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Home , Cast iron, Iron nitride, Nitriding

Metallographic preparation of nitrided and nitrocarburised components Application

Nitriding is a thermochemical process by Notes which the surface of a ferrous is en- riched with to improve the wear re- sistance of components. In nitrocarburising not only nitrogen but also small amounts of are involved in the process. The result is a nitrided layer consisting of the compound layer (“white layer”) and the diffusion zone immediately below the com- pound layer.

The and nitrocarburising process using gas or salt bath was developed in the early 20th century in Germany and the United States. The development of ion or plasma nitriding started in the 1930s but Nitrided , colour etched with Beraha’s reagent was not commercially used until the 1970s. All three nitriding methods have advantages dimensions and do not need costly finish- and the selection of a particular method ing work such as grinding or straightening depends on the specific application of the after nitriding. nitrided component. Nitriding and nitrocarburising are mainly Nitriding produces a hard, wear resistant used for ferrous components such as layer on carbon and low and , and rods in the cast . In addition, it considerably im- mechanical and automotive proves fatigue strength and by oxidizing industries. Other applications are cutting the nitrided surface, it enhances tools or large forming dies. parts, resistance. such as pump and houses, can also be nitrided. The main advantage of nitriding and nitro- Metallography of nitrided and nitrocar- carburising over other means of surface burised components is mainly used for is the low process temperature controlling the nitriding process through (500-600°C), preferably 580°C. Compo- examination of the layer. The white layer, nents can often be nitrided in the fully hard- diffusion zone and porous zone are meas- ened and tempered condition without the ured and evaluated. In addition, failure core properties being adversely affected. An analyses of used parts are carried out to additional advantage of the low temperature see if faulty material, surface deterioration process is the low risk of distortion. Conse- or the nitriding process were contributing quently the parts can be machined to final factors.

Difficulties in the preparation of nitrided parts: - Chipping and cracking in the layer - Edge-rounding Solution: - Proper mounting - Plane grinding with SiC paper - Fine grinding with diamond - Diamond polishing on hard cloths

Fig.1: Shrinkage gap between specimen 500x Fig. 2: Without edge retention, the layer is not in focus and mounting resin can cause flaking at high magnification of the nitrided layer and trap abrasives Formation and Nitriding processes composition of the and application of nitrided layer nitrided parts

At nitriding temperature the nitrogen dif- Fig. 4: Nitrocarburised after heating for 45 Before nitriding, the components have min/300°C, etched with 1% Nital showing need- fuses into the steel surface and reacts with les in the diffusion zone. to be thoroughly cleaned and degreased. iron, forming γ’ iron nitride (Fe4N), contain- Any surface contamination from grinding ing up to 6 wt % N. With increasing nitro- particles, oil or metal chips will result in gen the ε-phase (Fe3N) is formed, which an uneven formation of the nitrided layer. can absorb up to 11 wt % N. This can cause cracks in the coating which These two iron nitride phases, ε+γ’, form leads to flaking and corrosion (see Figs. 6 the compound layer, also called “white and 7) After cleaning, the parts are dried layer”, because it stays white when the steel and preheated and then transferred to the is etched with Nital. This compound layer actual nitriding environment. does not contain any metal but consists of The various nitriding processes can be dif- a non-metallic phase formed by iron and ferentiated mainly by their nitrogen source nitrogen that can be called a “nitride ce- and the energy supply. Salt bath-, gas- and ramic”. In the outer areas of the compound plasma nitriding have different advan- layer a porous zone can be found (see Fig. 5: Alloyed steel, nitrocarburised, etched with 1% tages regarding investment cost, process Nital, shows dark diffusion zone, and white compound Fig. 3). layer with dark oxide coat. time, environment, safety and quality. The properties of the resulting nitrided or The percentage of γ’ and ε-nitride depends the steel, such as , , nitrocarburised surface are in many cases on the carbon content of the steel: higher and tungsten. independent of the production process. carbon content promotes the formation of The required case depth is determined by ε, lower carbon content forms more γ’ iron Because of their sub-microscopically fine the application of the nitrided component nitride. distribution the in the diffusion and can be regulated through the nitriding zone of low carbon steels generally can not With an optical microscope the differen- temperature and time. be seen after etching of the metallographic tiation of ε and γ’ iron nitrides in the com- sample. However, after heating the sample pound layer is only possible by using very In the following paragraphs the different to temperature (200-400° C for special and difficult etching methods. nitriding processes are briefly described 15-30 min), the nitrogen in solid solution A correct analysis of the composition can and the application of the nitrided parts precipitates in the form of γ’ nitride needles. only be made by quantitative structural mentioned. These nitride needles can be etched so that x-ray analysis using deeply penetrating the diffusion zone becomes visible and its radiation. thickness can be measured (Fig. 4). The compound layer is relatively hard and On alloy steels the diffusion zone will be the increases with increasing con- etched dark with Nital, but the nitrides can tent of nitride forming alloying elements, not be resolved with an optical microscope at the same time the case depth decreases. (Fig. 5). Nitrided carbon steels have a surface hard- The thickness of the white layer and the dif- ness of 300-400 HV and alloyed steels from fusion zone depends on various parameters 700 HV to more than 1000 HV. of which the most important ones are time, Below the compound zone is the diffusion temperature and steel composition. The zone containing nitrogen in solid solution. white layer can be between 0-20 µm and In addition, it has stable metal nitrides the diffusion zone up to 0.8 mm, depending

formed by the various alloying elements of on the requirements of the application. =18 µm { Porous zone Compound layer ε-Nitride { γ ‘-Nitride

1000:1 Fig. 3: Details of composition of Enlarged • = Fe nitrided layer ° = Possible N-Positions Fig. 6 and 7: Impurities in the steel and on the surface can lead to faulty areas in the nitride layer and cause cracking or corrosion. Nitrocarburised cases are particularly resistant against abrasive wear, scuffing, sliding friction and corrosion. The porous surface can retain lubricants which adds to the running properties of, for instance, camshafts. (Fig. 8)

Salt bath nitrocarburising is a fast, flexible and economical process. Typical applica- tions are parts for the such as piston rods, crank and camshafts, Fig. 8: Salt bath nitrocarburised alloyed steel (16MnCr5), valves and . In addition, nitrocarbu- Fig.10: Gas nitrocarburised carbon steel, 580°C/1.5 hr. etched with 1% Nital, diffusion zone is etched dark, rised components are used in the aircraft compound layer with porous zone shows white. and off-shore industry and in mechanical engineering. bration and torsion. Components with bore Salt bath nitrocarburising holes, undercuts and cavities, for instance Salt bath nitrocarburizing is carried out in Gas nitriding and nitrocarburising gener- sintered steels, are suited for gas nitriding gas or electrically heated crucible furnaces. ally takes place in a sealed, bell-type nitrid- because the gas can easily enter every- The preferred material for the crucible is ing furnace which provides good gas cir- where as it circulates in the retort. . After preheating to 350°C, the culation. The process is mainly controlled plating or special resistant materials can be components are submerged into the salt- by the degree of dissociation of . used for masking areas that should not be bath, either hanging or lying in charging The ammonia gas reacts at 500-520°C with nitrided. racks, or as bulk material in the steel surface and decomposes, thereby Gas nitrided parts are typically machine or Inconel baskets. The salt bath consists releasing nascent nitrogen which diffuses spindles, pump housings, door of alkaline cyanate and alkaline carbonate. into the steel surface. As gas nitriding uses lock mechanisms, pump compo- Through oxidation and thermal reaction a lower temperature, process times are 40- nents and for gas compressors. with the immersed component surface, at 80 hrs. By adding gases containing carbon, nitriding temperature the alkaline cyanate gas nitrocarburising is also possible (Figs. Plasma nitriding and nitrocarburising is releases nitrogen and carbon which diffuse 9 and 10). As a consequence process carried out in a nitrogen - hydrogen at- into the surface of the component. Pure times are reduced. mosphere at 400-600°C and a pressure of nitriding is not possible with the salt bath approx. 50-500 Pa. For nitrocarburising, as small amounts of carbon will always The formation and properties of the com- gases containing carbon, such as meth- diffuse into the surface. The usual process pound layer and diffusion zone are similar ane, are added. The plasma is produced parameters are 90 min at 580°C. to those produced by salt bath nitriding. in a chamber with a high voltage However, the thickness of the compound whereby the work-piece acts as cathode The active nitrogen releasing agent of the zone can be more accurately controlled and the vacuum vessel as anode. salt bath is the alkaline cyanate. Through or even completely suppressed with gas Because nitrogen and hydrogen are the reaction of the cyanate ions the amount nitriding. This makes the steel surface very brought into the vacuum chamber as indi- of alkaline carbonate in the bath increases. hard but not brittle. Deep diffusion zones vidual gases, the ratio of nitrogen to hydro- By adding an organic polymer the optimal can be produced that have a beneficial gen can be controlled allowing variations cyanate content of the bath is replenished effect on mechanical strain caused by vi- of thickness and composition of the com- again. pound layer. Consequently, not only low carbon steels, but also austenitic steels, After nitrocarburising, in an powder metallurgy ferrous and oxidizing salt bath (380-420°C) produces refractory metals can be plasma nitrided. a black iron oxide (Fe3O4) on the surface. It fills the pores of the compound layer and acts as additional corrosion resistant pro- tection. After cooling to room temperature the components can be polished and then re-oxidised depending on the application.

Fig. 9: Alloyed steel (42CrMo4), gas nitrided 510°C/36 hrs, etched with 1% Nital, showing dark diffusion zone, white compound layer with porous zone. Difficulties during metallographic preparation of nitrided steels

The main problem during the preparation of nitrided components is the chipping of the porous zone and cracking of the compound zone (see Figs.1, 2 and 13). This damage is mostly introduced during the first grinding step. Incorrect mounting and long polishing with soft cloths results in rounded edges. As the evaluation of the coating is carried out with 1000x magni- fication, rounded edges lead to incorrect thickness measurements. Fig.11: Alloyed steel (16MnCr5N), plasma nitrocarburi- Fig.13: Cracks in the coating caused during preparation. sed, 570°C/ 6 hrs Recommendations for the preparation of nitrided steels forced resin is recommended (IsoFast, DuroFast). In addition, wrapping the sec- To avoid damage to the nitride layer, it is tioned specimen before mounting with a recommended that sectioning is carried thin, pure copper foil (0.05 mm) promotes out carefully on a cut-off machine with good edge retention.* water cooling, suitable for metallographic Also, the copper colour enhances the con- purposes. For cutting an aluminium oxide trast of the coating against the mounting wheel is selected according to the hardness resin, which is particularly helpful when of the nitrided component. This is usually a *The foil is stretched like a ribbon with the blade of a medium hard to soft cut-off wheel. pair of scissors, attached to the sample with instant glue, wrapped around and attached again. Then the excess foil is bent over the surface and pressed down tightly with Fig.12: Carbon steel, plasma nitrocarburised, 570°C/ Mounting: To avoid shrinkage gaps hot fingertip or nail. The copper foil on the sample surface is 6 hrs. Both nitride layers are without porous zone and compression mounting with a fibre rein- removed by the subsequent plane grinding. have a very fine surface finish.

Plasma nitriding gives a very fine surface finish, almost free of pores, that lends itself for a high polish (fig.11 and 12). Due to this low porosity plasma nitrided and nitrocarburised cam- and crank shafts are for instance used for high performance motors. As plasma nitriding allows a large variety of nitride layers, its fields of ap- plication are also varied. Examples are the surface treatment of forming dies such as large plastic dies and auto body blanking dies, tools for stainless steel Fig.14: (16MnCr5N) salt bath nitrocarbu- Fig.15: Same as Fig.14, mounted with copper foil, dark deep drawing, -, hot - and rised, mounted without copper foil. There is flaking oxide layer is clearly visible against the copper colour of extrusion dies. Other specific applications of the nitride layer and the thin oxide layer can not be foil. The layer is well preserved. distinguished from mounting resin. are corrosion resistant engine valves, high speed steel cutting tools and many applica- tions in mechanical engineering.

Fig.16: High alloy steel (X45CrSi9V), salt bath nitrocar- Fig.17: Same as Fig.16, mounted with copper foil, the burised, oxidised, etched with 1% Nital, diffusion zone compound layer can clearly be seen against the copper is etched dark, compound layer can not be distingu- foil and can be measured. ished from mounting resin. Grinding Table 1: Step PG FG Preparation method for nitrided steels

Surface SiC-paper 220# MD-Largo

DiaPro Suspension Allegro/Largo

Lubricant Water

working with oxidised components (com- rpm 300 150 pare Figs. 14 and 15, 16 and 17).

Grinding and polishing: Plane grinding is Force [N] 150 150 carried out with paper 180# and/or 200#. To ensure edge retention, it is Time As needed 5 min. important to fine grind with diamond on a fine grinding disc (MD-Largo). This is fol- lowed by a diamond polish on a synthetic Polishing silk cloth (MD-Dac), and a brief final polish with 1 µm diamond or colloidal silica. Step DP 1 DP 2 OP* The following method has given good, reproducible results for nitrided steels. The preparation data are for 6 mounted sam- Surface MD-Dac MD-Nap MD-Chem ples, 30 mm diameter, clamped in a holder. Depending on sample size and type of Suspension DiaPro Dac DiaPro Nap B OP-AA or OP-U coating the polishing times can be slightly modified. rpm 150 150 150 Etching: The nitrided coating is first ex- amined unetched to evaluate the porous Force [N] 180 150 90 zone and the shape and size of the pores. Etching with 1-3% Nital shows the white Time 6 min. 1 min. 1 min. compound layer and in alloyed steels the dark diffusion zone. For a good nitride case a very good base material is essential. *Optional step As an alternative to DiaPro polycrystalline diamond suspension P, Cracks, inclusions, banding or deformation 9 µm, 3 µm and 1 µm can be used together with blue lubricant. of the initial surface decisively influence the quality of the nitrided layer. Therefore it is also important to evaluate the structure of Summary The main problems when preparing nitri- the base material. Nitriding is a thermo chemical process ded layers are rounded edges and cracking In low carbon steels the diffusion zone by which the surface of a ferrous metal is or chipping of the layer during grinding can be identified by heating the sample at enriched with nitrogen to improve the wear and polishing. By carefully wrapping the 300°C / 45 min, and then etching with 1% and corrosion resistance of components. sample in a thin copper foil before hot Nital (see Fig. 4) Three nitriding methods can be used: salt compression mounting, this problem can bath nitrocarburising, nitriding and nitro- be overcome. For fine grinding silicon car- carburising in gas or plasma. Nitriding and bide paper or diamond on a fine grinding nitrocarburising result in the formation of disc can be used, whereby the latter results a very hard surface consisting of the com- in a better edge retention. The subsequent pound layer and the diffusion zone. Metal- diamond polishing needs to be carried out lography of nitrided layers is mainly used as long as needed to remove any damage for quality control, controlling the nitriding in the layer introduced by the preparation. process and for failure analysis.

Ferritic nodular cast iron, gas nitrided, 500x etched with 3% nital. Struers A/S Pederstrupvej 84 DK-2750 Ballerup, Denmark Phone +45 44 600 800 Fax +45 44 600 801 [email protected]

Authors USA and CANADA DEUTSCHLAND Struers Inc. Struers GmbH Elisabeth Weidmann, Anne Guesnier, 24766 Detroit Road Karl-Arnold-Strasse 13 B Struers A/S, Copenhagen, Denmark Westlake, OH 44145-1598 D- 47877 Willich Phone +1 440 871 0071 Telefon +49(02154) 486-0 Acknowledgement Fax +1 440 871 8188 Telefax +49(02154) 486-222 [email protected] [email protected] We wish to thank Durferrit GmbH, Mannheim, Germany, for the permission to reproduce photographs and SWEDEN ÖSTERREICH drawings. Struers A/S Struers GmbH We wish to thank Dr. Ulrich Baudis for his professional Smältvägen 1 Zweigniederlassung Österreich P.O. Box 11085 Ginzkeyplatz 10 support. We want to especially thank Ms. Sabine SE-161 11 Bromma A-5020 Salzburg Riesbeck for the sample preparation and generously Telefon +46 (0)8 447 53 90 Telefon +43 662 625 711 supplying the colour micrograph on page 1 and Figs. Telefax +46 (0)8 447 53 99 Telefax +43 662 625 711 78 2-17. [email protected] [email protected] We wish to thank Eltro GbmH, Baesweiler, Germany, for FRANCE SCHWEIZ supplying plasma nitrided sample material. Struers S.A.S. Struers GmbH We wish to thank Carl Gommann KG, Remscheid, 370, rue du Marché Rollay Zweigniederlassung Schweiz F- 94507 Champigny Weissenbrunnenstrasse 41 Germany, for supplying gas nitrided sample material. sur Marne Cedex CH-8903 Birmensdorf Téléphone +33 1 5509 1430 Telefon +41 44 777 63 07 Bibliography Télécopie +33 1 5509 1449 Telefax +41 44 777 63 09 [email protected] [email protected] Practical nitriding and ferritic mitrocarburizing, David Pye, ASM International, 2003 NEDERLAND/BELGIE CZECH REPUBLIC ASM Handbook, Vol. 9, Metallography and Struers GmbH Nederland Struers GmbH Mircostructure, ASM, 2004 Electraweg 5 Organizační složka NL-3144 CB Maassluis Milady Horákové 110/96 Wärmebehandlung von Eisenwerkstoffen, Nitrieren Tel. +31 (0) 10 599 72 09 CZ-160 00 Praha 6 – Bubeneč und Nitrocarburieren, D. Liedtke u.a., expert verlag, Fax +31 (0) 10 599 72 01 Tel: +420 233 312 625 Renningen, 2006 [email protected] Fax: +420 233 312 640 [email protected] Technologie der Salzschmelzen, Ulrich Baudis und BELGIQUE (Wallonie) Michael Kreutz, Die Bibliothek der Technik, Band 224, Struers S.A.S. POLAND verlag moderne industrie, 2001 370, rue du Marché Rollay Struers Sp. z.o.o. F- 94507 Champigny Oddział w Polsce sur Marne Cedex ul. Lirowa 27 Téléphone +33 1 5509 1430 PL-02-387 Warszawa Télécopie +33 1 5509 1449 Tel. +48 22 824 52 80 [email protected] Fax +48 22 882 06 43 [email protected] ­UNITED KINGDOM Struers Ltd. HUNGARY Erskine Ferry Road, Struers GmbH Old Kilpatrick Magyarországi fióktelep Glasgow, G60 5EU Puskás Tivadar u. 4 Phone +44 1389 877 222 H-2040 Budaörs Fax +44 1389 877 600 Phone +36 (23) 428-742 [email protected] Fax +36 (23) 428-741 [email protected] JAPAN Marumoto Struers K.K. SINGAPORE Takara 3rd Building Struers A/S 18-6, Higashi Ueno 1-chome 10 Eunos Road 8, Taito-ku, Tokyo 110-0015, #12-06 North Lobby Phone +81 3 5688 2914 Singapore Post Centre Fax +81 3 5688 2927 Singapore 408600 White layer with porous zone on 500x [email protected] Phone +65 6299 2268 low carbon steel Fax +65 6299 2661 [email protected] CHINA Struers (Shanghai) Ltd. Room 2705, Nanzheng Bldg. 580 Nanjing Road (W) CN - Shanghai 200041 Phone +86 (21) 5228 8811 Fax +86 (21) 5228 8821 [email protected]

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