- Email: [email protected]
Failure analysis of the repair procedure of an ore compactor roll
Engineering Failure Analysis 31 (2013) 195–202
Contents lists available at SciVerse ScienceDirect
Engineering Failure Analysis journal homepage: www.elsevier.com/locate/engfailanal
Failure analysis of the repair procedure of an ore compactor roll Thiago Figueiredo Azevedo, Cristiane Ramos dos Santos, Ricardo Estefany Aquino de Souza, Eduardo Kirinus Tentardini, Sandro Griza ⇑ Programa de Pós-graduação em Ciência e Engenharia de Materiais, Universidade Federal de Sergipe, São Cristóvão, Sergipe, Brazil
a r t i c l e
i n f o
Article history: Received 10 December 2012 Received in revised form 8 February 2013 Accepted 9 February 2013 Available online 18 February 2013 Keywords: Ore compactor roll Fatigue Press-fit assembly Welding
a b s t r a c t This paper deals with the failure analysis of a potassium ore compactor roll that experienced short term fracture in service after a repair procedure. The journal bearing of the shaft experienced wear after a long period in use. Afterwards, a surface deposition was performed in order to retrieve the roll functionality, but it culminated in the formation of radial cracks beginning just near the shaft curvature radius. This portion of the roll was then removed and replaced with a new shaft interference-fitted and welded into the roll. Moreover, after this new repair procedure, the roll experienced a short term fracture. Failure analyses were performed by liquid penetrant, fracture analysis, hardness testing and metallographic analysis. Numerical simulations were also carried out considering the last repair features. Fatigue failure nucleated near a singularity at the shaft curvature radius. This singularity was originated from the interference fit and welding procedures. Ó 2013 Elsevier Ltd. All rights reserved.
1. Introduction Components operating in load bearing condition undoubtedly wear out over time which is a common case found in shaft journal bearings, for example [1–3]. In many cases the surface damage enhanced by wear will contribute to the crack formation and it will propagate by fatigue. When the replacement of damaged components can cause extensive production losses, the strategy adopted can be the recovery of the wear surface by some deposition procedure or welding [4–7]. Although it is a palliative technique widely used, repairs can produce microstructural and geometric changes, as well as they can introduce residual stresses, all those factors which can decrease the fatigue performance of the component [8,9]. Interference fit is an alternative technique in the recovery of big shafts that experienced fracture. The stress state in an interference fit assembly is predicted according to the well-known Lamé solution [10]. The outer diameter of the shaft (inner cylinder) is compressed with a high stress, to set it in the other part (the outer cylinder). Shrinkage fit can be done in big parts, and it is evaluated by heating and expanding the outer cylinder diameter. The heated parts can be slipped together with a reduced axial load, and when they approach the equilibrium with the ambient temperature, their dimensional variations will create the desired interference to the frictional contact. Stress concentration on the shaft and on the ends of the outer cylinder emerges from the shrinkage cooling. The elastic strain enhanced by shrinkage promotes high normal and frictional loads between the two parts and this frictional load is able to transmit the torque from the shaft (the part driven by the motor) to the outer cylinder. For components operating under cyclic stresses it is needed to perform a careful analysis of the interference fit, since it is known that the fatigue strength will be lower due to fretting fatigue [11,12]. Therefore, in some cases the solution adopted is to weld both the interference fitted parts to decrease their relative movements.
⇑ Corresponding author. Tel.: +55 79 2105 6888; fax: +55 79 2105 6845. E-mail address: [email protected] (S. Griza). 1350-6307/$ - see front matter Ó 2013 Elsevier Ltd. All rights reserved. http://dx.doi.org/10.1016/j.engfailanal.2013.02.010
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Fig. 1. At left, cracks were found after the first repair (weld deposition). At right, the new shaft is the interference fitted in the roll machined hole. Afterwards, the chamfer was welded and the curvature radius was machined.
This paper deals with the failure of a potassium ore compactor roll which experienced sudden rupture in service after some maintenance actions. The roll underwent a repair by weld filler deposited over the journal bearing opposite to that coupled to the motor, due to the expected wear after long in service use. Following a short period of operation after this first repair, the presence of deeper radial cracks was detected through ultrasonic analysis (Fig. 1). So the roll experienced a last repair procedure which was carried out as follows: (a) machining to remove the cracked portion; (b) interference fit assembly of a new shaft end; (c) welding; (d) finish machining (Fig. 1). The purpose of this study is to analyze the failure occurred due to the last repair procedure, i.e., interference fit and welding. Fracture analysis, metallographic and microhardness tests were performed. Numerical simulation was also performed in an attempt to see the changes in the stresses provided for the repair procedure. 2. Materials and methods 2.1. Preliminary analysis The roll was manufactured from SAE 4340 steel, and it was quenched and tempered to a surface hardness of at least 39 HRC. The same specification was used for the replaced shaft manufacture. After the fracture occurrence, liquid penetrant technique was performed in both fracture surfaces. 2.2. Fracture analysis The fracture surface received a simple cleaning with water, liquid soap and a soft bristle brush. Macroscopic images of the fracture surface were performed by a digital camera (Canon EOS 1000D). A fracture sample was analyzed by scanning electron microscopy (SEM – Jeol Carry Scope JCM-5700). 2.3. Microstructure The metallographic sample was prepared to analyze the microstructure near a region identified previously as one of the several fracture nucleation points. Optical microscopy was applied (Carl ZEISS Axio SCOPE A.1). The sample was etched with Nital 2%. 2.4. Microhardness The metalographic sample was conducted to the Vickers (HV0.3) microhardness (Future Tech FM – 800). A microhardness profile was achieved from the deposition metal (DM) throughout the heat affected zone (HAZ) until reaching the base metal (BM). Altogether, 150 indentations were done. The profile initiated at 2 mm from the weld face and the subsequent indentations were evaluated with 0.3 mm distance. Approaching the HAZ, the distance was changed to 0.1 mm. When the profile approached the BM and the microhardness showed stabilization, the distance was changed again to 0.3 mm. 2.5. Finite element simulation Two different sketches were performed in the 2D numerical simulation. The simulations were performed in the linear elastic regime using structured hexagonal elements and quadratic geometry. The elastic modulus was 200 GPa and Poisson’s ratio 0.3. The first simulation was carried out on the original design of the roll (Fig. 2). The interference fit and welding were taken into account in the second simulation (Fig. 2). The roll was restricted in the medial cross section of the working zone
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Fig. 2. The hexagonal mesh, restriction in the medial cross section and the applied load. At left, original design of the roll and at right, repair simulation.
Table 1 The mesh quantification for the two models. Model
No. of elements
No. of nodes
Mesh size
Original Repair
10664 21643
3463 7002
15 10
(major diameter). The design load was applied on the end of the shaft. The interface between the two fitted parts was simulated in self contact, with 0.5 mm clearance. The mesh attributions are shown in the Table 1. The mesh refinement was evaluated for the repair simulation in the region of the singularity provided by the clearance approaching the weld (see Fig. 2). The endurance limit (rm) of 300 MPa for the SAE 4340 under rotating bending was considered as design criterion [13]. This endurance limit was applied in the Marine empirical law, according to the Eq. (1) [10]. The design stress in the roll curvature radius (rR) was obtained for a safety factor (SF = 2) and reducing factors due to surface (ka = 0.7) which is function of the design specifications of the surface hardness and finishing; cross section size (kb = 0.7) and stress concentration (kf = 0.6) which is function of the load and the curvature radius to diameter ratio. Therefore, the maximum Von Mises stress applied to the curvature radius due to the external load was 45 MPa.
rR ¼
rm ka kb kf SF
ð1Þ
The repair was modeled in the second 2D simulation, from the sketches of the two parts that reproduce both the interference fitted shaft and the remained roll. The friction coefficient of 0.15 was added. The interference was disregarded in the interface. This assumption excludes the residual stresses that occur prior to the welding. In practice, the interference can be significantly reduced in the region closer to the weld due to the stress relief imposed by the weld thermal supply. A ring reproducing the weld dimensions and the machined curvature radius was bonded onto the other two parts in their respective contact regions. 3. Results 3.1. Preliminary analysis The liquid penetrant analysis performed in both fracture surfaces revealed a discontinuity at the weld root only in the roll side (Fig. 3). 3.2. Fracture analysis Fracture surface shows rotating bending fatigue appearance. A sub-superficial perimeter that matches with the weld root shows multiple ratchet marks which are typical fatigue nucleation indicatives (Fig. 4). The crack progressed through the shaft on a transverse plane and also up through the throat of the weld. It can be seen a large and flat fatigue propagation surface. Beach marks can be seen on the propagation surface and a rough final rupture zone, closer to the center of the shaft whose appearance indicates a substantial torsional deformation. The presence of several ratchet marks regarded to rotating bending stresses in detail in the Fig. 5 and the extensive surface propagation indicate low external load and high stress concentration at the weld root. The SEM analysis showed a wrinkled fracture surface (Fig. 6) with absence of fracture micromechanisms. This aspect denotes that the crack faces in propagation were compressed together during the rotating bending loading.
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Fig. 3. Liquid penetrant results. At left, the absence of discontinuity in the shaft side; at right, discontinuity in the weld root of the roll side.
Fig. 4. The macrograph of the fracture surface is shown. The dashed white line indicates the metallographic plane. The letter ‘‘a’’ refers to the region that was examined in the SEM analysis. The image shows the appearance of rotating bending fatigue starting from sub-superficial ratchet marks along the weld root.
Fig. 5. The images show details of the ratchet marks closer to the weld root and some beach marks in the propagation surface.
3.3. Microstructure The microstructural analysis showed the aspects of the deposition metal (DM), heat affected zone (HAZ) and the base metal (BM) (Fig. 7). The analysis also showed that the fracture nucleation matches with the transition between DM and HAZ, at the weld root. The DM region shows dendritic structure with acicular ferrite and dispersed carbides (Fig. 8). The HAZ zone shows low temperature transformation microstructure making appear martensite plates (Fig. 9). The BM, in its turn, also shows low temperature transformation microstructure, but with more stable aspect, with ferritic matrix and dispersed carbides making appear bainite (Fig. 10).
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Fig. 6. SEM images showing substantial wrinkling and the absence of micromechanisms on the region (a) of the fracture surface showed in the Fig. 5.
Fig. 7. Cross section macrography of the shaft. The square shows the interface between the weld metal and HAZ, coincident region with the fracture initiation. Etchant: Nital 2%.
Fig. 8. The microstructure of the weld metal is constituted of dendrites of acicular ferrite and dispersed carbides.
3.4. Microhardness The microhardness profile showed a peak of 405 HV near the interface between the weld metal and HAZ. Microhardness stabilized near 250 HV in the base metal (Fig. 11). 3.5. Finite element simulation The simulation results for both original and repair models are shown respectively in Figs. 12 and 13. The maximum Von Mises stress of 45 MPa in the original design occurs in the compressed region of the curvature radius. This result is in agreement with the wrinkled appearance of the fracture surface. In the presence of the discontinuity near the weld root, there is a perceived stress concentration in the singularity and significant increase in the Von Mises stress to 475 MPa, reaching nearly 10 times the stress seen in the previous model.
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Fig. 9. Microstructure transformed in low temperature of the HAZ with appearance of tempered martensite plates.
Fig. 10. Microstructure of the base metal where it is observed the formation of ferrite and dispersed carbides.
Fig. 11. Vickers microhardness profile (HV0,3) from the weld metal throughout the HAZ and approaching the base metal.
4. Discussions In this study we performed the failure analysis of a repaired ore compactor roll. The root cause of the failure is attributed to the shaft wear in the coupling region of the journal bearing after a long period of use. This fact is predicted in the original design. The roll design optimization goes through improving the tribologic attributes, i.e., the wear strength of the journal bearing. However, the issue of this study is limited to highlight the failure causes after the second repair applied. An interference fit followed by weld procedure was performed to allow further ore compaction while waiting for the machining a new roll. However, the sudden failure occurred in short term. Metallographic analysis was not performed on the roll side to prevent further cuts that could make its reuse difficult. However, the penetrant liquid test was important to show that the failure initiation matches the discontinuity at the weld
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Fig. 12. The original model under flexion showed Von Mises stress of 45 MPa in the curvature radius.
Fig. 13. Stress concentration in the weld root due to the singularity enhanced by the repair.
root. The absence of revealed discontinuity on the other side, the bearing shaft, incriminates the weld root as the failure initiation region. The failure occurred by rotating bending fatigue initiated at a discontinuity in the weld root repair. The multiple ratchet marks found in this region and the evident large fatigue propagation surface indicate the discontinuity as important stress riser which contributed to the short term failure. Unfortunately, the weld and the interference fit interface evaluated in the second repair were all located near the curvature radius. This is the region expected to experience the highest stress in service, and it was confirmed by the numerical simulation of the original roll. There are several reports about shaft failures whose initiation is coincident with stress risers and weld procedures [14– 16]. In some cases, the welding is more prone to failure than the stress concentration achieved by section changes [17]. Welding can provides untempered martensite zones of higher hardness, low ductility and higher stress concentration of metallurgical nature due to the thermal shrinkage. These features create favorable conditions for the nucleation and propagation of fatigue cracks. In an attempt to minimize them, it is required a strict control of the procedures as pre-heating and cooling rate of the material during the welding. Heat treatment after welding is also an important requirement to the material stress relieve and to improve the mechanical behavior [18]. However, it is expected that the heat affected zone presenting fatigue properties be lower than the base metal. In the present study, the welding procedure promoted a hardness peak in the HAZ which denotes microstructural transformations in agreement with the previous thinking. The transition between the weld and the interference fit interface, in its turn, produces a singularity that acts as stress riser. Indeed, the numerical simulation showed an increase in the Von Mises stress related to this singularity. The numerical simulation result obviously should be considered carefully due to its simplicity. The actual form of the singularity was not
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necessarily reproduced since it is difficult to know the singularity curvature radius after the fracture. Furthermore, the simulation was performed in the linear elastic regime which does not consider the possible material plastic strain closer to the singularity. However, despite the limitation in predicting the exact stress magnitude at the singularity, notably the simulation showed the increase of the stress regarded to the fracture nucleation region. Finite element modeling suggested that stresses of the order of ten times the stress in the radius could be expected at the root of the weld. This level of stress would have been sufficient to lead to fatigue failure from this point and is consistent with the observations in this paper. To reduce the risk of recurrence, it is suggested that the repair is done by introducing a shaft of greatest diameter. It moves the welding area and therefore also the singularity for a region further away from the curvature radius. However, whatever the design change adopted, it should be previously analyzed, by numerical simulation for example, to check the resulting stress level. 5. Conclusion This study was enhanced to highlight the failure causes of a ore compactor roll which had undergone a repair procedure. The repair was evaluated from a press fit followed by welding procedure. The failure was due to rotating bending fatigue, nucleated in the weld root, coincident with the singularity provided from the press fit interface and welding. The finite element analysis was able to show that the singularity acted as stress riser, promoting the short term fatigue. Acknowledgements The authors would like to thank the financial support received from CAPES, CNPq and FINEP. References [1] [2] [3] [4] [5] [6] [7] [8] [9] [10] [11] [12] [13] [14] [15] [16] [17] [18]
Xiaolei Xu, Zhiwei Yu, Ding Hongxin. Failure analysis of a diesel engine gear-shaft. Eng Fail Anal 2006;13:1351–7. Xu Yu, Zhiwei Xiaolei. Failure analysis of a locomotive turbocharger main-shaft. Eng Fail Anal 2009;16:495–502. Bhattacharyya Sandip. Failure analysis of gas blower shaft of a blast furnace. Eng Fail Anal 2008;15:349–55. Desir Jean-Luc. Examples of repair welding of heavy machinery subject to breakage due to low frequency alternating stresses. Eng Fail Anal 2001;8:423–37. Rivas JS, Coronado JJ, Gómez AL. Tribological aspects for the shafts and bearings of sugar cane mills. Wear 2006;261:779–84. Sahraoui Tahar, Fenineche Nour-Eddine, Montavon Ghislain, Coddet Christian. Alternative to chromium: characteristics and wear behavior of HVOF coatings for gas turbine shafts repair (heavy-duty). J Mater Process Technol 2004;152:43–55. Brooman Eric W. Wear behavior of environmentally acceptable alternatives to chromium coatings: nickel-based candidates. Met Finish 2004;102(9):75–82. Savkovic´ Mile, Gašic´ Milomir, Petrovic´ Dragan, Zdravkovic´ Nebojša, Pljakic Radmila. Analysis of the drive shaft fracture of the bucket wheel excavator. Eng Fail Anal 2012;20:105–17. Atxaga G, Irisarri AM. Failure analysis of the end of a shaft of an engine. Eng Fail Anal 2010;17:714–21. Shigley JE, Mischk CR. Mechanical engineering design. 8th ed. McGraw-Hill Book Company; 2008. Lee Dong-Hyong, Kwon Seok-Jin, Ham Young-Sam, You Won-Hee. Characterization of fretting damage in a press-fitted shaft below the fretting fatigue limit. Procedia Eng 2010;2:1945–9. Alfredsson B. Fretting fatigue of a shrink-fit pin subjected to rotating bending: experiments and simulations. Int J Fatigue 2009;31:1559–70. ASM Handbook, vol. 1, Properties and selection: Irons, steels and high performance alloys, 10th ed., Metals Handbook, 1990. Asi Osman. Fatigue failure of a rear axle shaft of an automobile. Eng Fail Anal 2006;13:1293–302. Fernando Casanova, Failure analysis and redesign of a wagon wheel shaft for sugar cane transport, Dyna, Nro. 166, Medellin, April, 2011. p. 48–54. Fuller RW, Ehrgott Jr JQ, Heard WF, Robert SD, Stinson RD, Solanki K, et al. Failure analysis of AISI 304 stainless steel shaft. Eng Fail Anal 2008;15:835–46. Sattari-Far Iradj, Moalemi Mohammad. Failure of stainless digester shafts in a paper production plant. Eng Fail Anal 2003;10:675–82. Kou S. Welding metallurgy. 2nd ed. New Jersey: Wiley-Interscience Publication; 2002.
Contents lists available at SciVerse ScienceDirect
Engineering Failure Analysis journal homepage: www.elsevier.com/locate/engfailanal
Failure analysis of the repair procedure of an ore compactor roll Thiago Figueiredo Azevedo, Cristiane Ramos dos Santos, Ricardo Estefany Aquino de Souza, Eduardo Kirinus Tentardini, Sandro Griza ⇑ Programa de Pós-graduação em Ciência e Engenharia de Materiais, Universidade Federal de Sergipe, São Cristóvão, Sergipe, Brazil
a r t i c l e
i n f o
Article history: Received 10 December 2012 Received in revised form 8 February 2013 Accepted 9 February 2013 Available online 18 February 2013 Keywords: Ore compactor roll Fatigue Press-fit assembly Welding
a b s t r a c t This paper deals with the failure analysis of a potassium ore compactor roll that experienced short term fracture in service after a repair procedure. The journal bearing of the shaft experienced wear after a long period in use. Afterwards, a surface deposition was performed in order to retrieve the roll functionality, but it culminated in the formation of radial cracks beginning just near the shaft curvature radius. This portion of the roll was then removed and replaced with a new shaft interference-fitted and welded into the roll. Moreover, after this new repair procedure, the roll experienced a short term fracture. Failure analyses were performed by liquid penetrant, fracture analysis, hardness testing and metallographic analysis. Numerical simulations were also carried out considering the last repair features. Fatigue failure nucleated near a singularity at the shaft curvature radius. This singularity was originated from the interference fit and welding procedures. Ó 2013 Elsevier Ltd. All rights reserved.
1. Introduction Components operating in load bearing condition undoubtedly wear out over time which is a common case found in shaft journal bearings, for example [1–3]. In many cases the surface damage enhanced by wear will contribute to the crack formation and it will propagate by fatigue. When the replacement of damaged components can cause extensive production losses, the strategy adopted can be the recovery of the wear surface by some deposition procedure or welding [4–7]. Although it is a palliative technique widely used, repairs can produce microstructural and geometric changes, as well as they can introduce residual stresses, all those factors which can decrease the fatigue performance of the component [8,9]. Interference fit is an alternative technique in the recovery of big shafts that experienced fracture. The stress state in an interference fit assembly is predicted according to the well-known Lamé solution [10]. The outer diameter of the shaft (inner cylinder) is compressed with a high stress, to set it in the other part (the outer cylinder). Shrinkage fit can be done in big parts, and it is evaluated by heating and expanding the outer cylinder diameter. The heated parts can be slipped together with a reduced axial load, and when they approach the equilibrium with the ambient temperature, their dimensional variations will create the desired interference to the frictional contact. Stress concentration on the shaft and on the ends of the outer cylinder emerges from the shrinkage cooling. The elastic strain enhanced by shrinkage promotes high normal and frictional loads between the two parts and this frictional load is able to transmit the torque from the shaft (the part driven by the motor) to the outer cylinder. For components operating under cyclic stresses it is needed to perform a careful analysis of the interference fit, since it is known that the fatigue strength will be lower due to fretting fatigue [11,12]. Therefore, in some cases the solution adopted is to weld both the interference fitted parts to decrease their relative movements.
⇑ Corresponding author. Tel.: +55 79 2105 6888; fax: +55 79 2105 6845. E-mail address: [email protected] (S. Griza). 1350-6307/$ - see front matter Ó 2013 Elsevier Ltd. All rights reserved. http://dx.doi.org/10.1016/j.engfailanal.2013.02.010
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Fig. 1. At left, cracks were found after the first repair (weld deposition). At right, the new shaft is the interference fitted in the roll machined hole. Afterwards, the chamfer was welded and the curvature radius was machined.
This paper deals with the failure of a potassium ore compactor roll which experienced sudden rupture in service after some maintenance actions. The roll underwent a repair by weld filler deposited over the journal bearing opposite to that coupled to the motor, due to the expected wear after long in service use. Following a short period of operation after this first repair, the presence of deeper radial cracks was detected through ultrasonic analysis (Fig. 1). So the roll experienced a last repair procedure which was carried out as follows: (a) machining to remove the cracked portion; (b) interference fit assembly of a new shaft end; (c) welding; (d) finish machining (Fig. 1). The purpose of this study is to analyze the failure occurred due to the last repair procedure, i.e., interference fit and welding. Fracture analysis, metallographic and microhardness tests were performed. Numerical simulation was also performed in an attempt to see the changes in the stresses provided for the repair procedure. 2. Materials and methods 2.1. Preliminary analysis The roll was manufactured from SAE 4340 steel, and it was quenched and tempered to a surface hardness of at least 39 HRC. The same specification was used for the replaced shaft manufacture. After the fracture occurrence, liquid penetrant technique was performed in both fracture surfaces. 2.2. Fracture analysis The fracture surface received a simple cleaning with water, liquid soap and a soft bristle brush. Macroscopic images of the fracture surface were performed by a digital camera (Canon EOS 1000D). A fracture sample was analyzed by scanning electron microscopy (SEM – Jeol Carry Scope JCM-5700). 2.3. Microstructure The metallographic sample was prepared to analyze the microstructure near a region identified previously as one of the several fracture nucleation points. Optical microscopy was applied (Carl ZEISS Axio SCOPE A.1). The sample was etched with Nital 2%. 2.4. Microhardness The metalographic sample was conducted to the Vickers (HV0.3) microhardness (Future Tech FM – 800). A microhardness profile was achieved from the deposition metal (DM) throughout the heat affected zone (HAZ) until reaching the base metal (BM). Altogether, 150 indentations were done. The profile initiated at 2 mm from the weld face and the subsequent indentations were evaluated with 0.3 mm distance. Approaching the HAZ, the distance was changed to 0.1 mm. When the profile approached the BM and the microhardness showed stabilization, the distance was changed again to 0.3 mm. 2.5. Finite element simulation Two different sketches were performed in the 2D numerical simulation. The simulations were performed in the linear elastic regime using structured hexagonal elements and quadratic geometry. The elastic modulus was 200 GPa and Poisson’s ratio 0.3. The first simulation was carried out on the original design of the roll (Fig. 2). The interference fit and welding were taken into account in the second simulation (Fig. 2). The roll was restricted in the medial cross section of the working zone
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Fig. 2. The hexagonal mesh, restriction in the medial cross section and the applied load. At left, original design of the roll and at right, repair simulation.
Table 1 The mesh quantification for the two models. Model
No. of elements
No. of nodes
Mesh size
Original Repair
10664 21643
3463 7002
15 10
(major diameter). The design load was applied on the end of the shaft. The interface between the two fitted parts was simulated in self contact, with 0.5 mm clearance. The mesh attributions are shown in the Table 1. The mesh refinement was evaluated for the repair simulation in the region of the singularity provided by the clearance approaching the weld (see Fig. 2). The endurance limit (rm) of 300 MPa for the SAE 4340 under rotating bending was considered as design criterion [13]. This endurance limit was applied in the Marine empirical law, according to the Eq. (1) [10]. The design stress in the roll curvature radius (rR) was obtained for a safety factor (SF = 2) and reducing factors due to surface (ka = 0.7) which is function of the design specifications of the surface hardness and finishing; cross section size (kb = 0.7) and stress concentration (kf = 0.6) which is function of the load and the curvature radius to diameter ratio. Therefore, the maximum Von Mises stress applied to the curvature radius due to the external load was 45 MPa.
rR ¼
rm ka kb kf SF
ð1Þ
The repair was modeled in the second 2D simulation, from the sketches of the two parts that reproduce both the interference fitted shaft and the remained roll. The friction coefficient of 0.15 was added. The interference was disregarded in the interface. This assumption excludes the residual stresses that occur prior to the welding. In practice, the interference can be significantly reduced in the region closer to the weld due to the stress relief imposed by the weld thermal supply. A ring reproducing the weld dimensions and the machined curvature radius was bonded onto the other two parts in their respective contact regions. 3. Results 3.1. Preliminary analysis The liquid penetrant analysis performed in both fracture surfaces revealed a discontinuity at the weld root only in the roll side (Fig. 3). 3.2. Fracture analysis Fracture surface shows rotating bending fatigue appearance. A sub-superficial perimeter that matches with the weld root shows multiple ratchet marks which are typical fatigue nucleation indicatives (Fig. 4). The crack progressed through the shaft on a transverse plane and also up through the throat of the weld. It can be seen a large and flat fatigue propagation surface. Beach marks can be seen on the propagation surface and a rough final rupture zone, closer to the center of the shaft whose appearance indicates a substantial torsional deformation. The presence of several ratchet marks regarded to rotating bending stresses in detail in the Fig. 5 and the extensive surface propagation indicate low external load and high stress concentration at the weld root. The SEM analysis showed a wrinkled fracture surface (Fig. 6) with absence of fracture micromechanisms. This aspect denotes that the crack faces in propagation were compressed together during the rotating bending loading.
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Fig. 3. Liquid penetrant results. At left, the absence of discontinuity in the shaft side; at right, discontinuity in the weld root of the roll side.
Fig. 4. The macrograph of the fracture surface is shown. The dashed white line indicates the metallographic plane. The letter ‘‘a’’ refers to the region that was examined in the SEM analysis. The image shows the appearance of rotating bending fatigue starting from sub-superficial ratchet marks along the weld root.
Fig. 5. The images show details of the ratchet marks closer to the weld root and some beach marks in the propagation surface.
3.3. Microstructure The microstructural analysis showed the aspects of the deposition metal (DM), heat affected zone (HAZ) and the base metal (BM) (Fig. 7). The analysis also showed that the fracture nucleation matches with the transition between DM and HAZ, at the weld root. The DM region shows dendritic structure with acicular ferrite and dispersed carbides (Fig. 8). The HAZ zone shows low temperature transformation microstructure making appear martensite plates (Fig. 9). The BM, in its turn, also shows low temperature transformation microstructure, but with more stable aspect, with ferritic matrix and dispersed carbides making appear bainite (Fig. 10).
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Fig. 6. SEM images showing substantial wrinkling and the absence of micromechanisms on the region (a) of the fracture surface showed in the Fig. 5.
Fig. 7. Cross section macrography of the shaft. The square shows the interface between the weld metal and HAZ, coincident region with the fracture initiation. Etchant: Nital 2%.
Fig. 8. The microstructure of the weld metal is constituted of dendrites of acicular ferrite and dispersed carbides.
3.4. Microhardness The microhardness profile showed a peak of 405 HV near the interface between the weld metal and HAZ. Microhardness stabilized near 250 HV in the base metal (Fig. 11). 3.5. Finite element simulation The simulation results for both original and repair models are shown respectively in Figs. 12 and 13. The maximum Von Mises stress of 45 MPa in the original design occurs in the compressed region of the curvature radius. This result is in agreement with the wrinkled appearance of the fracture surface. In the presence of the discontinuity near the weld root, there is a perceived stress concentration in the singularity and significant increase in the Von Mises stress to 475 MPa, reaching nearly 10 times the stress seen in the previous model.
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Fig. 9. Microstructure transformed in low temperature of the HAZ with appearance of tempered martensite plates.
Fig. 10. Microstructure of the base metal where it is observed the formation of ferrite and dispersed carbides.
Fig. 11. Vickers microhardness profile (HV0,3) from the weld metal throughout the HAZ and approaching the base metal.
4. Discussions In this study we performed the failure analysis of a repaired ore compactor roll. The root cause of the failure is attributed to the shaft wear in the coupling region of the journal bearing after a long period of use. This fact is predicted in the original design. The roll design optimization goes through improving the tribologic attributes, i.e., the wear strength of the journal bearing. However, the issue of this study is limited to highlight the failure causes after the second repair applied. An interference fit followed by weld procedure was performed to allow further ore compaction while waiting for the machining a new roll. However, the sudden failure occurred in short term. Metallographic analysis was not performed on the roll side to prevent further cuts that could make its reuse difficult. However, the penetrant liquid test was important to show that the failure initiation matches the discontinuity at the weld
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Fig. 12. The original model under flexion showed Von Mises stress of 45 MPa in the curvature radius.
Fig. 13. Stress concentration in the weld root due to the singularity enhanced by the repair.
root. The absence of revealed discontinuity on the other side, the bearing shaft, incriminates the weld root as the failure initiation region. The failure occurred by rotating bending fatigue initiated at a discontinuity in the weld root repair. The multiple ratchet marks found in this region and the evident large fatigue propagation surface indicate the discontinuity as important stress riser which contributed to the short term failure. Unfortunately, the weld and the interference fit interface evaluated in the second repair were all located near the curvature radius. This is the region expected to experience the highest stress in service, and it was confirmed by the numerical simulation of the original roll. There are several reports about shaft failures whose initiation is coincident with stress risers and weld procedures [14– 16]. In some cases, the welding is more prone to failure than the stress concentration achieved by section changes [17]. Welding can provides untempered martensite zones of higher hardness, low ductility and higher stress concentration of metallurgical nature due to the thermal shrinkage. These features create favorable conditions for the nucleation and propagation of fatigue cracks. In an attempt to minimize them, it is required a strict control of the procedures as pre-heating and cooling rate of the material during the welding. Heat treatment after welding is also an important requirement to the material stress relieve and to improve the mechanical behavior [18]. However, it is expected that the heat affected zone presenting fatigue properties be lower than the base metal. In the present study, the welding procedure promoted a hardness peak in the HAZ which denotes microstructural transformations in agreement with the previous thinking. The transition between the weld and the interference fit interface, in its turn, produces a singularity that acts as stress riser. Indeed, the numerical simulation showed an increase in the Von Mises stress related to this singularity. The numerical simulation result obviously should be considered carefully due to its simplicity. The actual form of the singularity was not
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necessarily reproduced since it is difficult to know the singularity curvature radius after the fracture. Furthermore, the simulation was performed in the linear elastic regime which does not consider the possible material plastic strain closer to the singularity. However, despite the limitation in predicting the exact stress magnitude at the singularity, notably the simulation showed the increase of the stress regarded to the fracture nucleation region. Finite element modeling suggested that stresses of the order of ten times the stress in the radius could be expected at the root of the weld. This level of stress would have been sufficient to lead to fatigue failure from this point and is consistent with the observations in this paper. To reduce the risk of recurrence, it is suggested that the repair is done by introducing a shaft of greatest diameter. It moves the welding area and therefore also the singularity for a region further away from the curvature radius. However, whatever the design change adopted, it should be previously analyzed, by numerical simulation for example, to check the resulting stress level. 5. Conclusion This study was enhanced to highlight the failure causes of a ore compactor roll which had undergone a repair procedure. The repair was evaluated from a press fit followed by welding procedure. The failure was due to rotating bending fatigue, nucleated in the weld root, coincident with the singularity provided from the press fit interface and welding. The finite element analysis was able to show that the singularity acted as stress riser, promoting the short term fatigue. Acknowledgements The authors would like to thank the financial support received from CAPES, CNPq and FINEP. References [1] [2] [3] [4] [5] [6] [7] [8] [9] [10] [11] [12] [13] [14] [15] [16] [17] [18]
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