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Low-Temperature Carburization of Austenitic Stainless Steels S.R. Collins, P.C. Williams, and S.V. Marx, Swagelok CompanyA. Heuer, F. Ernst, and H. Kahn, Case Western Reserve University LOW-TEMPERATURE CARBURIZATIONis a gaseous carburization process performedat atmospheric pressure, at temperatures wherethe kinetics of substitutional diffusion are veryslow. Low-temperature carburization hardensthe surface of austenitic stainless steels throughthe diffusion of interstitial carbon, without theformation of carbides. The surface must be acti-vated, by modification and removal of the natu-rally occurring chromia layer, for the process towork. The process results in a hardened diffu-sional case on the surface, typically 20 to 35 m mthick, with carbon content at the surface inexcess of 10 at.%. The process is conformal andcan be applied to finished components withoutchange in dimension. Property enhancementsinclude: Increased surface hardness : Close to thesurface, a microhardness value of approxi-mately HV 1200 is obtained, which corre-sponds to greater than 70 HRC. Residual compressive stress : At the surface,values in excess of 2 GPa (300 ksi) havebeen measured by X-ray diffraction. Retained ductility : Scanning electron micros-copy of surfaces of tensile bars tested to fail-ure show deformation characteristics typicalof austenite (i.e., intersecting slip bands) andthe absence of cracking.This combination of increased surface hardness,compressivesurfacestress,andretainedductilityleads to improved performance in applicationsdemanding resistance to wear, erosion, andfatigue. Additionally, corrosion resistance to avariety of chloride-containing media also hasshown dramatic improvement, due to carbon-enhanced passivity of the resulting surface. Overview Case hardening is a widely used industrialprocess for enhancing the surface hardness of metal and alloy components. In a typicalcommercial process, a carburizing gas contactsthe workpiece at elevated temperature and car-bon atoms diffuse into the metal surface. Hard-ening occurs through the formation of carbideprecipitates. Gas carburization normally isaccomplished at 950 C (1700 F) or above,because most steels must be heated to thesetemperatures to transform to the high-temperatureaustenitic (face-centered cubic, or fcc) structurepreferable for carbon infusion. This structure ispreferable because it enables higher diffusivityand higher solubility limits than the ferritic(body-centered cubic, or bcc) structure.Austenitic stainless steels are among themost ductile and most corrosion resistant of allferrous alloys. However, they have only modesthardness compared to other steels, limitingtheir performance in many applications. Thesesteels typically contain 10 to 18 wt% Cr and8 to 14 wt% Ni; the chromium addition providescorrosion resistance, while the nickel additionstabilizes the desirable austenitic structure atroom temperature. These alloys are valued for their combined corrosion resistance and ductil-ity. In many technical applications, however,their performance, lifetime, and aestheticappearance would be improved if their surfaceswere hardened. Unfortunately, hardening byconventional carburization is carried out at ele-vatedtemperatures,wheretheformationofchro-mium-bearing carbides cannot be avoided, andchromium depletion from the austenite matrixcan compromise corrosion resistance. Addition-ally, the levels of hardness that can be achievedare limited by the resulting microstructure of the case, a distribution of chromium-rich car-bides in a chromium-depleted matrix. Hardnessvalues of 50 to 60 HRC are achieved under theseconditions,attheexpenseofcorrosionresistanceof the alloy.Low-temperature carburization hardens thesurface of chromium-containing austeniticalloys,in particular austenitic stainless steels. The pro-cess involves activation of the surface followedby a gas-phase carburization treatment, per-formed at temperatures low enough to avoid theformation of carbides (350 to 550 C, or 660 to1020 F), for a sufficient time (typically 20 to60 h) to allow carbon diffusion to occur.The result is a carbon-rich, uniform (single-phase) and highly conformal compositionallygraded surface layer ranging from 10 to 40 m mthick, with a near-surface hardness that canapproach HV 1200 (over 70 HRC). Because thetreatment occurs at low temperature and phasetransformations are avoided, parts do not distortor change dimensions. Carbon concentrations atthe surface of treated 316 stainless steel compo-nentshavebeenverifiedbyavarietyofanalyticalmethodstobeon theorder of12 to15at.%;up to20 at.% has been demonstrated on treated super-austenitic steels. The case remains austenitic andretains its ductility. Parts can be bulk handled,enabling processing of high numbers of parts.In this article, examples shown are results for 316 stainless steel, although a substantial bodyof data has been generated for other industriallyimportant alloys, such as superaustenitic stain-less steels, precipitation-hardening stainlesssteels, duplex alloys, nickel-base alloys, andcobalt-base alloys (Ref 1–3). Background and CompetingTechnologies Methods for hardening stainless steel surfaceshave been researched since at least the 1980s(Ref 4, 5). These methods include liquid sodiumand cyanide salt bath treatments, plasma nitrida-tion and carburization (Ref 6, 7), ion implanta-tion, and gaseous atmospheric heat treatments.All of these methods will provide a hardeneddiffusion case on stainless steels with varyingperformance characteristics. However, commer-cial scale-up has been elusive for many of thesemethodsduetotechnologicalbarriers.Forexam-ple, liquid sodium activates and readily transferscarbon to the stainless steel, but a scale-up fromthe laboratory to a production process presentsa significant hurdle for safety and handlingreasons. Plasma and ion methods are line-of-sight ASM Handbook , Volume 4D, Heat Treating of Irons and Steels J. Dossett and G.E. Totten, editorsCopyright # 2014 ASM International W All rights reservedasminternational.org methods. They effectively activate the surfacebut are not able to provide a uniform case for workpieces with complex shapes, especially inbatch processing.Recent survey articles have reviewed theworldwide state of low-temperature nitridingand carburizing of austenitic stainless steels(Ref 1, 8). In addition, several academic confer-ences have been held in the past two decades onthe topic of low-temperature thermochemicalsurface treatments of stainless steels (Ref 9–13).On the commercial front, several entities adver-tise low-temperature hardening processes for stainless steels, as shown in Table 1. It shouldbe noted that only two of these processes(SAT12 and NV Pionite) are known to be gas-phase low-temperature carburization, while theentity responsible for a third carbon-impartingmethod (Kolsterising) is entirely silent on howit is achieved. The process and results discussedin this article are specific to the SwagelokSAT12 treatment. Commercial Application Since 1999, low-temperature carburizationhas been applied to the ferrules of the Swageloktube fitting, as shown in Fig. 1. In recognitionof the development and commercialization of the surface-hardening technology in the tube fit-ting application, Swagelok received the ASMInternational Engineering Materials Achieve-ment Award in 2006 (Ref 14). Swagelok has acaptive heat treating facility that batch processesand bulk handles millions of ferrules per year, innominal sizes ranging from 6 mm to 1 inch. Theprocessisperformedinspecialty-builtcarburiza-tion furnaces with enhanced gas handling cap-abilities, as shown in Fig. 2. Due to the use of dryHClandCOintheprocess,thefurnacesmustbe leak-tight to atmosphere. Process Considerations Successful low-temperature carburization of stainless steels and other chromium-containingalloys depends on the alignment of severalprocessing parameters. These include activationof the surface, proper surface preparation,selection and condition of the alloy to be car-burized, treatment temperature, and carburizingatmosphere. Figure 3 shows a typical processcycle. Activation Stainless steels and other chromium-containing alloys obtain their “stainless” charac-teristic from the chromium-rich oxide that formson the exposed surface. This oxide layer formswhen a fresh surface is exposed to an oxygen-containing medium, such as air, and will formafter very short time exposures. The chromialayer on the surface of stainless steel constitutesa major hurdle to low-temperature carburizationbecause it blocks the inward diffusion of carbon.In order to diffuse the carbon atoms into thestainless steel from the surface, the chromiumoxidelayermustberemoved,oratleastmodifiedto become carbon-transparent. This step gener-ally is known as surface activation or depassiva-tion. Several methods have been developed(Ref 15–19) that eliminate the inhibiting effectsof the oxide and activate the surface for efficienttransport of carbon from a conventional carbur-izing atmosphere into the stainless steel matrix.As noted earlier, some processes, such as plasmaor liquid sodium treatments, readily activate apassive stainless steel surface by removing theoxide by sputtering. In commercial productionprocessing, activation can be achieved by expos-ing the furnace load to a halide-containing gasmixture such as nitrogen trifluoride or hydrogenchloride and nitrogen at atmospheric pressure.Hydrochloric acid is effective as an activatinggas for a wide range of chromium-containingcorrosion-resistant austenitic alloys, includingstainless steels and nickel-base alloys, such asalloys 625 and 825. Processing Temperature Ranges and the Concept of Paraequilibrium Low-temperature carburization has a limitedtemperature processing range, usually 350 to550 C (1020 F). At the lower processing tem-perature, the process becomes very slow andless economically feasible. Lower temperaturesmean longer times for the same case thickness.At the upper end, it becomes increasingly diffi-cult to avoid the formation of carbides, whichdepend in part on the thermally-assisted mobil-ity of sustitutional alloying elements, such aschromium, in the iron matrix.Traditionally, carburization is carried out athigh temperature to maximize the solubility andtherateofinterstitialsolutediffusion.Oncoolingto room temperature, however, a major part of the solute atoms will precipitate as carbidephases. Figure 4 shows the appropriate time-temperature-transformation (TTT) diagram for the heat treatment of an austenitic stainless steelwithconventionalcarbon contents(Ref 20).After high-temperature carburization ( T > 950 C, or 1740 F), precipitation of carbides can beavoided only by extremely high cooling rates(path A ). Furthermore, it is easy to exceed thesolubility limit at these temperatures and formcarbides during the carburization process itself.At the cooling rates typical of industrial pro-cesses, however, the path will cross the carbide“nose” and precipitation will occur (path B ).Similar problems are encountered in nitridationofthesesteels.Carburizationatlowtemperaturesproved capable of hardening the surface of aus-teniticstainlesssteelwithoutformingphasesthatwould deplete the matrix of chromium, andthereby degrade corrosion resistance. In thisprocess, the formation of carbides is kineticallysuppressed (path C ). This results in an extremelyhigh or “colossal” carbon supersaturation, infact far higher than can be achieved by high-temperature carburization.At moderate temperatures, substitutionalsolutes such as chromium and nickel diffusemuch more slowly in austenitic steels than dointerstitial solutes such as carbon. At 450 to500 C (840 to 930 F), the diffusivity of chro-mium is on the order of 10 –21 m 2 /s (Ref 21),whereasthecarbondiffusivityatthistemperatureisintherange10 –16 to10 –17 m 2 /s(Ref22–24).Thisfactorof10 4 to10 5 differenceindiffusioncoeffi-cients enables homogeneous carburization inaustenitic stainless steels (and other fcc alloys) Table 1 Commercially available processes for low-temperature nitriding and carburizing of stainless steels Entity Process nameInterstitial hardeningelementTemperatureMethod Applications Comments C F University of Birmingham,U.K.LTPN N < 450 < 842 Plasma . . . . . . LTPC C < 550 < 1022 Plasma . . . . . . Bodycote, U.K. Kolsterising C Trade secret Trade secret Hardware, watch cases . . . Nivox2 N < 400 < 752 Plasma Control rods used in nuclear reactorsBodycote acquired Nutrivid(France) 2010Nivox4 and Nivox LH C < 460 < 860 Plasma . . . Nihon Parkerizing, Japan Palsonite N + C 450–490 842–914 Cyanide saltbathSmall bore weapon components . . . Airwater Ltd., Japan NV Super Nitriding N 300–400 572–752 Gas Flatware, hardware Fluoride activationNV Pionite C < 500 < 932 Gas Hardware, watch cases Fluoride activationSwagelok Company, U.S.A. SAT12 C 380–550 716–1022 Gas Tube fitting ferrules, hardware HCl activationNitrex Metal Technologies,CanadaNitreg-S N . . . . . . Gas Piston rings, pitch gears . . . Expanite A/S, Denmark Expanite N + C . . . . . . Gas . . . . . . 452 / Heat Treated High-Alloy Steels under conditions where the formation of chromium-rich carbides is suppressed by insuffi-cient diffusion kinetics for substitutional solutes.In other words, carburization is possible belowthenoseofthecarbidecurveontheTTTdiagramshowninFig.4.Attherelevanttemperatures,thesystem is not able to approach a complete ther-modynamic equilibrium, because local differ-ences in the chemical potentials of the metalatoms cannot be equilibrated by transport. Onlynonuniformities of the chemical potentials of the interstitial solutes can be equilibrated. Suchlimitedequilibrationofsome,butnotall,compo-nents in a metallic system is called “paraequili-brium.” This can occur at temperatures wheresubstitutional solutes are effectively immobile,whereas interstitial solutes such as carbon candiffuseoverappreciabledistanceswithinreason-ably short times. Paraequilibrium in austeniticstainless steels allows for vastly increased solu-bility limits for carbon. Carburizing Atmosphere Several carburizing gas species will impartcarbon to the active surface, with varying levelsof efficiency at the processing temperaturesrequired for low-temperature carburization.These include acetylene and carbon monoxide,among others. Pack carburization also has beendemonstrated. A side effect of the high carbonpotential needed for processing can be the gen-eration of soot, which then has to be removedafter processing using standard aqueous clean-ing methods. Candidate Alloys for Treatment Several commercially-available corrosion-resistantalloyscanbetreatedbylow-temperaturecarburization. The process has been applied to300 series stainless steels, in particular 316 stain-less steel, with great effectiveness. Optimal treat-ment is obtained on single-phase fcc or fullyaustenitic alloys containing high fractions of chromium and nickel. Under paraequilibriumconditions chromium, with its high affinity for Fig. 2 Pit furnaces for low-temperature carburization. Courtesy of the Swagelok Company Low-temperature carburizationCooling T e m p e r a t u r e , ° C Time, hSurface activationHCl + N 2 , 250 ° C3 h20–30 h3 h3 hCo + H 2 + N 2 , 470 ° C(880 ° F)(480 ° F) Fig. 3 Typical process cycle for low-temperaturecarburization of austenitic stainless steels. Ittakes up to three days to perform the carburizationprocess, in contrast to minutes with typical heat treatingprocesses. Fig. 1 Commercial application of low-temperature carburization: Swagelok Tube Fitting. Courtesy of the SwagelokCompany Fig. 4 Time-temperature-transformation (TTT) diagram of carbide formation in austenitic stainless steels. Source: Ref 20 Low-Temperature Carburization of Austenitic Stainless Steels / 453 carbon,enablesadramaticincreaseofthesolubil-ity limit of carbon (Ref 25, 26). Nickel, althoughnot a carbide-forming element, appears toenhance the carbon levels that can be obtainedby suppressing carbide precipitation (Ref 27).Austenitic stainless steels can contain lessdesirable metastable phases, such as ferrite(bcc) and strain-induced martensite (body-cen-tered tetragonal, or bct). Ferrite can be retainedin austenitic stainless steels if not sufficientlyhot worked or homogenized. Strain-inducedmartensite can occur on the surface of suscepti-ble alloys that have been heavily worked or machined. Carburization treatment of ferrite or martensite at temperatures usually applied toaustenite results in a severe loss of corrosionresistance. The mechanism appears to be thecreation of surface carbides in the ferrite phase.Optimal treatment by low-temperature carburi-zation occurs when these phases are not pres-ent; ferrite can be reduced or eliminated byannealing, and strain-induced martensite canbe removed via electropolishing.Other alloys have been treated with varyinglevels of success; these include superausteniticstainless steels (AL6XN, 254SMO), precipitation-hardening iron-base superalloys (A286), nickel-base corrosion-resistant alloys (alloy 625, alloy718, alloy 825, Hastelloy C), precipitation-hardening stainless steels (17-4PH, 15-5PH,13-8Mo), and duplex stainless steels (2205 and2507). Product Forms and Surface Condition Low-temperature carburization has been suc-cessfully applied to several product forms,including plate, sheet, foil, wire, machined com-ponents, castings, forgings, powder, powderedmetal (P/M) and metal injection molded (MIM)components. Surfaces must be clean and freeof scale, oil, or other residues so that the activa-tion step can proceed. The mechanical surfacecondition needed for a successful treatmenthas been evaluated based on performancecharacteristics of the surface after treatment.Acceptable surface roughness parameters weredetermined from cyclic polarization tests per-formed on samples with different finishes.A 120 grit finish or better hardly limits the cor-rosion response in cyclical polarization testing.Rougher surfaces may show metastable pitting(Ref 28). The 120-grit finish typically is muchrougher than the finish obtained through stan-dard machining practices.TheactivationstepillustratedinFig.3isinter-rupted with a short carburization cycle. This pro-cessing feature enables a deeper and moreuniformcase,apparentlybecauseitprovidesbet-ter surface activation. Observations by research-ers of corrosion in flue gases (Ref 29) suggestthat the presence of soot enhances activation of passive surfaces. Additional research on the sec-ond carburization cycle showed that a rampedcarbon potential was needed to achieve the hard-ness and depth required, and to minimize theformation of soot that would need to be removedin postprocessing. Quality Control Methods for Evaluating Treatment Whenparts are treated correctly, application of the low-temperature carburization process shouldbe transparent to the end user. In other words, theparts should visually appear as they did prior totreatment. Because only a layer comprising thefirst 25 m m or so below the surface is affected bythe treatment, it can be difficult to determine if acomponent has been treated without resorting todestructive methods and optical microscopy.Two qualitative methodsfor examining treatmenthave been used: scratch hardness using calibratedfiles, and immersion insodium hypochloritesolu-tions. These methods interrogate the case byexamining improvements in hardness and corro-sion response to a chloride-containing environ-ment. Although they can identify a differencebetween a treated and nontreated surface, theyare not specific enough to be used for anythingbut a screening test. For detailed evaluation of case properties, quantitative methods providingenhanced spatial resolution are preferable. Theseinclude light optical microscopy of metallo-graphic cross sections, depth profiles of nano- or microhardness, and composition-depth profilesobtainedthroughglow-dischargeopticalemissionspectrometry(GD-OES)orcalibratedAugerelec-tron spectroscopy (AES) using a scanning Auger microprobe (SAM) on a cross section. Thesemethodsaredestructive,inthatacomponentmustbesectionedtoevaluate the case.However,quan-titative comparative data is obtained and can beused to develop metrics for the case. Microstructure of theLow-Temperature Carburized Layer The low-temperature carburized layer on aus-tenitic steel has been characterized using opticalmetallography. On a sectioned and etched speci-men, the case appears as a relatively featureless,etch-resistant surface layer of approximately10 to 40 m m. This layer is a diffusion gradientof carbon in solid solution in the austenitematrix. The structure of the case has been evalu-ated by X-ray diffraction, and has been shown tobe expanded austenite. The lattice parameter of the austenite expands from 0.360 to 0.372 nmafter treatment (Ref 30). Other researchers, par-ticularly those working in low-temperaturenitriding processes, have called this structureS-phase (Ref 31–33). However, there is no phasetransformation and the layer does not represent anew phase. Expanded austenite is formed whenlarge amounts of either nitrogen or carbon(or both) are dissolved in the surface of an aus-tenitic stainless steel, forming a supersaturatedsolid solution without the precipitation of chro-mium-containing nitrides or carbides. Data for treated 316L stainless steel are shown in Fig. 5(Ref34).Thesurfacewaselectropolishedfordif-ferent times to progressively remove the case.The analysis used X-ray diffraction (XRD) todetermine the lattice parameter and evaluateresidual stresses, and to identify the possiblepresence of carbides arising from carburization.The shift in the peaks to lower 2 y values for lesser amounts removed (higher concentrationsof carbon) indicates the increasing lattice expan-sion. Carbon concentration profiles usingAuger electron spectroscopy (AES) and energy-dispersive spectrometry (EDS) show carbon (111) γ (200) γ 6 µ m0121200800400 H V 2 5 0102030405000145290102030405011 µ m I n t e n s i t y 19 µ m24 µ mUntreatedspecimen40 (a) (b) 2 θ , degreeDepth from surface, µ mCarburizedsurface4244464850528 X c , a t . % σ 1 1 , G P a σ 1 1 , k s i 40 − 1 − 2 Fig. 5 Microstructural evaluation of low-temperature carburized 316 stainless steel. (a) X-ray diffraction (XRD) of different depths within the case, obtained by serial removal of the surface via electropolishing. Note thepeak shift to the left from nontreated specimen to carburized surface, indicating lattice expansion. No peaksassociated with carbides are evident in the case. (b) Microhardness profile as a function of depth. Measurementswere taken from multiple components from a single process run. Superimposed curves of carbon concentration(X C , at.%) and residual compressive stress ( s 11 , GPa) are obtained from the XRD spectra shown in (a). Source: Ref 34 454 / Heat Treated High-Alloy Steels
