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Failure analysis of a locomotive turbocharger main-shaft
Engineering Failure Analysis 16 (2009) 495–502
Contents lists available at ScienceDirect
Engineering Failure Analysis journal homepage: www.elsevier.com/locate/engfailanal
Failure analysis of a locomotive turbocharger main-shaft Xu Xiaolei *, Yu Zhiwei College of Electromechanics and Material Engineering, Dalian Maritime University, Dalian, Liaoning 116026, PR China
a r t i c l e
i n f o
Article history: Received 12 March 2008 Accepted 6 June 2008 Available online 20 June 2008 Keywords: Main-shaft Fillet radius Rotation bending fatigue Failure analysis
a b s t r a c t A failure investigation has been conducted on a locomotive turbocharger main-shaft. The fracture position is located at a sharp edged groove between two journals with different cross-sectional area. Fractography investigation indicates that the rotating bending fatigue is the dominant failure mechanism of the main-shaft. Detailed metallurgical analysis indicates the root fillet region of the groove had subjected to abnormal high-temperature resulting from the intense friction between the bearing-sleeve and the assisted pushing bearing, which makes the fatigue strength of the root fillet region of the groove decrease. Over-small root fillet radius of the groove by which stress concentration is created facilitates the fatigue cracks to initiate in the root fillet region. Ó 2008 Elsevier Ltd. All rights reserved.
1. Background It was reported that a locomotive turbocharger main-shaft fractured, which had serviced for 140,000 km before failure. The failed main-shaft was made of 42CrMo steel. The hardness of core material is specified as HB260-300. Some journals of main-shaft are specified to be induction-quenched to obtain the surface hardness of HRC56-62. The paper describes the careful fractographic and metallurgical investigation on the failed main-shaft, and the damage of the bearing-sleeve assembled with main-shaft was examined. The possible reasons for failure were assessed.
2. Investigation methods The chemical composition of the failed main-shaft material was determined by spectroscopy chemical analysis method. The microstructure in various regions was observed by SEM. The fracture surfaces were analyzed by visual and SEM observation to study the failure mechanism. Microhardness profiles were made on an MH-6 Vikers meter with a load of 1000 g to determine the effective hardening depth. According to the Chinese standard (GB5617-85) [1] when the hardness value of the position measured is equal to HVHL = 0.8HVMS, the depth from the position with HVHL to the surface is defined as the effective induction hardening depth, where HVHL is defined as the limited hardness and HVMS as the lowest surface hardness of hardened region specified by technical specification (in present work, required lowest surface hardness of hardened region is HRC56, corresponding to HVMS = 620, so HVHL = 496).
* Corresponding author. Tel.: +86 0411 84729613; fax: +86 0411 84728670. E-mail address: [email protected] (X. Xiaolei). 1350-6307/$ - see front matter Ó 2008 Elsevier Ltd. All rights reserved. doi:10.1016/j.engfailanal.2008.06.007
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3. Results 3.1. Macroscopic features 3.1.1. Main-shaft The fractured main-shaft and the damaged bearing sleeve assembled with the main-shaft are shown in Fig. 1, along with an un-fractured main-shaft to show clearly the groove between journals H and I. The groove is of 5.0 mm depth with a radius of 0.5 mm according to the specification. Journals J, F, H, E are demanded to be induction-quenched. The fracture position is located at a sharp edged groove between the journals I and H (marked in Fig. 1). No observable bending occurred on the
Fig. 1. Fractured main-shaft and broke bearing sleeve.
Fig. 2. Surface damage morphology.
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failed main-shaft. The surface of the journal I close to the fracture and the located surface region of the journals H and F exhibit dark-brown or blue color without showing metal glossy like other journals (Figs. 1 and 2). It is suggested that the regions had subjected to high-temperature oxidation in operating of the main-shaft. The diameter of the various journals has no measurable change compared with the specification. However, serious wear, scraping and cupper adhesion appear on the surface of the journal I close to the fracture (Fig. 2). The macrograph of the fracture surfaces is shown in Fig. 3. Beach marks showing fatigue crack propagation [1] can be observed on the fracture surface. By tracing back the beach marks, it was found that the fatigue cracks initiated at the root fillet of the groove between journals I and H and at various points along the shaft circumference, and propagated radially, as shown by arrows in Fig. 3. The root fillet of the groove from which the cracks originated attributes to a sharp notch. One of
Fig. 3. Macro morphology of the fracture surface.
Fig. 4. Surface damage morphology of the bearing sleeve.
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the most common areas of crack initiation is at the stress concentration occurring at the sharp changes in cross-journal area of the shaft [2–3]. Final fracture region is situated at the location of about 1/2 radius of the fracture and the area is one fourth of the whole fracture area (marked in Fig. 3). The orientation of the final fracture region deviated slightly from the propagation direction of the main crack. It is suggested that the rotating-bending fatigue fracture is the dominant failure mechanism of the fractured main-shaft [2], and the load which the main-shaft bore is greater before fracturing. 3.1.2. Bearing sleeve Before disassembling, the bearing sleeve had engaged with the journal H tightly. Wide and deep circumferential wear marks appear on the external surface and serious break occurred on the bearing sleeve (Fig. 4). In severe wear region, the external diameter of the sleeve is only 41.95 mm, which means the maximum wear thickness is about 3 mm (45.0 mm is the specified value of the external diameter). More than 10 strips longitudinal cracks were found in the serious worn region (Fig. 4). Some cracks are throughout the serious worn region and un-worn region without deviating. It is indicated that the longitudinal cracks appearing on the external surface of the bearing sleeve formed earlier than wear marks did, and the friction force between the bearing sleeve and assisted pushing bearing was greater. Because the bearing sleeve was assembled at journal H and the drive force of the main-shaft comes from the turbo-disk, the bearing sleeve and journal H would not turn after the main-shaft fractured. It is concluded that the wear on the external surface of the bearing sleeve had taken place before the main-shaft fractured. 3.2. Microscopic features SEM observations on the fracture surfaces reveal that no obvious metallurgical inclusions were found in the crack origin region. Deep machining marks and wear marks resulting from the contact of the matched fractures each other can be observed on crack origin region (Fig. 5a). Fatigue striations were observed in crack propagation regions (Fig. 5b).
Fig. 5. SEM observation on fracture surface (a) crack origin region and (b) crack propagation region.
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Table 1 Composition analysis results (wt%) Element
Core of failed main-shaft
42CrMo
C Si Mn P S Cr Mo Fe O Cu Al Pb Sn Zn
0.40 0.22 0.68 0.011 0.007 0.98 0.16 Balance
0.38–0.45 0.20–0.40 0.50–0.80 60.04 60.04 0.90–1.20 0.15–0.25 Balance
Region A 1.28 0.38 0.52 0.25 0.34 0.60 44.77 2.74 42.56 1.16 0.47 0.74 4.20
Fig. 6. Longitudinal section macro-morphology of groove between journals I and H.
3.2.1. Chemical composition Table 1 gives the chemical composition of the failed main-shaft, along with the specified chemical composition. It can be seen that the failed main-shaft was fabricated from 42CrMo steel. The chemical composition of oxidation surface (region A of the journal I) was determined by EDAX (Table 1). It can be seen that the higher contents O, Cu, Sn, Pb, Zn appear in the region A. According to the chemical composition of the oxidation surface and the width of the wear marks, it can be deduced that the wear marks in region A resulted from the assisted pushing bearing. It is suggested that intense friction between the surface of journal I close to the fracture and the assisted pushing bearing led to severe oxidation of the region. 3.3. Microstructure examination Longitudinal section microstructure in the interface region of journals I and H were observed. Two quench-hardening regions exhibiting crescent-shape appear on the journal I close to the fracture (Figs. 6, 7a), which just correspond to the serious wear regions A and B on journal I. The microstructure of the hardening region is composed of martensite (Fig. 7b). In fact, journal I is not specified to be induction-quenched, so the appearance of the hardening region on the journal I results from friction-quenching. The core microstructure is composed of tempered sorbite (Fig. 7c), which is the normal morphology of this grade steel.
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Fig. 7. SEM observations on microstructure in various regions: (a) low-powered morphology in B region showing macro-hardening layer, (b) microstructure in B region showing martensite and (c) core microstructure showing sorbite.
The geometry measurement of the groove between journals H and I was conducted (Fig. 8). It can be seen that the root fillet radius of the groove is 0.26 mm, which is smaller than the specified value (0.50 mm). 3.4. Hardness examinations Surface hardness of various journals and core hardness (average of the three measurements) are shown in Table 2. It can be seen that the hardness values of the journal J and E correspond to the specified value, but the hardness values of journals H and F are HRC4-7 lower than the technical specification. The core hardness value of the failed main-shaft is within the specified range.
501
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Fig. 8. Observation on groove showing root fillet.
Table 2 Surface hardness (HRC) and core hardness (HB) of the failed main-shaft Examination position
Average value of three readings
Journal Journal Journal Journal Journal Core
57.0 49.0 51.0 61.7 33.3 HB262
J H F E I
Specified value HRC56-62
HB260-300
Fig. 9. Microhardness profiles of various regions.
The micro-hardness profiles from the surface to the interior in different regions of the journals H and I were measured (Fig. 9). It can be seen that the effective induction hardening depth of locations 1, 2, 3 on the journal H is much lower than the specified depth (P2.4 mm). However, the hardening depth in the region A and B of the journal I is about 0.50 mm, although the induction-quenching process was not conducted on the journal I. The hardness examination corresponds to the microstructure observation. 4. Analysis on failure causes From the observations and analysis in Section 3, it is inferred that the chemical composition and the core hardness are within the range of the technical specification. No obvious metallurgical inclusions were found in the crack origin and crack propagation regions. The surface hardness and the effective induction hardening depth on the journal H are lower than the specifications. The hardening layer appears on the surface of the journal I which is not demanded to be induction-quenched.
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The analysis of the fracture surface and metallurgical examinations allow some hypotheses to be proposed concerning the processes that lead to the final failure of the component. 4.1. Failure sequence The wear of the bearing sleeve occurred first, and then the main-shaft fractured, which can be confirmed as follows: Because turning of the main-shaft is driven by the turbo-disk, it is impossible for the end of the main-shaft (journals H F, E) to turn after the main-shaft fractured, and the serious wear can not occur on the bearing sleeve surface. Moreover, rotating-bending fatigue fracture is the dominant failure mechanism of the main-shaft and the deviation direction of the instantaneous fracture region is identical with the turning direction of the main-shaft. It is suggested that the friction force on the surface of the journals I and H is the drive force resulting in the fatigue fracture of the main-shaft. It can be concluded that the serious wear and oxidation on the surface of journals I and H had taken place before the main-shaft fractured. 4.2. Failure causes By normal heat treatment process, surface hardness of journals J, H, F and E which are specified to be induction-quenched should not have greater difference. However, the surface hardness values of the journals H and F are lower than the values of journals J and E. From the appearance of oxidation film and serious wear morphology on the journals I, H and F, it can be deduced that the journals H and F subjected to high temperature. In terms of the surface hardness HRC48, the surface temperature of the journal H attained to about 400 °C, which means the surface region of the journal H was tempered at about 400 °C. And in terms of presence of the hardening layer in the regions A and B of journal I, the temperature attained to 800 °C in the located regions on the surface of the journal I. In general, the operating temperature conditions can not produce the overheating, additionally, the longitudinal cracks on the external surface of the bearing sleeve throughout the worn and unworn regions without changing orientation suggests that the longitudinal cracks formed earlier and the friction force between the bearing sleeve and the assisted pushing bearing was greater. It may be deduced that the heat source has been from the friction between the bearing sleeve and the assisted pushing bearing. Importantly, this implies that the overheating must be in the root fillet region of the groove between journals H and I, which would decrease intensely the fatigue strength of the fillet region. Moreover, over-small root fillet radius of the groove increase the stress concentration lever to facilitate the fatigue cracks to initiate in the fillet region of the groove and created the conditions responsible for the rotation-bending fatigue fracture of the main-shaft. 5. Conclusions The wear of the bearing sleeve took place first, and then the main-shaft fractured. Rotating-bending fatigue fracture is the dominant failure mechanism of the main-shaft. Under the abnormal operating of turbocharger, a greater friction force was produced between the bearing sleeve and the assisted pushing bearing to induce high temperature. The fatigue strength of the root fillet region of the groove between journals I and H would be intensely decreased resulting from the high temperature. Additionally, the over-small fillet radius increases the stress concentration lever to facilitate the fatigue cracks to initiate in the fillet region of the groove and creates the conditions responsible for the rotation-bending fatigue fracture of the main-shaft. References [1] Metal handbook. Fractography and atlas of fractographys, vol. 9., 8th ed. Metals Park (OH): American Society for Metals; 1974. [2] ASM Metals handbook. Failure analysis and prevention, vol. 11, 9th ed.; 1985. [3] Berndt F, van Bennekom A. Pump shaft failures a compendium of case studies. Eng Fail Anal 2001;8:135–44.
Contents lists available at ScienceDirect
Engineering Failure Analysis journal homepage: www.elsevier.com/locate/engfailanal
Failure analysis of a locomotive turbocharger main-shaft Xu Xiaolei *, Yu Zhiwei College of Electromechanics and Material Engineering, Dalian Maritime University, Dalian, Liaoning 116026, PR China
a r t i c l e
i n f o
Article history: Received 12 March 2008 Accepted 6 June 2008 Available online 20 June 2008 Keywords: Main-shaft Fillet radius Rotation bending fatigue Failure analysis
a b s t r a c t A failure investigation has been conducted on a locomotive turbocharger main-shaft. The fracture position is located at a sharp edged groove between two journals with different cross-sectional area. Fractography investigation indicates that the rotating bending fatigue is the dominant failure mechanism of the main-shaft. Detailed metallurgical analysis indicates the root fillet region of the groove had subjected to abnormal high-temperature resulting from the intense friction between the bearing-sleeve and the assisted pushing bearing, which makes the fatigue strength of the root fillet region of the groove decrease. Over-small root fillet radius of the groove by which stress concentration is created facilitates the fatigue cracks to initiate in the root fillet region. Ó 2008 Elsevier Ltd. All rights reserved.
1. Background It was reported that a locomotive turbocharger main-shaft fractured, which had serviced for 140,000 km before failure. The failed main-shaft was made of 42CrMo steel. The hardness of core material is specified as HB260-300. Some journals of main-shaft are specified to be induction-quenched to obtain the surface hardness of HRC56-62. The paper describes the careful fractographic and metallurgical investigation on the failed main-shaft, and the damage of the bearing-sleeve assembled with main-shaft was examined. The possible reasons for failure were assessed.
2. Investigation methods The chemical composition of the failed main-shaft material was determined by spectroscopy chemical analysis method. The microstructure in various regions was observed by SEM. The fracture surfaces were analyzed by visual and SEM observation to study the failure mechanism. Microhardness profiles were made on an MH-6 Vikers meter with a load of 1000 g to determine the effective hardening depth. According to the Chinese standard (GB5617-85) [1] when the hardness value of the position measured is equal to HVHL = 0.8HVMS, the depth from the position with HVHL to the surface is defined as the effective induction hardening depth, where HVHL is defined as the limited hardness and HVMS as the lowest surface hardness of hardened region specified by technical specification (in present work, required lowest surface hardness of hardened region is HRC56, corresponding to HVMS = 620, so HVHL = 496).
* Corresponding author. Tel.: +86 0411 84729613; fax: +86 0411 84728670. E-mail address: [email protected] (X. Xiaolei). 1350-6307/$ - see front matter Ó 2008 Elsevier Ltd. All rights reserved. doi:10.1016/j.engfailanal.2008.06.007
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X. Xiaolei, Y. Zhiwei / Engineering Failure Analysis 16 (2009) 495–502
3. Results 3.1. Macroscopic features 3.1.1. Main-shaft The fractured main-shaft and the damaged bearing sleeve assembled with the main-shaft are shown in Fig. 1, along with an un-fractured main-shaft to show clearly the groove between journals H and I. The groove is of 5.0 mm depth with a radius of 0.5 mm according to the specification. Journals J, F, H, E are demanded to be induction-quenched. The fracture position is located at a sharp edged groove between the journals I and H (marked in Fig. 1). No observable bending occurred on the
Fig. 1. Fractured main-shaft and broke bearing sleeve.
Fig. 2. Surface damage morphology.
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497
failed main-shaft. The surface of the journal I close to the fracture and the located surface region of the journals H and F exhibit dark-brown or blue color without showing metal glossy like other journals (Figs. 1 and 2). It is suggested that the regions had subjected to high-temperature oxidation in operating of the main-shaft. The diameter of the various journals has no measurable change compared with the specification. However, serious wear, scraping and cupper adhesion appear on the surface of the journal I close to the fracture (Fig. 2). The macrograph of the fracture surfaces is shown in Fig. 3. Beach marks showing fatigue crack propagation [1] can be observed on the fracture surface. By tracing back the beach marks, it was found that the fatigue cracks initiated at the root fillet of the groove between journals I and H and at various points along the shaft circumference, and propagated radially, as shown by arrows in Fig. 3. The root fillet of the groove from which the cracks originated attributes to a sharp notch. One of
Fig. 3. Macro morphology of the fracture surface.
Fig. 4. Surface damage morphology of the bearing sleeve.
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the most common areas of crack initiation is at the stress concentration occurring at the sharp changes in cross-journal area of the shaft [2–3]. Final fracture region is situated at the location of about 1/2 radius of the fracture and the area is one fourth of the whole fracture area (marked in Fig. 3). The orientation of the final fracture region deviated slightly from the propagation direction of the main crack. It is suggested that the rotating-bending fatigue fracture is the dominant failure mechanism of the fractured main-shaft [2], and the load which the main-shaft bore is greater before fracturing. 3.1.2. Bearing sleeve Before disassembling, the bearing sleeve had engaged with the journal H tightly. Wide and deep circumferential wear marks appear on the external surface and serious break occurred on the bearing sleeve (Fig. 4). In severe wear region, the external diameter of the sleeve is only 41.95 mm, which means the maximum wear thickness is about 3 mm (45.0 mm is the specified value of the external diameter). More than 10 strips longitudinal cracks were found in the serious worn region (Fig. 4). Some cracks are throughout the serious worn region and un-worn region without deviating. It is indicated that the longitudinal cracks appearing on the external surface of the bearing sleeve formed earlier than wear marks did, and the friction force between the bearing sleeve and assisted pushing bearing was greater. Because the bearing sleeve was assembled at journal H and the drive force of the main-shaft comes from the turbo-disk, the bearing sleeve and journal H would not turn after the main-shaft fractured. It is concluded that the wear on the external surface of the bearing sleeve had taken place before the main-shaft fractured. 3.2. Microscopic features SEM observations on the fracture surfaces reveal that no obvious metallurgical inclusions were found in the crack origin region. Deep machining marks and wear marks resulting from the contact of the matched fractures each other can be observed on crack origin region (Fig. 5a). Fatigue striations were observed in crack propagation regions (Fig. 5b).
Fig. 5. SEM observation on fracture surface (a) crack origin region and (b) crack propagation region.
X. Xiaolei, Y. Zhiwei / Engineering Failure Analysis 16 (2009) 495–502
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Table 1 Composition analysis results (wt%) Element
Core of failed main-shaft
42CrMo
C Si Mn P S Cr Mo Fe O Cu Al Pb Sn Zn
0.40 0.22 0.68 0.011 0.007 0.98 0.16 Balance
0.38–0.45 0.20–0.40 0.50–0.80 60.04 60.04 0.90–1.20 0.15–0.25 Balance
Region A 1.28 0.38 0.52 0.25 0.34 0.60 44.77 2.74 42.56 1.16 0.47 0.74 4.20
Fig. 6. Longitudinal section macro-morphology of groove between journals I and H.
3.2.1. Chemical composition Table 1 gives the chemical composition of the failed main-shaft, along with the specified chemical composition. It can be seen that the failed main-shaft was fabricated from 42CrMo steel. The chemical composition of oxidation surface (region A of the journal I) was determined by EDAX (Table 1). It can be seen that the higher contents O, Cu, Sn, Pb, Zn appear in the region A. According to the chemical composition of the oxidation surface and the width of the wear marks, it can be deduced that the wear marks in region A resulted from the assisted pushing bearing. It is suggested that intense friction between the surface of journal I close to the fracture and the assisted pushing bearing led to severe oxidation of the region. 3.3. Microstructure examination Longitudinal section microstructure in the interface region of journals I and H were observed. Two quench-hardening regions exhibiting crescent-shape appear on the journal I close to the fracture (Figs. 6, 7a), which just correspond to the serious wear regions A and B on journal I. The microstructure of the hardening region is composed of martensite (Fig. 7b). In fact, journal I is not specified to be induction-quenched, so the appearance of the hardening region on the journal I results from friction-quenching. The core microstructure is composed of tempered sorbite (Fig. 7c), which is the normal morphology of this grade steel.
500
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Fig. 7. SEM observations on microstructure in various regions: (a) low-powered morphology in B region showing macro-hardening layer, (b) microstructure in B region showing martensite and (c) core microstructure showing sorbite.
The geometry measurement of the groove between journals H and I was conducted (Fig. 8). It can be seen that the root fillet radius of the groove is 0.26 mm, which is smaller than the specified value (0.50 mm). 3.4. Hardness examinations Surface hardness of various journals and core hardness (average of the three measurements) are shown in Table 2. It can be seen that the hardness values of the journal J and E correspond to the specified value, but the hardness values of journals H and F are HRC4-7 lower than the technical specification. The core hardness value of the failed main-shaft is within the specified range.
501
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Fig. 8. Observation on groove showing root fillet.
Table 2 Surface hardness (HRC) and core hardness (HB) of the failed main-shaft Examination position
Average value of three readings
Journal Journal Journal Journal Journal Core
57.0 49.0 51.0 61.7 33.3 HB262
J H F E I
Specified value HRC56-62
HB260-300
Fig. 9. Microhardness profiles of various regions.
The micro-hardness profiles from the surface to the interior in different regions of the journals H and I were measured (Fig. 9). It can be seen that the effective induction hardening depth of locations 1, 2, 3 on the journal H is much lower than the specified depth (P2.4 mm). However, the hardening depth in the region A and B of the journal I is about 0.50 mm, although the induction-quenching process was not conducted on the journal I. The hardness examination corresponds to the microstructure observation. 4. Analysis on failure causes From the observations and analysis in Section 3, it is inferred that the chemical composition and the core hardness are within the range of the technical specification. No obvious metallurgical inclusions were found in the crack origin and crack propagation regions. The surface hardness and the effective induction hardening depth on the journal H are lower than the specifications. The hardening layer appears on the surface of the journal I which is not demanded to be induction-quenched.
502
X. Xiaolei, Y. Zhiwei / Engineering Failure Analysis 16 (2009) 495–502
The analysis of the fracture surface and metallurgical examinations allow some hypotheses to be proposed concerning the processes that lead to the final failure of the component. 4.1. Failure sequence The wear of the bearing sleeve occurred first, and then the main-shaft fractured, which can be confirmed as follows: Because turning of the main-shaft is driven by the turbo-disk, it is impossible for the end of the main-shaft (journals H F, E) to turn after the main-shaft fractured, and the serious wear can not occur on the bearing sleeve surface. Moreover, rotating-bending fatigue fracture is the dominant failure mechanism of the main-shaft and the deviation direction of the instantaneous fracture region is identical with the turning direction of the main-shaft. It is suggested that the friction force on the surface of the journals I and H is the drive force resulting in the fatigue fracture of the main-shaft. It can be concluded that the serious wear and oxidation on the surface of journals I and H had taken place before the main-shaft fractured. 4.2. Failure causes By normal heat treatment process, surface hardness of journals J, H, F and E which are specified to be induction-quenched should not have greater difference. However, the surface hardness values of the journals H and F are lower than the values of journals J and E. From the appearance of oxidation film and serious wear morphology on the journals I, H and F, it can be deduced that the journals H and F subjected to high temperature. In terms of the surface hardness HRC48, the surface temperature of the journal H attained to about 400 °C, which means the surface region of the journal H was tempered at about 400 °C. And in terms of presence of the hardening layer in the regions A and B of journal I, the temperature attained to 800 °C in the located regions on the surface of the journal I. In general, the operating temperature conditions can not produce the overheating, additionally, the longitudinal cracks on the external surface of the bearing sleeve throughout the worn and unworn regions without changing orientation suggests that the longitudinal cracks formed earlier and the friction force between the bearing sleeve and the assisted pushing bearing was greater. It may be deduced that the heat source has been from the friction between the bearing sleeve and the assisted pushing bearing. Importantly, this implies that the overheating must be in the root fillet region of the groove between journals H and I, which would decrease intensely the fatigue strength of the fillet region. Moreover, over-small root fillet radius of the groove increase the stress concentration lever to facilitate the fatigue cracks to initiate in the fillet region of the groove and created the conditions responsible for the rotation-bending fatigue fracture of the main-shaft. 5. Conclusions The wear of the bearing sleeve took place first, and then the main-shaft fractured. Rotating-bending fatigue fracture is the dominant failure mechanism of the main-shaft. Under the abnormal operating of turbocharger, a greater friction force was produced between the bearing sleeve and the assisted pushing bearing to induce high temperature. The fatigue strength of the root fillet region of the groove between journals I and H would be intensely decreased resulting from the high temperature. Additionally, the over-small fillet radius increases the stress concentration lever to facilitate the fatigue cracks to initiate in the fillet region of the groove and creates the conditions responsible for the rotation-bending fatigue fracture of the main-shaft. References [1] Metal handbook. Fractography and atlas of fractographys, vol. 9., 8th ed. Metals Park (OH): American Society for Metals; 1974. [2] ASM Metals handbook. Failure analysis and prevention, vol. 11, 9th ed.; 1985. [3] Berndt F, van Bennekom A. Pump shaft failures a compendium of case studies. Eng Fail Anal 2001;8:135–44.