Surface & Coatings Technology 200 (2005) 1830–1836www.elsevier.com/locate/surfcoat

New developments in thermo-chemical diffusion processes

Winfried Gräfen ⁎, Bernd Edenhofer

Ipsen International GmbH, Flutstraße 78, Kleve, Germany

Available online 25 October 2005

Abstract

Thermo-chemical diffusion processes like carburising, nitriding and boronizing play an important part in modern manufacturing technologies.They exist in many varieties depending on the type of diffusing element used and the respective process procedure. The most important industrialheat treatment process is case-hardening, which consists of the thermo-chemical diffusion process carburising or its variation carbonitriding,followed by a subsequent quench. The latest developments of using different gaseous carburising agents and increasing the carburisingtemperature are one main area of this paper.

The other area is the evolvement of nitriding and especially the ferritic nitrocarburising process by improved process control and newlydeveloped process variations using carbon, nitrogen and oxygen as diffusing elements in various process steps. Also, boronizing and specialthermo-chemical processes for stainless steels are discussed.© 2005 Published by Elsevier B.V.

Keywords: Carburising/carbonitriding; Low-pressure carburising/carbonitriding; High-temperature carburising; Nitriding potential control; Ferritic nitrocarburising(fnc); Ferritic oxi-nitrocarburising (fonc); Solution nitriding; Plasma-nitriding

1. Introduction

In thermo-chemical diffusion processes, elements likecarbon, nitrogen or boron are diffused into metal surfaces inorder to enhance the surface properties and the strength ofmetallic components.

In modern heat treatment furnaces, the diffused elementsusually originate from gases reacting at high temperatures withthe metallic surfaces. This can be a pure thermal and chemicalreaction as a consequence of the thermal dissociation of thegases. An increase of the reaction velocity can be achieved inutilizing an electric field in order to ionize the reaction gas(plasma) resulting in largely increased mass transfer.

The industrial thermo-chemical diffusion processes existingtoday are known under the names carburising, nitriding andboronizing. They exist since many decades and have evolvedwith time to precisely controlled and reliable processes as partof the total manufacturing process of metal, especially steelcomponents.

In the last few years, a number of new developments andimprovements in different areas have helped to increase the

⁎ Corresponding author.E-mail address: wgr@ipsen.de (W. Gräfen).

0257-8972/$ - see front matter © 2005 Published by Elsevier B.V.doi:10.1016/j.surfcoat.2005.08.107

importance of diffusion processes, leading to metallic compo-nents with higher endurance capability.

2. Carburising

The dominating carburising technology today is the gaseouscarburising process using endothermic gas as carrier gas and ahydrocarbon gas, like natural gas, propane, lpg or others, asenrichment gas for achieving high carbon potentials. Also,methanol diluted with nitrogen can be fed into the furnace,creating at elevated temperatures a carrier gas inside the furnacesimilar to endothermic gas.

The most economical gassing process is the direct feed ofa fuel (hydrocarbon gas) plus an oxidizing gas (air, carbondioxide or water) into the furnace and creating a CO- and H2-containing carburising atmosphere inside the furnace [1].Certain requirements like sufficiently high furnace tem-perature, strong gas circulation, furnace muffle, etc., need toexist in the furnace for a successful utilization of this in situgassing technique called Supercarb® [2]. Therefore, yearsago, this process was limited to batch furnaces like pitfurnaces and sealed quench furnaces. In the meantime, theSupercarb® process is also used in all types of continuousfurnaces like mesh-belt furnaces, rotary hearth furnaces and in

Fig. 1. Mean carbon flux values (g/m2 h) for different carburising processes.

Fig. 3. Carbon and nitrogen profiles of a steel 30CrMo4 after low-pressurecarbonitriding at 880 °C.

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the last 4 years also in specially adapted pusher furnaces [3].The savings in gas consumption using Supercarb® can bevery high.

3. Low-pressure carburising

Even more process gas can be saved when hydrocarbongases totally without an oxidizing gas are directly introducedinto carburising furnaces. In this case, the carbon transfer is adirect result of the decomposition of the hydrocarbon into freecarbon and hydrogen. Because of the high carbon availabilityof hydrocarbon gases, such a process only works with a highdilution of the hydrocarbon gases, or a utilization of thehydrocarbon gases at low pressures. The last version is thewell-known low-pressure carburising process.

In the 1980s and 1990s, the main hydrocarbon gas used forlow-pressure carburising was propane, despite its inherentdeficiencies of furnace sooting and non-uniform carburising[4,5].

In the last 5 years, the hydrocarbon gas acetylene hastaken the dominant role in low-pressure carburising. Thereason is its extraordinary carburising power with on theaverage 10% more carbon transferred compared to propane(Fig. 1) as well as its much more uniform carburisingcapability, especially on complicated work piece geometries,and finally the fact that vacuum furnaces run with acetylenedo not show any soot formation if run at a pressure below 10mbar [5,6].

The main advantages of low-pressure carburising are theincreased mass transfer resulting in reduced process times,

Fig. 2. Cycle for low-pressure carbonitriding.

improved layer uniformity, no internal oxidation, increasedstress resistance and better surface quality (in connection withgas quenching) [7].

4. Low-pressure carbonitriding

Until recently, a deficiency still existed, and this was theinability to do a carbonitriding process at low pressure.

With plasma carburising, it has been possible for about30 years to carbonitride using methane or propane in theboost phases and nitrogen gas in the diffuse phases [8].This procedure is not possible with low-pressure carburising,as nitrogen gas starts to dissociate thermally only above1000 °C.

Lately, however, a method was developed using ammonia atlow pressures in the diffuse phases, or in most cases in the lastdiffuse phase, in order to transfer nitrogen next to carbon intothe steel surface (Fig. 2).

Adjusting the time and temperature ratio of the ammoniautilization vs. the acetylene utilization allows the production ofdefined carbon and nitrogen surface contents. In this way,relatively low surface nitrogen contents of, e.g., 0.3 wt.% (Fig.3) or very high surface nitrogen contents of close to 0.7 wt.%(Fig. 4) can be produced [9].

The advantage of carbonitriding vs. carburising is that acarburised microstructure with an increased content of nitrogenhas a higher temperature resistance, an increased hardenability,improved wear resistance, and also in some instances a higherload-carrying capability [10].

Fig. 4. Carbon and nitrogen profiles of a steel 15CrNi6 after low-pressurecarbonitriding at 930/820 °C.

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5. High-temperature carburising

Another trend in the last few years is the increased utilizationof higher carburising temperatures with the main goal to reducecycle times and thus save costs.

Fig. 5 shows curves of carburising depths vs. carburisingtimes for four different carburising temperatures of 880, 930,980 and 1050 °C for gaseous carburising in endothermic gasand different carbon potentials. In this diagram, the time savingfor different carburising depths in using higher carburisingtemperatures can be seen. For example, for a carburising depthof 1.2 mm, the holding time on temperature can be reduced from400 to 220 to 115 min by increasing the carburising temperaturefrom 930 to 980 and further to 1050 °C.

Naturally, the utilization of higher carburising temperaturesof above 1000 °C can also be done with low-pressurecarburising, as can be seen in Fig. 1. Vacuum furnaces for low-pressure carburising are even more adapted for highertemperatures because the material used for the furnace liningand the furnace heating elements are usually graphite, which hasvery high temperature resistance. But also atmosphere furnacesfor gas carburising are today increasingly used for high-temperature carburising, as the table in Fig. 6 shows.

Thus, even in sealed quench furnaces, temperatures of 1015°C and 1020 °C are used today industrially, and also pusherfurnaces have gone up to 980 °C [11]. This is due to theincreased use of newly developed silicon carbide materials forhearth, muffles and especially radiant tubes.

The main problems remaining with high-temperaturecarburising is the grain growth of existing case-hardening steelsand the reduced lifetime of grids and baskets.

Fig. 5. Influence of temperature and carbon potential on carburising depthand cycle duration (Aufkohlungstiefe=carburising depth; Aufkohlungs-dauer=carburising time).

6. Nitriding

6.1. Control of the nitriding potential

The state of the art of nitriding in ammonia or dilutedammonia gas is to control the nitriding potential. The nitridingpotential is defined as:

KN ¼ pðNH3ÞpðH2Þ3=2

This definition is a direct consequence of the ammoniadissociation reaction:

NH3 X ½N� þ 3=2H2

By choosing the respective nitriding potential, nitrogen-richcompound layers of the ε-nitride, nitrogen-poor compoundlayers of the γ'-nitride as well as totally compound-layer-freenitrided surfaces can be produced.

The so-called Lehrer diagram gives good guidelines alsofor industrial steels, what type of compound layer to expect forthe respective nitriding potentials controlled in the furnaces[12].

For controlling the nitriding potential, it is necessary tomeasure either the ammonia content or the hydrogen content ofthe atmosphere. This can be done with infrared or other gasanalysers. The state of the art is, however, to measure thenitriding potential continuously on-line directly inside thefurnace with a hydrogen sensor called HydroNit® [13]. Thissensor, the scheme of which is shown in Fig. 7, is capable ofmeasuring directly the partial pressure of hydrogen inside thenitriding furnace using a measuring tube of a special materialcapable of being permeable only to hydrogen gas.

7. Ferritic nitrocarburising

In ferritic nitrocarburising (fnc), both nitrogen and carbon aretransferred into the steel surface to produce a nitrogen andcarbon containing ε-compound layer.

The gas used for this process therefore is a mixture ofammonia gas and a carbon-carrying gas. Standard industriallyused gases are a mixture of ammonia and endothermic gas(50:50) or a mixture consisting of ammonia plus CO2 (5%) andnitrogen gas (45%) [14,15].

In these gas mixtures, the nitrogen transfer depends on theammonia dissociation just like in nitriding. The carbon transferis caused by the CO–hydrogen reaction:

COþ H2 X ½C� þ H2O

with gases with high CO-content (endothermic gas) deliveringmuch more carbon than those with low CO-content (CO2).

The main problem with the nitrocarburising atmospheresproduced by these two gas mixtures is that the carbon contentand the nitrogen content in the compound layer cannot beadjusted independently of each other. The carbon transferincreases with higher hydrogen content, which at the same time,however, lowers the nitriding potential. Thus, automatically

Furnace type Parts Load wgt. Temp. Atmo- Quenching (kg) (°°C) sphere

RTQPF-4(5)- bearing 300-400 1020 N2-M + direct GRM components propane

TF-2-25-GRR chain 1.600 1015 endogas+ slow coolbushings natural gas

PP-50x50x50- gears 150-200 920- endogas + direct 20-G 980 propane

Fig. 6. Industrial applications of high-temperature carburising furnaces.

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compound layers produced in ferritic nitrocarburising with thetwo gas mixtures mentioned above will have a low carboncontent if a high nitrogen content is produced and vice versa(Fig. 8) [16].

With a new method developed in the last few years, it ispossible to produce ε-compound layers in ferritic nitro-carburising which have at the same time a high nitrogen contentas well as a high carbon content. J. Wünning had already in1977 shown that the strongest carbon-transferring gases innitrocarburising next to endothermic gas are hydrocarbon gasesand especially propane [17].

The new method developed [18] splits the nitrocarburisingcycle in two parts, with the first part run in ammonia plusCO2 and nitrogen in order to produce a high nitrogen contentin the compound layer. The second part is run in a gasmixture consisting of ammonia and propane (plus nitrogen)(Fig. 9).

8. Ferritic oxi-nitrocarburising

Oxi-nitriding has also been known since the 1970s and wasnoted for faster surface reactions and higher nitrogen transfer[19]. It never gained much importance, as in pure classicalnitriding in ammonia gas, the growth of the compound layer

Fig. 7. Principle of the HydroNit®-Sensor.

was already sufficiently fast, and the goal in those days wasmore to restrict its thickness than to improve it.

With the short cycle times of ferritic nitrocarburising and theproblem with sometimes bothered surface reactions due topassive oxide layers on the surface of the steel components, theimportance of the utilization of oxygen in the first part of an fnccycle was noticed about 3 years ago [20].

This led to the development of the ferritic oxi-nitro-carburising process with air being added to the nitridingatmosphere inside the furnace during the last part of the heatingcycle and the first part of the nitrocarburising cycle. H.-J. Spiesexamined this effect and found that high oxidizing potentials areneeded in order to transform the passive oxide layer into anitrogen permeable layer of iron oxide [21].

The ferritic oxi-nitrocarburising treatment is favourably usedfor higher alloyed materials, like, e.g., hot and cold working toolsteels and also especially stainless steels, as the example of thesteel X5CrNi 18-10 (DIN 1.4301) in Fig. 10 demonstrates.

9. Special processes for stainless steels

Stainless steels, if treated with normal nitriding or car-burising processes, lose most of their corrosion resistance due tothe formation of chromium nitrides or carbides.

Fig. 8. Chemical composition of ε-compound layer produced by differentnitriding and carburising potentials [16].

Fig. 11. Microstructure of the steel X2CrNiMo 18-14-3 (DIN 1.4435) afterplasma-carburising at 350 °C [22].

Fig. 9. Special two-step fnc cycle resulting in ε-layers with large nitrogen andcarbon content.

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By the development of new low-temperature or high-temperature processes, this deficiency can be overcome.

10. Plasma-carburising of austenitic steels

Lowering the carburising temperature to values whichprohibit the formation of chromium carbides (Cr23C6), i.e., totemperatures below 400 °C, can produce a thin shallow surfacelayer supersaturated with carbon with a large hardness increaseand basically no loss of corrosion resistance.

Fig. 11 shows as an example the microstructure of the steelX2CrNiMo 18-14-3 (DIN 1.4435) after plasma carburising for96 h at 350 °C, having produced a carburised layer of 25 μm

Fig. 10. Structure and hardness profile of an oxi-nitrocarburised austeniticstainless steel X5CrNi 18-10 (DIN 1.4301).

thickness with a hardness of approx. 1150 HV and a carboncontent of approx. 3 wt.% [22]. Because of the low temperature,there are hardly any dimensional changes involved with thisprocess.

11. Plasma-nitriding of austenitic steels

The formation of the chromium nitrides CrN and Cr2N canbe avoided by nitriding at temperatures below 470 °C leading toa shallow (10–30 μm) nitrogen super-saturated diffusion layerof high hardness (approx. 1100 HV) [23].

The structure of such a layer produced on the steel 314L afterplasma-nitriding at 400 °C is shown in Fig. 12 [23]. Thisprocess is used in different areas of food processing equipment,chemical industry, nuclear power plants, etc.

12. Solution nitriding

The low-temperature processes of plasma-carburising andplasma-nitriding have the disadvantage that the thickness of thediffusion layers produced are extremely shallow, in economicaltimes reaching not much above 20 μm.

A new developed process is able to overcome thisdeficiency and to produce hardened layers on stainless steelswith thicknesses of up to 1 and even 2 mm without any loss ofcorrosion resistance.

This process uses the capability of stainless steels to dissolvenitrogen at temperatures above 1000 °C to a large extent

Fig. 12. Microstructure of the steel 314L after plasma-nitriding at 400 °C [23].

Fig. 13. Typical Fe2B-layer produced by plasma-boronizing in an argon–hydrogen–trimethylborate gas mixture at 1000 °C for 16 h at 1 mbar [28].

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without formation of chromium nitrides. This process wasdeveloped theoretically and in the laboratory by Professor H.Berns [24]. The industrialization of the solution nitridingtechnology SolNit® was done in a joint co-operation betweenProfessor Berns, Ipsen International and Härterei Gerster AG,Switzerland [25,26].

13. Boronizing

In boronizing, boron atoms diffuse into the surface of steelsand form compound layers of FeB and Fe2B of appreciablethickness (50–200 μm).

The industrial utilization of boronizing is mainly donewith boron-containing pastes or granules [27] with limitedimportance because of the labour-intensive procedure of pasteapplication and removal.

Gas boronizing and plasma boronizing promised to bemuch cleaner and more economical processes. Their in-dustrial utilization, however, is almost non-existent due tothe poisonous character of the gaseous donor media, likediborane, borontrichloride or boron trifluoride.

Fig. 14. Scheme of a flexible

Because of this, a development project was started a fewyears ago at the Institut für Werkstofftechnologie Bremen(IWT) to develop a novel boronizing process using harmlessboron-containing precursors. Successful results were achievedusing trimethylborate B(OCH3)3 and exciting it with a plasma.Fig. 13 shows a typical Fe2B-layer produced by plasma-boronizing in an argon–hydrogen–trimethylborate gas mixtureat 1000 °C for 16 h at 1 mbar [28].

More lately, the also harmless precursor triethylboran B(C2H5)3 proved to be an excellent boron source as well [29].Thus, it can be expected that these plasma-boronizing processesusing precursors will soon develop to fully fledged industrialprocesses.

14. Innovative equipment for thermo-chemical diffusionprocesses

In the past, thermo-chemical diffusion processes werecarried out in batch furnaces (pit, bell, chamber) or steppedrespectively continuous furnaces with limited process andquenching flexibility. In the last few years, an innovative cellconcept of furnaces integrating atmosphere, vacuum (lowpressure) and plasma processes into one heat treatment lineand leaving each load the choice for quenching in oil, water,polymer or gases, was developed [30]. Fig. 14 shows such amultiple cell system called mult-i-cell®, where a type of shuttlesystem called VacMobil® (a travelling vacuum furnace initself) transfers the load from cell to cell, until a whole heattreatment sequence like, e.g., pre-heating, austenitizing,quenching, tempering, nitriding (gas or plasma) and cooling, isfinished.

At the end of the heat treatment sequence, the load haspassed through a plurality of furnace cells (six in the examplementioned above) without ever having been in contact with airand without any necessity of the subsequent load to passthrough the same cells or same sequence. Thus, an ultimateflexible heat treatment installation is now available for high-quality industrial manufacturing of metallic components.

open mult-i-cell® system.

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15. Conclusions

The examples mentioned above of carburising, carbo-nitriding, ferritic nitrocarburising and boronizing represent onlya limited amount of the development work on thermo-chemicaldiffusion processes of the last few years. They prove, however,that thermo-chemical diffusion processes are clearly on theadvance. Only due to their increased capabilities, the deve-lopment of higher stressed motor engine components, carsuspension parts, drive shafts and gear components is madepossible, frequently in conjunction with a respective wear-resistant or low-friction surface coating produced in pvd-installations [31].

References

[1] W. Göhring, C.H. Luiten, Heat Treat. Met. 4 (1980) 79.[2] B. Edenhofer, Heat Treat. Met. 3 (1995) 55.[3] A. Nayak, Experiences in Using the Ipsen Supercarb® Process for Case-

Hardening of Automobile Components in a Pusher Furnace. Ipsen ONTOP, vol. 6, 2003, p. 8.

[4] C.F. Luiten, F. Limque, F. Bless, Carburising in Vacuum Furnaces, HeatTreatment (1979 May 22–24) Birmingham, UK.

[5] M. Lohrmann, W.M. Gräfen, D. Herring, J. Greene, Heat Treat. Met. 2(2002) 39.

[6] W. Gräfen, B. Edenhofer, Heat Treat. Met. 4 (1999) 79.[7] B. Edenhofer, Advancement in Case-Hardening Technology for Automo-

tive Components, MTEC-IFHTSE Conference, Bangkok/Thailand: Janu-ary 27–29, 2003.

[8] B. Edenhofer, HTM 28 (1973) 165 (in German).[9] W. Gräfen, Low-pressure carbonitriding using acetylene and ammonia—a

novel diffision process for case-hardening. HTM (in press).

[10] E. Meinhard, TZ Met.bearb. 76 (10) (1982) 23 (in German).[11] B. Edenhofer, H. Handel, HTM 57 (5) (2002) 357 (in German).[12] E. Lehrer, Z. Elektrochem. 36 (6) (1930) 383 (in German).[13] M. Lohrmann, Heat Treat. Met. 3 (2001) 53.[14] C.H. Luiten, Z. Wirtsch. Fertig. 68 (9) (1973) 482 (in German).[15] J. Wünning, HTM 29 (1) (1974) 42 (in German).[16] J. Kunze, Nitrogen and Carbon in Iron and Steel Thermo-Dymanics,

Akademie-Verlag, Berlin, 1990, p. 90.[17] J. Wünning, Z. Wirtsch. Fertig. 72 (3) (1977) 152 (in German).[18] H. Kotzott, Use of Hydrocarbons in Nitrocarburising to Achieve an ε-rich

Compound Layer. Ipsen Customer Meeting, Düsseldorf, May 4, 2000.[19] Z. Rogalski, et al., Application of Oxynitriding to a New Grade of High-

Speed Steel Designed for Thermochemical Treatment. Heat Treatment '76.Stratford-upon-Avon/UK, May 6–7, The Metals Society, London, 1976.

[20] W. Lerche, B. Edenhofer, HTM 57 (4) (2002) 240 (in German).[21] H.-J. Spies, F. Vogt, HTM 52 (6) (1997) 342 (in German).[22] D. Günther, et al., HTM 56 (2) (2001) 74 (in German).[23] J.P. Lebrun, L. Poirier, Solutions to Improve the Surface Hardness of

Stainless Steels Without Loss of Corrosion Resistance. ATS Congress,Paris la Vilette (2002 June).

[24] H. Berns, S. Siebert, HTM 49 (2) (1994) 123 (in German).[25] H. Berns, R.L. Juse, J.W. Bouwman, B. Edenhofer, Heat Treat. Met. 2

(2000) 39.[26] R. Zaugg, The Use of the SolNit®-Process (Solution Nitriding) for

Stainless Steels in Vacuum Furnaces with High-Pressure Gas Quenching.Ipsen ON TOP, 7(1), 2004, p. 15.

[27] W. Fichtl, HTM 33 (1) (1978) 13 (in German).[28] A. Küper, H.-R. Stock, P. Mayr, HTM 56 (2001) 95 (in German).[29] N. Kaghaev, H.-R. Stock, P. Mayr, HTM 58 (2003) 243 (in German).[30] W. Gräfen, F. Bless, T. Kreuzaler, Heat Treat. Met. 3 (2003) 68.[31] W.-D.Münz, Integration of Hard PVD-Coatingwith Thermo-Chemical and

Plasma-assisted Pre-treatment Processes. Ipsen ON TOP, 6(2), 2003, p. 12.