Christiansen, Thomas Lundin

Somers, Marcel A. J.

Low-temperature surface hardening of stainless steel provides the required performance properties without affecting corrosion resistance

Online Research Database In Technology

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J.Published in:Advanced Materials & ProcessesPublication date:2013Document VersionPublisher's PDF, also known as Version of recordLink back to DTU OrbitCitation (APA):Christiansen, T. L., & Somers, M. A. J. (2013). HTPro: Low-temperature Surface Hardening of Stainless Steel.Advanced Materials & Processes, (November-December), 52-53.ADVANCED MATERIALS & PROCESSES • NOVEMBER-DECEMBER 201352Stainless steels rely on the presence ofchromium in solid solution, which allowsthe development and maintenance of apassive layer at the surface. Nitriding, car-burizing, and nitrocarburizing are gener-ally not considered good practice, becauseprocessing in the conventional tempera-ture range between 490° and 950°C leadsto chromium nitride and chromium car-bide precipitation. While this provides ahardening effect, it is highly detrimentalto corrosion properties. Since the mid-1980s, several processes were developedthat enable low-temperature surface hard-ening of stainless steel at temperaturesbelow 440°C.The first deliberate surface hardening ofstainless steel was achieved by a processknown today as Kolsterizing[1], a methodostensibly inspired by corrosion phenom-ena observed in liquid-metal fast breederreactors[2, 3]. About the same time, seminalwork by Zhang and Bell[4] on plasma ni-triding of stainless steel was published.Throughout the 80s and 90s, the develop-ment of low-temperature surface harden-ing of stainless steel relied largely onplasma-based techniques, while in the past10 years in particular, gaseous processingwas developed and commercialized.This article describes fundamental andtechnological aspects of low-temperaturesurface hardening (LTSH) of stainless steel.The results shown are taken from the au-thors’ research during the past 15 years.LTSH principlesThe TTT diagram in Fig. 1 demonstratesthe combination of allowable treatmenttime at low temperature before precipita-tion of Cr-based nitrides or carbides oc-curs. In this temperature range,interstitially dissolved nitrogen and carboncan diffuse over a relatively long distance,while substitutional dissolved metallic ele-ments can be considered stationary. Con-sequently, nitride or carbide developmentproceeds so slowly that a nitrogen or car-bon rich case free of chromium-nitrides/carbides develops. In the earlydays of LTSH, the case produced was con-sidered a new phase, dubbed the S phase[4].Recent research shows that no new phasedevelops, but rather LTSH of austeniticstainless steels produces a case that is es-sentially a solid solution of high amountsof nitrogen and/or carbon in austenitewhere interstitial atoms group aroundchromium atoms[5–7]. Therefore, it is incor-rect to refer to the case produced as Sphase; expanded austenite is preferred.The hardening effect that occurs by dis-solving nitrogen and carbon at low tem-perature in stainless steel is not due tonitride or carbide formation. Rather, so-lution of high amounts of interstitialatoms in the austenite lattice provides ef-fective hardening.Process technology and applicationsPlasma processes, apart from the propri-etary Kolsterizing process, have a uniqueadvantage over gaseous processing, be-cause surface activation (removal of thepassive film through sputtering) is an in-herent step of such treatments. The (tem-porary) removal of the passive layer isnecessary to allow surface penetration ofnitrogen and carbon from the nitriding/carburizing atmosphere.Gaseous processing enables the highestflexibility, as well as straightforward mon-itoring and control. For a long time, it ap-peared that gaseous processing of stainlesssteel was possible only by in-situ removalof the passive layer in aggressive halo-genides[8, 9], or after ex-situ deposition of ametal layer promoting dissociation of thegas components and protecting the sur-face against (re)passivation during storageand treatment[10, 11]. Later, robust gaseoustreatments were developed based on gasmixtures that can both remove the passivelayer and provide the nitrogen/carbon tothe stainless steel surface[12–14]. Expanite, acompany co-founded by the authors andThomas Strabo Hummelshøj, works ex-clusively with gas mixtures that have thisdual ability.Figure 2a shows the case produced duringgaseous nitriding of austenitic stainlesssteel. The corresponding nitrogen contentprofile, hardness, and residual stress levelare shown in Fig. 3. Dissolution of a hugeamount of nitrogen leads to an apprecia-ble increase in surface hardness and highcompressive residual stresses, which arisedue to austenite lattice expansion in the ni-trided case. High surface hardness con-tributes to improved wear and gallingperformance, while residual stress en-hances fatigue performance. During ni-triding of austenitic stainless steel, analmost featureless case develops at the sur-face (Fig. 2a), indicating that the zone ismore difficult to dissolve by the etchingreagent than the unaffected austenite.Similar results are obtained with carburiz-ing (Fig. 2b), although less carbon can bedissolved resulting in lower increase inhardness and residual stress. The choice ofnitriding or carburizing depends on theapplication, as both processes have advan-LOW-TEMPERATURE SURFACE HARDENING OF STAINLESS STEEL LOW-TEMPERATURE SURFACE HARDENING OF STAINLESS STEEL PROVIDES THE REQUIRED PERFORMANCE PROPERTIES WITHOUT AFFECTING CORROSION RESISTANCE.Thomas L. Christiansen and Marcel A.J. SomersTechnical University of Denmark, Lyngby16HTPROFig. 1 — TTT diagram of austenite with ahigh nitrogen or carbon content. A low-temperature treatment of long durationor a high temperature treatment (>1050°C)combined with fast cooling can be appliedto prevent development of chromium nitrides/carbides.High-temperature solution nitriding/carburizingChromium nitrides/carbidesAusteniteLow temperature surface hardeninglog tTnov HTPro_am&p master template 2010 QX6.qxt  11/11/2013  4:07 PM  Page 52ADVANCED MATERIALS & PROCESSES • NOVEMBER-DECEMBER 2013 53tages. For example, dissolved nitrogen hasa positive effect on corrosion resistance(e.g., pitting). Carburizing produces an ad-vantageous shallow case-core transitionbecause the affinity of chromium for car-bon is not as high as for nitrogen. By com-parison, nitriding yields a relatively sharpcase-core transition. A gradual transitionin hardness/composition can be tailoredby adopting gaseous nitrocarburizing orthe two-stage process of carburizing fol-lowed by nitriding[15]. These processes pro-duce a hardened case consisting of a hardzone of nitrogen-expanded austenite anda zone of carbon-expanded austenite un-derneath (Fig. 2c).For the case of heavy surface loading,austenite load bearing capacity should beenhanced further. The simplest solution isto prolong the duration of the nitriding/nitrocarburizing treatment. However, thisenhances the risk for precipitation ofchromium nitrides and associated loss ofcorrosion resistance. In such demandingapplications, the low-temperature surfacehardening treatment can be preceded by ahigh-temperature solution nitriding treat-ment[16], which dissolves a relatively lowamount of nitrogen into austenite up to adepth of several millimeters. Cooling fromthe solution nitriding temperature shouldbe done carefully to prevent developmentof chromium nitride (Cr2N or CrN) pre-cipitation (see Fig.1).Alloy grades other than austenitic stainlesssteels can be treated. Most stainless steeltypes including austenitic, ferritic, duplex,martensitic, and precipitation-hardening(PH) grades can develop a surface case ofnitrogen and/or carbon-expanded austen-ite by undergoing gaseous nitriding, car-burizing, and nitrocarburizing treatments.Expanded austenite can also be formed inother types of (similar) alloy systems, suchas many Ni-base alloys. For example, Ni-base superalloys such as the Nimonic se-ries can be nitrided, but low temperatures(360–400°C) are required to suppress for-mation of unwanted CrN[17]. Also, marten-sitic and austentic PH steels can be nitridedand simultaneously bulk hardened[18, 19].Summary and outlookSurface hardening of stainless steel can beachieved by low temperature nitriding, car-burizing, and nitrocarburizing by transfor-mation of the surface into nitrogen and/orcarbon-expanded austenite. Gaseous pro-cessing provides a high degree of tailorabil-ity of the hard surface case enablingtailoring of materials properties, and there-fore, performance. Most stainless steels andsimilar alloy systems can be surface hard-ened by means of gaseous processing.Today’s stainless steel alloys treated usingLTSH are designed for purposes otherthan surface hardening. New stainlesssteel alloys with compositions tailored foroptimal LTSH will further expand the ap-plicability of low-temperature surfacehardening. HTPROReferences1. B.H. Kolster, VDI-Berichte, 506, p107–113, 1983.2. B.H. Kolster and A.J. Rogers, Corrosionand Mass Transfer, AIME, p 252–264, 1973.3. F.B. Litton and A. E Morris, J. Less-Com-mon Metals, 22, p 71–82, 1970.4. Z.L. Zhang and T. Bell, Surf. Engrg., 1(2), p131–136, 1985.5. T. Christiansen and M.A.J. Somers, ScriptaMaterialia, 50, p 35–37, 2004.6. J. Oddershede, et al., Scripta Materialia,62, p 290–293, 2010.7. J. Oddershede, et al., Steel Research Intl.,82(10), p 1248–1254, 2011.8. M. Tahara, et al., European patents EP 0588 458 B1 and EP 0 787 817 A2.9. S. Collins and P. Williams, Adv. Matls. &Proc., 164, p 32–33, 2006.10. S.V. Marx and P.C. Williams, Europeanpatent EP 1 095 170 B1.11. M.A.J. Somers, T. Christiansen, and P.Møller, European patent EP 1 521 861 B1.12. M.A.J. Somers, T.L. Christiansen, Europ-ean patent EP 1 910 584 A1, 2005.13. T.L. Christiansen, T.S. Hummelshøj, andM.A.J. Somers, Surf. Engrg.,27, p 602–608, 2011.14. T.L. Christiansen, T.S. Hummelshøj, andM.A.J. Somers, WO 2011 009463 A1. 15. T. Christiansen and M.A.J. Somers, Surf.Engrg., 27, p 445–455, 2005.16. T.L. Christiansen, T.S. Hummelshøj, andM.A.J. Somers: PCT application PCT/DK2012/050139, 2011.17. K.M. Eliasen, T.L. Christiansen, andM.A.J. Somers, Surf. Engrg., 26, Vol 4, p248–255, 2010.18. R.B. Frandsen, T. Christiansen, andM.A.J. Somers, Surf. & Coat. Tech., 200(16/17), p 5160–5169, 2006.19. F.A.P. Fernandes, T. L. Christiansen, andM.A.J. Somers, Heat Treat & Surf. Engrg.Conf. & Expo, Chennai Trade Centre, Chen-nai, India, May 2013.For more information:Marcel A.J. Somers,Prof. and Section Head – DTU MechanicalEngineering, Technical University of Den-mark, Produktionstorvet Bldg. 425, 2800Kgs. Lyngby, Denmark; somers@mek.dtu.dk;www.mek.dtu.dk.HTPRO17Fig. 2 — Cross sections of (a) AISI 316 after nitriding at 445°C for 22 h in a gas mixture containing 60% NH3 and 40% H2, (b) AISI 316 carburized in acetylene at 520°C for 3 h (the transition from core to hardened case is more diffuse than for nitriding), and (c) cold-worked AISI304 nitrocarburized at 420°C for 19 h (the nitrocarburized case is subdivided in a zone of nitrogen-expanded austenite and a zone of carbon-expanded austenite below).(a)    (b)                                                                                     (c)Fig. 3 — Nitrogen content, hardness, andresidual stress in AISI 316 after nitriding at445°C for 22 h in a gas mixture containing60% NH3 and 40% H2.0         5        10        15       20       25Depth, mm0   -2   -4   -6   -8 1´03Stress, MPa1400   1200   1000   800   600   400   200Hardness HV10, (MPa)10   8   6   4   2   0Nitrogen content, wt%nov HTPro_am&p master template 2010 QX6.qxt  11/8/2013  1:49 PM  Page 53