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Metallurgical analysis of failed gear
Engineering Failure Analysis 9 (2002) 359–365 www.elsevier.com/locate/engfailanal
Metallurgical analysis of failed gear Abhay K. Jha *, V. Diwakar Material Characterisation Division, Materials and Metallurgy Group, Vikram Sarabhai Space Centre, Trivandrum 695 022, India Received 8 January 2001; accepted 3 February 2001
Abstract Bronze is used for the fabrication of special purpose gears in mechanical transmission systems. Recently, fracture of gear teeth occurred when a gear was in use. The fractured gear teeth were subjected to detailed analysis using standard metallurgical techniques. The results revealed that corrosive wear at the root fillet caused pitting, intense localised plastic strain and folds, leading to crack formation. Advancement of the crack took place under the successive stress repetitions to which the gear was subjected, causing the tooth to fail by fatigue. # 2002 Elsevier Science Ltd. All rights reserved. Keywords: Gear-tooth failures; Wear; Corrosion; Fatigue; Casting defects
1. Introduction Gears are used to transmit motion and power. Usually, due to their high strength-to-weight ratio and relatively low cost, steels are used as gear materials. For special purpose gears, other materials are also in use. Bronze is one of the materials used for such gears. Tin bronze was used to fabricate one such gear and subsequently a few gear teeth were found broken after the gear had been in use for some time. A schematic diagram of the gear and location of failure are shown in Fig. 1. The failed gear teeth were subjected to detailed metallurgical analysis using standard metallographic techniques.
2. Observations The failed gear teeth were subjected to detailed chemical analysis and metallographic analysis using the optical (OM) and electron microscopic (SEM) techniques. Chemical analysis revealed that the material contained 12.7% Sn, 0.30% Ni, 0.10% Zn and balance copper. Visual observation revealed that failure in all the failed teeth occurred from the root fillet region. The surface at the fracture surface edge revealed flow of material with formation of folds. The gear load
* Corresponding author. Tel.: +91-0471-563237; fax: +91-0471-415348. E-mail address: [email protected] (A.K. Jha). 1350-6307/02/$ - see front matter # 2002 Elsevier Science Ltd. All rights reserved. PII: S1350-6307(01)00010-3
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contact surface was free of any obvious surface deterioration. The fracture surface was subjected to stereo microscopic observation. A flat surface extending a few hundred microns was seen on the fracture surface and thereafter the granular features were present. While viewing under the scanning electron microscope (SEM), at the root fillet location (the location at which the tooth experiences the maximum bending stress) folds associated with flakes were seen (Fig. 2). The flake morphology is shown in Fig. 3. Fragmentation of flakes was also noticed (Fig. 4a and b). A crack was seen (Fig. 5) very close to a fold line and running
Fig. 1. Schematic of gear tooth and location of failure.
Fig. 2. SEM photomicrograph showing root fillet region with corrosive wear and folds associated with flakes.
Fig. 3. SEM photomicrograph showing flake morphology.
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across the root radius. At the root fillet region and very near to the edge of the fracture surface, signs of corrosive wear patches were seen (Fig. 6a), causing pitting of the contacting tooth surface (Fig. 6b). Clear evidence of mechanical abrasion was also noticed (bottom region. Fig. 7) followed by a flat region of fracture surface. This flat region was in general featureless except for a few corrosive wear patches (Fig. 7) distributed
Fig. 4. (a) and (b) SEM photomicrograph showing fragmentation of flakes.
Fig. 5. SEM photomicrograph showing presence of crack at root fillet region.
Fig. 6. SEM photomicrograph showing (a) corrosive wear at root fillet and pitting, (b) pitting at high magnification.
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uniformly. The remaining area of the fracture surface revealed the presence of shrinkage cavities within which dendritic lobes are clearly seen (Fig. 8a and b). Secondary cracks were also seen at a few locations (Fig. 9). The gear tooth was cut along a plane perpendicular to the load contact surface and polished using standard metallographic techniques. The surface was polished up to a diamond finish and etched with
Fig 7. SEM photomicrograph showing abraison at root fillet region and flat featureless region with few corrosive wear patches.
Fig. 8. SEM fractograph showing (a) shrinkage cavity and (b) dendrite lobes within the cavity.
Fig. 9. SEM fractograph showing presence of secondary crack.
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chromic acid. Microstructures at various locations of the gear tooth are typical of a cast structure as shown in Fig. 10. Fig. 10a revealed the microstructure at the core. Folds at the root fillet were clearly seen associated with plastic flow of material as revealed by deformed grains (Fig. 10b). Secondary cracks were also seen (Fig. 10c and d and e).
Fig. 10. Optical micrographs showing microstructures at different locations, 75.
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3. Discussion The failure has taken place at the root fillet region of the gear. This is the location where the tooth bending stress is maximum. Corrosive wear patches as observed, presumably caused by chemical reaction of lubricants, or of contaminants such as water or acids with the gear tooth surface. These corrosive wear patches result in pitting as evidenced in Fig. 6. Observation of similar corrosive wear on gear teeth associated with pitting has been reported earlier [1]. At the root fillet region, near corrosive wear patches, intense plastic deformation has taken place, which results in flaking of the material. This is a type of mechanical wear under localised plastic deformation associated with the action of combined rolling and sliding [1]. The state of stress produced by rolling and sliding contact is concentrated in a small volume of material and produces intense plastic strain. This strain accumulates locally as the same volume of the material is stressed with each load cycle, until a crack is initiated [2]. The presence of folds, cracks and fragmentation of flakes have revealed the severity of plastic deformation under the applied load. A metallographically polished specimen when viewed under the optical microscope revealed a dendritic microstructure along with clear evidence of folds at the root fillet associated with grain flow under plastic deformation. This supports the idea of fold formation due to accumulated plastic strain under the repetitive applied load. This causes multiple cracks to form at and below the surface, which grow and join as facilitated by the presence of shrinkage cavities. The presence of secondary cracks on the fracture surface supports this, hypothesis. Usually crack advancement due to successive loading leaves the signature of finely spaced parallel marks called fatigue striations on the fracture surface. In the present case, striations are not distinctly visible. One possible explanation is either that the stress level is close to the fatigue limit of the material or the crack growth rate is high (10 4 in per cycle or more). At a high crack growth rate, striations become wavy and develop a rough front. A large plastic zone exists in front of the crack, which causes extensive secondary cracking [3]. Flat region as a result of such large plastic zone has been reported earlier [4]. Indications of abrasion are seen at the root fillet region, indicating the presence of abrasive particles in the lubricant, which may be dirt, sand, impurities in the oil or metal detached from the tooth surface (fragmented flakes). Other factors causing abrasion are load intensity, temperature and rubbing speed. Poor surface finish and low viscosity of the lubricant also cause abrasion. Dendrite lobes as seen within the shrinkage cavities in the fracture surface are typical of ‘‘free surface’’ within the cavities/voids after the available liquid metal solidified. A high pouring temperature during casting results in this phenomenon [5]. Observations made so far are clear indications of corrosive wear, pitting at root fillets, shrinkage cavities and accumulation of intense plastic strain within small volumes of material under repetitive applied load. Accumulation of plastic strain results in subsurface shear and tensile stresses. The intense plastic strain so accumulated in a small volume of material introduces a high order of subsurface shear and tensile stress with the ultimate result of cracking. Progressive advancement of the crack as revealed by the presence of corrosive wear on a flat region of the fracture surface is facilitated by the presence of shrinkage cavities. The presence of a flat region at the beginning of the fracture process and multiple cracks are the result of the advancement of such cracks under a large number of stress repetitions.
4. Conclusion Intense plastic strain accumulated in a small volume of material due to localised rolling and sliding contact, causing folds, flakes and cracking. Fatigue growth caused crack advancement. Further, the presence of corrosion wear, pits and shrinkage cavitites facilitated the cracking process.
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Acknowledgements The authors wish to express their gratitude to Dr. T.S. Lakshamanan, Head, MCD and Dr. K.V. Nagarajan, Group Director, MMG for technical discussions and suggestions at various stages of the analysis. The permission granted by Shri G. Madhavan Nair, Director, VSSC for publishing the paper is also gratefully acknowledged.
References [1] [2] [3] [4] [5]
Metals hand book, vol. 10, 8th ed. American Society for Metals. Metals Park, OH, 1975, p. 514. ASM hand book, vol. 19, Fatigue and Fracture. ASM International, Materials Park, OH 1996, p. 332. Metals hand book, vol. 10, 8th ed. American Society for Metals, Metals Park, Ohio, 1975, p. 98. Metals hand book, vol. 12, 9th ed. 1987, p. 298. Metals hand book, vol. 12. 9th ed. 1987, p. 405.
Metallurgical analysis of failed gear Abhay K. Jha *, V. Diwakar Material Characterisation Division, Materials and Metallurgy Group, Vikram Sarabhai Space Centre, Trivandrum 695 022, India Received 8 January 2001; accepted 3 February 2001
Abstract Bronze is used for the fabrication of special purpose gears in mechanical transmission systems. Recently, fracture of gear teeth occurred when a gear was in use. The fractured gear teeth were subjected to detailed analysis using standard metallurgical techniques. The results revealed that corrosive wear at the root fillet caused pitting, intense localised plastic strain and folds, leading to crack formation. Advancement of the crack took place under the successive stress repetitions to which the gear was subjected, causing the tooth to fail by fatigue. # 2002 Elsevier Science Ltd. All rights reserved. Keywords: Gear-tooth failures; Wear; Corrosion; Fatigue; Casting defects
1. Introduction Gears are used to transmit motion and power. Usually, due to their high strength-to-weight ratio and relatively low cost, steels are used as gear materials. For special purpose gears, other materials are also in use. Bronze is one of the materials used for such gears. Tin bronze was used to fabricate one such gear and subsequently a few gear teeth were found broken after the gear had been in use for some time. A schematic diagram of the gear and location of failure are shown in Fig. 1. The failed gear teeth were subjected to detailed metallurgical analysis using standard metallographic techniques.
2. Observations The failed gear teeth were subjected to detailed chemical analysis and metallographic analysis using the optical (OM) and electron microscopic (SEM) techniques. Chemical analysis revealed that the material contained 12.7% Sn, 0.30% Ni, 0.10% Zn and balance copper. Visual observation revealed that failure in all the failed teeth occurred from the root fillet region. The surface at the fracture surface edge revealed flow of material with formation of folds. The gear load
* Corresponding author. Tel.: +91-0471-563237; fax: +91-0471-415348. E-mail address: [email protected] (A.K. Jha). 1350-6307/02/$ - see front matter # 2002 Elsevier Science Ltd. All rights reserved. PII: S1350-6307(01)00010-3
360
A.K. Jha, V. Diwakar / Engineering Failure Analysis 9 (2002) 359–365
contact surface was free of any obvious surface deterioration. The fracture surface was subjected to stereo microscopic observation. A flat surface extending a few hundred microns was seen on the fracture surface and thereafter the granular features were present. While viewing under the scanning electron microscope (SEM), at the root fillet location (the location at which the tooth experiences the maximum bending stress) folds associated with flakes were seen (Fig. 2). The flake morphology is shown in Fig. 3. Fragmentation of flakes was also noticed (Fig. 4a and b). A crack was seen (Fig. 5) very close to a fold line and running
Fig. 1. Schematic of gear tooth and location of failure.
Fig. 2. SEM photomicrograph showing root fillet region with corrosive wear and folds associated with flakes.
Fig. 3. SEM photomicrograph showing flake morphology.
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across the root radius. At the root fillet region and very near to the edge of the fracture surface, signs of corrosive wear patches were seen (Fig. 6a), causing pitting of the contacting tooth surface (Fig. 6b). Clear evidence of mechanical abrasion was also noticed (bottom region. Fig. 7) followed by a flat region of fracture surface. This flat region was in general featureless except for a few corrosive wear patches (Fig. 7) distributed
Fig. 4. (a) and (b) SEM photomicrograph showing fragmentation of flakes.
Fig. 5. SEM photomicrograph showing presence of crack at root fillet region.
Fig. 6. SEM photomicrograph showing (a) corrosive wear at root fillet and pitting, (b) pitting at high magnification.
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uniformly. The remaining area of the fracture surface revealed the presence of shrinkage cavities within which dendritic lobes are clearly seen (Fig. 8a and b). Secondary cracks were also seen at a few locations (Fig. 9). The gear tooth was cut along a plane perpendicular to the load contact surface and polished using standard metallographic techniques. The surface was polished up to a diamond finish and etched with
Fig 7. SEM photomicrograph showing abraison at root fillet region and flat featureless region with few corrosive wear patches.
Fig. 8. SEM fractograph showing (a) shrinkage cavity and (b) dendrite lobes within the cavity.
Fig. 9. SEM fractograph showing presence of secondary crack.
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chromic acid. Microstructures at various locations of the gear tooth are typical of a cast structure as shown in Fig. 10. Fig. 10a revealed the microstructure at the core. Folds at the root fillet were clearly seen associated with plastic flow of material as revealed by deformed grains (Fig. 10b). Secondary cracks were also seen (Fig. 10c and d and e).
Fig. 10. Optical micrographs showing microstructures at different locations, 75.
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3. Discussion The failure has taken place at the root fillet region of the gear. This is the location where the tooth bending stress is maximum. Corrosive wear patches as observed, presumably caused by chemical reaction of lubricants, or of contaminants such as water or acids with the gear tooth surface. These corrosive wear patches result in pitting as evidenced in Fig. 6. Observation of similar corrosive wear on gear teeth associated with pitting has been reported earlier [1]. At the root fillet region, near corrosive wear patches, intense plastic deformation has taken place, which results in flaking of the material. This is a type of mechanical wear under localised plastic deformation associated with the action of combined rolling and sliding [1]. The state of stress produced by rolling and sliding contact is concentrated in a small volume of material and produces intense plastic strain. This strain accumulates locally as the same volume of the material is stressed with each load cycle, until a crack is initiated [2]. The presence of folds, cracks and fragmentation of flakes have revealed the severity of plastic deformation under the applied load. A metallographically polished specimen when viewed under the optical microscope revealed a dendritic microstructure along with clear evidence of folds at the root fillet associated with grain flow under plastic deformation. This supports the idea of fold formation due to accumulated plastic strain under the repetitive applied load. This causes multiple cracks to form at and below the surface, which grow and join as facilitated by the presence of shrinkage cavities. The presence of secondary cracks on the fracture surface supports this, hypothesis. Usually crack advancement due to successive loading leaves the signature of finely spaced parallel marks called fatigue striations on the fracture surface. In the present case, striations are not distinctly visible. One possible explanation is either that the stress level is close to the fatigue limit of the material or the crack growth rate is high (10 4 in per cycle or more). At a high crack growth rate, striations become wavy and develop a rough front. A large plastic zone exists in front of the crack, which causes extensive secondary cracking [3]. Flat region as a result of such large plastic zone has been reported earlier [4]. Indications of abrasion are seen at the root fillet region, indicating the presence of abrasive particles in the lubricant, which may be dirt, sand, impurities in the oil or metal detached from the tooth surface (fragmented flakes). Other factors causing abrasion are load intensity, temperature and rubbing speed. Poor surface finish and low viscosity of the lubricant also cause abrasion. Dendrite lobes as seen within the shrinkage cavities in the fracture surface are typical of ‘‘free surface’’ within the cavities/voids after the available liquid metal solidified. A high pouring temperature during casting results in this phenomenon [5]. Observations made so far are clear indications of corrosive wear, pitting at root fillets, shrinkage cavities and accumulation of intense plastic strain within small volumes of material under repetitive applied load. Accumulation of plastic strain results in subsurface shear and tensile stresses. The intense plastic strain so accumulated in a small volume of material introduces a high order of subsurface shear and tensile stress with the ultimate result of cracking. Progressive advancement of the crack as revealed by the presence of corrosive wear on a flat region of the fracture surface is facilitated by the presence of shrinkage cavities. The presence of a flat region at the beginning of the fracture process and multiple cracks are the result of the advancement of such cracks under a large number of stress repetitions.
4. Conclusion Intense plastic strain accumulated in a small volume of material due to localised rolling and sliding contact, causing folds, flakes and cracking. Fatigue growth caused crack advancement. Further, the presence of corrosion wear, pits and shrinkage cavitites facilitated the cracking process.
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Acknowledgements The authors wish to express their gratitude to Dr. T.S. Lakshamanan, Head, MCD and Dr. K.V. Nagarajan, Group Director, MMG for technical discussions and suggestions at various stages of the analysis. The permission granted by Shri G. Madhavan Nair, Director, VSSC for publishing the paper is also gratefully acknowledged.
References [1] [2] [3] [4] [5]
Metals hand book, vol. 10, 8th ed. American Society for Metals. Metals Park, OH, 1975, p. 514. ASM hand book, vol. 19, Fatigue and Fracture. ASM International, Materials Park, OH 1996, p. 332. Metals hand book, vol. 10, 8th ed. American Society for Metals, Metals Park, Ohio, 1975, p. 98. Metals hand book, vol. 12, 9th ed. 1987, p. 298. Metals hand book, vol. 12. 9th ed. 1987, p. 405.