machine design, Vol.3(2011) No.2, ISSN 1821-1259 pp. 91-94

*Correspondence Author’s Address: IMR Institut, Patrijaha Dimitrija 7, Belgrade, Serbia, imrkb@eunet.rs

Original scientific paper

NITRIDING AS A WAY FOR INCREASING DINAMIC STRENGTH OF GRAY CAST IRON GEARS

Radinko GLIGORIJEVIC1, * - Jeremija JEVTIC2 - Djuro BORAK3

1,2,3 IMR Institut, Belgrade, Serbia

Received (30.03.2011); Revised (17.05.2011); Accepted (27.06.2011) Abstract: Lubricating oil pump gears made of grey cast iron, SL 20 (Grade 14- BS 1452) intended for use in highspeed diesel engine have been parallelly nitrided (550oC, 25 h) in plasma N2/H2/CH4 gaseous mixture and in NH3 gas using conventional procedure. The process of plasma nitriding has been performed so as to enable obtaining of compound layer type e (bonding zone with a mono-phase) which was confirmed by x-ray and microstructure testing. Nitriding in NH3, however, gives compound layer type bonding zone with duplex phase (g` +e) which is less resistant to wear than the mono-phase because it is brittle and porous and often prone to cracking and scaling. In addition, plasma nitriding affects dimensions to a lesser degree and offers better surface quality if compared with conventional nitriding which results in separation of rough graphite clusters - ``pimply``of surface. Keywords: nitriding, gears, cast iron 1. INTRODUCTION For the application of cheaper and lighter of materials with better damping properties for the development of certain engine components, it is necessary to develop technological processes for attaining and satisfactory fatigue strength of such materials. One of cheaper materials with good damping characteristics, which are used to create dynamic, less loaded gear is cast gray iron. Its main drawback is the small dynamic strength. One way of increasing dynamic strength cast iron is nitriding, which is next to the dynamic strength increases the hardness, wear resistance and galling and resistance to corrosion and higher temperatures [1,2,3,4]. Compared to the low carbon steels, the effects of nitriding of grey cast iron are much lower. Specificity nitriding grey cast iron is reflected in the significant structural inhomogeneity of the nitrided layer. The essential differences compared to steel are: 1. surface hardness and depth of the grey cast iron is

lower than that of steel, 2. the temperature increase rate of absorption of nitrogen

in the gas nitriding of grey cast iron is not increased in that regard as with steel, which reflects the fact that the diffusion coefficient of nitrogen in the e` and a - phases of Fe-N equilibrium system, decreases with increasing the carbon content,

3. The dimensional changes of gray iron parts are higher than those of steel, depending on the initial structure, especially the relation of ferrite / pearlite, ways of getting cast as the nitriding temperature.

In gas nitriding of grey cast iron coming up graphite separation to the surface of the sample and the surface becomes rough. Therefore it is necessary subsequently grinding, after which the surface remains small holes-“recesses”. Just these two basic reasons - separation of

graphite and greater dimensional changes were the cause for dispute resolution seeking nitriding gear oil pump diesel engine in the plasma gas mixture N2/H2/CH4 and to compare it with the gears nitrided standard procedure - in the ammonia. 2. BASIC INFORMATION ABOUT THE

PROCESS OF PLASMA NITRIDING Thermochemical process ingestion of carbon and / or nitrogen in the steel/iron surface machine parts and components, is done traditionally, using, solid, liquid and gaseous environment for the transfer of it. Discovery of a plasma environment for the implementation of thermophysical processes, significant effects in the surface layers related to wear, corrosion resistance and dynamic properties.

Fig.1. Characteristic areas of glow discharge in rarefied gas

Radinko Gligorijevic, Jeremija Jevtic, Djuro Borak: Nitriding as a Way for Increasing Dinamic Strength of Gray Cast Iron Gears; Machine Design, Vol.3(2011) No.2, ISSN 1821-1259; pp. 91-94

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Industrial application of plasma can be realized in the form of glow discharge gas which comes to applying the potential difference between two metal electrodes placed in the court with rarefied gas, which is illustrated in fig 1. In order sizes mbar pressures and voltage of a few hundred volts, electrons emitted from the cathode, increase your energy at the expense of the established electric field, reaches a value of energy that enables them to collisions with atoms of present gas and excitating them and / or ionizing, resulting in the appearance of intense light by which this discharge its name glow discharge. The field of gas discharges in which the electron energy only for excitation enough and / or ionization of atoms present gas zone is called the cathode drop and is the main subject of interest for the application of plasma nitriding. Thanks to the present electric field, ions of gas (resulting in the border area of cathode drop) achieve significant energy before they collide with atoms of cathode`s surface. As a result of such collisions in the field of gas discharges to emit electrons (which is precisely to maintain a discharge process), and atoms and ions of the cathode surface constituent atoms. Part of the kinetic energy of ions will be presented to the surface in the form of heat, which allows controlled heating of the cathode to a temperature that is directly dependent on gas pressure, the potential difference and the properties of the cathode. Constituent atoms and / or ions of cathode material in the field of cathode drop, which is characterized by extreme reactivity, come in many chemical reactions, after which there is a depositing rezultantnih aggregation on the very surface of the cathode, where during the process of surface diffusion leads to a number of chemical reactions that result incorporating aggregation have deposited with the cathode surface. Basic information technology, about plasma nitriding, description of physico-chemical processes and description of equipment in which plasma nitriding was performed are given in the papers [5,6,7]. 3. EXPERIMENTAL Oil pumps (rotary) gears are made of gray iron castings, quality SL 20 (Grade 14- BS 1452), hardness HB = 120-128, and ferrite-pearlite structure (> 50% ferrite) Fig.2.

Fig.2. Microstructure of the gears of SL.20

The distribution of graphite shows Fig.3

Fig.3. The shape and distribution of graphite As the gears surface hardness was small a group of gears were nitrided 25 h in NH3 at 530°C, a second group of ones were nitrIted in plasma gas mixture N2/H2/CH4 25 h at 550°C. Before and after nitriding were measured hardness and dimensions of gear and the external (Ds = 90 mm) - 6 seats and the inner diameter (Du = 70 and Du1 = 42 mm) - in six places and thickness (b = 12.91mm), Fig.4.

Fig.4.The appearance of paired gears

4. RESULTS AND DISCUSSION

Dimensional change of the outer diameter Ds- gears's shows fig. 5. (Each column shows the value of the 6 measurements on 2 gear. It is observed that the absolute changes in dimensions much higher when nitriding in NH3 as compared to plasma nitriding. The same apply for the inside diameter (Du and Du1 - fig. 4.) Fig. 6 and fig.7. In Fig. 6. and 7 it is noticed that all the group gear does not behave similarly. Namely, some gear group had a reduction in internal diameter, and in others an increase. The cause of this behavior was not examined, although one of them may be higher residual stresses. Thus we can conclude that the dimensions change gears much higher when nitriding in NH3 than in plasma. It must be said that the process of nitriding in the plasma so driven to get a mono-phase compound layer type ε (Fig.9), which has good resistance to abrasion and corrosion.

Radinko Gligorijevic, Jeremija Jevtic, Djuro Borak: Nitriding as a Way for Increasing Dinamic Strength of Gray Cast Iron Gears; Machine Design, Vol.3(2011) No.2, ISSN 1821-1259; pp. 91-94

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Fig. 5. Change outer gears diameter Ds nitreded in NH3 and plasma

Fig.6. Changes inner gears diameter (Du) nitrided in NH3 and plasma

Fig.7. Changes inner gears diameter (Du1) nitrided in

NH3 and plasma Figure 8. also confirms the superiority of plasma nitriding of a classic - in NH3, with which it is evident that increasing the thickness of gear ("swelling". ") twice as the nitriding in NH3 than in plasma. For this there are several reasons of which the most important mechanism and types of formation phase and surface effects.

Fig.8. Change the thickness of gears nitrided in NH3 and plasma

Radinko Gligorijevic, Jeremija Jevtic, Djuro Borak: Nitriding as a Way for Increasing Dinamic Strength of Gray Cast Iron Gears; Machine Design, Vol.3(2011) No.2, ISSN 1821-1259; pp. 91-94

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As the surface appearance of the plasma nitriding intensified to be in it results in greater depth reinforced zone and surface hardness (Fig. 10, curve 1) of the analogue in classical nitration (NH3), curve 2, Fig. 10.

Fig.9. Microstructure gear tooth surface layer nitrided in plasma gas mixtures N2/H2/CH4 at 550 0 C, 25 h.

Fig.10. Microhardness depth profile of gear nitrided in plasma (curve 1) and ammonia (curve 2)

Fig.11 . Microstructure surface layer of gear teeth nitrided in NH3, at 530 C and 25 h

When nitriding in NH3 formed two-phase compound layer, ie. (g` +e) phase (Fig. 11). This duplex phase is very brittle and often the cause of formation cracks which later

spread in service and lead to fracture. These deficiencies can be avoided in plasma nitriding. Besides the quality of surface nitrided in plasma was significantly better than those of nitrided in NH3. Nitriding in NH3 is coming up to separate grarnulated graphite which gives the surface "pimpled" look [4] and requests its refinement. This phenomenon has not on the plasma nitriding, which is very important for economic reasons introduction of this technology. The reason for the lack of graphite on the surface certainly lies in the fact that every second impinging more than 1015 energ. particles fall on the cm2 (which is a straight monolayer surface target). In interactions with the surface atoms, ions can be accelerated its impulse delivered to, so-called, cascaded collisions, whose the final result, the outbreak of the atoms of the target in vacuum. This so-called sputtering process and is one of the important new features in the plasma nitriding technology. 5. CONCLUSION On the base obtained results it can be concluded: 1. Nitriding is a way for increasing dynamic strength

of grey cast iron. 2. Gas nitriding of gray cast iron coming up graphite

separation to the surface and it become rough. This phenomenon is absent in plasma nitriding.

3. Dimensional change of cast iron gears are much higher when nitrided in NH3 than in plasma.

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1983, 3 [2] LAHTIN, J., KOGAN, D.; Azotirovanie stali,

Mashinostroenie, Moscow 1976 [3] TOSIC, M.; GLIGORIJEVIC, R.; Plasma Surface

Engineering, vol.2, DGM; Informationgeselshaft 1989, p.903

[4] GLIGORIJEVIC, R.; JEVTIC, J.; The importance of material matching to Wear Resistance, Balkantrieb`96, Thesolaniki 1996

[5] GLIGORIJEVIC, R.; TOSIC, M.; TERZIC, I.; Plasma-carbonitriding of Nodular Iron, III JUSTOM, Proceeding II, p.237-247, N.Sad 1983.

[6] TERZIC,I.;TOSIC, M.; Equipement for Plasma-nitriding of steel parts in Plasma Glow Dischage, II JUSTOM, Proceeding II, p.135, N.Sad.1983.

[7] TOSIC, M.; TERZIC, I.; GLIGORIJEVIC, R.; Plasma nitriding of steel in Plasma Glow Dischage - Technology with great posssibility, XXVIII Yugoslav conference for ETAN, 1984.