ROLLING CONTACT FATIGUE WEAR OF NITRIDING AUSTEMPERED DUCTILE IRON (ADI)-ADI DISCS Carlos Rafael Figueroa Hernández Francisco Urbano Ordoñes Doctor Professors – Dep. de Tecnología. Facultad de Ingeniería Mecánica. Instituto Superior Politécnico José Antonio Echeverría. Calle 127 s/n. Apartado 6028. Habana 6. Marianao. Ciudad de La Habana. Cuba João Telésforo Nóbrega de Medeiros –
[email protected] Doctoral Student of Surface Phenomena Laboratory – PME-EPUSP – S. Paulo-SP – Brazil Professor of Mechanical Eng. Department - UFRN – Natal – RN - Brazil Adelci Menezes de Oliveira –
[email protected] Doctoral Student of Surface Phenomena Laboratory – PME-EPUSP – S. Paulo-SP – Brazil Engineer of PETROBRAS – Rio de Janeiro - RJ Amilton Sinatora –
[email protected] Deniol Katsuki Tanaka –
[email protected] Associate Professors of Mechanical Engineering Department. Polytechnic School of São Paulo University Av. Professor Mello Moraes 2231, 05508-900. São Paulo - Brazil. Abstract. This paper presents a study on the behavior of nitrided austempered ductile iron (ADI) under contact fatigue tests. ADI was austenitized at 900 0C for 2 hours and austempered at 380 oC for 2 hours. Later, the ADI was nitrided at 570oC for a time of 6 hours to provide white layers fro m 5 to 6 µm deep. The contact fatigue tests were carried out using a disc test machine until a defect was detected in the signal from a vibration sensor. Hertzian pressures of 1.73, 1.78, 2.04, 2.46, 2.61 and 2.70 were used during the tests. The phase comp osition of nitride layer was determined using X-ray analysis with Cu K α radiation, which detected the presence of the εand γ′ phases. The pitting, spalls and cracks that appeared on the surface were observed in a scanning electron microscope (SEM).The results indicated that the white nitrided layer on ADI disappeared during the contact fatigue tests, keeping the transition zone inalterable. Also it was confirmed the decrease of the endurance limit of nitrided ADI for the lowest Hertzian pressures. Moreover it was detected that the microstructure of nitrided ADI shows a strong influence in the formation of the surface damages. Keywords: Contact fatigue – Nitrided layer – ADI – Wear – Thin films
1. Introduction Nowadays, the use of austempered ductile iron (ADI) in mechanical components for the automotive industry is becoming increasingly important, since this alloy has some advantages with respect to steel, such as: improved machinability, lower density with comparable strength, lower sensibility to surface defects, noise reduction due to the higher damping capacity imparted by the presence of graphite, lower material costs with improved dimensional stability prior to machining. These remarkable advantages of ADI have allowed its selection for the manufacture of transmission gears (Tartera, 1985; Harding, 1984, Haseeb et alii, 2000). Usually, transmission gears are subjected to high dynamic loads, cyclic loads, surface wear and contact fatigue, which is the main cause of serious surface damages, such as pittings and spalls. These defects are a result of the formation of surface or subsurface initial cracks, which grow under repeated contact loading (Glodez, 1999; Aldfredsson, 2000). Under certain conditions, the cracks become large enough for unstable growth to occur, which results in the formation of leaving void spaces known as pittings or spalls. Some thermochemical treatments to improve the contact fatigue resistance of several alloys have been recommended. Recently (Vatavuk, 1997), the nitriding process on austempered ductile iron has been studied to provide a compromise between the toughness of the bainitic ferrite with stabilized austenite in the structure of the metal matrix and the high wear resistance that this treatment confer. In this case, the nitriding process was used, which results in a white layer composed by the nitrides Fe2-3 N and Fe4 N. The nitriding process of ADI can promote aging to occur, but it has been demonstrated that the hardness of the ductile iron alloyed with copper and molybdenum and austempered at 3800 C remains practically unchanged when aged at 570o C (Ordoñez, 1998). Also, the same author recognized that the phenomenon of precipitation of carbides might occur in the metal matrix, which can reduce the toughness of ADI. The main objective of this work was to study the contact fatigue behavior of nitrided ADI. The contact fatigue test was used to determine the life of components made with this alloy under different Hertzian pressures. Besides, this work studies the main mechanisms that allow the formation of surface damages.
2. Experimental procedure The specimens of ADI were obtained in a charge consisting of ductile iron scrap and steel, which was melted in a medium frequency induction furnace and was alloyed with copper and molybdenum. Later, the liquid metal was inoculated by addition of Fe-0.75%Si. The main characteristics of cast material can be observed in Table 1. The specimens were machined to provided discs with a contact geometry that, during fatigue tests, provided a distribution of Hertzian pressures similar to the contact of two spheres (Rx=Ry =76 mm). An average surface roughness Ra= 0,4 µm (axial direction) and Ra= 0,2 µm surface finish (circumferential direction) was reached at the running track after machining. Table 1. Results of as cast analysis of ADI C Si 3.52 2.94 Brinell Hardness HB 280 ± 6
Mn 0.2
Chemical composition ( % mass ) P S Mg 0.03 0.02 0.048 Microstructure Pearlite > 90%
Mo Cu 0.29 1.42 % of Nodules 7
To obtain an adequate homogenization of the austenitic structure, the disc-specimens were austenitized at 900o C in a furnace with controlled atmosphere for 2 hours. Later, they were quenched in a salt bath at 380o C for 2 hours, to provide a structure of upper bainite with residual austenite, which considerably improves the toughness of ADI. The nitriding process was carried out at 570o C using dissociated (30%) ammonia (NH3 ) for 6 hours. The microstructural characterization of the nitrided layer, as well as the core of the ADI, was conducted using optical and scanning electron microscopy techniques (SEM). The analysis indicated the presence of a white layer with average thickness of 5.5 µm. The microhardness of the surface and subsurface was determined using a load of 50g for 10 seconds. The maximum hardness reached on the surface was 670 HV. The phase composition of the nitrided layer was obtained by means of X-ray diffraction with Cu Kα radiation and angles in the range 2θ = 25 – 120 0 . The phases γ′ and ε were revealed with diffraction peaks with indices hkl (111), (200), (101) and (220). The contact fatigue behavior of austempered ductile iron was determined with a disc-testing machine (PLINT & Partners LTD, TE73HS/HT), whose underlying principle (Figure1) consists in two discs rotating together in loaded edgeto-edge contact.
Figure 1. Functioning of discs test machine (schematic) The contact tests were carried out at 5,000- 5,050 cycles/min under dry conditions, with applied pressures (P0 max) of 1.73, 1.78, 2.04, 2.41, 2.46 and 2.70 GPa. These values were calculated with Equations 1-5 (Boresi, 1993), where: P = Load (N), a = Semi-axes of Hertzian contact ellipse (m), P0 = Hertzian pressure (Pa), Em = Harmonic mean elastic Modulus (Pa), D1,2 = Diameter of discs (m), ν = Poisson coefficient of ADI, E1 =Young’s modulus of nitrided layer (Pa), E2 = Young’s modulus of ADI (Pa).
a = 0.721[P (η η 1 + η 2 ).2.D1 D2 /(D1 + D2 )]1/3
(1)
P0 = 1.5P / π .a2
(2)
η 1 = ( 1-ν ν 1 2 ) / Em
(3)
η 2 = ( 1-ν ν 2 2 ) / Em
(4)
Em = 2 E1 E2 /(E1 + E2 )
(5)
An average elastic modulus of 158 GPa was used for the ADI (Harding, 1984), and a value of 221 GPa was used for the nitrided layer. Loads of 14600, 13190, 11100, 6300, 4200 and 3900 (N) and a Poisson coefficient of 0.275 were used to determine Pmax. Each disc pair was monitored using the vibratory signal response emitted by a sensor, which was calibrated to terminate the test once the vibration indicated those macroscopic defects such as pittings, cracks, spalls had been formed. The analysis of the morphology of the defects formed during the tests was conducted in the cross and longitudinal sections of the central part of the running track of the discs. Scanning electron microscopy techniques with secondary electrons and acceleration voltage of 25 kV were used. The mass loss was used as a criterion of surface wear and was measured using a scale with resolution of 0.01 g. 3. Results and discussion 3. 1. Heat treatment and nitriding process In Fig. 2, a transition zone with thickness on the order of 110 µm appeared in the cross section of the nitrided layer. For this magnification it was impossible to see the white layer, which was characterized by the X-ray diffraction pattern (Figure 3).
Figure 2. Transition zone of nitride layer 200 X
Figure 3. Diffraction patterns of X-ray of the surface of nitride ADI
This analysis reveled the presence of the nitrides Fe2-3 N and Fe4 N. Both phases are formed for a definite case of depth on the order of 6 µm as could be observed in the scanning electron microscope (Figure 4). It could be observed also that this white layer was around the nodules of graphite that appear at the surface and near at the surface. This behavior was probably due to the preferential diffusion that exists at the channel formed between the metal matrix and the wall of graphite nodules. A significant aspect, it was the growth of the grain of ADI in the diffusion zone from a probably recrystallization process of the ferrite phase during the diffusion of nitrogen atoms (Figure 5). Below diffusion zone it can be observed the matrix of ADI after the nitriding process formed by ferrite and carbides as a product of the decomposition of residual austenite and bainite, which were obtained during the austempering treatment.
The results from the microhardness tests (HV0.050) on the cross section of the nitrided layer (Figures 2 , 4, 5, 6) showed a little difference between the diffusion zone and the core of nitrided ADI. This is due to the fact that the hardness of the core increased after the aging during the nitriding process as a result of the coalescence of bainite and the formation of carbide clusters. The main difference was encountered between the white layer and the core. This behavior can be considered normal due to a concentration gradient between the surface and the core of metal (Figures 2, 4, 5, 6). The maximum value of microhardness (670 HV) was found on the surface of the metal as a result of the presence of the nitrides Fe2-3 N and Fe4 N.
Figure 4. White layer and transition zone 1000 X
Figure. 5. Growth of grain and presence of nitrides around the nodules 500X
Figure 6. Microhardness profile of nitrided layer
3. 2. Contact fatigue tests The metalographic analysis of the surface after the contact fatigue tests showed that the nitrided layer disappeared under action of the surface contact stress, probably due to the brittle behavior of these phases. However the diffusion zone was present during the tests. (Lajtin, 1989) considered very important the presence of this zone composed by the mixture γ′ and αn (nitride ferrite) phases, which are tougher than the nitrides and can resist satisfactorily the action of cyclic loads. In Figure 7 it can be observed that the delamination process was present during the tests of contact fatigue. Probably this mechanism is a typical phenomenon of the contact of nitrided ADI against nitride ADI under dry conditions. This behavior in this alloy has not yet been reported by the literature. Suh (1973) considered that this phenomenon is preceded by the formation of a net of microcracks, which probably appeared as a result of dislocation motion under the presence of surface stresses. Medeiros et al. (2000) presented a review about the actuating mechanisms on contact fatigue, but they included only AISI 52100 steel surfaces in that study.
Figure 7. Delamination process on the the surface of ADI. 300 X
Figure 8. Influence of graphite nodules in the formation of microspall. 1000 X
Figure 9. Surface cracks forming angles of 30o 800 X
Figure 10. Cracks parallel to the surface and influence of defects 800X
The platelets of surface metal detached due to the delamination wear process are transformed in debris, which might act as abrasive particles above all in the first stage when the nitride layer is present. Besides the presence of nodules on the surface might have a strong influence in the formation of cracks and pittings, which can act as nucleation sites of these defects (Figure8). In this case, the nodule acts as a void space near the surface, where the part of metals that exist between the nodule and the surface breaks under action of contact pressures. An analysis of the cross and longitudinal sections of the running track showed that cracks (Figure 9) appear near the surface forming angles from 20 to 30o , but some may continue to propagate parallel to the surface, creating a fatigue crack and finally a spalling crater (Figure 10). These spalls may also start as a microcrack at a small distance below the surface. In this micrograph can be observed clearly the nucleation process of cracks from a defect that exist near the surface. These subsurface cracks appear also due to the presence of the maximum shear stress (Lajtin, 1989) in this zone and the influence of the cast defects and graphite nodules. They act as nucleation sites of cracks (Figure 11), probably due to the stress induced in the vicinity by effect of deformation during contact fatigue or by advancing main crack.
Figure 11. Nucleation of cracks induced by the deformed graphite nodules 800 X
Figure 12. Nucleation and crack connected by the presence of nodules 1000 X
This behavior can be compared with a work made by Dommarco (1998), in which it was confirmed that the nodules act as natural defects where cracks easily nucleate. The influence of graphite nodules as nucleation sites is much more significant when they are deformed by the presence of surface stress.
Figure 13. Results of discs tests on nitrided ADI.
Figure 14. The effect of load on wear of nitrided ADI.
Also, it could be observed that the subsurface crack propagation connected graphite nodules (Figure 12). These fracture mechanisms might permits that the graphite raise to the surface and act as a solid lubricant. The contact fatigue behavior of nitrided ADI can be observed in the Figure 13 for several loads. In this diagram, it is possible see that the endurance limit significantly decreased at low loads. This behavior has not yet been reported in the literature. Probably this phenomenon occurred due to the presence of dynamic factors associated with the geometry and balancing of the discs when low loads were acting. Also it is probable that an increasing of the vibration levels as result of a low index of plasticity between the nitrided layers of both specimens activated the vibration sensor and the test machine stopped. The parameters and results of the contact tests are presented in Table 2. Making a comparison between the results of this work and those obtained by Ross, Harding and Cooper (1986) for ADI without nitriding process and lubrication condition, it was demonstrated the positive influence of the nitride layer on ADI and the possible presence of solid lubrication mechanism by the graphite. In this work was obtained a high life for the nitride ADI in comparison with the work made by Ross et al. (1986).
Table 2. Result of discs tests on nitrided austempered ductile iron (ADI) Specimen pair 1 2 3 4 5 6
Load (kN) 3.9 4.2 6.3 11.1 13.19 14.6
P0 max (GPa) 1.73 1.78 2.04 2.41 2.46 2.70
Max shear stress (GPa) 0.58 0.59 0.68 0.82 0.87 0.90
Depth of Max Shear Stress (mm) 0.66 0.68 0.77 0.93 0.99 1.02
Cycles x 106 0.75 3.31 9.87 4.73 9.62 0.58
Loss of mass (grams) 0.14 0.26 0.79 2.09 3.06 2.59
The variation of mass loss as a function of load showed a linear relationship, as observed in Figure 14. A similar situation was found in the evaluation of bainitic steel (Bayer, 1994).
Table 3. Regression analysis for the contact stress (Y) and the mass loss (X) during the fatigue tests of the nitrided ADI Polynomial regression for Figure 13
Linear regression for Figure 14
Y = A + B1*X + B2*X^2 + B3*X^3 + B4*X^4 + + B5*X^5
Y=A+B*X
Parameter Value Error -----------------------------------------------------A 8.38122 0 B1 -13.49438 0 B2 7.28953 0 B3 -1.60934 0 B4 0.15633 0 B5 -0.00554 0 -----------------------------------------------------R-Square (COD) SD N P Value -----------------------------------------------------1 0 6
