EFFECT OF GAS NITRIDING PROCESS ON TRIBOLOGICAL PROPERTIES OF CONSTRUCTIONAL STEEL

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1 T R I B O L O G I A 105 Jan SENATORSKI *, Jan TACIKOWSKI *, Ewa KASPRZYCKA **, BOGDAN BOGDAŃSKI *, Krzysztof CZARNECKI *** EFFECT OF GAS NITRIDING PROCESS ON TRIBOLOGICAL PROPERTIES OF CONSTRUCTIONAL STEEL WPŁYW PROCESU AZOTOWANIA GAZOWEGO NA WŁAŚCIWOŚCI TRIBOLOGICZNE STALI KONSTRUKCYJNYCH Key words: thermal and chemical treatment, nitriding, tribological properties Słowa kluczowe: obróbka cieplno-chemiczna, azotowanie, właściwości tribologiczne Abstract The paper presents the results of examinations of the structure and tribological properties of nitrided layers produced on constructional steels with use of controlled gas nitriding. The nitriding layers structure, hardness, and tribological properties were determined. Tribological properties of the layers * ** Instytut Mechaniki Precyzyjnej w Warszawie, ul. Duchnicka 3, Warszawa, Polska. Politechnika Warszawska, Wydział BMiP w Płocku, Instytut Inżynierii Mechanicznej, ul. Łukasiewicza 17, Płock, Polska. *** Studia doktoranckie, Politechnika Warszawska, Wydział BMiP w Płocku.

2 106 T R I B O L O G I A were performed by means of taper-three roll test. It has been proven that the application of the gas nitriding process to constructional steels significantly improved their resistance to friction wear. INTRODUCTION Gas nitriding of steel through the introduction of nitrogen atoms into the surface layer of the metal by means of the diffusion process is used to improve the fatigue life of steel and its resistance to friction wear and jamming of mating components [L. 1 6]. The nitriding process represents a low-temperature ( ºC) thermal and chemical treatment technology that is widely applied in industry to various products, mainly made of steel, both to single units and to mass or serial production, and its popularity is permanently growing. It is a cost-effective alternative to widespread high temperature technologies ( ºC) thermal and chemical treatment methods, such as carburizing or carbonitriding. The lower temperatures of the gas nitriding process make it possible to reduce deformation and undesired changes of workpiece dimensions as compared to carburizing and carbonitriding. Gas nitriding technology leads to the development of a diffusive layer on the metal surface, where the layer thickness is up to 0.6mm and its hardness reaches as much as 1300 HV, which makes the parts more resistant to wear due to friction, seizure, and fatigue impacts [L ]. The current trends in industrial application of the gas nitriding indicate that that the process itself and its variations are growing in popularity and importance, since they improve the resistance of steel products and machinery components to corrosion [L ]. Moreover, it is a rather inexpensive process with low consumption of thermal power. This study focused on the determination of friction wear resistance demonstrated by the 18HGT grade steel (18ChGT on standard GOST) that used to be surface hardened by gas carburizing as well as the 38HMJ grade steel (~41CrAlMo-7-10 on standard EN) that is typically treated by nitriding. DEVELOPMENT OF PROTECTIVE CASE HARDENED SURFACES Nitrided surfaces were developed on samples made of the 18HGT and 38HMJ grade steels. The technology assumed short-term treatment in the controlled atmosphere of the NH 3 -N 2 type that enables the formation of hardened surfaces with the desired surface concentration of nitrogen. The nitriding processes were carried out at the temperature of 530ºC for 6 hours. To improve the properties of the parent steel, the samples were subjected to the processes of thermal treatment, namely quenching at 860ºC and tempering at 600ºC for 2 hours.

3 T R I B O L O G I A 107 INVESTIGATION METHODOLOGY Nitrided case hardened surfaces were developed on samples with the dimensions of 8 x 35 mm. The mating conical samples with the dimensions 23/12x13 mm were made of the 45 steel grade with thermal treatment up to the hardness of 30HRC. The tests of friction wear were carried out within the arrangement of three cylinders and a cone at the friction speed of 0.58 m/s and a unit holding pressure of 200 MPa according to the PN-83/H standard [L. 14]. The overall duration of friction impact was assumed as 100 minutes, which corresponded to the friction run of 3470 m. The wear was measured after each 10 minutes of the test while simultaneous increasing of the load for subsequent test periods in proportion to the surface worn to maintain the initial unit holding pressure. Tests were stopped when any symptoms of seizure occurred. The investigations were carried out under the terms of combined friction cycles with lubrication by means of oil drops, where a Lux 10 oil lube was used and the oil consumption was 30 oil drops per minute. INVESTIGATION RESULTS Development of a protective case hardened surface Microstructures of nitrided surfaces developed on the 18HGT grade steel as disclosed on metallographic polished sections of samples after preliminary nital etching are shown in Fig. 1. The microstructure of the sample that was not subjected to thermal treatment before the nitriding process is shown in Fig. 1a, while Fig. 1b presents the sample that had been thermally treated prior to nitriding. An area with content of carbonitrides and nitrides + γ` with the thickness of 8 µm is visible on both pictures just next to the case hardened surface. Deeper within the surface one can find an area of solid nitrogen in Fe. A similar structure was observed for samples made of the 38HMJ grade steel. The total thickness of the nitrided surface was assessed based on the hardness distribution across the surface section under the assumption that the surface thickness is considered as the distance between the sample s surface and the location where the hardness is 50 HV1. For the 18HGT grade steel, the thickness was 160 µm with 100 µm for the 38HMJ grade. The surface thickness of the nitrided sample made of 18HGT was 835 HV 0.1 and 1300 HV 0.5 for the 38HMJ grade steel.

4 108 T R I B O L O G I A ` a b Fig. 1. Microstructure of nitrided specimens made of the 18HGT grade steel: a) without toughening before nitriding, b) toughening before nitriding (hardening, 860 C; tempering, 600 C, 2 h). Etching 2%HNO 3. Magn. x 250 Rys. 1. Mikrostruktura azotowanych próbek ze stali 18HGT: a) bez ulepszania cieplnego przed azotowaniem, b) ulepszanie cieplne przed procesem azotowania (hartowanie, 860 C; odpuszczanie, 600 C, 2 h). Traw. 2%HNO 3. Pow. x 250 Tribological properties of case hardened surfaces Tribological properties of a nitride surface developed on the 18HGT grade steel samples are shown on the 3D graph that demonstrates the progress of surface wear as a function of the friction run (Fig. 2). The graph is linked to figures that refer to the wear intensity and distribution of hardness in surface layers and their structure. Similarly, Fig. 3 presents the tribological characteristics of nitrided surfaces as a function of friction and the holding pressure for the nitrided samples made of the 38HMJ grade steel.

5 T R I B O L O G I A 109 Fig. 2. Tribological characteristics of a nitrided surface developed on 18HGT grade steel in relation to the steel structure: a) linear as a function of friction run and distance from the specimen surface, b) wear intensity as a function of distance from the specimen surface, c) distribution of hardness in the surface, d) structure of the case hardened surface Rys. 2. Charakterystyka tribologiczna warstwy azotowanej na stali 18HGT w powiązaniu z jej strukturą: a) zużycie liniowe w funkcji drogi tarcia i odległości od powierzchni, b) intensywność zużycia w funkcji odległości od powierzchni, c) rozkład twardości w warstwie, d) struktura warstwy Fig. 3. Tribological characteristics of nitrided surface applied on the 38HMJ grade steel: Linear wear as a function of friction run and the distance from the specimen surface Rys. 3. Charakterystyka tribologiczna warstwy azotowanej na stali 38HMJ; zużycie liniowe w funkcji drogi tarcia i odległości od powierzchni

6 110 T R I B O L O G I A CONCLUSIONS Nitrided surfaces applied to samples made of the 18HGT and 38HMJ grade steels demonstrated higher overall wear and higher wear intensity within the adjacent surface layers (Fig. 3, 4). However, these layers are initially covered by a layer of carbonitrides and nitrides + γ` with high surface hardness, but under the high unit holding pressures of 200 MPa and 300 MPa, that protective layer is quickly worn during the grinding-in process. Therefore, the determined figures of wear intensity refer to the layer located below the protective carbonitrides and nitrides, and it is the layer that manifests much lower hardness. The lowest linear wear and wear intensity are attributable to the layer with the highest hardness (0.04 mm from the surface). For deeper layers, in pace with a decrease in the layer hardness, the linear wear and wear intensity increases up to accelerated wear at the depth of 0.2 mm as soon as 60 minutes of the test progress. REFERENCE 1. Zyśk J., Kasprzycka E.: Zależność niektórych własności mechanicznych od stężenia węgla w warstwach węgloazotkowych typu ε na stalach niestopowych. Metaloznawstwo i Obróbka Cieplna 1974, nr 7, s Zyśk J., Tacikowski J., Kasprzycka E.: Characteristic features of shorttime nitrided layers formed on steels, [w:] Metallkunde und Wärmebehandlung, Karl-Marx-Stadt 1975, 10, Zyśk J., Tacikowski J., Kasprzycka E.: Podatność wybranych stali stopowych na azotowanie, [w:] Obróbka cieplna i cieplno-chemiczna w atmosferach regulowanych i w próżni, Wyd. IMP, Warszawa 1979, s Zyśk J., Tacikowski J., Kasprzycka E.: Die Nitrierbarkeit ausgewählter legierter Stähle. Härterei-Technische Mitteilungen 1979, nr 6, s Zyśk J., Tacikowski J., Kasprzycka E.: Behaviour of Different Steel Grades to Nitriding, [w:] Progress in The Field of the Heat Treatment Technologies and Equipment. Bucharest, Rumunia Zyśk J., Tacikowski J., Kasprzycka E.: Charakterystyki warstw azotowanych na różnych stalach stopowych. Metaloznawstwo i Obróbka Cieplna 1980, nr 46, s Małdziński L.: Termodynamiczne, kinetyczne i technologiczne aspekty wytwarzania warstwy azotowanej na żelazie i stalach w procesach azotowania gazowego. Wydawnictwo Politechniki Poznańskiej, Seria Rozprawy Nr 373, Poznań Ratajski J.: Wybrane aspekty współczesnego azotowania gazowego pod kątem sterowania procesem. Wyd. Politechnika Koszalińska, Koszalin Małdziński L., Tacikowski J.: Concept of an economical and ecological process of gas nitriding of steel, HTM, 9/2006, 61Band, p Zyśk J.: Rozwój azotowania gazowego stopów żelaza. Wyd. Instytut Mechaniki Precyzyjnej, Seria Monografie IMP, Warszawa 2008.

7 T R I B O L O G I A Michalski J.: Charakterystyki i obliczenia atmosfer do regulowanego azotowania gazowego stali. Wyd. Instytut Mechaniki Precyzyjnej, Seria Monografie IMP, Warszawa Senatorski J.: Empiryczna ocena materiałów na ślizgowe węzły trące. Wyd. IMP, seria Monografie IMP, Warszawa Senatorski J.: Podnoszenie tribologicznych właściwości materiałów przez obróbkę cieplną i powierzchniową. Wyd. IMP, seria Monografie IMP, Warszawa PN-83/H Próba tarcia w układzie: 3 wałeczki stożek, Warszawa Streszczenie W pracy omówiono wyniki badań dotyczących właściwości tribologicznych warstw azotowanych wytwarzanych na stalach konstrukcyjnych 18HGT i 38HMJ w procesie regulowanego azotowania gazowego. Tribologiczne właściwości otrzymanych warstw oceniano z wykorzystaniem testu trzy wałeczki stożek. Badania właściwości tribologicznych próbek ze stali 18HGT i 38HMJ z warstwami azotowanymi wykazały, że zastosowanie azotowania gazowego wpływa w sposób istotny na zwiększenie ich odporności na zużycie przez tarcie.