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Fatigue failure of helical compression spring in coke oven batteries
Engineering Failure Analysis 10 (2003) 291–296 www.elsevier.com/locate/engfailanal
Fatigue failure of helical compression spring in coke oven batteries B. Ravi Kumar*, Swapan K. Das, D.K. Bhattacharya Materials Characterisation Division, National Metallurgical Laboratory, Jamshedpur-831007, India Received 12 August 2002; accepted 15 October 2002
Abstract The failure of a helical compression spring employed in coke oven batteries was analysed. Microstructural analysis and hardness measurements did not show any degradation of the spring material. Surface corrosion product was analysed by XRD and SEM–EDS. Sulphur and chlorine bearing compounds were detected. Macrofractography of the fracture surface revealed beach marks, indicating fatigue as the mode of fracture and surface pits. It was established that the spring failed due to corrosion fatigue. # 2003 Elsevier Science Ltd. All rights reserved. Keywords: Spring; Fatigue; Residual stress; Corrosion; Coke oven
1. Introduction A large variety of springs operate under frequently varying loads and deflections. These springs fail by fracture, although the maximum operating stress levels are not only well below the ultimate strength but even the elastic limit of the material. Such failures are invariably fatigue failures. Fatigue failure can often be unpredictable and it is difficult for the spring designer to set safe working materials property and stress limits [1]. In order to ensure long life under fatigue loading conditions, it is generally necessary to design for comparatively low maximum stresses. Since fatigue fracture is controlled by surface condition and is assisted by tensile stresses, techniques such as surface hardening and shot peening are introduced to improve life. Fatigue failures are progressive cracking failures, caused by tensile stress only since it would be difficult to conceive of cracking due to compressive stresses [2,3]. It must not be overlooked that the torsional stresses which occur in helical coil springs can be resolved into principal tensile and compressive components, and these tensile components can also cause cracking. In the present study, a failed helical spring used in coke oven batteries has been examined for the cause of failure. The manufacturer of the spring supplied a virgin spring along with the failed one for comparison of the properties between failed and virgin springs. The spring has four active coils and was used to compensate * Corresponding author. Tel.: +91-657-270027; fax: +91-657-270527. E-mail address: [email protected] (B.R. Kumar). 1350-6307/03/$ - see front matter # 2003 Elsevier Science Ltd. All rights reserved. PII: S1350-6307(02)00075-4
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B.R. Kumar et al. / Engineering Failure Analysis 10 (2003) 291–296
for expansion or contraction in the coke oven battery during heating of the coal. Due to variations in the dimension of the battery the spring was under fatigue loading. The springs have been in general failing well before their stipulated service life. In this failure analysis, the causes of the premature failure of the springs have been investigated.
2. Testing procedure and results 2.1. Visual examination of general physical features Visual examination of the failed spring was carried out to inspect the general overall condition of the spring. The spring has four active coils and fracture took place in the fourth active coil from the bottom of the spring as shown in Figure 1(a). The fracture surface extended at an angle to the spring wire axis from near the outer diameter (OD) to towards the inner diameter (ID) of the spring. The surface of the spring was covered with a thick layer of weakly adhering corrosion product, Figure 1(a). No useful discernible fracture surface feature was visible due to the presence of corrosion product, Figure 1(b). 2.2. Chemical analysis Specimens were cut from the virgin and failed components (close to the failed surface in the latter) and analysed for material composition by Direct Reading Spectrometer. The compositions of both virgin and failed springs were within the specification limits of 51CrMoV4 spring steel grade (Table 1). 2.3. Surface examination (A) Surface corrosion product analysis The thick corrosion product on the surface was carefully removed from the service exposed spring. It was analysed for phases and chemistry by X-ray diffraction (XRD), and energy dispersive spectroscopy (EDS) attached to the scanning electron microscope (SEM). XRD phase analysis of the corrosion product indicated Fe3S4, Fe9S10, FeOC1.NH3 and iron oxide. Elementa1 analysis by EDS supported the above findings by identifying sulphur and chlorine in the same sample (Figure 2).
Fig. 1. (a) Photograph of the helical compression spring showing fracture, (b) magnified image of the fracture surface. A thick layer of corrosion product cover can be seen in both photographs.
293
B.R. Kumar et al. / Engineering Failure Analysis 10 (2003) 291–296 Table 1 Chemical analysis of the spring material C
Mn
Si
S
P
Cr
Mo
V
0.50
0.91
0.24
0.03
0.03
1.0
0.19
0.1
(B) Fracture surface analysis The fracture surface of the failed spring was cut and cleaned to remove the top corrosion product layer from its surface to reveal the subsurface fracture features if any. Surface cleaning revealed reasonably good fracture features. Macrofractography of the fracture surface was conducted using a stereomicroscope. A typical characteristic feature of the fatigue fracture in the form of beach marks was found on the failed surface. This is indicated by arrow marks in Figure 3(a) and (b). A careful assessment of the fracture surface was made to find the origin and path of the fracture. This revealed crack initiation near the OD of the spring. This site is marked in Figure 3(a) as ‘A’. A magnified view is shown in Figure 3(b). The crack appeared to have grown at an angle to the wire axis from near the OD towards the ID. The final fast fracture path is indicated by ‘F’ in the figure. No SEM fractography studies were possible even after cleaning due to the presence of the corroded layer. The subsurface immediately below the corrosion layer was examined to evaluate its condition. A very rough surface with pit like features was observed, Figure 3(c). These could possibly be corrosion pits generated during degradation of the spring surface.
Fig. 2. EDX plot showing presence of sulphur and chlorine in the corrosion product.
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B.R. Kumar et al. / Engineering Failure Analysis 10 (2003) 291–296
Fig. 3. (a) Beach marks (indicated by arrows) and crack initiation (A) and fast fracture (F) along the path indicated, (b) magnified view of the same, (c) subsurface condition with corrosion pits (spots).
2.4. Metallography Specimens were cut perpendicular to the spring wire axis as thin discs from both virgin and failed springs (near the fracture surface from the latter). Specimens were polished and etched using 2% nital as per standard metallography techniques to reveal microstructures. SEM examination of the microstructure was conducted. A very fine tempered martensitic structure was revealed in both specimens (Figure 4). Both springs appeared microstructurally similar. Specimens were scanned across the cross section of the spring rod to find any variation in microstructure from outer periphery to centre. A nearly uniform microstructure was found in both specimens. Further, no undesirable inclusions were detected in the microstructure, indicating a microstructurally healthy material. 2.5. Mechanical properties (A) Hardness A hardness survey was made across the cross section of the metallographically prepared sample on both virgin and failed specimens. A very close match of hardness in both failed and virgin springs was found. The virgin and failed samples showed hardness values of 44RC and 47RC, respectively. Here again no cross sectional variation of hardness was detected. This compares well with the earlier findings of microstrutural homogeneity.
B.R. Kumar et al. / Engineering Failure Analysis 10 (2003) 291–296
295
Fig. 4. SEM micrograph of tempered martensite in specimens of (a) failed spring and (b) virgin spring.
(B) Residual stress analysis X-ray diffraction was employed to determine residual stresses in both virgin and failed springs [4,5]. High compressive stresses of the order of 273 MPa along the wire length and 338 MPa in the circumferential direction of the spring wire were found in the virgin spring. Stress measurements on the failed spring were carried out after the removal of corrosion product. In general, stresses of very small magnitude ranging from 23 MPa to 43 MPa were detected. The decrease in the stress magnitude may be due to the removal of the compressed surface layer by extensive corrosion. The presence of tensile stresses of low level is bad for fatigue resistance.
4. Discussion The failure analysis is divided into Part I and Part II. Part I: In this part, materials related aspects of the failure are looked at. From the results of chemical analysis, microstructural and hardness studies it is found that the material conforms to the spring manufacturer’s specifications. A uniform tempered martensitic microstructure across the cross section of the spring rod has been observed. No microstructural degradation or presence of inclusions have been noticed. The absence of both of these factors indicates that there is no loss in the fatigue resistance of the material. Therefore, material degradation or improper materials properties have been eliminated. Part II: Here the service/environmental conditions are considered. Results of macrofractography indicated beach marks on the fracture surface, a typical characteristic feature of fatigue fracture. Failure by fatigue occurs over a time period. Such failures are generally not related to service overload failures. Hence the possibility of any overloading of the component is ruled out in this case. As per the user’s specification, the spring was designed to perform at ambient temperature. The results of microstructure and hardness studies of the failed spring did not show any degradation that could be correlated to high temperature during service. Both microstructure and hardness were similar to the virgin spring. The presence of a highly porous corrosion product on the spring surface has been considered. The phase and elemental analysis of this product by XRD and SEM revealed sulphur and chlorine bearing compounds. This indicates the existence of a corrosive environment at the plant. The presence of sulphur and chlorine increases the propensity for corrosion of steels and causes excessive material loss leading to section thinning and development of surface pits [6,7]. Surface pitting and reduced cross sections due to dissolution of iron in the presence of the corrosive environment can act as stress concentration sites in components subjected to load. These defects would lead to a reduction in the fatigue resistance of the
296
B.R. Kumar et al. / Engineering Failure Analysis 10 (2003) 291–296
material [8,9]. Fatigue crack initiation could take place at these areas at loads well below the yield point of the material and finally cause failure of the component by fatigue [10]. In the present situation, extensive corrosion of the spring was observed. The subsurface immediately below the corrosion product has shown some surface pits. The presence of subsurface corrosion pits along with sulphur and chlorine bearing corrosion products indicate corrosion fatigue as the main cause of failure. This conclusion is supported by the residual stress results showing a marked decrease or reversal of surface residual compressive stress.
5. Conclusion The most probable cause of failure of the helical compression springs was corrosion fatigue accentuated by loss of surface residual compressive stress.
Acknowledgements The authors are grateful to Professor S.P. Mehrotra, Director, National Metallurgical Laboratory, for giving permission to publish this work.
References [1] [2] [3] [4] [5] [6] [7] [8]
Berry WR. Spring design a practical treatment. London: Emmott & Company Limited; 1961. Kovac M. Materials Science and Technology 1994;10(5):384–8. Larsson M, Melander A, Nordgren A. Materials Science and Technology 1993;9(3):235–45. Residual Stress Measurement by X-ray Diffraction. Report: SAE J84a, Society of Automotive Engineerings, USA; 1971. Noyan IC, Cohen JB. Residual Stress Measurement By X-ray Diffraction and Interpretation. New York: Springer Verlag; 1987. Kane RD. International Metals Reviews 1985;30(6):291–301. Tomlinson WJ, Meades DM. Corrosion 1991;47(4):269–71. Zhou X, Ke W, Zang Q. Conference: Corrosion and Corrosion Control for Offshore and Marine construction, Xiamen, China, 1988. Beijing: International Academic Publishers; 1988. [9] Kondo Y. Corrosion 1989;45(1):7–11. [10] Crooker TW, Leis BN, editors. Corrosion fatigue: mechanics, metallurgy, electrochemistry, and engineering. ASTM; 1983.
Fatigue failure of helical compression spring in coke oven batteries B. Ravi Kumar*, Swapan K. Das, D.K. Bhattacharya Materials Characterisation Division, National Metallurgical Laboratory, Jamshedpur-831007, India Received 12 August 2002; accepted 15 October 2002
Abstract The failure of a helical compression spring employed in coke oven batteries was analysed. Microstructural analysis and hardness measurements did not show any degradation of the spring material. Surface corrosion product was analysed by XRD and SEM–EDS. Sulphur and chlorine bearing compounds were detected. Macrofractography of the fracture surface revealed beach marks, indicating fatigue as the mode of fracture and surface pits. It was established that the spring failed due to corrosion fatigue. # 2003 Elsevier Science Ltd. All rights reserved. Keywords: Spring; Fatigue; Residual stress; Corrosion; Coke oven
1. Introduction A large variety of springs operate under frequently varying loads and deflections. These springs fail by fracture, although the maximum operating stress levels are not only well below the ultimate strength but even the elastic limit of the material. Such failures are invariably fatigue failures. Fatigue failure can often be unpredictable and it is difficult for the spring designer to set safe working materials property and stress limits [1]. In order to ensure long life under fatigue loading conditions, it is generally necessary to design for comparatively low maximum stresses. Since fatigue fracture is controlled by surface condition and is assisted by tensile stresses, techniques such as surface hardening and shot peening are introduced to improve life. Fatigue failures are progressive cracking failures, caused by tensile stress only since it would be difficult to conceive of cracking due to compressive stresses [2,3]. It must not be overlooked that the torsional stresses which occur in helical coil springs can be resolved into principal tensile and compressive components, and these tensile components can also cause cracking. In the present study, a failed helical spring used in coke oven batteries has been examined for the cause of failure. The manufacturer of the spring supplied a virgin spring along with the failed one for comparison of the properties between failed and virgin springs. The spring has four active coils and was used to compensate * Corresponding author. Tel.: +91-657-270027; fax: +91-657-270527. E-mail address: [email protected] (B.R. Kumar). 1350-6307/03/$ - see front matter # 2003 Elsevier Science Ltd. All rights reserved. PII: S1350-6307(02)00075-4
292
B.R. Kumar et al. / Engineering Failure Analysis 10 (2003) 291–296
for expansion or contraction in the coke oven battery during heating of the coal. Due to variations in the dimension of the battery the spring was under fatigue loading. The springs have been in general failing well before their stipulated service life. In this failure analysis, the causes of the premature failure of the springs have been investigated.
2. Testing procedure and results 2.1. Visual examination of general physical features Visual examination of the failed spring was carried out to inspect the general overall condition of the spring. The spring has four active coils and fracture took place in the fourth active coil from the bottom of the spring as shown in Figure 1(a). The fracture surface extended at an angle to the spring wire axis from near the outer diameter (OD) to towards the inner diameter (ID) of the spring. The surface of the spring was covered with a thick layer of weakly adhering corrosion product, Figure 1(a). No useful discernible fracture surface feature was visible due to the presence of corrosion product, Figure 1(b). 2.2. Chemical analysis Specimens were cut from the virgin and failed components (close to the failed surface in the latter) and analysed for material composition by Direct Reading Spectrometer. The compositions of both virgin and failed springs were within the specification limits of 51CrMoV4 spring steel grade (Table 1). 2.3. Surface examination (A) Surface corrosion product analysis The thick corrosion product on the surface was carefully removed from the service exposed spring. It was analysed for phases and chemistry by X-ray diffraction (XRD), and energy dispersive spectroscopy (EDS) attached to the scanning electron microscope (SEM). XRD phase analysis of the corrosion product indicated Fe3S4, Fe9S10, FeOC1.NH3 and iron oxide. Elementa1 analysis by EDS supported the above findings by identifying sulphur and chlorine in the same sample (Figure 2).
Fig. 1. (a) Photograph of the helical compression spring showing fracture, (b) magnified image of the fracture surface. A thick layer of corrosion product cover can be seen in both photographs.
293
B.R. Kumar et al. / Engineering Failure Analysis 10 (2003) 291–296 Table 1 Chemical analysis of the spring material C
Mn
Si
S
P
Cr
Mo
V
0.50
0.91
0.24
0.03
0.03
1.0
0.19
0.1
(B) Fracture surface analysis The fracture surface of the failed spring was cut and cleaned to remove the top corrosion product layer from its surface to reveal the subsurface fracture features if any. Surface cleaning revealed reasonably good fracture features. Macrofractography of the fracture surface was conducted using a stereomicroscope. A typical characteristic feature of the fatigue fracture in the form of beach marks was found on the failed surface. This is indicated by arrow marks in Figure 3(a) and (b). A careful assessment of the fracture surface was made to find the origin and path of the fracture. This revealed crack initiation near the OD of the spring. This site is marked in Figure 3(a) as ‘A’. A magnified view is shown in Figure 3(b). The crack appeared to have grown at an angle to the wire axis from near the OD towards the ID. The final fast fracture path is indicated by ‘F’ in the figure. No SEM fractography studies were possible even after cleaning due to the presence of the corroded layer. The subsurface immediately below the corrosion layer was examined to evaluate its condition. A very rough surface with pit like features was observed, Figure 3(c). These could possibly be corrosion pits generated during degradation of the spring surface.
Fig. 2. EDX plot showing presence of sulphur and chlorine in the corrosion product.
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B.R. Kumar et al. / Engineering Failure Analysis 10 (2003) 291–296
Fig. 3. (a) Beach marks (indicated by arrows) and crack initiation (A) and fast fracture (F) along the path indicated, (b) magnified view of the same, (c) subsurface condition with corrosion pits (spots).
2.4. Metallography Specimens were cut perpendicular to the spring wire axis as thin discs from both virgin and failed springs (near the fracture surface from the latter). Specimens were polished and etched using 2% nital as per standard metallography techniques to reveal microstructures. SEM examination of the microstructure was conducted. A very fine tempered martensitic structure was revealed in both specimens (Figure 4). Both springs appeared microstructurally similar. Specimens were scanned across the cross section of the spring rod to find any variation in microstructure from outer periphery to centre. A nearly uniform microstructure was found in both specimens. Further, no undesirable inclusions were detected in the microstructure, indicating a microstructurally healthy material. 2.5. Mechanical properties (A) Hardness A hardness survey was made across the cross section of the metallographically prepared sample on both virgin and failed specimens. A very close match of hardness in both failed and virgin springs was found. The virgin and failed samples showed hardness values of 44RC and 47RC, respectively. Here again no cross sectional variation of hardness was detected. This compares well with the earlier findings of microstrutural homogeneity.
B.R. Kumar et al. / Engineering Failure Analysis 10 (2003) 291–296
295
Fig. 4. SEM micrograph of tempered martensite in specimens of (a) failed spring and (b) virgin spring.
(B) Residual stress analysis X-ray diffraction was employed to determine residual stresses in both virgin and failed springs [4,5]. High compressive stresses of the order of 273 MPa along the wire length and 338 MPa in the circumferential direction of the spring wire were found in the virgin spring. Stress measurements on the failed spring were carried out after the removal of corrosion product. In general, stresses of very small magnitude ranging from 23 MPa to 43 MPa were detected. The decrease in the stress magnitude may be due to the removal of the compressed surface layer by extensive corrosion. The presence of tensile stresses of low level is bad for fatigue resistance.
4. Discussion The failure analysis is divided into Part I and Part II. Part I: In this part, materials related aspects of the failure are looked at. From the results of chemical analysis, microstructural and hardness studies it is found that the material conforms to the spring manufacturer’s specifications. A uniform tempered martensitic microstructure across the cross section of the spring rod has been observed. No microstructural degradation or presence of inclusions have been noticed. The absence of both of these factors indicates that there is no loss in the fatigue resistance of the material. Therefore, material degradation or improper materials properties have been eliminated. Part II: Here the service/environmental conditions are considered. Results of macrofractography indicated beach marks on the fracture surface, a typical characteristic feature of fatigue fracture. Failure by fatigue occurs over a time period. Such failures are generally not related to service overload failures. Hence the possibility of any overloading of the component is ruled out in this case. As per the user’s specification, the spring was designed to perform at ambient temperature. The results of microstructure and hardness studies of the failed spring did not show any degradation that could be correlated to high temperature during service. Both microstructure and hardness were similar to the virgin spring. The presence of a highly porous corrosion product on the spring surface has been considered. The phase and elemental analysis of this product by XRD and SEM revealed sulphur and chlorine bearing compounds. This indicates the existence of a corrosive environment at the plant. The presence of sulphur and chlorine increases the propensity for corrosion of steels and causes excessive material loss leading to section thinning and development of surface pits [6,7]. Surface pitting and reduced cross sections due to dissolution of iron in the presence of the corrosive environment can act as stress concentration sites in components subjected to load. These defects would lead to a reduction in the fatigue resistance of the
296
B.R. Kumar et al. / Engineering Failure Analysis 10 (2003) 291–296
material [8,9]. Fatigue crack initiation could take place at these areas at loads well below the yield point of the material and finally cause failure of the component by fatigue [10]. In the present situation, extensive corrosion of the spring was observed. The subsurface immediately below the corrosion product has shown some surface pits. The presence of subsurface corrosion pits along with sulphur and chlorine bearing corrosion products indicate corrosion fatigue as the main cause of failure. This conclusion is supported by the residual stress results showing a marked decrease or reversal of surface residual compressive stress.
5. Conclusion The most probable cause of failure of the helical compression springs was corrosion fatigue accentuated by loss of surface residual compressive stress.
Acknowledgements The authors are grateful to Professor S.P. Mehrotra, Director, National Metallurgical Laboratory, for giving permission to publish this work.
References [1] [2] [3] [4] [5] [6] [7] [8]
Berry WR. Spring design a practical treatment. London: Emmott & Company Limited; 1961. Kovac M. Materials Science and Technology 1994;10(5):384–8. Larsson M, Melander A, Nordgren A. Materials Science and Technology 1993;9(3):235–45. Residual Stress Measurement by X-ray Diffraction. Report: SAE J84a, Society of Automotive Engineerings, USA; 1971. Noyan IC, Cohen JB. Residual Stress Measurement By X-ray Diffraction and Interpretation. New York: Springer Verlag; 1987. Kane RD. International Metals Reviews 1985;30(6):291–301. Tomlinson WJ, Meades DM. Corrosion 1991;47(4):269–71. Zhou X, Ke W, Zang Q. Conference: Corrosion and Corrosion Control for Offshore and Marine construction, Xiamen, China, 1988. Beijing: International Academic Publishers; 1988. [9] Kondo Y. Corrosion 1989;45(1):7–11. [10] Crooker TW, Leis BN, editors. Corrosion fatigue: mechanics, metallurgy, electrochemistry, and engineering. ASTM; 1983.