Jan SENATORSKI , Jan TACIKOWSKI , Paweł MĄCZYŃSKI *

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1 TRIBOLOGIA 3/2017 p Jan SENATORSKI *, Jan TACIKOWSKI *, Paweł MĄCZYŃSKI * Wear Resistance Characteristics of Thermo- -Chemically Treated Structural Steels Charakterystyki odporności na zużycie przez tarcie obrobionych cieplno- chemicznie stali konstrukcyjnych Key words: precipitation hardening, wear resistance, nitriding, carburizing, steel. Abstract Słowa kluczowe: Streszczenie The article presents results of investigations of wear resistance by friction, employing the 3 cylinder cone method, of selected structural steels subjected to given thermo-chemical treatment, i.e. nitriding, carburizing, and precipitation hardening after nitriding. The investigated steels were C45, 21NiCrMo2, 18HGT, and 41Cr4. These materials, after thermo-chemical treatment undergo metallurgical characteristics of diffusion layers formed on steel. It was observed that proportionality exists between their wear resistance and the value of surface unit loading. Moreover, the friction wear properties of these layers exhibited certain differences, depending on their microstructure and chemistry. utwardzanie wydzieleniowe, odporność na zużycie, azotowanie, nawęglanie, stal. Artykuł ten przedstawia wyniki badań odporności na zużycie, wykorzystując metodę 3 wałeczki stożek, dotyczące wybranych stali konstrukcyjnych poddanych określonej obróbce cieplno-chemicznej, tj. azotowaniu, nawęglaniu i utwardzaniu wydzieleniowemu po azotowaniu. Badanymi stalami były: C45, 21NiCrMo2, 18HGT i 41Cr4. Stale te po obróbce cieplno-chemicznej podlegają ocenie metaloznawczej warstw dyfuzyjnych wytwarzanych na stali. Zauważa się ich proporcjonalną zależność pomiędzy odpornością na zużycie a wartością nacisku jednostkowego powierzchni. Jednakże własności tarciowo-zużyciowe wyraźnie różnią się, zależąc od ich mikrostruktury i budowy chemicznej. Introduction Of all the thermo-chemical processes, the ones which developed in a most dynamic manner are those run in controlled atmospheres, primarily including carburizing and nitriding [L. 1, 2]. Several research projects carried out in Poland [L. 3] and abroad [L. 4] unequivocally demonstrated the absolute necessity of the evaluation of the service properties of steels surface hardened by different technological methods. The most significant role in these evaluations is that of tribological testing [L. 5]. In the realm of surface hardening, the various techniques of forming diffusion layers constitute one of the most effective ways of obtaining a surface layer that reduce the effects of wear due to friction [L. 6]. The technique of evaluation of sliding friction nodes in laboratory conditions is relatively well-developed, and literature describes several machines and equipment used in determining tribological characteristics of such nodes [L. 7]. However, many of these developed methods were found to be insufficient from the point of view of value and ranges of wear parameters, as well as sensitivity, to evaluate diffusion layers obtained by means of thermo-chemical treatment. These thermoschemical processes constitute an effective means of exerting an active influence on the sliding friction node enabling enhancement of its service life [L. 8]. One of the most interesting methods of comparing tribological properties of materials and of the technological method used is the 3 cylinder-cone test [L. 9, 10]. Investigations of tribological properties of surface hardened materials constituting friction nodes are the subject of the current report and such investigations were conducted by the said method. Moreover, the * Institute of Precision Mechanics, ul. Duchnicka 3, Warszawa, Poland.

2 172 TRIBOLOGIA 3/2017 objects of the investigations are thermo-chemically treated structural steels, including those carburized and nitrided [L. 11]. The last variation consisted of treatment in a nitriding process modified by quenching and tempering [L. 12]. Carburized layers were formed in processes conducted in controlled atmospheres, enabling the obtaining of layers with a determined surface concentration of C [L. 13]. Steels designated for wear resistance tests were subjected to ferritic or austenitic nitriding in order to vary their microstructure prior to strengthening. Such strengthening was attained by austenitization of the nitrided steel, followed by rapid cooling and aging at temperatures of the order of 100 C [L. 14, 16]. Experimental Steel grades selected and types of diffusion layers For the thermo-chemical processes enabling the formation of diffusion layers to be investigated, certain structural steels were selected, especially those suitable for the obtaining of such layers. Those steel grades, as well as the types of layers obtained, are put together in Table 1, while Table 2 lists conditions of layer formation and their heat treatment. The carburizing process was carried out in a controlled atmosphere allowing the obtaining of a diffusion layer with a predicted surface concentration of carbon. Table 1. Materials investigated and types of thermo-chemical treatment Tabela 1. Badane materiały i rodzaje obróbek cieplno-chemicznych Lp. Steel grade (current designation)/new designation/ Type of thermo-chemical treatment 1 (21NiCrMo2)/8620/ Carburizing 2 (41Cr4)/40H/ Nitriding 3 (- ) / 18HGT/ Nitriding 4 (C45) / 1045/ Nitriding 5 (C45) / 1045/ Precipitation hardening after nitriding Table 2. Conditions of formation of diffusion layers and heat treatment Tabela 2. Warunki wytwarzania warstw dyfuzyjnych i obróbki cieplnej No. Type of thermochemical treatment Steel grade Temperature T [ C] 1 Carburizing 21NiCrMo Process parameters Time Medium τ [h] Controlled atmosphere with carbon potential 0.85% 2 Austenitic nitriding (A A ) Controlled C45 atmosphere based 3 Ferritic nitriding (A f ) on NH 3 4 Nitriding 18HGT Controlled atmosphere based on 5 Nitriding 41Cr NH 3 N Precipitation hardening after nitriding Nitriding (N) Precipitation hardening after nitriding Austenitizing (A) Precipitation hardening after nitriding Aging (S) C Controlled atmosphere based on NH 3 N 2 Water quenching after austenitization Comments Quenching from carburizing temperature, tempering at 180 C for 2h Prior hardening 860C tempering 600 C, 3h

3 TRIBOLOGIA 3/ The process of the nitriding of medium carbon or low alloy steels yields a thin layer of nitrides of approx μm and a hardness of the order of HV0.1. The substrate to this layer is constituted by a zone of a solution of nitrogen in α iron with a hardness of HV, while the hardness of the core does not exceed 250 HV. Strengthening of the substrate of nitride layers, as well as of the core, can be achieved by austenitization of the nitrided steel, i.e. by rapid cooling and aging at low temperatures of the order of 100 C [L. 12]. This kind of treatment causes an atrophy of the nitride zone due to the diffusion of nitrogen in the direction of the core and the formation in its place of mixed microstructures consisting of nitro-carbon martensite, additionally strengthened by aging. Characteristics of layers The layers formed for testing, whose characteristics are listed in Table 3, met requirements of thickness and hardness, usually recommended in industrial practice. Table 3. Characteristics of diffusion layers Tabela 3. Charakterystyka warstw dyfuzyjnych No. Type of thermo-chemical treatment Steel grade Process parameters T [ C] / τ [h] Layer thickness [mm] Surface hardness HV1 1 Carburizing 21NiCrMo2 930/ Austenitic (A A ) 630/ Nitriding C45 3 Ferritic (A f ) 580/ HGT 570/ Nitriding 5 41Cr4 530/ Precipitation Nitriding (N) 630/ hardening after Austenitizing (A) 740/0.5 C45 (1045) 8 nitriding Aging (S) 100/1 839 The carburized layer on 21NiCrMo2 grade steel, with martensitic microstructure, although being the thickest, did not, at the same time, exhibit the highest surface hardness. In this respect, it was exceeded by nitrided layers formed on low alloy steel, as well as on 1045 grade steel precipitation hardened, preceded by nitriding. Such a combination of nitriding of carbon steel with subsequent austenitising, followed by rapid quenching, enables the hardening of the substrate to the nitride layer as well as of the core [L. 12]. Metallurgical analysis and wear resistance testing of layers Investigations relating to metallurgical characteristics of layers were focussed on problems of the effect of the following: The microstructure and thickness of layers, The atmosphere employed in the process, and Process parameters applied in the thermo-chemical treatments. In the evaluation of effects of the processes carried out, the following methods were applied: optical and electron microscopy, as well as x-ray microanalysis. Investigations related to tribological characteristics comprised wear tests, carried out on samples of the 3 cylinder-cone systems [L. 9, 15]. These wear tests were run maintaining approximately fixed unit pressures at a selected level. The counter-specimen was, in all cases, C45 (1045) grade steel, quenched and tempered to approx. 30 HRC, from which the conical counterspecimens were made. The overall time of the test was 100 min, while the friction velocity v was 60 rad/s. The depth of the wear scar was measured every 10 min during an interruption of the test. These sliding wear tests were conducted with lubrication by metered Lux -10 oil. Linear wear was characterized by total wear z l [μm], as well as by wear rate I l [μm/m]. Results of tribological testing are shown in Figs. 1 and 2. They constituted a service-oriented criterion of rating of the effects of precipitation hardening after nitriding. Versions of this hardening, which were diversified from the point of view of microstructure and thickness of the original nitrided layer and differing by austenitising parameters, were selected, based on metallurgical evaluations. Results of testing of tribological properties of carbon steel, which were nitrided and precipitation hardened after nitriding, were compared with those obtained after carburizing, as shown in Fig. 3.

4 174 T R I B O L O G I A 3/2017 Fig. 1. A comparison of linear wear of precipitation hardened C45 (1045) grade steel, and the effect of the initial nitrided layer. Ferritic nitriding (Af) and austenitic nitriding (AA). Symbols used on plots: thickness of nitrided layer gmp/gbr/gc. Labelling (Ic): 1 50 MPa; MPa Rys. 1. Porównanie zużycia liniowego stali C45 utwardzanej wydzieleniowo. Wpływ pierwotnej warstwy azotowanej. Azotowanie ferrytyczne (Af) i austenityczne (AA). Zapis na wykresach grubość warstwy azotowanej gmp/gbr/gc. Oznaczenia (Ic): 1 50 MPa; MPa

5 T R I B O L O G I A 3/ Fig. 2. A comparison of linear wear of C45(1045) grade steel, nitrided (A) and precipitation hardened after nitriding (A + H + S). Tests were conducted at the surface and at a depth of 0.02mm. Labelling: 1 50 MPa; MPa Rys. 2. Porównanie zużycia liniowego stali C45 azotowanej (A) i utwardzanej wydzieleniowo po azotowaniu (A + H + S). Badania na powierzchni i w odległości 0,02 mm od powierzchni. Oznaczenia: 1 50 MPa; MPa

6 176 T R I B O L O G I A 3/2017 Fig. 3. A comparison of linear wear of carburized 8620, nitrided 18HGT and 40H steels, and precipitation hardened C45 (1045) steel. Labelling: 1 50 MPa; MPa, MPa Rys. 3. Porównanie zużycia liniowego nawęglanej stali 8620, azotowanych stali 18HGT i 40H oraz utwardzanej wydzieleniowo stali 45. Oznaczenia: 1 50 MPa; MPa, MPa The investigations carried out showed that, in a process comprising controlled nitriding, followed by austenitization, rapid quenching, and aging, it is possible to produce an alloy (Fe-M)-C-N in the region of the former nitrided layer and to harden it. A significant role in shaping of the microstructure of the subsurface zone is played by the original nitrided layer. It constitutes storage of nitrogen, which diffuses in the direction of the steel s core, and it determines the level and distribution of nitrogen concentration.

7 TRIBOLOGIA 3/ Basic hardening effects usually occur below the original former nitride layer. Besides the initially formed nitrided layer, a significant factor affecting hardening is constituted by the parameters of the austenitising process. The investigations carried out here showed that the biggest hardening effect with respect to carbon steel can be achieved by austenitising at a temperature of the order of 740 C. Carbon steels, initially nitrided in a controlled process, exhibit, after precipitation hardening, very good resistance to friction wear, clearly superior to that after nitriding only, and that is true not only of the surface but also of deeper-lying zones. The best initial microstructures were found to be nitride layers with a thickness of 1 20 µm, obtained in a controlled ferritic nitriding process, or thin nitride layers (< 10 μm) containing braunite, or braunite layers, obtained in a controlled austenitic nitriding process. Conclusions 1. Precipitation hardening of low carbon steel, following its nitriding, enhanced its resistance to friction wear. 2. By appropriate design of the nitrided layer through the nitriding process, it is possible, with subsequent hardening, to achieve lower total linear wear, as well as reduced wear rate in comparison with steel that has been nitrided only. 3. Thanks to precipitation hardening of prior nitrided steel, the zone of good wear resistance reaches several times deeper than after nitriding only. 4. Tribological properties of medium carbon steel precipitation hardened after nitriding are more beneficial than those of nitrided only low alloy steels, as well as those of carburized steels. References 1. Burakowski T., Senatorski J., Tacikowski J.: Badania odporności na zużycie azotowanych, nawęglanych i chromowanych warstw dyfuzyjnych. (Investigations of wear resistance of nitrided, carburized and chromized diffusion layers) Postępy Technologii Maszyn i Urządzeń. Vol. 3 4/90, p Burakowski T., Senatorski J., Tacikowski J.: Comparison of sliding wear resistance of carburized and nitrided layers on 18HGT steel. Surface Engineering, t. 3, no. 3, 1987, p Senatorski J.: Instrumentalna analiza zużycia tribologicznego materiałów utwardzonych powierzchniowo. (Instrumental analysis of tribological wear of surface hardened materials). Zagadnienia Eksploatacji Maszyn. Z 4 (96), 1993, p Habig K.M.: Reibung und Verschleiss von gehärten, nitrierten und borierten Stahl Gleitparungen in Luft und Vakuum. Zeitschrift für Metallunde 1984, no. 8, p Senatorski J.: Einfluss verchiedener Diffusionsschiechlen auf das verschleissverhalten von Eisenwerkostoffen. Ifl Mitteilungen, no. 6, 1986, p Senatorski J., Tacikowski J., Kasprzycka E., Bogdański B.: Appication of Nitriding to improve tribological properties of constructional steel. Engineering & Automation Problems, No 4, 2013, p Piekoszewski W., Szczerek M., Wiśniewski M.: Metody oceny właściwości eksploatacyjnych materiałów węzłów tarcia. (Methods of evaluation of service properties of materials for friction nodes) ZEM z. 1, 1990, p Burakowski T., Senatorski J., Tacikowski J.: Aktualny stan i perspektywy przemysłowego stosowania warstw dyfuzyjnych o wysokich własnościach tribologicznych. (Current state and perspectives of industial application of diffusion layers with superior tribological properties) Metaloznawstwo i Obróbka Cieplna, no , 1982, p PN-83/H Próba tarcia w układzie: 3 wałeczki stożek. (Wear test by the cone-3 cylinder system). 10. Senatorski J., Tacikowski J., Mączyński P.: Analiza odporności na zużycie przez tarcie warstw dyfuzyjnych w świetle próby 3 wałeczki stożek. (An analysis of wear resistance of diffusion layers, as shown by the 3 cylinder-cone test). Inżynieria Powierzchni, no. 4, 2015, p Senatorski J., Tacikowski J., Mączyński P.: Badania porównawcze odporności na zużycie przez tarcie azotowanych i nawęglanych stali konstrukcyjnych. (Comparative investigations of wear resistance of nitrided and carburized structural steels). Tribologia, no. 3, 2015, p Tacikowski J.: Opracowanie procesu utwardzania wydzieleniowego stopów (Fe, Mn) C N. (Development of precipitation hardening process of (Fe, Mn) C N) alloys. Problemy Eksploatacji, no. 6, 1995, p Burakowski T., Senatorski J., Tacikowski J.: Vergleich des Verschleiβwiderstandes nach Einsatzhärten und Nitrieren. HTM, 1985, nr 40, s

8 178 TRIBOLOGIA 3/ Senatorski J., Tacikowski J., Trojanowski J.: Ocena wpływu utwardzania cieplnego warstwy azotowej na właściwości tribologiczne stali węglowych. (An evaluation of the influence of thermal hardening of a nitrided layer on the tribological properties of carbon steels). Tribologia, no. 3, 1994, p Senatorski J., Tacikowski J., Liliental W.: Tribological properties of Nitrided layers, as determined by cone three-cylinder wear test. Tribology Transactions v. 41, 1998, 2, p Hanisch T., Diem A., Lingenhole K., Surberg C.H.: Influence of different steels on the tribological behavior of nitrocarburized layers. Proceedings of the 2nd European Conference on Tribology Ecotrib 2009 Pisa, Italy , vol. 2, p