Combination Electron Beam Hardening and Nitriding

Abstract. Combined heat treatments, known as duplex or hybrid technologies too, allow newstructure/property relations of layer matrix compounds. The sequence of treatment is of basic importance. There are within reach both, properties and property gradients, which are impossible, using the respective single treatment processes alone and also not by a simple addition of effects of the single processes.

A good technical progress is given in case of combination thermochemical treatment with

high energy beam surface hardening, in particular electron beam hardening. After a historical survey (milestones) about combination of thermochemical with thermal (surface) heat treatment technologies, the paper deals with the principles of the combined electron beam hardening after nitriding and - vice versa - electron beam hardening before nitriding and there effects on microstructure and properties. Typical examples for industrial application are discussed.

In this field of heat treatment the further development is focused to combinations of high energy beam hardening (electron or laser beam) with hard coating.

Electron Meets Nitrogen

Combination Electron Beam Hardening and Nitriding

Extract from a publication in honour of Prof. Dr.-Ing. habil. Heinz-Joachim Spies to his 75th birthday from Prof. Dr.-Ing. habil. Rolf Zenker

1. Milestones of Combination Thermochemical with Thermal (Surface) Treatment

The interest for combination of thermochemical with thermal heat treatment was waked up by a publication of the year 1983 [1]. In this paper a general survey of different possibilities of combining thermal, thermochemical and thermomechanical heat treatment was given.

One year later first paper about a duplex treatment, the nitriding (N) of laser (LB) treated low alloyed steel was published by Bell and Bloyce [2]. The first paper dealing with the vice versa process laser treatment after nitrocarburizing (NC) was published by Zenker and Zenker [3].

In 1986 was given out a basic scientific work [4] in which was shown that the physical and chemical processes in thermochemical (nitrocarburized) layers are nearly independent of the kind of energy transfer (laser, resistance heating) [5].

More detailed investigations were carried out and new combinations [6] came in the focus of scientific interests, e.g. nitriding (nitrocarburizing) with induction hardening [7, 8].

Results of the combination nitriding (nitrocarburizing) with EBH were published for the first time in 1992 [9].

The basic investigations in the field of combination nitriding (N) with electron beam hardening (EBH) were forced [10-12] and so the first industrial application in the tool industry, the combined EBH+N of extrusion screws, could be offered in 1995 [13].

At the same time successful results of promising combination of boriding with EB hardening (B+EBH) were published [10, 12].

The permanently growing basic knowledge and the continuously developed technological know how in the complex processes combined N+EBH or EBH+N in 1998 the first industrial application in the mechanical engineering (EBH of nitrocarburized rolling elements) and in 2006 the first mass production of an automotive power train component (special camshaft) were started [14, 15].

The materials scientific basic knowledge and the technical technological know how is available now. So the door for a wide field of application for an advantageous industrial use of combined thermochemical and EB surface treatment is opened up.

2. Introduction

Hard surface layers often cannot show advantages of their good hardness, strength and wear resistance, because of their bordered thickness, especially when they are deposited on relatively soft materials so that an additional thermal treatment of the base material before or after thermochemical treatment is necessary.

Surface treatment technologies with high energy beams like electron beam (EB) or laser beam (LB) offer a good and modern alternative to the mostly used bulk heat treatment technologies for producing effective supporting layers beneath the hard surface layer.

The energy deposition is - especially in case of EB - precisely programmable. So it is possible, to limit the heat treatment (energy transfer) exactly only to the highly loaded areas and up to the depth, where a transformation hardening is necessary. Therefore, the bulk materials are not heated up to critical temperatures as a result of the heat treatment. The thermal load of the overall component is minimized and thus distortion can also be reduced or avoided.

A combination of these modern thermal surface hardening processes with a thermochemical treatment (carburizing, nitriding, nitrocarburizing, boriding) brings into account very technical advantageous effects in relation to the loading conditions.

The general aim of combined surface treatment technologies is to improve the resistance against complex tribological loads [1, 4, 5 13, 18-20], among others:

- sliding wear in connection with high pressure intensity (typical for forming and cutting tools, as well as high loaded automotive components)

- sliding wear and contact fatigue (typical for components, e. g. bearings, gear wheels, cams)

In certain cases of combined surface treatment novel effects arise which are not attainable by any single process [1, 18-21]. Therefore, great importance will be attached to these combined, also called duplex or hybrid processes [2, 5, 19-22].

Not only the matrix materials, the kind of thermochemical treatment and thermal surface treatment as well as the process parameters of the single treatments are of importance, but also the sequence of the processes, e. g. whether the thermal surface hardening process is carried out before or after the thermochemical treatment. The sequence of the single treatment processes of thermochemical and thermal surface treatment is determined by:

- processing temperature of thermochemical treatment

- surface hardening temperature

- properties of layer (thermal stability)

- properties of bulk material (tempering stability).

The following presented results are related exclusive to the combination nitriding and EB hardening.

3. Principles and Applications

3.1 Combination of Nitriding and EB Hardening (N+EBH)

As well is known, wear resistance is improved by nitriding significantly, but only up to a physically and economically limited depth. There exist different contributions of compound and diffusion layer to the property changes.

The principle of the combination N+EBH is to produce a specific complex property gradient in addition to good surface properties (hardness, wear and corrosion resistance). In result of the transformation of the kinetic energy of electrons to thermal energy during the interaction with metallic materials, the compound layer is partially or completely transformed in dependence on energy input , so that the thickness of the seam of pores grows.

Because EB processes are carried out in a vacuum chamber and because the EB processing cycles are very short (0.1 to some s) the nitrogen diffusion is limited. Mostly a complete transformation is not desired so that an exact dosage of energy input must be realized.

The heat is transported by heat conduction to the interior of the material. Because of the high cooling rates (up to 104 Ks-1) the microstructure is martensitically transformed by self quenching. Because of the enrichment of the diffusion layer with nitrogen after N+EBH the hardness is higher than after nitriding and also after EB hardening.

So by additional local EB hardening of nitrided steels an increase in the local load potential results also due to the "depth effect" of the energy beam, e.g. the martensitic transformation up to a required depth (<1.0...(1.5) mm).

In case of unalloyed and low alloyed steels with a relatively low tempering stability and the need for both a large depth effect and a wear resistant nitriding layer, the combined N+EBH technology is preferred. With regard to the metallurgical compatibility, EBH is only useful as a subsequent treatment. The surface compound layer, which is decisive for the tribological properties, should not be influenced or specifically transformed in order to enhance the run-in behaviour.

The application of N+EBH technology is known for components, especially in the automotive industry and in mechanical engineering (e.g. cams, shafts, bolts). [18, 24].

Likewise the applications in series production for combined N+EBH are used for components subjected to complex local load conditions.

3.2 Combination of EB Hardening and Nitriding (EBH + N)

The combined EBH + N technology is available for a wide scale of industrial applications. For extrusion tools, as well as for cold forming dies, these combined thermal/thermochemical surface technologies are introduced in industrial production.

In every case, the parts are locally high loaded (abrasive/sliding wear in connection with high local compression and partially with corrosion also). Besides the good layer properties, a further advantage of this surface treatment combination is the generally small distortion of the tools and components as a result of the only local interacting EB (energy transfer field) during the complex thermal processing. Because the nitriding is carried out as the second step, the compound layer is not thermal influenced by a following thermal process and therefore not transformed, so that the good stability of the compound layer against corrosion is maintained also. The hard EBH layer gives an additional load support for the

compound layer.

For this combination the steels must be characterized by a good tempering stability, so that the following nitriding at temperatures up to 450 °C does not change the properties of the EB hardened layers.

4. Summary and Outlook

The combination of surface technologies nitriding and EB hardening (prior or subsequent to the thermochemical process) allows new structure/property relations of the layer matrix compounds. Properties and property profiles are attainable, which are impossible to realize with the single surface treatment processes alone or a simple addition of effects of the single processes.

Both combined technologies EBH+N and N+EBH are introduced for several industrial applications. Some general examples were discussed in the presented paper. Scientific knowledge and technological know how for further industrial application is available in a wide

scale.

The development in the field of combined surface treatment was continued and expended on new variants, especially hard coating (HC) and EBH or LBH [16-18, 22, 25-27]. The main effect of these treatment combinations is in particular the supporting effect of the EB hardening layer to the very hard but thin and mostly brittle surface layer. The application fields of these technologies are directed to local high loaded components and tools.

5. Literature

[1] Zenker, R.: Kombinierte thermochemisch-thermische Wärmebehandlung. In: 1. Wärmebehandlungstagung, 11.-13.05.1983, Karl-Marx-Stadt, Neue Hütte 28(1983), 10, pp. 379-385

[2] Bell, T.; Bloyce, A.: Nitriding laser treated titanium bearing low alloyed steel. In: Heat Treatment 1984, London, 02.-04.04.1984

[3] Zenker, R.; Zenker, U.: Kombination Karbonitrieren/Laserstrahlhärten. Eine neue Variante der Randschichtwärmebehandlung. In: Neue Hütte 31(1986), 11, pp. 407-413

[4] Zenker, R.: Beitrag zur Entwicklung neuer Wärmebehandlungstechnologien in Verbindung mit hohen Erwärmungsgeschwindigkeiten. TU Bergakademie Freiberg, Habilitation, 1986

[5] Zenker, R.: Kombinierte thermochemische/Hochgeschwindigkeitswärmebehandlung. Einige Grundlagen und Behandlungsergebnisse. In: 2. Wärmebehandlungstagung: 18.04.1984, Karl-Marx-Stadt, Neue Hütte 31(1986), 1, pp. 1-6

[6] Zenker, R.; Zenker, U.: Laser beam hardening of a nitrocarburized steel containing 0.5% C and 1% Cr. In: Surface Engineering 5(1989), 1, pp. 45-54

[7] Keßler, O.; Hoffmann, F.; Mayr, P.: Systematik der Kombinationsverfahren. HTM 52(1997), 3, pp. 150-155

[8] Keßler, O.: Kombinationsverfahren zur Randschichtbehandlung von Stählen -Stoffeigenschaftsändernde und Beschichtungsverfahren. Aachen: Shaker Verlag, 2003. zgl. Habilitationsschrift, Universität Bremen, 2003

[9] Spies, H.-J.; Pompe, W.; Zenker, R.: Innovationen auf dem Gebiet der Randschichttechnik. In: Ingenieur-Werkstoffe 4(1992), pp. 42-46

[10] Spies, H.-J.; Zenker, R; Bernhard, K.: Duplex-Randschichtbehandlung von metallischen Werkstoffen mit Elektronenstrahltechnologien. In: HTM 53(1998), 4, pp. 222-227

[11] Zenker, R.; Buschbeck, H.: Kombination einer thermochemischen Behandlung mit einer Elektronenstrahl-Randschichtbehandlung. In: Materialforschung - neue Werkstoffe, Symposium des BMFT, Würzburg: 2.-4.11.1994, 1994, pp. 634-642

[12] Spies, H.-J.; Zenker, R.; Bernhard, K.: Obrabotka Powierschniowa materialow metalowych metoda duplex z. wykorzystaniem wiazki elektronowej. In: Iwzynieria Powierzchni (2000), 4, pp. 32-39

[13] Zenker, R., Spies H.-J.: 15 Jahre industrielle Anwendung der Elektronenstrahl-Randschichtbehandlung, 57. Härterei-Kolloquium Wiesbaden 2001

[14] Zenker, R.: Structure and properties as a result of electron beam surface treatment, Advanced Engineering Materials 6(2004)7,pp 581-588

[15] Zenker, R.: Modern thermal electron beam processes - research results and industrial application. In: Metallurgia Italiana 101(2009), 4, pp. 55-62

[16] Zenker, R.; Spies, H.-J., Buchwalder, A.; Sacher, G.: Combination of thermal surface treatment by high energy beams with thermochemical treatment and hard protective coating - State of the art. In: 15th IFHTSE Congress, Wien, 26.-28.09.2006

[17] Zenker, R.; Spies, H.J.; Buchwalder, A.; Sacher, G.: Combination of high energy beam processing with thermochemical treatment and hard protective coating: state of the art. In: International Heat Treatment & Surface Engineering 4(2007), 12, pp. 152-155

[18] Sacher, G.; Zenker, R.; Spies, H.-J.: Duplex treatment of tools and components: previous or subsequent electron beam hardening of thermochemically-treated and PVD hard-coated steels for tools and components. In: Materials and Manufacturing Processes 24(2009), pp. 800-805

[19] Bell, T.: Surface Engineering Past, Present, Future. In: Surface Engineering 6(1990), 1, pp. 1-40

[20] Matthews, A.; Leyland, A.: Hybrid technologies in surface engineering. In: Surface and Coatings Technology 71(1995), pp. 88-92

[21] Spies, H.-J.: Erhöhung des Verschleißschutzes von Eisenwerkstoffen durch die Duplex-Randschichttechnik. Stahl und Eisen 117(1997), 6, pp. 45-52

[22] Zenker, R.; Sacher, G.; Buchwalder, A.; Liebich, J.; Reiter, A.; Häßler, R.: Hybrid technology hard coating - electron beam surface hardening. In: Surface and Coatings Technology 202(2007), pp. 804-808

[23] Wurms, R.: How much valve train variability really makes sense. In: Variable Ventilsteuerung, Essen: 13.-14.02.2007

[24] Zenker, R. Elektronenstrahl-Randschichtbehandlung - Innovative Technologien für höchste industrielle Ansprüche. Munich: Pro-beam AG & Co. KgaA, 2003.

[25] Zenker, R.: Electron beam surface technologies - new developments and state of the application on an industrial scale. In: "NETSU SHORI" (Journal of the Japan Society for Heat Treatment) as Proceedings of the 17th IFHTSE Congress, Kobe, 27.- 30.10.2008 (in press)

[26] Sacher, G., Zenker, R., Frenkler, N., Kimme, T.: Kombinierte Randschichtwärmebehandlung - PVD-Hartstoffbeschichtung in Verbindung mit dem Elektronenstrahl- oder Laserstrahlhärten, HTM 64(2009)1, pp. 20-27

[27] Sacher, G., Buchwalder, A., Zenker, R.: Durcissement combinant un revetement dur et un dursissement par trempe après chauffage par faisceau d electrons ou faisceau laser. In: Traitement Thermique 392(2009)1, pp. 25-28

Thanks:

The author thanks all friends, colleagues and collaborators for the cooperation and support of the interesting and successful research and development work in the field of combined heat treatment about more than 25 years.

In case of further questions please contact:

Zenker Consult, Mittweida and TU Bergakademie Freiberg, Institute of Materials Engineering, Germany