J Fail. Anal. and Preven. (2012)12: 427-437 DOI10.1007/sl1668-0129583-z TECHNICAL ARTICLE-PEER-REVIEWED Failure analysis of fatigue fracture on the Outer ring of a Cylindrical Roller bearing in an Air Blower Motor Zhi-Qiang Yu. Zhen-Guo Yang Submitted: I November 2011/in revised form: 19 March 2012/Published online: 30 May 2012 C ASM International 2012 Abstract In order to identify the causes of a fracture roller, etc. are the most widely used type of bearings incident that occurred on the ring of a cylindrical because of their excellent tradeoff between size, cost, and oller bearing within an air blower motor, diverse charac- performance [1]. Therein, the cylindrical roller bearing, terization analyses were conducted his study. prototype of which in the form of logs with a large stone Metallurgical structures and chemical compositions of the block on them could even date back to ancient Egyptian era, bearings matrix materials were inspected by metallo- possesses its popularity in applications that are always graphic microscope and photoelectric direct reading involved with heavy radial loading and spectrometer. Scanning electron microscope and energy tion, such as power generation, oil field, and mining dispersive spectroscope were applied to detect the micro- industries. Concretely, a cylindrical roller bearing generally copic morphologies and micro-area compositions on the consists of four parts(Fig. 1),i.e. the inner ring, the outer fracture surfaces and contact surfaces. The compositions ring, the cage, and several cylinder-shaped rollers which are and thermal properties of the lubrication medium, the actually the reasons for the name cylindrical to originate grease, were also examined. Analysis results revealed that Indeed, cylindrical roller bearings have some specific interaction between dry friction and impact both led by the advantages, e.g., the avoidance of fretting which is usu- degraded grease due to decomposition and oxidation at ally encountered in ball bearings [2]. However, failure elevated temperatures, which resulted in serious wear of incidents still occurred on them different the outer cage, was the main cause that orig- unexpected causes. Gerdun et al. [3] analyzed a failure inated and propagated the fatigue cracks on the corners of case of one railway freight wagon wheel that was brought the outer ring, and eventually resulted in the fracture. about by the fatigue fracture on the inner rings of the Finally, countermeasures and suggestions have been bearings. Li et al. [4] even reported that a failed aero- engine was the result of fracture of the cage rivets of one cylindrical roller bearing. Moreover, it is a fact that the Keywords Bearing failure. Failure analysis. Fatigue greases applied between rollers and rings for providing brication are also key factors in ensuring the safe operation of bearings [5, 6]. Prashad [7] testified that the failure of some cylindrical roller bearings in alternators Introduction was just relevant to the chemical decomposition of the grease. Thus, thorough investigation on failure cases of Rolling-element bearings with diverse variants including cylindrical roller bearings, particularly, those aroused by ball, needle, cylindrical roller, spherical roller, tapered failed greases, should be paid special attention to for prevention of recurring incidents In this article, a fracture incident on the outer ring of a Z-QYu()·ZG.Yang Department of Materials Science, Fudan University, Shanghai cylindrical roller bearing that was sealed at the driving end 200433, People's Republic of China of an air blower motor that had a rotation speed of 990 rpm e-mail:yuzhiqiang@fudan.edu.cn in the electric power unit was reported and analyzed Spring
TECHNICAL ARTICLE—PEER-REVIEWED Failure Analysis of Fatigue Fracture on the Outer Ring of a Cylindrical Roller Bearing in an Air Blower Motor Zhi-Qiang Yu • Zhen-Guo Yang Submitted: 1 November 2011 / in revised form: 19 March 2012 / Published online: 30 May 2012 ASM International 2012 Abstract In order to identify the causes of a fracture incident that occurred on the outer ring of a cylindrical roller bearing within an air blower motor, diverse characterization analyses were conducted in this study. Metallurgical structures and chemical compositions of the bearing’s matrix materials were inspected by metallographic microscope and photoelectric direct reading spectrometer. Scanning electron microscope and energy dispersive spectroscope were applied to detect the microscopic morphologies and micro-area compositions on the fracture surfaces and contact surfaces. The compositions and thermal properties of the lubrication medium, the grease, were also examined. Analysis results revealed that interaction between dry friction and impact both led by the degraded grease due to decomposition and oxidation at elevated temperatures, which resulted in serious wear of the outer ring and the cage, was the main cause that originated and propagated the fatigue cracks on the corners of the outer ring, and eventually resulted in the fracture. Finally, countermeasures and suggestions have been proposed. Keywords Bearing failure Failure analysis Fatigue Fracture Introduction Rolling-element bearings with diverse variants including ball, needle, cylindrical roller, spherical roller, tapered roller, etc. are the most widely used type of bearings because of their excellent tradeoff between size, cost, and performance [1]. Therein, the cylindrical roller bearing, prototype of which in the form of logs with a large stone block on them could even date back to ancient Egyptian era, possesses its popularity in applications that are always involved with heavy radial loading and high-speed operation, such as power generation, oil field, and mining industries. Concretely, a cylindrical roller bearing generally consists of four parts (Fig. 1), i.e., the inner ring, the outer ring, the cage, and several cylinder-shaped rollers which are actually the reasons for the name cylindrical to originate. Indeed, cylindrical roller bearings have some specific advantages, e.g., the avoidance of fretting which is usually encountered in ball bearings [2]. However, failure incidents still occurred on them owing to different unexpected causes. Gerdun et al. [3] analyzed a failure case of one railway freight wagon wheel that was brought about by the fatigue fracture on the inner rings of the bearings. Li et al. [4] even reported that a failed aeroengine was the result of fracture of the cage rivets of one cylindrical roller bearing. Moreover, it is a fact that the greases applied between rollers and rings for providing lubrication are also key factors in ensuring the safe operation of bearings [5, 6]. Prashad [7] testified that the failure of some cylindrical roller bearings in alternators was just relevant to the chemical decomposition of the grease. Thus, thorough investigation on failure cases of cylindrical roller bearings, particularly, those aroused by failed greases, should be paid special attention to for prevention of recurring incidents. In this article, a fracture incident on the outer ring of a cylindrical roller bearing that was sealed at the driving end of an air blower motor that had a rotation speed of 990 rpm in the electric power unit was reported and analyzed. Z.-Q. Yu (&) Z.-G. Yang Department of Materials Science, Fudan University, Shanghai 200433, People’s Republic of China e-mail: yuzhiqiang@fudan.edu.cn 123 J Fail. Anal. and Preven. (2012) 12:427–437 DOI 10.1007/s11668-012-9583-z
J Fail. Anal and Preven.(2012)12: 427-437 The face material of the bearing with 18 rolling-elements side of the raceway of the detached failed bearing, turned was GCrl5 bearing steel. The out to become agglomerated, semisolid, and heavy machined brass cage (MA/C3). The lubrication Meanwhile, the outer ring of the bearing presented the was molybdenum disulfide(MoS2)lithium grease. During fracture failure. Hence, in order to confirm the actual its operation, the bearing suddenly failed when its opera- causes of this failure, a variety of characterization methods tional temperature exceeded the warning limit of 70C. were successively conducted according to our previous Afterward, the lubricating grease, which was found on the experiences [8]. Photoelectric direct reading spectrometer, metallographic microscope, and Rockwell hardness(HRC) tester were employed to inspect, respectively, the chemical compositions, the metallographic structures, and the hard- ness of the matrix materials of the failed roller bearing Meanwhile, scanning electron microscope (SEM) and energy dispersive spectroscope(EDS)were used to analyze the microscopic morphologies along with the micro-area compositions of the fracture surface and the contact surface of the bearings outer ring. Furthermore, Fourier transform outer r infrared spectroscopy(FTIR), Raman spectroscopy(RS thermogravimetric analysis (TGA), and x-ray diffraction (XRD)were utilized to characterize the degradation extent of the greases applied within the roller bearing. Based on conclu Isions were put forward hypothesizing that this fracture failure was brought about Fig. 1 3D schematic diagram of a cylindrical roller bearing by both the unqualified matrix materials and the degraded fracture position (c) (d) comer Fig. 2 External appearances of the fracture on the failed roller bearing:(a) total morphology, (b) magnification of front face, (c)magnification of side face, and(d) fractograph
The face material of the bearing with 18 rolling-elements was GCr15 bearing steel. The cage was a combined machined brass cage (MA/C3). The lubrication medium was molybdenum disulfide (MoS2) lithium grease. During its operation, the bearing suddenly failed when its operational temperature exceeded the warning limit of 70C. Afterward, the lubricating grease, which was found on the side of the raceway of the detached failed bearing, turned out to become agglomerated, semisolid, and heavy. Meanwhile, the outer ring of the bearing presented the fracture failure. Hence, in order to confirm the actual causes of this failure, a variety of characterization methods were successively conducted according to our previous experiences [8]. Photoelectric direct reading spectrometer, metallographic microscope, and Rockwell hardness (HRC) tester were employed to inspect, respectively, the chemical compositions, the metallographic structures, and the hardness of the matrix materials of the failed roller bearing. Meanwhile, scanning electron microscope (SEM) and energy dispersive spectroscope (EDS) were used to analyze the microscopic morphologies along with the micro-area compositions of the fracture surface and the contact surface of the bearing’s outer ring. Furthermore, Fourier transform infrared spectroscopy (FTIR), Raman spectroscopy (RS), thermogravimetric analysis (TGA), and x-ray diffraction (XRD) were utilized to characterize the degradation extent of the greases applied within the roller bearing. Based on these analytic results, conclusions were put forward hypothesizing that this fracture failure was brought about Fig. 1 3D schematic diagram of a cylindrical roller bearing by both the unqualified matrix materials and the degraded Fig. 2 External appearances of the fracture on the failed roller bearing: (a) total morphology, (b) magnification of front face, (c) magnification of side face, and (d) fractograph 428 J Fail. Anal. and Preven. (2012) 12:427–437 123
J Fail. Anal. and Preven. (2012)12: 427-437 Table 1 Chemical compositions of the failed roller bearing (wt %o) Elemen Si Mo Outer ring 0.85l 0.314 0.007 0.009 0.269 0.137 1.751 0.186 0032 0.187 GB/T GCr15095-1050.15-0.35<0.025<0.0250.25-0.450301.401.65<0.10<0.25 AISIE521000.98-1.100.15-0.30<0.0250.025025-0.45 1.30-1.60 (b)R (c) Fig 3 Metallographic structures of the failed roller bearing:(a) outer ring, x500.(b)roller, x500, and(e)cage, x10 greases owing to excessive operational temperature of the exhibits relatively smooth, but contains some traces of bearing. Achievements of this article have critical impor- friction, Fig. 2d. tance in prevention of such failures on the cylindrical roller ngs running under similar operatic Matrix Material Examination Chemical Compositions Experimental and Results Chemical compositions of the matrix materials of the outer Visual observation ring and the roller of this failed roller bearing are listed in Table l, and are also compared with the requirements of Figure 2a presents the location of this fracture, i.e., on the GCr15 bearing steel in GB/T 18254-2002 standard of China bottom of the outer ring of the roller bearing. Moreover, it (approximately equals to AISI E52100). It is obvious that can be learnt from Fig. 2b, c that this fracture is straight, except silicon, other elements in the roller are all qu even crossing the thickness of the bearing. Its fractograph according to the specifications. However, as for the outer Spring
greases owing to excessive operational temperature of the bearing. Achievements of this article have critical importance in prevention of such failures on the cylindrical roller bearings running under similar operation conditions. Experimental and Results Visual Observation Figure 2a presents the location of this fracture, i.e., on the bottom of the outer ring of the roller bearing. Moreover, it can be learnt from Fig. 2b, c that this fracture is straight, even crossing the thickness of the bearing. Its fractograph exhibits relatively smooth, but contains some traces of friction, Fig. 2d. Matrix Material Examination Chemical Compositions Chemical compositions of the matrix materials of the outer ring and the roller of this failed roller bearing are listed in Table 1, and are also compared with the requirements of GCr15 bearing steel in GB/T 18254-2002 standard of China (approximately equals to AISI E52100). It is obvious that except silicon, other elements in the roller are all qualified according to the specifications. However, as for the outer Table 1 Chemical compositions of the failed roller bearing (wt.%) Element C Si S P Mn Ni Cr Mo Cu Outer ring 0.851 0.314 0.007 0.009 0.269 0.137 1.751 0.193 0.186 Roller 0.964 0.583 0.009 0.011 1.064 0.125 1.464 0.032 0.187 GB/T GCr15 0.95–1.05 0.15–0.35 B0.025 B0.025 0.25–0.45 B0.30 1.40–1.65 B0.10 B0.25 AISI E52100 0.98–1.10 0.15–0.30 B0.025 B0.025 0.25–0.45 1.30–1.60 Fig. 3 Metallographic structures of the failed roller bearing: (a) outer ring, 9500, (b) roller, 9500, and (c) cage, 9100 J Fail. Anal. and Preven. (2012) 12:427–437 429 123
J Fail. Anal and Preven.(2012)12: 427-437 ring, its carbon content does not meet the requirement, while of the matrix materials of the outer ring and the roller are its chromium content exceeds the requirement In terms of displayed in Fig. 3. As the heat treatment conditions of the the former one, it is common sense that steel hardness is GCr15 steel used in the bearing are quenching and temper proportional to the carbon content in it; as to the latter one, ing, it can be observed that both the materials are composed higher chromium content will bring about more retained of acicular martensites and evenly distributed carbides. austenites, which are apt to deform under stresses in the However, more grain boundaries of the retained austenites microstructure after heat treatment. Consequently, owing to were present in the outer ring, Fig. 3a. This phenomenon is these two factors, it can be concluded that hardness of the consistent with the results of the chemical compositional outer ring of the failed bearing is intrinsically unqualified. analysis. Besides, the cage consisted of both acicular a phase As for the cage, its chemical compositions are Zn 40% (white)and B phase(black), Fig 3c, showing the typical (wt %), Cu 59%0, and Pb 1%, in accordance with the AstM microstructure of casting brass alloy C37800 lead brass requiremen Hardness Survey Metallographic Structures Table 2 lists the hardness results of the outer ring and the Etched in agent of picric acid (2, 4, 6-trinitrophenol)1.0 g, roller. It clearly reveals that the outer ring possesses lower HCI5.0 mL, and ethanol 100 mL, metallographic structures hardness than GCr15 specification, which further testifies the results of its composition. Table 2 Hardness of the failed roller bearing(hrc) Test position Outer ring Roller Specification SEM and EDS Analyses 65.3 Fracture Surface 60.2 65.5 First, fracture surface of the outer ring was observed Average using SEM. As shown in Fig 4a, dissociation steps along so w spot Moon Set woo fiae Fig 4 SEM morphologies of the fracture surface:(a)total morphology, (b) trace of friction, (c) trace of squeeze, and(d) actinomorphou
ring, its carbon content does not meet the requirement, while its chromium content exceeds the requirement. In terms of the former one, it is common sense that steel hardness is proportional to the carbon content in it; as to the latter one, higher chromium content will bring about more retained austenites, which are apt to deform under stresses in the microstructure after heat treatment. Consequently, owing to these two factors, it can be concluded that hardness of the outer ring of the failed bearing is intrinsically unqualified. As for the cage, its chemical compositions are Zn 40% (wt.%), Cu 59%, and Pb 1%, in accordance with the ASTM C37800 lead brass requirement. Metallographic Structures Etched in agent of picric acid (2,4,6-trinitrophenol) 1.0 g, HCl 5.0 mL, and ethanol 100 mL, metallographic structures of the matrix materials of the outer ring and the roller are displayed in Fig. 3. As the heat treatment conditions of the GCr15 steel used in the bearing are quenching and tempering, it can be observed that both the materials are composed of acicular martensites and evenly distributed carbides. However, more grain boundaries of the retained austenites were present in the outer ring, Fig. 3a. This phenomenon is consistent with the results of the chemical compositional analysis. Besides, the cage consisted of both acicular a phase (white) and b phase (black), Fig. 3c, showing the typical microstructure of casting brass alloy. Hardness Survey Table 2 lists the hardness results of the outer ring and the roller. It clearly reveals that the outer ring possesses lower hardness than GCr15 specification, which further testifies the results of its composition. SEM and EDS Analyses Fracture Surface First, fracture surface of the outer ring was observed using SEM. As shown in Fig. 4a, dissociation steps along Table 2 Hardness of the failed roller bearing (HRC) Test position Outer ring Roller Specification 1 60.2 65.3 65–66 2 60.3 65.8 3 60.2 65.5 Average 60.2 65.5 Fig. 4 SEM morphologies of the fracture surface: (a) total morphology, (b) trace of friction, (c) trace of squeeze, and (d) actinomorphous fracture origin 430 J Fail. Anal. and Preven. (2012) 12:427–437 123
J Fail. Anal. and Preven. (2012)12: 427-437 with dimples were present on the fracture surface, which surface, which is also depicted in Fig. 2d. This shows represents a typical morphology of quasi-dissociation actually the distinct evidence of the fracture origin, and fracture. Furthermore, traces of mutual friction and will be concretely explained as follows ueeze can be also found on the fracture surface. as shown in Fig 4b, c, respectively. These two phenomena Contact Surface on the Outer ring may have resulted from the contact between the two counterfaces of the outer ring after fracture. It should be The contact surface between the outer ring and the roller particularly noted from Fig 4d that actinomorphous also was then analyzed under SEM and EDS. Figure 5a fringes have originated from the corner of the fracture displays neat machining fringes on the fresh surface of the (a) (d) 2.003.04.00s,006,0D7.00n,09, 1..。2.。03.004.00s0G.DO7.B.0分.o Fig 5 SEM morphologies and EDS of the contact surface on the outer ring:(a) normal morphology, (b) lar trenches, (c)concave, (d adhered substances (e) eDs of site nd (f eds of site B Spring
with dimples were present on the fracture surface, which represents a typical morphology of quasi-dissociation fracture. Furthermore, traces of mutual friction and squeeze can be also found on the fracture surface, as shown in Fig. 4b, c, respectively. These two phenomena may have resulted from the contact between the two counterfaces of the outer ring after fracture. It should be particularly noted from Fig. 4d that actinomorphous fringes have originated from the corner of the fracture surface, which is also depicted in Fig. 2d. This shows actually the distinct evidence of the fracture origin, and will be concretely explained as follows. Contact Surface on the Outer ring The contact surface between the outer ring and the roller also was then analyzed under SEM and EDS. Figure 5a displays neat machining fringes on the fresh surface of the Fig. 5 SEM morphologies and EDS of the contact surface on the outer ring: (a) normal morphology, (b) irregular trenches, (c) concave, (d) adhered substances (e) EDS of site A, and (f) EDS of site B J Fail. Anal. and Preven. (2012) 12:427–437 431 123