RESEARCH WORKS REGARDING THE INFLUENCE OF THE NITRIDED LAYER STRUCTURAL PROPERTIES ON THE WEAR BEHAVIOUR OF THE TURBO-BLOWERS PINIONS AND GEAR WHEELS

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1 September 23 RESEARCH WORKS REGARDING THE INFLUENCE OF THE NITRIDED LAYER STRUCTURAL PROPERTIES ON THE WEAR BEHAVIOUR OF THE TURBO-BLOWERS PINIONS AND GEAR WHEELS Adriana Preda 1, Dan Levcovici 1, Sanda Maria Levcovici 2, Carmen Papadatu 2 1 S.C. UZINSIDER Engineering S.A. Galaţi, 2 Dunărea de Jos University of Galaţi, Romania research@uzineng.ro ABSTRACT This paper shows the results of the research works on the nitrided parts subject to heavy stresses. Experiments were performed on specimens of 3 alloy steel grades taken from pinions and gear wheels used at a turbo-blower. The results of the laboratory analyses (spectrometry, metallography, hardness measurements and X-ray diffractometry) showed the interdependence between the nature and sequence of the nitrided layer structural constituents and the chemical composition of the basic material. KEYWORDS: pinion, turbo-blower, nitride layer. 1. INTRODUCTION The metallurgical equipment consists of assemblies and subassamblies of parts subjected to heavy fatigue and wear stresses. Such a case, typical of the turbo-blowers running as speed multipliers gaining on the output shaft a speed of 23. rpm, is that of the gear made up of a gear whell and a double teeth pinion. The parts are subjected to a thermochemical treatment of nitriding in order to with stand to such stresses. This paper aims to analyse the behaviour ot three nitrided parts made upt of different materials, placed out of operation at distinct periods of time. Laboratory analyses as the chemical composition, hardness measurements, metallography and X-ray diffractometry were performed on the specimens sampled from the respective parts in order to indentify the structural constituents and the interdependence between mechanical and structural characteristics. The summarised results showed the interdependence between the nitrided layer structural constituents and the wear behaviour of the analysed parts. 2. THEORETICAL ASPECTS REGARDING THE NITRIDED LAYER STRUCTURAL PROPERTIES The nitriding process is a thermal treatment of enriching with nitrogen the surfaces of the wear and corrosion stressed parts. Diffusion and interaction of the nitrogen with the basic material lead to structural constituents whose nature determines a major hardness of the nitrided layer. These processes condition the formation of two distinct areas: the area of the chemical compositions and the area of the diffusion. The chemical combination area consists of two phases: - the ε phase a solid solution based on the chemical compound of Fe 3 N, rich in nitrogen (8.2 to 11.2%N) having a hexagonal compact (HC), crystal lattice, is highly resistant to wear, [1], [2] and corrosion; - the γ phase a solid solution based on the Fe 4 N compound, presents a centred-face cube (C.F.C.) crystal lattice, has a lower nitrogen solubility (5.7 to 6.1%N), showing very high values of hardness and tenacity. The diffusion area is composed of: - the phase (the nitrided ferite) a nitrogen connate solid solution in Fe crystallizing in centred volume cube (C.V.C.) lattice, showing a maximum solubility of.11% at a temperature of 59 C; - the γ phase (the nitrided austenite) a nitrogen connate solid solution in Feγ (C.F.C.), having a maximum nitrogen solubility of 2.8% at a temperature of 65 C. The transformation of γ - martensite with nitrogen (a suprasatured solution of nitrogen in Fe), showing very high values of hardness, takes place from the nitriding temperature, in case of a rapid cooling of austenite with nitrogen (γ).

2 24-26 September The nature and sequence of the nitrided layer structural constituents, the hardness and the depth of skin, are determined both by the chemical composition of the chosen material and by the technological parameters of the thermochemical treatment [1, 2, 3]. In case of hard worn out parts [4], the skin structure selection has to observe the three layered rule as it is advised by the tribology studies: a. a thin layer of.2 to.4mm, not so rigid, showing a crystal lattice separate to the basic (hexagonal compact) material, avoiding adhesive wear; b. a very tough and flexible layer, not breakable or deformable under very high stresses; c. a basic material showing adequate hardness, mechanical strength as well as a suitable tenacity in order to avoid material cracking under stress. Figure 1 depicts the layer hardness in their sequence. The gear wheel made of this material had worked for the three months in the same working conditions as the 1 part. HV a (ε) b (γ )' c () 3. LABORATORY TESTS The specimens sampled from the parts made up of different materials, 1, cod 2 and 3, were analysed. The results of the spectrochemical hardness and microhardness tests are shown in table 1. The 1 sample is made up of a material recommended for nitriding. The pinion manufactured from this type of material was used in service for over three years, showing high values of hardness both for the nitrided layer as well as for the basic material. The microhardness registers a lower value at the tooth point rather than at the intermediate layer placed on a depth of.2mm, confirming the presence of the ε phase. The 2 sample is executed of a make good recommended material, with no special nitriding performances. The Cr (a conductive element to the nitrided layer) content is low, under 1.%, while the Ni (an element with no influence upon the nitrided layer) content is high, of over 1.%. The hardness of this material is lower than the 1 sample, both is the nitrided area, as well as in the basic material. a b c Layer tickness Fig. 1 Hardness of the worn out skin. The 3 sample is manufactured from a face hardening recommended material. Further to a make good treatment, low carbon ferrite content determines the occurrence of a martensite poor in carbon which changes in a sorbite with a low carbide content, when a high tempering occurs. The structure shown by the basic material has a low mechanical strength, fact that produces a quick fatigue and the cracking of the material due to high cyclical stresses. The pinion manufactured from this type of material had registered the lowest hardness of the basic material, working in service (three weeks only). Microstructures enhaced that the serquence and the composition at the nitrided layer differs form one material to another (figures 2, 3, 4). The white layer (ε) with very good wear resistance continuously shows up on the teeth flank from 1, is missing from 2 and shows up on very small area from 3. Part material/ Standard CrMoV9 DIN 1652/4/9 4CrNi12 STAS 791/88 18MnCr11 STAS 791/88 Table 1 Analysis of chemical composition and hardness. Hardness, Microhardness Chemical composition, [%] HV [dan/mm 2 ] basic flank depth nitrided tooth tooth C Si Mn Cr Mo Ni material.2 [mm] area point basic

3 September 23 Fig. 2 Code 1. Pinion. Crest of thread. ( 2). 2% nital attack. Fig. 3 Code 2. Gear. Crest of thread. ( 2). 2% nital attack. Fig. 4 Code 3. Pinion. Flanck of tooth. ( 2). 2% nital attack. Code 1 Code Fe 3 N (11) 9 8 Fe (11) Fe (11) Fe 4 N (2) Fe 3 N (12) Fe (2) Fe 3 N (11) Fe 4 N (22) Fe 3 N (13) Fe 3 N (21) Fe4 N (311) Fe 3 N (4) Fe (22) Fe 3 N (11) Fe 4 N (2) Fe 3 N (12) Fe (2) Fe 3 N (13) Fe 4 N (311) Fe 3 N (4) Fe (22) Diffraction angle 2 θ [ o ] Fig. 5 Variation of diffraction lines intensity depends Diffraction angle 2 [ o θ ] Fig. 6 Variation of diffraction lines intensity depends

4 24-26 September Fe 3 N (11) Fe (11) Fe 4 N (2) Fe 3 N (12) Fe (2) Code 3 Fe 3 N (11) Fe 4 N (22) Fe 3 N (13) Diffraction angle 2 [ o θ ] Fig. 7 Variation of diffraction lines intensity depends Quantitative ε phase ratio, [%] HV mat. = 415; ε = 51% HV mat. =358; ε = 19% HV mat. = 322; ε = 29% Fe 4 N (311) Fe 3 N (4) Fe (22) Table 2 Structural components analysis Detected phase Crystal lattice Quantitative ratio [%] 1 ε-fe 3 N HC CFC 51 2 CFC 47 ε-fe 3 N HC 19 3 ε-fe 3 N HC 29 CFC 41 Legend: HC compact hexagonal; CFC centered sides cube. The macroscopic aspect of 1 and 3 pinions when wasted is given in figure 8. The running behaviour of the analyzed parts is mainly presented in table 3. 3 years 3 months 3 weeks Running time Fig. 8 Variation graphic of running time depending Detecting the existent phases into the nitride layer made by X ray diffraction of 2. DRON diffractometer, using CoK (figures 5, 6, 7). Structural compounds found into the nitrided layers, their quantitative ratios as well as the crystalline network are given in Table 2. Code 3 Pinion, although with a higher ε phase ratio (29%) as against 2 (19%) was made of a complete unsuitable material (18MnCr11 in order to be cemented), that made the piece cracking in three weeks running only. Fig. 9 Code 1 and 3 pinions when wasted. 4. CONCLUSIONS 1. The running behaviour of gears and pinions working in hard conditions depend on the nitrided layer structural characteristics as well as on the basic material quality. 2. The sequence, nature and quantitative ratio of the structural components have a firm character in the waste behaviour of the nitrided parts. 3. ε phase with HC crystalline lattice into the nitrided layer presence, differed to the basic material (C.V.C.) in large enough quantitative ratio, allows a very good waste resistance. Table 3 Analysis of running behaviour depending on physical and structural characteristics. HV hardness Phase quantitative in nitrided basic Detected phase ratio [%] area material Running time ε-fe 3 N 51 3 years ε-fe 3 N months ε-fe 3 N weeks

5 September Using an unsuitable basic material leads to an increase of the cracking susceptibility of the pinions and gears in running, even in the condition of a large enough ε phase quantitative ratio in the nitrided layer. 5. Choosing a material containing alloying elements with favorable influences upon the compactivity, hardness and thickness of the nitrided layer (CrMoV), allows a well waste behaviour of the turbo-blowers parts. 6. Achieving a proper nitriding treatment that allow making a compact and hard layer where ε phase wear resistant to be in a greater proportion than γ phase hard and ductile, allows rising the wear resistance of the pieces. REFERENCES 1. Cartiş Ioan Gh., Tratamente Termochimice, (Termochemical treatment). Editura Facla, Timişoara, Dulămiţă T., Tratamente termice şi termochimice, (Thermal and termochemical treatment) Editura Didactică şi Pedagogică, Bucureşti, Lakhtin Yu, Aspectul metalelor şi prelucrarea termică a metalelor. (Aspect and thermal treatments of metals) Moscova, * * * Manual Industrial de l usure et de grippage CETIM, Centre Tehnique des Industries Mécaniques, Franţa, 1973.