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Study of a torsion spring fracture
Engineering Failure Analysis 98 (2019) 150–155
Contents lists available at ScienceDirect
Engineering Failure Analysis journal homepage: www.elsevier.com/locate/engfailanal
Study of a torsion spring fracture ⁎
Valles González María Pilar , García-Martínez María, Pastor Muro Ana
T
National Institute of Aerospace Technology (INTA), Ctra. Torrejón-Ajalvir Km.4, 28850 Torrejón de Ardoz, Madrid, Spain
A R T IC LE I N F O
ABS TRA CT
Keywords: Failure analysis Microstructures Surface finish Fatigue Cracks
One flange of a spring belonging to an actuator drive that worked under torsion has been fractured. It is placed next to the electric motors whose mission is to move the drive manually through a hexagonal key. The whole equipment is an extensor-retraction actuator consisting in two electric motors that run a gear. This gear pulls a chain moving a telescopic beam. It was confirmed that there was no signs of anomalous working previous to the fracture. Some roughness requirements were mandatory in the first wire of the spring as well as both spring flanges, thus two springs (used and new) were compared. Several tests were performed in order to determine the cause of the fracture such as a chemical and mechanical characterization of the material, a fractographic analysis of the fracture surface, and the determination of the spring treatment state using metallographic and microscopic techniques. Additional observations were carried out in the other flange in order to detect possible defects. It was verified that the material corresponds to a 300 maraging steel type IV in a solubilization and ageing heat treatment state. The fracture surface presented fractographic characteristics of a typical progressive fracture. It was concluded that although the steel type and treatment state is correct, the roughness of specific zones is not well finished and, may have generated a crack that have nucleated in a notch of the first wire of the spring inner side surface. The crack may be produced by a pinned particle or any other surface defect that afterwards may propagate by a fatigue mechanism.
1. Introduction Maraging steels with high nickel content, like type 300, present a unique combination of high resistance, excellent fracture toughness with sufficient ductility, being very useful for applications such as rocket and missile engine cases, springs, actuators, landing gear components, etc. [1,2]. They achieve their best properties by applying a heat treatment with ageing precipitation after their martensitic transformation. They are very dimensionally stable and do not require protective atmospheres during the heat treatment [3]. Its precipitates are intermetallic compounds formed by the combination of Ni and Co plus alloying elements like Mo and Ti. These steels offer an attractive alternative to high and medium C tool steels because they have neither corrosion problems due to the high concentration of Ni and the absence of carbides, nor quenching cracks because of their low C content [4]. The fractured spring is a helical torsion spring, compound by a body with several wires and two flanges at the end of each extreme. This spring works under bending moments and this dynamic load usually leads to fatigue failures. The main causes of fatigue failures are material defects, surface imperfections, inadequate heat treatments, corrosion or decarburization. Under service conditions, the position of maximum effort is the internal surface of a spring active wire, thus the wire surface is very sensitive to material imperfections that provide stress concentration points that may initiate fatigue cracks. Besides, it is well known that in this type of spring, the fracture appears in the transition zone between the body and the first wire of the spring [5]. ⁎
Corresponding author. E-mail address: [email protected] (M.P. Valles González).
https://doi.org/10.1016/j.engfailanal.2019.01.075 Received 21 November 2018; Received in revised form 21 January 2019; Accepted 23 January 2019 Available online 24 January 2019 1350-6307/ © 2019 Published by Elsevier Ltd.
Engineering Failure Analysis 98 (2019) 150–155
M.P. Valles González et al.
Table 1 Chemical composition (% weight). Fe
C
Mn
Si
Ni
Co
Mo
Ti
Al
P
S
Base
˂0,02
0,03
0,01
18,7
8,8
4,8
0,72
0,11
< 0,01
< 0,005
Several studies have been carried out to verify the influence of the spring surface state in the material fatigue behaviour, concluding that the finishing state has to be taken into consideration, because the tested fatigue polished samples presented a longer life than the ones that were tested directly after the machining [6]. This work presents the followed procedure to determine the cause of the fracture of one of the spring flange under torsion loads. 2. Materials and methods. experimental section The chemical analysis of the spring material, shown in Table 1, was carried out by an X-ray fluorescence equipment Panalytical, model PW2404, and the fusion and combustion techniques LECO equipments. This material composition corresponds to a 300 maraging steel type IV. A hardness test was carried out in one of the spring wires far from the fracture area, using a durometer Galileo Rockwell, resulting in a hardness of 52,5 ± 0,5 HRC. The metallographic samples were obtained mounting the zones of interest in conductive resin and after polishing, they were etched with Vilella etching agent (1 g picric acid, 5 ml HCl in 100 ml of ethanol). Their microstructure was observed in an optical microscope Leica MEF4M and, using the software for image analysis Leica QUANTIMET, the grain size was determined according to the planimetric method following the ASTM E112 standard. To observe the fracture surface, a stereo microscope Leica Wild M10 and a scanning electron microscope Jeol 840 equipped with an X-ray dispersive energies microanalysis system were used. The Ra roughness parameter was determined using a Taylor-Hobson equipment, model Form Talysurf serial 2. 3. Results and discussion 3.1. Visual observation and macrofractographic study The spring was fractured in one of the flanges (Fig. 1a). It was also observed that the last wire containing the fracture presented a brown coloration. See Fig. 1b. Four zones have been marked in the fractured surface of the flange (Fig. 2a). Zone 1 corresponds to the side inner surface of the wire, zone 2 corresponds to the inner surface, zone 3 corresponds to the side external surface and zone 4 corresponds to the external surface. Considering the macrofractographic signs, zone 1 (Fig. 2b) may correspond to the initial fracture zone because some cracks parallel to the fracture surface were observed next to it (Fig. 2d). Fracture surfaces present several bright crushed zones on them. Considering its aspect, the surface fracture may correspond to a fatigue fracture whose origin would be in the inner side surface of the first spring wire, next to the flange. As it is shown in Fig. 3, the other flange of the spring was also inspected, detecting a rubbed zone in the elbow area, high surface damage in the external side surface and two cracks. One of them has been originated in the defect placed in the inner side surface of the first wire near to the fillet and it has propagated along the external side surface.
Fig. 1. Side and top view of the fractured spring. Fig. 1a) fracture flange. Fig. 1b) last wire brown coloration. (For interpretation of the references to colour in this figure legend, the reader is referred to the web version of this article.) 151
Engineering Failure Analysis 98 (2019) 150–155
M.P. Valles González et al.
Fig. 2. a) Flange fracture surface. b) Zone 1. Origin zone. c) Fracture surface of the last spring wire. d) Cracks parallel to the fracture.
Fig. 3. a) Damage in the flange without fracture in the elbow area. b) Presence of cracks c) Origin of a crack in a surface defect. d) Another crack.
152
Engineering Failure Analysis 98 (2019) 150–155
M.P. Valles González et al.
Fig. 4. a) Longitudinal and cross section of the spring wire, containing the fracture and the decarburized area. b) Crack localized in the marked zone area in 4a) that begins in a surface defect of the inner part. c) Decarburized area.
3.2. Microstructural characterization Metallographic samples were prepared from a longitudinal and a cross section of a spring wire near the fracture. In Fig. 4, the longitudinal section showed a crack nucleated in a surface defect in the inner part of the wire. In addition, a possible decarburization in the external part of the wire has been observed. Fig. 5 shows the microstructure of the spring material obtained from the longitudinal section of a wire placed far from the fracture. It is martensitic in a dendritic substructure, as expected from a spring steel heat treated with a solubilization and ageing process. After observing a possible decarburization area (Fig. 4a), a Vickers hardness (HV 0,1Kg) scan was performed from the external to the inner part in the spring wire near the fracture. The test was carried out using a microdurometer Future FM-7 and the results are presented in Fig. 6. According to this graph, a decrease in hardness from the wire surface to 0,5 mm of thickness approximately occurs, confirming the material decarburization in that area. 3.3. Microfractographic study Both fracture surfaces presented a lot of crushing, therefore the less damaged surface containing the flange was observed. The fracture surface was divided in four zones for inspection. See Fig. 7a. Zone 1 presents the fracture origin morphology, so it was examined along lines denoted as A and B. No clear signs of the origin of the fracture were determined, although fatigue striations were presented at the beginning of the crack propagation area. As the fracture progresses these microfractographical signs are mixed with voids that correspond to partial fractures by overload (Fig. 7d). These microfractographical signs are typical of a progressive fracture in this type of steels. Some ductile fatigue striations are present in the crack propagation area with voids in the final zone where the fracture has produced by static overload. 3.4. Roughness determination A roughness value of 64 μin, equivalent to 16,256 μm, is required, except for some areas, such as the first wire and both flanges, where the value is restricted to 16 μin, equivalent to 0,40,640 μm. Considering these values, a microscopic observation in these and a roughness measure in a new spring were carried out. In the microscopic observation, several surfaces defects were observed like voids, scratches from the grinding, SiC embedded
Fig. 5. a) Martensitic microstructure in a dendritic substructure in a zone far from the fracture. b) Martensitic laths in the austenitic grains. 153
Engineering Failure Analysis 98 (2019) 150–155
M.P. Valles González et al.
VICKERS HARDNESS SCAN LONGITUDINAL SECTION NEXT TO THE FRACTURE 800 700 600
HV 0,1 Kg
500 400 300 200 100 0
0.0
0.2
0.4
0.6
0.8
1.0
1.2
1.4
1.6
1.8
Distance from external surface (mm) Fig. 6. a) Vickers hardness scan graph.
Fig. 7. a) Fracture surface containing the flange. It is divided in 4 zones and the line inspections are marked. b) Possible origin of the fracture. c) Fatigue striations. d) Transition zone between striations and voids.
particles and Cu and Zn enriched particles that may correspond to a brass tool. All these defects lead to think that the surface preparation was not correct. In Fig. 8, one area of the flange and the surface appearance is shown. Considering the Ra permitted values mentioned above, the roughness is adequate in the external surfaces of wire 1 and 2 next to the flanges, but it is higher than the required in both surfaces of the flanges. The Ra roughness measures determined by the standard UNE-ISO 4827 are shown in Table 2. 4. Conclusions The spring has been manufactured with a 300 maraging steel type IV and presents a martensitic microstructure in a dendritic substructure with a hardness value of 52 HRC. These values correspond to a solubilization and ageing heat treatment. A dark/brown discoloration has been observed in the first wire of the spring with a decarburization area of 0,5 mm of thickness. Probably, it has been produced by friction after the fracture because no previous functional problem was confirmed. 154
Engineering Failure Analysis 98 (2019) 150–155
M.P. Valles González et al.
Fig. 8. a) Inner surface of one of the flanges. A line where the roughness measures are carried out is marked. b) Surface appearance where some scratches and rests of the employed material are shown. Table 2 Roughness measures. Area
Ra(μm)
Inner flange 1 External flange 1 Wire 1 next to flange Wire 2 next to flange Inner flange 2 External flange 2 Wire 1 next to flange Wire 2 next to flange
0,5436 13,113 0,0933 0,1921 0,5707 10,193 0,1692 0,0984
1 1
2 2
In the unbroken flange zone some cracks were detected, observing that one of them was originated in a defect of the inner surface of the first wire next to the fillet, and it has propagated along its external side surface. The roughness inspection of the inner and external surfaces of a new spring revealed that the special requirements regarding the surface finishing has not been satisfied. Roughness values exceeded the limits on the inner surface as well as on the external surface of both flanges. Macrofractographic and microfractographic signs revealed that the fracture was originated in a critical zone of the spring, next to the fillet, between the first wire and the flange. The origin was localized in the inner side surface of the first wire of the spring and the fracture was completed by static overload. Despite the type of steel and heat treatment fulfilled the material requirements, the roughness finishing of the critical areas was not correct. This fact may lead to origin a crack in a notch of the lateral inner surface of the first spring wire, produced by the presence of a particle or other surface defect that, afterward, it may be propagated by a fatigue mechanism. Acknowledgements Authors want to thank all Metallic Materials Area staff and especially, to the Microanalytical and Microstructural Characterization Laboratory. This research did not received any specific grant from funding agencies in the public, commercial or not-for-profit sectors. References [1] Wei Wang, Wei Yan, Qiqiang Duan, Yiyin Shan, Zhefeng Zhang, Ke Yang, Study on fatigue property of a new 2.8 GPa grade maraging steel, Mater. Sci. Eng. A 527 (2010) 3057–3063. [2] K.V. Rajkumar, B.P.C. Rao, Characterization of aging behaviour in M250 grade maraging steel using eddy current non-destructive methodology, Mater. Sci. Eng. A 464 (2007) 233–240. [3] A.T.I. Technical Data Sheet, VERSION 1 (2012) 1–10. [4] M. Stanford, K. Kibble, M. Lindop, D. Mynors, C. Durnall, An Investigation into Fully Melting a Maraging Steel Using Direct Metal Laser Sintering (DMLS), Steel Research Inst. 79 Special Edition Metal Forming Conference, 2008, pp. 847–852. [5] Y. Prawoto, M. Ikeda, S.K. Manville, A. Nishikawa, Design and failure modes of automotive suspension springs, Eng. Fail. Anal. 15 (2008) 1155–1174. [6] R. Fujckaz, C. Nolan, J. Kapp, The Effects of Surface Treatment on Torsional Fatigue Failure in Recoil Springs, Technical Report ARCCB-TR-92045. US ARMY ARMAMENT RESEARCH, 1992.
155
Contents lists available at ScienceDirect
Engineering Failure Analysis journal homepage: www.elsevier.com/locate/engfailanal
Study of a torsion spring fracture ⁎
Valles González María Pilar , García-Martínez María, Pastor Muro Ana
T
National Institute of Aerospace Technology (INTA), Ctra. Torrejón-Ajalvir Km.4, 28850 Torrejón de Ardoz, Madrid, Spain
A R T IC LE I N F O
ABS TRA CT
Keywords: Failure analysis Microstructures Surface finish Fatigue Cracks
One flange of a spring belonging to an actuator drive that worked under torsion has been fractured. It is placed next to the electric motors whose mission is to move the drive manually through a hexagonal key. The whole equipment is an extensor-retraction actuator consisting in two electric motors that run a gear. This gear pulls a chain moving a telescopic beam. It was confirmed that there was no signs of anomalous working previous to the fracture. Some roughness requirements were mandatory in the first wire of the spring as well as both spring flanges, thus two springs (used and new) were compared. Several tests were performed in order to determine the cause of the fracture such as a chemical and mechanical characterization of the material, a fractographic analysis of the fracture surface, and the determination of the spring treatment state using metallographic and microscopic techniques. Additional observations were carried out in the other flange in order to detect possible defects. It was verified that the material corresponds to a 300 maraging steel type IV in a solubilization and ageing heat treatment state. The fracture surface presented fractographic characteristics of a typical progressive fracture. It was concluded that although the steel type and treatment state is correct, the roughness of specific zones is not well finished and, may have generated a crack that have nucleated in a notch of the first wire of the spring inner side surface. The crack may be produced by a pinned particle or any other surface defect that afterwards may propagate by a fatigue mechanism.
1. Introduction Maraging steels with high nickel content, like type 300, present a unique combination of high resistance, excellent fracture toughness with sufficient ductility, being very useful for applications such as rocket and missile engine cases, springs, actuators, landing gear components, etc. [1,2]. They achieve their best properties by applying a heat treatment with ageing precipitation after their martensitic transformation. They are very dimensionally stable and do not require protective atmospheres during the heat treatment [3]. Its precipitates are intermetallic compounds formed by the combination of Ni and Co plus alloying elements like Mo and Ti. These steels offer an attractive alternative to high and medium C tool steels because they have neither corrosion problems due to the high concentration of Ni and the absence of carbides, nor quenching cracks because of their low C content [4]. The fractured spring is a helical torsion spring, compound by a body with several wires and two flanges at the end of each extreme. This spring works under bending moments and this dynamic load usually leads to fatigue failures. The main causes of fatigue failures are material defects, surface imperfections, inadequate heat treatments, corrosion or decarburization. Under service conditions, the position of maximum effort is the internal surface of a spring active wire, thus the wire surface is very sensitive to material imperfections that provide stress concentration points that may initiate fatigue cracks. Besides, it is well known that in this type of spring, the fracture appears in the transition zone between the body and the first wire of the spring [5]. ⁎
Corresponding author. E-mail address: [email protected] (M.P. Valles González).
https://doi.org/10.1016/j.engfailanal.2019.01.075 Received 21 November 2018; Received in revised form 21 January 2019; Accepted 23 January 2019 Available online 24 January 2019 1350-6307/ © 2019 Published by Elsevier Ltd.
Engineering Failure Analysis 98 (2019) 150–155
M.P. Valles González et al.
Table 1 Chemical composition (% weight). Fe
C
Mn
Si
Ni
Co
Mo
Ti
Al
P
S
Base
˂0,02
0,03
0,01
18,7
8,8
4,8
0,72
0,11
< 0,01
< 0,005
Several studies have been carried out to verify the influence of the spring surface state in the material fatigue behaviour, concluding that the finishing state has to be taken into consideration, because the tested fatigue polished samples presented a longer life than the ones that were tested directly after the machining [6]. This work presents the followed procedure to determine the cause of the fracture of one of the spring flange under torsion loads. 2. Materials and methods. experimental section The chemical analysis of the spring material, shown in Table 1, was carried out by an X-ray fluorescence equipment Panalytical, model PW2404, and the fusion and combustion techniques LECO equipments. This material composition corresponds to a 300 maraging steel type IV. A hardness test was carried out in one of the spring wires far from the fracture area, using a durometer Galileo Rockwell, resulting in a hardness of 52,5 ± 0,5 HRC. The metallographic samples were obtained mounting the zones of interest in conductive resin and after polishing, they were etched with Vilella etching agent (1 g picric acid, 5 ml HCl in 100 ml of ethanol). Their microstructure was observed in an optical microscope Leica MEF4M and, using the software for image analysis Leica QUANTIMET, the grain size was determined according to the planimetric method following the ASTM E112 standard. To observe the fracture surface, a stereo microscope Leica Wild M10 and a scanning electron microscope Jeol 840 equipped with an X-ray dispersive energies microanalysis system were used. The Ra roughness parameter was determined using a Taylor-Hobson equipment, model Form Talysurf serial 2. 3. Results and discussion 3.1. Visual observation and macrofractographic study The spring was fractured in one of the flanges (Fig. 1a). It was also observed that the last wire containing the fracture presented a brown coloration. See Fig. 1b. Four zones have been marked in the fractured surface of the flange (Fig. 2a). Zone 1 corresponds to the side inner surface of the wire, zone 2 corresponds to the inner surface, zone 3 corresponds to the side external surface and zone 4 corresponds to the external surface. Considering the macrofractographic signs, zone 1 (Fig. 2b) may correspond to the initial fracture zone because some cracks parallel to the fracture surface were observed next to it (Fig. 2d). Fracture surfaces present several bright crushed zones on them. Considering its aspect, the surface fracture may correspond to a fatigue fracture whose origin would be in the inner side surface of the first spring wire, next to the flange. As it is shown in Fig. 3, the other flange of the spring was also inspected, detecting a rubbed zone in the elbow area, high surface damage in the external side surface and two cracks. One of them has been originated in the defect placed in the inner side surface of the first wire near to the fillet and it has propagated along the external side surface.
Fig. 1. Side and top view of the fractured spring. Fig. 1a) fracture flange. Fig. 1b) last wire brown coloration. (For interpretation of the references to colour in this figure legend, the reader is referred to the web version of this article.) 151
Engineering Failure Analysis 98 (2019) 150–155
M.P. Valles González et al.
Fig. 2. a) Flange fracture surface. b) Zone 1. Origin zone. c) Fracture surface of the last spring wire. d) Cracks parallel to the fracture.
Fig. 3. a) Damage in the flange without fracture in the elbow area. b) Presence of cracks c) Origin of a crack in a surface defect. d) Another crack.
152
Engineering Failure Analysis 98 (2019) 150–155
M.P. Valles González et al.
Fig. 4. a) Longitudinal and cross section of the spring wire, containing the fracture and the decarburized area. b) Crack localized in the marked zone area in 4a) that begins in a surface defect of the inner part. c) Decarburized area.
3.2. Microstructural characterization Metallographic samples were prepared from a longitudinal and a cross section of a spring wire near the fracture. In Fig. 4, the longitudinal section showed a crack nucleated in a surface defect in the inner part of the wire. In addition, a possible decarburization in the external part of the wire has been observed. Fig. 5 shows the microstructure of the spring material obtained from the longitudinal section of a wire placed far from the fracture. It is martensitic in a dendritic substructure, as expected from a spring steel heat treated with a solubilization and ageing process. After observing a possible decarburization area (Fig. 4a), a Vickers hardness (HV 0,1Kg) scan was performed from the external to the inner part in the spring wire near the fracture. The test was carried out using a microdurometer Future FM-7 and the results are presented in Fig. 6. According to this graph, a decrease in hardness from the wire surface to 0,5 mm of thickness approximately occurs, confirming the material decarburization in that area. 3.3. Microfractographic study Both fracture surfaces presented a lot of crushing, therefore the less damaged surface containing the flange was observed. The fracture surface was divided in four zones for inspection. See Fig. 7a. Zone 1 presents the fracture origin morphology, so it was examined along lines denoted as A and B. No clear signs of the origin of the fracture were determined, although fatigue striations were presented at the beginning of the crack propagation area. As the fracture progresses these microfractographical signs are mixed with voids that correspond to partial fractures by overload (Fig. 7d). These microfractographical signs are typical of a progressive fracture in this type of steels. Some ductile fatigue striations are present in the crack propagation area with voids in the final zone where the fracture has produced by static overload. 3.4. Roughness determination A roughness value of 64 μin, equivalent to 16,256 μm, is required, except for some areas, such as the first wire and both flanges, where the value is restricted to 16 μin, equivalent to 0,40,640 μm. Considering these values, a microscopic observation in these and a roughness measure in a new spring were carried out. In the microscopic observation, several surfaces defects were observed like voids, scratches from the grinding, SiC embedded
Fig. 5. a) Martensitic microstructure in a dendritic substructure in a zone far from the fracture. b) Martensitic laths in the austenitic grains. 153
Engineering Failure Analysis 98 (2019) 150–155
M.P. Valles González et al.
VICKERS HARDNESS SCAN LONGITUDINAL SECTION NEXT TO THE FRACTURE 800 700 600
HV 0,1 Kg
500 400 300 200 100 0
0.0
0.2
0.4
0.6
0.8
1.0
1.2
1.4
1.6
1.8
Distance from external surface (mm) Fig. 6. a) Vickers hardness scan graph.
Fig. 7. a) Fracture surface containing the flange. It is divided in 4 zones and the line inspections are marked. b) Possible origin of the fracture. c) Fatigue striations. d) Transition zone between striations and voids.
particles and Cu and Zn enriched particles that may correspond to a brass tool. All these defects lead to think that the surface preparation was not correct. In Fig. 8, one area of the flange and the surface appearance is shown. Considering the Ra permitted values mentioned above, the roughness is adequate in the external surfaces of wire 1 and 2 next to the flanges, but it is higher than the required in both surfaces of the flanges. The Ra roughness measures determined by the standard UNE-ISO 4827 are shown in Table 2. 4. Conclusions The spring has been manufactured with a 300 maraging steel type IV and presents a martensitic microstructure in a dendritic substructure with a hardness value of 52 HRC. These values correspond to a solubilization and ageing heat treatment. A dark/brown discoloration has been observed in the first wire of the spring with a decarburization area of 0,5 mm of thickness. Probably, it has been produced by friction after the fracture because no previous functional problem was confirmed. 154
Engineering Failure Analysis 98 (2019) 150–155
M.P. Valles González et al.
Fig. 8. a) Inner surface of one of the flanges. A line where the roughness measures are carried out is marked. b) Surface appearance where some scratches and rests of the employed material are shown. Table 2 Roughness measures. Area
Ra(μm)
Inner flange 1 External flange 1 Wire 1 next to flange Wire 2 next to flange Inner flange 2 External flange 2 Wire 1 next to flange Wire 2 next to flange
0,5436 13,113 0,0933 0,1921 0,5707 10,193 0,1692 0,0984
1 1
2 2
In the unbroken flange zone some cracks were detected, observing that one of them was originated in a defect of the inner surface of the first wire next to the fillet, and it has propagated along its external side surface. The roughness inspection of the inner and external surfaces of a new spring revealed that the special requirements regarding the surface finishing has not been satisfied. Roughness values exceeded the limits on the inner surface as well as on the external surface of both flanges. Macrofractographic and microfractographic signs revealed that the fracture was originated in a critical zone of the spring, next to the fillet, between the first wire and the flange. The origin was localized in the inner side surface of the first wire of the spring and the fracture was completed by static overload. Despite the type of steel and heat treatment fulfilled the material requirements, the roughness finishing of the critical areas was not correct. This fact may lead to origin a crack in a notch of the lateral inner surface of the first spring wire, produced by the presence of a particle or other surface defect that, afterward, it may be propagated by a fatigue mechanism. Acknowledgements Authors want to thank all Metallic Materials Area staff and especially, to the Microanalytical and Microstructural Characterization Laboratory. This research did not received any specific grant from funding agencies in the public, commercial or not-for-profit sectors. References [1] Wei Wang, Wei Yan, Qiqiang Duan, Yiyin Shan, Zhefeng Zhang, Ke Yang, Study on fatigue property of a new 2.8 GPa grade maraging steel, Mater. Sci. Eng. A 527 (2010) 3057–3063. [2] K.V. Rajkumar, B.P.C. Rao, Characterization of aging behaviour in M250 grade maraging steel using eddy current non-destructive methodology, Mater. Sci. Eng. A 464 (2007) 233–240. [3] A.T.I. Technical Data Sheet, VERSION 1 (2012) 1–10. [4] M. Stanford, K. Kibble, M. Lindop, D. Mynors, C. Durnall, An Investigation into Fully Melting a Maraging Steel Using Direct Metal Laser Sintering (DMLS), Steel Research Inst. 79 Special Edition Metal Forming Conference, 2008, pp. 847–852. [5] Y. Prawoto, M. Ikeda, S.K. Manville, A. Nishikawa, Design and failure modes of automotive suspension springs, Eng. Fail. Anal. 15 (2008) 1155–1174. [6] R. Fujckaz, C. Nolan, J. Kapp, The Effects of Surface Treatment on Torsional Fatigue Failure in Recoil Springs, Technical Report ARCCB-TR-92045. US ARMY ARMAMENT RESEARCH, 1992.
155