams and additionalcompositiondata potentialrangeaccordingto the Lehrerdiagramin Fig.3. for the Fe-N-Cphasesin compound layerslo-13,24, presuming The first more-or-lesscomplete Fe-N-Cdiagram in the l o c a le q u i l i b r i u ma t p h a s eb o u n d a r i e sa n d t h e c o m p o u n d temperaturerange of practicalinterestfor nitrocarburising layer/ diffusion zone interface.These "calculated" Fe-N-C was proposed by Naumann and Langenscheidrs on the diagramspermittedthe coexistenceof e-phaseand s-phase basis of their extensivework on iron powders equitibrated at 848K (575"C),implyingthat no e+0+y'three-phaseregion in flowing NH3/H2/COgas mixturesl' (Fig.6). Clearly, the occurs. presenceof carbon in Fe-N-Cphasesstabilisesthe com- The results of the modelling of the Fe-N-Cdiagram by position of e-carbonitridetowards lower total interstitial Slyckeet a124, Du12and Kunzel3are shown for the e-phase atom contents than for the case where only nitrogen is boundarieswith y', o and 0 (cementite)phasesin Fig.7.fhe dissolved.The developmentof y'-phaseis suppressedby large spread among the resultsof the various evaluations the presenceof carbon. indicatesthe sensitivityof the calculations for the input data Nevertheless, the phasediagram in Fig.6indicatesthat no and the assumptionsmade. equilibriumcan exist betweenferriteand epsilonphase,for The main reasonfor the relativelypoor descriptionof the the temperatureconcerned.In later experimentalwork, it thermodynamicsof Fe-N-Cphasesis the lack of accurate was observedthat, for compound layersdevelopedduring nitrogen and carbon absorption isotherms for Fe-N-C nitrocarburising at temperaturesof about 848K(575.C),the phases.Themajorityof the data used for the evaluationof e-phasedid occur directlyadjacentto the diffusionzonele-22,the calculatedFe-N-Cphase diagram refer to a transition which would not be permitted by the phase diagram *The occurrence phasein thephasediagrampresented of theaustenite proposedfor this temperature,provided local equilibrium in Fig.6is omitted,because austenite hasnotbeenreportedto develoo applied (Fig.6).Fufthermore,the e-phase in compound at 848K(575"0.

Ferc Fig.6.Isothermal sectionofthe Fe-N-C phasediagram at 848K(575"C14.Note the relatively large solubility range of €-phasefor both nitrogenand carbon as comparedto the small homogeneity rangesofy'and cementitephases. Thisisothermal sectionis the result of an interpolation between measurementsat 823K(550"C)and 873K (600"C). Theoccurrenceof the austenitephase at this temperatureis hypothetical.

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nitrogencontent(at.%) HeatTreatmentof Metals2000.4

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andNitrocarburising Evolution Layer; a Comparison of theStates of Knowledge of Nitriding Thermodynamics, Kinetics andMicrostructural of theCompound

Fig.7.Parl of the Fe-N-Cphase diagramshowing the homogeneityrangeof t-phase at 848K(575'C),as proposedby Slyckeet al2a,Du12and Kunzel3 on the basisof thermodynamic modelling.In orderto avoid confusionin the graph, only the diagram calculatedby Du12is given with all phase boundaries; the annotationof monophase, two-phaseand three-phase regionsappliesto this evaluation.For the diagrams proposedby Slyckeet al2aand by Kunzel3,only the de+a, €/€+{ and €/€+0phase boundariesare given.

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N-content (at.o/c)

state as occurring in a growing layer during nitrocarburising. This transition state can be far from r e p r e s e n t a t i v ef o r e q u i l i b r i u m , p a r l i c u l a r l yi f n o l o c a l equilibrium occurs at interfaces,as a consequenceof relativelvslow kineticsof the surface/interJace reactions. I n a d d i t i o n ,t h e s i m p l i f y i n g a s s u m p t i o n sm a d e i n t h e evaluationsof thermodvnamicdata in Refs.24, 12 and 13 were different, which can partly explain the large differencesbetweenthe calculatede-phasefields.By way of example, the y' and 0 phases were considered to be s t o i c h i o m e t r i pc h a s e si n R e f . 1 3 l.t i s h i g h l y q u e s t i o n a b l e whether these assumptionsare justified.Thehomogeneity widths in terms of N and C contents,respectively, may well be narrow.However,the chemicalpotentialrangeswherein the / and cementitephasesare stableare far from narrow (cf. the isothermal width of the y' region in the Lehrer d i a g r a mi n F i g . 3 l . A n o t h e r m a j o r o m i s s i o n ,i n t h e t h e r m o d y n a m i cm o d e l s used so far for the evaluation of the calculatedFe-N-C d i a g r a m ,i s t h e a s s u m p t i o no f a r a n d o md i s t r i b u t i o no f t h e interstitially-dissolved N and C atoms over (paft of) the availableinterstitialsites with resoectto the iron lattice. Rathet both long-rangeorder (LRO)as well as short-range order (SRO)among interstitially-dissolved atoms are to be expected,and were indeedreporledto occurmore than half However,the limited data that referto a a centurya9o25,26. stateof imposedequilibriumbetweenthe gas phaseand an Fe-N-C ohase do not allow the verification of more sophisticated thermodvnamic models for interstitial solutions.

DEVELOPMENT OF THE COMPOUND LAYER Nucleation ln gaseousnitridingof iron,the first phasedevelopingat the which nucleatesupon exceedingthe sudace is y'-FeoN,-*, m a x i m u m s o l u b i l i t yo f n i t r o g e n i n i r o n i n t h e r e g i o n adjacent to the surface.The incubation time for the nucleationof y'-nitridedependson the competitionbetween the flux of nitrogen atoms arriving at the surfaceand the fluxesof nitrogenatoms leavingthe surface(Fig.8/Theflux o f a r r i v i n g n i t r o g e n a t o m s i s c o n t r o l l e db y a m m o n i a dissociation;the fluxes of nitrogen atoms leaving the surfaceare due to the developmentand desorptionof N, (cf.Reaction(Ic))and solid-statediffusionof nitrogenatoms into the bulk.A ouantitativeestimateof the incubationtime for the nucleation o f y ' o n p u r e i r o n ,u s i n gt h e e x p e r i m e n t a l datafor the kineticsof the sudacereactionsas oresentedin Ref. 27 and the diffusivityof nitrogen in ferrite as given in Ref.28,is presentedin Fig. 9 (detailsof the calculationare given in Ref. 29; cf. Ref.30for comparison).Evidently,the desorption of nitrogen from the sudace has a relatively small effect on the incubationtime for nucleation. Experimental work on the nucleation of y' on ferrite indicated that the incubation times given in Fig.9 are realisticestimatesand that a oreoxidationof the sur{ace p r i o r t o n i t r i d i n gc a n r e d u c et h e a c t u a l i n c u b a t i o nt i m e dramatically2s. ln gaseous nitrocarburising,the nucleation of the phases constituting the compound layer depends on the competition of the sur{ace reactions for ammonia

(Fin R Thp

2.16 at 8481(575"C),the amount of cementitein the compound layerincreasedfor the rangeof treatmenttimes investigated. Forthe NC2series,where the highestnitrogenactivitywas applied,the compound layer consistedof an e sub-layerat the surfaceand a "( sub-layer adjacentto the substrateupon prolongednitrocarburising, a n a l o g o u st o a c o m p o u n d l a y e r o b t a i n e da f t e r g a s e o u s nitriding.Thenitrogenand carbon depth distributionsafter 240 minutesof nitrocarburising indicatethat the amount of nitrogenin e-phaseadjacentto the sudaceincreaseswith a1 and the carboncontentdecreasescorrespondingly(Fig.14b and c). Oualitatively, the activitiesof nitrogenand carbonin the gas p h a s e , a , a n d a c , h a v e t h e f o l l o w i n g i n f l u e n c eo n t h e microstructural evolutionof the compound layer: O f o r a 6 < 2 . 1 6( a t 8 4 8 1 ( 5 7 5 " C ) : T h ei g h e rt h e v a l u eo f a * the sooner the e and y' phases develop, the sooner cementiteis dissolvedand, moreover,the smalleris the amount of the y'-phaserelativeto the e-phasein the compound layer upon prolonged nitrocarburising. Furthermore,even for a high a., a high a, leadsto the developmentof a bilayerstructure(eon top of y'),within the range of nitrocarburising times investigated. P o s s i b l ya s i m i l a rl a y e rs e q u e n c ed e v e l o p sf o r t h e N C 1 and NC3 seriesif nitrocarburisingis prolongedbeyond the times investigatedin the presentwork (i.e.4 hours). A high aNpromotesboth the uptakeof nitrogenfrom the gas and the associatedredistributionof nitrocarburising c a r b o nt a k e n u p i n a n e a r l i e rs t a g eo f n i t r o c a r b u r i s i n g (cf. Fig. 14band cfor the NC2series). O for a, > 2.16 (at 8481C575"C):The cementitephasewas always present in the compound layer (for the nitrocarburisingtimes investigated). The appearanceof the y'-phase is first observed after relatively long nitroc a r b u r i s i n g( 1 h o u r ) .T h e h i g h a 6 c o m b i n e d w i t h t h e moderateaN suppressesthe developmentof y'-nitride. The nitrogencontentin the compound layeris relatively low due to the persistenceand groMh of cementite. HeatTreatmentof Metals2000.4

M.A.J.Somers

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Fig.l3. (a)Micrographsof cross-sectionsof the compound layers formed in the NCl series(Table1) for times indicatedin minutes.Positivephase contrastmicroscopy:€-phaseappearslight grey, y'-phaseappearsdark, cementite (0) appearswhite22.EPMA nitrogen (b) and carbon (c) depth profiles for NCI seriesafter nitrocarburisingtimes indicatedin legenf,22.

1

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