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Metallurgical investigation of failure analysis in industrial machine components
Materials Today: Proceedings xxx (xxxx) xxx
Contents lists available at ScienceDirect
Materials Today: Proceedings journal homepage: www.elsevier.com/locate/matpr
Metallurgical investigation of failure analysis in industrial machine components K. Krishnakumar ⇑, Arockia Selvakumar School of Mechanical and Building Sciences, VIT University, Chennai 600127, India
a r t i c l e
i n f o
Article history: Received 22 July 2019 Accepted 13 September 2019 Available online xxxx Keywords: Surface spalling Fatigue propagation Scanning electron microscopy Quasi cleavage Chains Bearings
a b s t r a c t Bearings and chains are important industrial machine components used in wide range of heavy cyclical load applications. Main aim of this research work is to conduct a metallurgical investigation of failure analysis in failed bearing and chain parts. The failure analysis is carried out by chemical analysis, optical metallography, hardness analysis and scanning electron microscopy (SEM) analysis. In failed bearing inner race ways ridged marks were perceived due to electrical pitting and outer race ball groove worn-out on one side due to axial thrust load. Bearing balls have surface spalling due to contact fatigue. In chain failed bush multiple crack fronts are observed in longitudinal and circumferential. Crack geometry and fracture surface indicates failure through impact fatigue mode. The fracture surface indicated quasi cleavage fracture mode near the crack origins. The results in both investigations suggest that the major common causes of failures are due to over load, fatigue, wear and unsuitable material selection. MoS2coating, thin dense chromium coating and thin hard titanium nitride coating in all bearing components has been suggested to further improve the wear resistance and fatigue strength. Material SAE 8620 alloy steel for chain bush has been suggested to further improve life to withstand in the impact load conditions. Ó 2019 2019 Elsevier Ltd. All rights reserved. Selection and peer-review under responsibility of the scientific committee of the International conference on Materials and Manufacturing Methods.
1. Introduction Bearings support the rotating elements identical as axles and shafts in the mechanical structure. Basics parts of bearing are inner race, outer race, cage, ball and roller [1,2]. Bearings allow the motions as follows: Radial, linear, spherical and hinge. Roller chains transmit the power by means cyclical tension load during engaged with sprocket [3]. Basics parts of roller chain are inner plate, outer plate, bush, roller and pin. Plates should have the good tensile strength and dynamic strength; bush should be resist to wearing and withstanding dynamic shock load. Roller is subject to impact load during engaged with sprocket. Pins are subject to bending and shear forces during transmitting the load to link plate [4]. The most common failures of bearing and roller chains are over load, fatigue, wear and corrosion. Main aim of this research work is to conduct a metallurgical investigation of failure analysis in failed bearing and chain parts. The following methods are used for metallurgical failure analysis. They are chemical analysis, optical ⇑ Corresponding author. E-mail address: [email protected] (K. Krishnakumar).
metallography, hardness analysis and scanning electron microscopy (SEM) analysis. 2. Case study: failure analysis of bearing The bearing is failed during service. Visually there were no grease seepage and no marks due to heat trace, corrosion and mechanical harms on the outer surface of the bearing. Fig. 1(a) and (b) shows the ball bearing in assembled and dismantle condition. 2.1. Materials and methods 2.1.1. Chemical analysis Optical emission spectrometer is used to determine the chemical composition as shown in the Table 1. Outer race, inner race and ball from failed bearing are taken for chemical analysis. It is found that all components are made up of AISI 52100 steel is a high carbon, chromium containing low alloy steel [5,6]. It has high hardness, extreme wear resistance, and outstanding corrosion
https://doi.org/10.1016/j.matpr.2019.09.071 2214-7853/Ó 2019 2019 Elsevier Ltd. All rights reserved. Selection and peer-review under responsibility of the scientific committee of the International conference on Materials and Manufacturing Methods.
Please cite this article as: K. Krishnakumar and A. Selvakumar, Metallurgical investigation of failure analysis in industrial machine components, Materials Today: Proceedings, https://doi.org/10.1016/j.matpr.2019.09.071
2
K. Krishnakumar, A. Selvakumar / Materials Today: Proceedings xxx (xxxx) xxx
Fig. 1. (a) Ball bearing; (b) Ball bearing after dismantled condition.
Table 1 Chemical composition of the bearing parts as compared to the related grade (% wt). Part No
Element
1 Outer race 2 Inner race 3 Ball AISI 52100 Specification
C%
Mn%
P%
S%
Si%
Cr%
0.99 0.98 0.99 0.95–1.05
0.35 0.41 0.33 0.25–0.45
<0.001 0.011 <0.001 0.027 max
0.012 <0.001 <0.001 0.025 max
0.25 0.18 0.28 0.15–0.35
1.52 1.48 1.51 1.30–1.65
resistance in acid atmosphere, surface finishing and dimensional precision. In Table 1 chemical specifications of AISI 52100 are definite and observation of components are also specified.
overheat surface of the balls are damaged. After damage the contact fatigue and surface spalling occurs respectively. 2.2. Discussion
2.1.2. Hardness measurement and metallography All bearing parts surface and core hardness are tested by using Rockwell harness tester. Core and surface hardness of all the parts are with between the ranges of 60 and 63 HRC. All parts are typical of through-hardened. In inner race ridged marks are observed due to electrical pitting [7] are shown in the Fig. 2(a). Outer race of the ball bearing is cut in to two half and black slush grease was cleaned from the bearing parts for further investigation. In outer race, wear on one side of the groove is observed due axial thrust load are shown in the Fig. 3(a). In bearing ball lesser circular spots are shown in the surface spalling due rolling contact fatigue [8] as shown in Fig. 3(b). 2.1.3. Scanning electron microscopy SEM photograph of inner race Fig. 2(b). showing the small spots with several digs in the increased magnification of destructions due to electrical pitting. SEM photograph of ball Fig. 4(a) and (b) showing the contact fatigue propagation marks with higher magnification. In ball surface small amount of materials are removed and these type of fracture is stated as spalling. Spalling will occurs in all sliding parts in bearing like balls, inner and outer races. Due to
The chemical analysis of failed bearing parts suggests that the outer race, inner race and balls materials confirms to AISI 52100 specifications, which is in common, the specifications recommended for such applications. All bearing parts through hardened, which are usually in the range of 60– 63 HRC. Normally load, comes vertically from the shaft to bearing. So the wear should be uniformly in the ball groove surface of outer race. Visually in failed outer race which was worn out on one side of the ball path. This evident on outer race confirms the axial thrust load. While electrical current passing through the bearing parts, and sparks are generated to fuse the surface of inner race. Further it will develop a corrugated surface, and should be avoided the flow of current by insulation. By SEM analysis in inner race ridged marks, corrugated patches were found. Typically these types of effects specified as electrical pitting. These electrical pitting turn to roughen the rolling contact surfaces and it leads to mechanical damages. In balls surfaces circular spots are found. Small amounts of material also removed from the circular spots. In SEM analysis crack propagation marks are found around the boundaries. The morphology of the fractured surface indicated that ball is overheated at the instance
Fig. 2. (a) Inner race with arrows showing ridged marks (magnification – 10); (b) SEM image of bearing inner race showing a small spot with several digs in increased magnification due to electrical pitting. (magnification – 400).
Please cite this article as: K. Krishnakumar and A. Selvakumar, Metallurgical investigation of failure analysis in industrial machine components, Materials Today: Proceedings, https://doi.org/10.1016/j.matpr.2019.09.071
K. Krishnakumar, A. Selvakumar / Materials Today: Proceedings xxx (xxxx) xxx
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Fig. 3. (a) Outer race ball groove width and wear on one side of ball pathway due to axial thrust load. (magnification – 8); (b) Bearing ball showing the surface spalling due to contact fatigue and spots are circular. (magnification – 16).
Fig. 4. (a) SEM images of ball showing the interface fatigue propagation in outer edges (magnification – 25); (b) Area 1 shown in Fig. 4(a) at higher magnification of fatigue propagation marks (magnification – 100).
of fracture. Due to electrical pitting and over heat ball surfaces are damaged. The presence of beach marks in balls also suggests fatigue failure. After damage the rolling contact fatigue and surface spalling occurred. 2.3. Conclusions 1. The through hardened on AISI 52100 material given the fatigue and wear resistance has not satisfactory for this application. 2. Outer race ball groove is not worn out uniformly throughout the surface. It worn out on one side of the groove due to axial thrust load. 3. Inner race groove have the corrugated patches, small spots with several digs. Due to improper insulation the electric current passed through the bearing. The inner race failure confirms which is due to electrical pitting. 4. In balls beach marks were found in the edges of the surface. Surface spalling occurred in the surface contact area. Around the surface spalling beach marks were found. The ball failure confirms which is due rolling contact fatigue. 2.4. Recommendations The following measures can be taken to avoid the failure of the bearing. 1. To prevent from electrical pitting damage, avoid the flow of electric current by using slip ring or insulation bearing with proper grounding. 2. To avoid the axial thrust load, larger bearing with higher capacity can be used reduce contact loads.
3. If possible use high hardness material than AISI 52100 for higher bearing capacity. Another option is used to suggest MoS2 coating, thin dense chromium coating and thin hard Titanium nitride [9] coating in all bearing components. This coating will can enhance the wear resistance and protect the surface as well as lubricate. 3. Case study: failure analysis of chain bush This chain bush is failed in service. It is reported that bush is the part of roller chain assembly. Basically roller chain consists of inner link plate, outer link plate, pin, bush and roller as shown in Fig. 5 (a). Pins are press fitted with outer link plate, bushes are press fitted with inner link plate and roller can be rotate freely. 3.1. Materials and methods 3.1.1. Visual examination In failed bush multiple crack fronts are observed. The crack initiated from the inner surface of the bush as shown in Fig. 5(c) and it growth circumferentially in both ends due to interference fit with inner link plates. After circumferential crack the crack extended to longitudinal mode as shown in Fig. 5(b). Significant wear also observed in inner surface of the bush due to contact with pin. Crack geometry and fracture surface indicates failure through impact fatigue mode. 3.1.2. Chemical analysis Chemical composition of the failed bush is analysed by Optical Emission Spectroscopy and made up of AISI 1020 specifications
Please cite this article as: K. Krishnakumar and A. Selvakumar, Metallurgical investigation of failure analysis in industrial machine components, Materials Today: Proceedings, https://doi.org/10.1016/j.matpr.2019.09.071
4
K. Krishnakumar, A. Selvakumar / Materials Today: Proceedings xxx (xxxx) xxx
Fig. 5. (a) Roller chain; (b) Crack geometry of the failed bush; (c) Crack initiation and growth direction (magnification – 25).
Table 2 Chemical composition of bush as compared to the related grade (% wt). Sample
C%
Mn%
P%
S%
Si%
#1 AISI 1020 Specification
0.2 0.18–0.23
0.45 0.30–0.60
0.005 0.04 max
0.024 0.05 max
0.026 0.035 max
[10], see Table 2. This type of material is typically case hardened for drive applications.
3.1.3. Hardness measurement and metallography Hardness measurements are carried out on metallographically polished sample. Hardness throughout the wall thickness is measured by rockwell hardness tester. Hardness readings are taken in 15 N: Rockwell superficial 15 kgf. Surface hardness was 90.7 HR15N, case depth in inner diameter was 0.4 mm and case depth in outer diameter was 0.45 mm. Microstructure of bush at the case is fine tempered martensite as shown in the Fig. 6(a) and at core is low carbon martensite as shown in the Fig. 6(b).
3.1.4. Scanning electron microscopy The fracture surface indicated quasi cleavage fracture [11] mode near the crack origins. Quasi cleavage fracture shown in Fig. 6(c) initiates at the high stress point and the fracture converts to crack progression as intergranular failure and it will discharge the hydrogen pressure. Typically the quasi cleavage crack will occurs in the tempered martensitic structured steel. Quasi cleavage will followed in high strength materials with a combination of cleavage and microvoid coalescence. Basically cleavage fracture failure performs in owing to low temperature, solid cold worked components, great level of limitations and quick impact load. In bush, cleavage fracture occurred due to sudden impact load from roller during engaged with sprocket tooth.
Fig. 6. (a) Microstructure of bush at the case showing the fine tempered martensite; (b) Core showing the low carbon martensite; (c) SEM micrograph showing quasi cleavage fracture mode near to the crack origin.
Please cite this article as: K. Krishnakumar and A. Selvakumar, Metallurgical investigation of failure analysis in industrial machine components, Materials Today: Proceedings, https://doi.org/10.1016/j.matpr.2019.09.071
K. Krishnakumar, A. Selvakumar / Materials Today: Proceedings xxx (xxxx) xxx
3.2. Discussions The chain link plates are interference fitted with failed bush at both ends. The chemical analysis of the failed bush materials confirms to AISI 1020 specifications, which is low hardenability and low tensile carbon steel. It is a typically case hardened material, which bush surface hardness is 90.7 HR15N. Case depth in inner diameter is 0.4 mm and outer diameter is 0.45 mm. In microscopic examination of the bush found that the case was fine tempered martensite and the core was low carbon martensite structure. Optical method evident that the crack initiation from inner surface of the bush as shown in Fig. 5(c). The crack growth circumferentially in both ends due to interference fit with link plates. After circumferentially crack it extends to longitudinal mode. The crack geometry confirms that the failure is due to impact load. Normally chain in running condition the chain wrapped with the sprocket. During cyclic tensile load in chain the load transmit from sprocket teeth to roller. Again the load transmits from roller to bush. Due roller and bush impact load the bush is failed. 3.3. Conclusions 1. The bush was case hardened AISI 1020 material given strength is not sufficient for impact load in this application. Microstructure of the case was fine tempered martensite and the core was low carbon martensite structure. High strength alloy elements like nickel, chromium and molybdenum are not present in AISI 1020 material. 2. Multiple crack fronts are observed. Due to cyclic load the fatigue crack was originated from the inner surface and propagated circumferentially and extends to longitudinal mode. 3. By SEM analysis fracture surface indicated quasi cleavage fracture mode near the crack origins due to sudden impact load transferred from roller, during engaged with sprocket teeth. 3.4. Recommendations The chain bush SAE 8620 alloy steel with fine tempered martensitic microstructure is desirable for this high impact load application. Presence of nickel 0.4–0.7%, chromium 0.4–0.6%, molybdenum 0.15–0.25% in SAE 8620 material [12] will confi-
5
dently enhance wear resistance and withstand in impact load conditions. SAE 8620 have high surface wear resistance, good core strength and toughness. When carburized, hardened and tempered, it develops a hard case of 60 to 63 HRC. Chain with SAE 8620 bush is more suitable for tough environment applications like trencher, combine harvester, baler, rotavator, construction loader and mining drill rigs. SAE 8620 bush can be manufacturing through cold forging process. Material cost of SAE 8620 is 30% more than AISI 1020. Compared to performance the cost is negligible. References [1] K. Gurumoorthy, Arindam Ghosh, Failure investigation of a taper roller bearing: a case study. Case studies, Eng. Fail. Anal. 1 (2013) 110–114. [2] R.K. Upadhyay, A. Kumaraswamidhas, Md. SikandarAzam, Rolling element bearing failure analysis: a case study, Case Stud. Eng. Fail. Anal. 1 (2013) 1–17. [3] James C. Conwell, G.E. Johnson, Experimental investigation of link tension and roller-sprocket impact in roller chain drives, Mech. Mach. Theory 3 (1996) 533–544. [4] American Chain Association, Standard Handbook of Chains: Chains for Power Transmission and Material Handling, 2006. [5] Umamaheswari Madopothula, Ramesh Babu Nimmagadda, Vijayaraghavan Lakshmanan, Assessment of white layer in hardened AISI 52100 steel and its prediction using grinding power, Mach. Sci. Technol. 2 (2017) 299–319. [6] Chang-Soon Lee, In-Gyu Park, Young-Shik Pyoun, In-Shik Cho, In-Ho Cho, Jin Park, Rolling contact fatigue characteristics of Sae52100 by ultrasonic nanocrystal surface modification technology, Int. J. Mod. Phys. B 24 (2010) 3065–3070. [7] S. Raadnui, S. Kleesuwan, Electrical pitting wear debris analysis of greaselubricated rolling element bearings, Wear 271 (2011) 1707–1718. [8] Farshid Sadeghi, Behrooz Jalalahmadi, Trevor S. Slack, Nihar Raje, Nagaraj K. Arakere, A review of rolling contact fatigue, J. Tribol. 131 (2009) 041403– 41411. [9] Can Emrah Yaldiz, Renno Veinthal, Andre Gregor, Kyriakos Georgiadis, Mechanical properties of thin hard coatings on TiC-NiMo substrates, Estonian J. Eng. 15 (2009) 329–339. [10] Diana Maritza Marulanda Cardona, Jittraporn Wongsa-Ngam, Hernando Jimenez, Terence G. Langdon, Effects on hardness and microstructure of AISI 1020 low-carbon steel processed by high-pressure torsion, J. Mater. Res. Technol. 6 (2017) 355–360. [11] Motomichi Koyama, Eiji Akiyama, Takahiro Sawaguchi, Kazuyuki Ogawa, Irina V. Kireeva, Yury I. Chumlyakov, Kaneaki Tsuzaki, Hydrogen-assisted quasicleavage fracture in a single crystalline type 316 austenitic stainless steel, Corros. Sci. 75 (2013) 345–353. [12] Munzali Musa, Abubakar Gambo Mohammed, Auwal Muhammad, Wear properties of boron added high strength low alloy (HSLA) SAE 8620 steel, J. Met. Mater. Miner. 28 (2018) 22–29.
Please cite this article as: K. Krishnakumar and A. Selvakumar, Metallurgical investigation of failure analysis in industrial machine components, Materials Today: Proceedings, https://doi.org/10.1016/j.matpr.2019.09.071
Contents lists available at ScienceDirect
Materials Today: Proceedings journal homepage: www.elsevier.com/locate/matpr
Metallurgical investigation of failure analysis in industrial machine components K. Krishnakumar ⇑, Arockia Selvakumar School of Mechanical and Building Sciences, VIT University, Chennai 600127, India
a r t i c l e
i n f o
Article history: Received 22 July 2019 Accepted 13 September 2019 Available online xxxx Keywords: Surface spalling Fatigue propagation Scanning electron microscopy Quasi cleavage Chains Bearings
a b s t r a c t Bearings and chains are important industrial machine components used in wide range of heavy cyclical load applications. Main aim of this research work is to conduct a metallurgical investigation of failure analysis in failed bearing and chain parts. The failure analysis is carried out by chemical analysis, optical metallography, hardness analysis and scanning electron microscopy (SEM) analysis. In failed bearing inner race ways ridged marks were perceived due to electrical pitting and outer race ball groove worn-out on one side due to axial thrust load. Bearing balls have surface spalling due to contact fatigue. In chain failed bush multiple crack fronts are observed in longitudinal and circumferential. Crack geometry and fracture surface indicates failure through impact fatigue mode. The fracture surface indicated quasi cleavage fracture mode near the crack origins. The results in both investigations suggest that the major common causes of failures are due to over load, fatigue, wear and unsuitable material selection. MoS2coating, thin dense chromium coating and thin hard titanium nitride coating in all bearing components has been suggested to further improve the wear resistance and fatigue strength. Material SAE 8620 alloy steel for chain bush has been suggested to further improve life to withstand in the impact load conditions. Ó 2019 2019 Elsevier Ltd. All rights reserved. Selection and peer-review under responsibility of the scientific committee of the International conference on Materials and Manufacturing Methods.
1. Introduction Bearings support the rotating elements identical as axles and shafts in the mechanical structure. Basics parts of bearing are inner race, outer race, cage, ball and roller [1,2]. Bearings allow the motions as follows: Radial, linear, spherical and hinge. Roller chains transmit the power by means cyclical tension load during engaged with sprocket [3]. Basics parts of roller chain are inner plate, outer plate, bush, roller and pin. Plates should have the good tensile strength and dynamic strength; bush should be resist to wearing and withstanding dynamic shock load. Roller is subject to impact load during engaged with sprocket. Pins are subject to bending and shear forces during transmitting the load to link plate [4]. The most common failures of bearing and roller chains are over load, fatigue, wear and corrosion. Main aim of this research work is to conduct a metallurgical investigation of failure analysis in failed bearing and chain parts. The following methods are used for metallurgical failure analysis. They are chemical analysis, optical ⇑ Corresponding author. E-mail address: [email protected] (K. Krishnakumar).
metallography, hardness analysis and scanning electron microscopy (SEM) analysis. 2. Case study: failure analysis of bearing The bearing is failed during service. Visually there were no grease seepage and no marks due to heat trace, corrosion and mechanical harms on the outer surface of the bearing. Fig. 1(a) and (b) shows the ball bearing in assembled and dismantle condition. 2.1. Materials and methods 2.1.1. Chemical analysis Optical emission spectrometer is used to determine the chemical composition as shown in the Table 1. Outer race, inner race and ball from failed bearing are taken for chemical analysis. It is found that all components are made up of AISI 52100 steel is a high carbon, chromium containing low alloy steel [5,6]. It has high hardness, extreme wear resistance, and outstanding corrosion
https://doi.org/10.1016/j.matpr.2019.09.071 2214-7853/Ó 2019 2019 Elsevier Ltd. All rights reserved. Selection and peer-review under responsibility of the scientific committee of the International conference on Materials and Manufacturing Methods.
Please cite this article as: K. Krishnakumar and A. Selvakumar, Metallurgical investigation of failure analysis in industrial machine components, Materials Today: Proceedings, https://doi.org/10.1016/j.matpr.2019.09.071
2
K. Krishnakumar, A. Selvakumar / Materials Today: Proceedings xxx (xxxx) xxx
Fig. 1. (a) Ball bearing; (b) Ball bearing after dismantled condition.
Table 1 Chemical composition of the bearing parts as compared to the related grade (% wt). Part No
Element
1 Outer race 2 Inner race 3 Ball AISI 52100 Specification
C%
Mn%
P%
S%
Si%
Cr%
0.99 0.98 0.99 0.95–1.05
0.35 0.41 0.33 0.25–0.45
<0.001 0.011 <0.001 0.027 max
0.012 <0.001 <0.001 0.025 max
0.25 0.18 0.28 0.15–0.35
1.52 1.48 1.51 1.30–1.65
resistance in acid atmosphere, surface finishing and dimensional precision. In Table 1 chemical specifications of AISI 52100 are definite and observation of components are also specified.
overheat surface of the balls are damaged. After damage the contact fatigue and surface spalling occurs respectively. 2.2. Discussion
2.1.2. Hardness measurement and metallography All bearing parts surface and core hardness are tested by using Rockwell harness tester. Core and surface hardness of all the parts are with between the ranges of 60 and 63 HRC. All parts are typical of through-hardened. In inner race ridged marks are observed due to electrical pitting [7] are shown in the Fig. 2(a). Outer race of the ball bearing is cut in to two half and black slush grease was cleaned from the bearing parts for further investigation. In outer race, wear on one side of the groove is observed due axial thrust load are shown in the Fig. 3(a). In bearing ball lesser circular spots are shown in the surface spalling due rolling contact fatigue [8] as shown in Fig. 3(b). 2.1.3. Scanning electron microscopy SEM photograph of inner race Fig. 2(b). showing the small spots with several digs in the increased magnification of destructions due to electrical pitting. SEM photograph of ball Fig. 4(a) and (b) showing the contact fatigue propagation marks with higher magnification. In ball surface small amount of materials are removed and these type of fracture is stated as spalling. Spalling will occurs in all sliding parts in bearing like balls, inner and outer races. Due to
The chemical analysis of failed bearing parts suggests that the outer race, inner race and balls materials confirms to AISI 52100 specifications, which is in common, the specifications recommended for such applications. All bearing parts through hardened, which are usually in the range of 60– 63 HRC. Normally load, comes vertically from the shaft to bearing. So the wear should be uniformly in the ball groove surface of outer race. Visually in failed outer race which was worn out on one side of the ball path. This evident on outer race confirms the axial thrust load. While electrical current passing through the bearing parts, and sparks are generated to fuse the surface of inner race. Further it will develop a corrugated surface, and should be avoided the flow of current by insulation. By SEM analysis in inner race ridged marks, corrugated patches were found. Typically these types of effects specified as electrical pitting. These electrical pitting turn to roughen the rolling contact surfaces and it leads to mechanical damages. In balls surfaces circular spots are found. Small amounts of material also removed from the circular spots. In SEM analysis crack propagation marks are found around the boundaries. The morphology of the fractured surface indicated that ball is overheated at the instance
Fig. 2. (a) Inner race with arrows showing ridged marks (magnification – 10); (b) SEM image of bearing inner race showing a small spot with several digs in increased magnification due to electrical pitting. (magnification – 400).
Please cite this article as: K. Krishnakumar and A. Selvakumar, Metallurgical investigation of failure analysis in industrial machine components, Materials Today: Proceedings, https://doi.org/10.1016/j.matpr.2019.09.071
K. Krishnakumar, A. Selvakumar / Materials Today: Proceedings xxx (xxxx) xxx
3
Fig. 3. (a) Outer race ball groove width and wear on one side of ball pathway due to axial thrust load. (magnification – 8); (b) Bearing ball showing the surface spalling due to contact fatigue and spots are circular. (magnification – 16).
Fig. 4. (a) SEM images of ball showing the interface fatigue propagation in outer edges (magnification – 25); (b) Area 1 shown in Fig. 4(a) at higher magnification of fatigue propagation marks (magnification – 100).
of fracture. Due to electrical pitting and over heat ball surfaces are damaged. The presence of beach marks in balls also suggests fatigue failure. After damage the rolling contact fatigue and surface spalling occurred. 2.3. Conclusions 1. The through hardened on AISI 52100 material given the fatigue and wear resistance has not satisfactory for this application. 2. Outer race ball groove is not worn out uniformly throughout the surface. It worn out on one side of the groove due to axial thrust load. 3. Inner race groove have the corrugated patches, small spots with several digs. Due to improper insulation the electric current passed through the bearing. The inner race failure confirms which is due to electrical pitting. 4. In balls beach marks were found in the edges of the surface. Surface spalling occurred in the surface contact area. Around the surface spalling beach marks were found. The ball failure confirms which is due rolling contact fatigue. 2.4. Recommendations The following measures can be taken to avoid the failure of the bearing. 1. To prevent from electrical pitting damage, avoid the flow of electric current by using slip ring or insulation bearing with proper grounding. 2. To avoid the axial thrust load, larger bearing with higher capacity can be used reduce contact loads.
3. If possible use high hardness material than AISI 52100 for higher bearing capacity. Another option is used to suggest MoS2 coating, thin dense chromium coating and thin hard Titanium nitride [9] coating in all bearing components. This coating will can enhance the wear resistance and protect the surface as well as lubricate. 3. Case study: failure analysis of chain bush This chain bush is failed in service. It is reported that bush is the part of roller chain assembly. Basically roller chain consists of inner link plate, outer link plate, pin, bush and roller as shown in Fig. 5 (a). Pins are press fitted with outer link plate, bushes are press fitted with inner link plate and roller can be rotate freely. 3.1. Materials and methods 3.1.1. Visual examination In failed bush multiple crack fronts are observed. The crack initiated from the inner surface of the bush as shown in Fig. 5(c) and it growth circumferentially in both ends due to interference fit with inner link plates. After circumferential crack the crack extended to longitudinal mode as shown in Fig. 5(b). Significant wear also observed in inner surface of the bush due to contact with pin. Crack geometry and fracture surface indicates failure through impact fatigue mode. 3.1.2. Chemical analysis Chemical composition of the failed bush is analysed by Optical Emission Spectroscopy and made up of AISI 1020 specifications
Please cite this article as: K. Krishnakumar and A. Selvakumar, Metallurgical investigation of failure analysis in industrial machine components, Materials Today: Proceedings, https://doi.org/10.1016/j.matpr.2019.09.071
4
K. Krishnakumar, A. Selvakumar / Materials Today: Proceedings xxx (xxxx) xxx
Fig. 5. (a) Roller chain; (b) Crack geometry of the failed bush; (c) Crack initiation and growth direction (magnification – 25).
Table 2 Chemical composition of bush as compared to the related grade (% wt). Sample
C%
Mn%
P%
S%
Si%
#1 AISI 1020 Specification
0.2 0.18–0.23
0.45 0.30–0.60
0.005 0.04 max
0.024 0.05 max
0.026 0.035 max
[10], see Table 2. This type of material is typically case hardened for drive applications.
3.1.3. Hardness measurement and metallography Hardness measurements are carried out on metallographically polished sample. Hardness throughout the wall thickness is measured by rockwell hardness tester. Hardness readings are taken in 15 N: Rockwell superficial 15 kgf. Surface hardness was 90.7 HR15N, case depth in inner diameter was 0.4 mm and case depth in outer diameter was 0.45 mm. Microstructure of bush at the case is fine tempered martensite as shown in the Fig. 6(a) and at core is low carbon martensite as shown in the Fig. 6(b).
3.1.4. Scanning electron microscopy The fracture surface indicated quasi cleavage fracture [11] mode near the crack origins. Quasi cleavage fracture shown in Fig. 6(c) initiates at the high stress point and the fracture converts to crack progression as intergranular failure and it will discharge the hydrogen pressure. Typically the quasi cleavage crack will occurs in the tempered martensitic structured steel. Quasi cleavage will followed in high strength materials with a combination of cleavage and microvoid coalescence. Basically cleavage fracture failure performs in owing to low temperature, solid cold worked components, great level of limitations and quick impact load. In bush, cleavage fracture occurred due to sudden impact load from roller during engaged with sprocket tooth.
Fig. 6. (a) Microstructure of bush at the case showing the fine tempered martensite; (b) Core showing the low carbon martensite; (c) SEM micrograph showing quasi cleavage fracture mode near to the crack origin.
Please cite this article as: K. Krishnakumar and A. Selvakumar, Metallurgical investigation of failure analysis in industrial machine components, Materials Today: Proceedings, https://doi.org/10.1016/j.matpr.2019.09.071
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3.2. Discussions The chain link plates are interference fitted with failed bush at both ends. The chemical analysis of the failed bush materials confirms to AISI 1020 specifications, which is low hardenability and low tensile carbon steel. It is a typically case hardened material, which bush surface hardness is 90.7 HR15N. Case depth in inner diameter is 0.4 mm and outer diameter is 0.45 mm. In microscopic examination of the bush found that the case was fine tempered martensite and the core was low carbon martensite structure. Optical method evident that the crack initiation from inner surface of the bush as shown in Fig. 5(c). The crack growth circumferentially in both ends due to interference fit with link plates. After circumferentially crack it extends to longitudinal mode. The crack geometry confirms that the failure is due to impact load. Normally chain in running condition the chain wrapped with the sprocket. During cyclic tensile load in chain the load transmit from sprocket teeth to roller. Again the load transmits from roller to bush. Due roller and bush impact load the bush is failed. 3.3. Conclusions 1. The bush was case hardened AISI 1020 material given strength is not sufficient for impact load in this application. Microstructure of the case was fine tempered martensite and the core was low carbon martensite structure. High strength alloy elements like nickel, chromium and molybdenum are not present in AISI 1020 material. 2. Multiple crack fronts are observed. Due to cyclic load the fatigue crack was originated from the inner surface and propagated circumferentially and extends to longitudinal mode. 3. By SEM analysis fracture surface indicated quasi cleavage fracture mode near the crack origins due to sudden impact load transferred from roller, during engaged with sprocket teeth. 3.4. Recommendations The chain bush SAE 8620 alloy steel with fine tempered martensitic microstructure is desirable for this high impact load application. Presence of nickel 0.4–0.7%, chromium 0.4–0.6%, molybdenum 0.15–0.25% in SAE 8620 material [12] will confi-
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dently enhance wear resistance and withstand in impact load conditions. SAE 8620 have high surface wear resistance, good core strength and toughness. When carburized, hardened and tempered, it develops a hard case of 60 to 63 HRC. Chain with SAE 8620 bush is more suitable for tough environment applications like trencher, combine harvester, baler, rotavator, construction loader and mining drill rigs. SAE 8620 bush can be manufacturing through cold forging process. Material cost of SAE 8620 is 30% more than AISI 1020. Compared to performance the cost is negligible. References [1] K. Gurumoorthy, Arindam Ghosh, Failure investigation of a taper roller bearing: a case study. Case studies, Eng. Fail. Anal. 1 (2013) 110–114. [2] R.K. Upadhyay, A. Kumaraswamidhas, Md. SikandarAzam, Rolling element bearing failure analysis: a case study, Case Stud. Eng. Fail. Anal. 1 (2013) 1–17. [3] James C. Conwell, G.E. Johnson, Experimental investigation of link tension and roller-sprocket impact in roller chain drives, Mech. Mach. Theory 3 (1996) 533–544. [4] American Chain Association, Standard Handbook of Chains: Chains for Power Transmission and Material Handling, 2006. [5] Umamaheswari Madopothula, Ramesh Babu Nimmagadda, Vijayaraghavan Lakshmanan, Assessment of white layer in hardened AISI 52100 steel and its prediction using grinding power, Mach. Sci. Technol. 2 (2017) 299–319. [6] Chang-Soon Lee, In-Gyu Park, Young-Shik Pyoun, In-Shik Cho, In-Ho Cho, Jin Park, Rolling contact fatigue characteristics of Sae52100 by ultrasonic nanocrystal surface modification technology, Int. J. Mod. Phys. B 24 (2010) 3065–3070. [7] S. Raadnui, S. Kleesuwan, Electrical pitting wear debris analysis of greaselubricated rolling element bearings, Wear 271 (2011) 1707–1718. [8] Farshid Sadeghi, Behrooz Jalalahmadi, Trevor S. Slack, Nihar Raje, Nagaraj K. Arakere, A review of rolling contact fatigue, J. Tribol. 131 (2009) 041403– 41411. [9] Can Emrah Yaldiz, Renno Veinthal, Andre Gregor, Kyriakos Georgiadis, Mechanical properties of thin hard coatings on TiC-NiMo substrates, Estonian J. Eng. 15 (2009) 329–339. [10] Diana Maritza Marulanda Cardona, Jittraporn Wongsa-Ngam, Hernando Jimenez, Terence G. Langdon, Effects on hardness and microstructure of AISI 1020 low-carbon steel processed by high-pressure torsion, J. Mater. Res. Technol. 6 (2017) 355–360. [11] Motomichi Koyama, Eiji Akiyama, Takahiro Sawaguchi, Kazuyuki Ogawa, Irina V. Kireeva, Yury I. Chumlyakov, Kaneaki Tsuzaki, Hydrogen-assisted quasicleavage fracture in a single crystalline type 316 austenitic stainless steel, Corros. Sci. 75 (2013) 345–353. [12] Munzali Musa, Abubakar Gambo Mohammed, Auwal Muhammad, Wear properties of boron added high strength low alloy (HSLA) SAE 8620 steel, J. Met. Mater. Miner. 28 (2018) 22–29.
Please cite this article as: K. Krishnakumar and A. Selvakumar, Metallurgical investigation of failure analysis in industrial machine components, Materials Today: Proceedings, https://doi.org/10.1016/j.matpr.2019.09.071