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Influence of manufacturing process on fatigue resistance of high strength steel bolts for connecting rods
Journal Pre-proofs Influence of manufacturing process on fatigue resistance of high strength steel bolts for connecting rods A. Acri, S. Beretta, F. Bolzoni, C. Colombo, L.M. Vergani PII: DOI: Reference:
S1350-6307(19)30689-2 https://doi.org/10.1016/j.engfailanal.2019.104330 EFA 104330
To appear in:
Engineering Failure Analysis
Received Date: Revised Date: Accepted Date:
17 May 2019 25 September 2019 11 November 2019
Please cite this article as: Acri, A., Beretta, S., Bolzoni, F., Colombo, C., Vergani, L.M., Influence of manufacturing process on fatigue resistance of high strength steel bolts for connecting rods, Engineering Failure Analysis (2019), doi: https://doi.org/10.1016/j.engfailanal.2019.104330
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© 2019 Published by Elsevier Ltd.
Influence of manufacturing process on fatigue resistance of high strength steel bolts for connecting rods
A. Acri1, S. Beretta2, F. Bolzoni2, C. Colombo3, L.M. Vergani3 1
2
Brugola OEB Industriale S.p.A., Piazza Giovanni XXIII 36, 20851 Lissone (MB), Italy [email protected]
Politecnico di Milano, Chemistry, Material and Chemical Engineering Department ‘‘Giulio Natta’’, Via Mancinelli 7, 20131 Milan, Italy [email protected], [email protected] 3
Politecnico di Milano, Department of Mechanical Engineering, Via La Masa 1, 20156 Milan, Italy [email protected], [email protected]
Abstract: The connecting rod (or conrod) is an important component of an internal combustion engine. An eventual failure of the bolts clamping the big end of conrods can cause failure of the engine; for this reason, they are addressed as “critical fasteners”. Bolts manufacturers must guarantee steel purity and avoid any surface defects, thread forming laps, surface decarburization, surface phosphate diffusion or hydrogen embrittlement risk in order to increase their reliability. This paper is focused on coating treatments of bolts and on their effect on fatigue life. An experimental testing campaign has been performed to investigate the effect of Mn-phosphate coating and micro Zn- phosphate coating on fatigue performances of the bolts, compared with the not coated ones. Particular attention is paid to the surface preparation of the bolt (i.e. by acid pickling or sandblasting) before the final phosphate coating operation. Diffusible and total hydrogen has been measured on all the bolts to verify the possible influence of hydrogen embrittlement.
KEYWORDS: Conrod bolt; high strength steel; coating; fatigue; hydrogen embrittlement.
1.
INTRODUCTION
Internal combustion engine connecting rod (or conrod) is a critical component with high volume production. It connects the piston to the rotating crankshaft, transmitting the thrust generated from combustion, and it is subjected to complex loading. It undergoes high cyclic loads of the order of one billion cycles, which range from high compressive loads due to combustion, to high tensile loads due to inertia. Therefore, durability of this component is of critical importance. There are structural and fatigue analyses of connecting rods available in the literature [1–3]. Although these studies show that, from a design point of view, the weak components of a conrod are not the bolts, still under certain circumstances a fatigue failure may occur to on the bolts, leading to the failure of conrod and engine [4]. Typical phenomena investigated in the literature, which can result in bolt failure, are: -
forming laps [4] or surface cracks [5];
-
surface decarburization [6,7];
-
surface phosphorus diffusion, occurring during heat treatment when the steel is coated with phosphate prior to cold heading [8,9];
-
hydrogen embrittlement (HE) [10–14] and stress corrosion cracking (SCC) [15,16].
Avoiding these phenomena during bolt production process will allow the compliance of the bolts with international standards and Original Equipment Manufacturers (OEMs) demands. Due to these requirements and the critical safety importance of connecting rod integrity, the bolts clamping the big end of a connecting rod are addressed as “critical fasteners”. The manufacturing process plays a basic role with respect to the reliability of the bolts. In the present paper we focus on the treatments needed to coat the bolts. Bolts coating aims at obtaining adequate tribological properties required during engine assembly and working conditions. As illustrated in [17], a phosphate coating with lubricating oil is a good combination which guarantees long-lasting adequate friction properties. Before coating the bolts, it is necessary to remove the surface oxides that can be found after the quenching and tempering treatment. Chemical or mechanical cleaning (i.e. respectively acid pickling or sandblasting) can be used. In the present paper we focus on two different coatings for the bolts: manganese phosphate (MP) coating after chemical cleaning (i.e. acid pickling) of the bolt surface, and micro-zinc phosphate (ZP) coating after sandblasting cleaning of the bolt surface. Hence, the main difference between these two considered treatments is the surface cleaning process before the final bolt phosphate coating operation.
Our aim consists in investigating the effect of these two alternative coating treatments on bolts’ fatigue life and failure modes. Moreover, a further specific goal of the research is to assess if the chemical cleaning and/or the coating operations of bolt surface can cause the presence of hydrogen in concentration sufficient to induce an embrittling effect. Therefore, we performed an extensive experimental campaign on conrod bolts made of high strength steel. Three series of bolts, having the same geometry, were subjected to different manufacturing processes to obtain the series: 1) without coating, 2) with a manganese phosphate (MP) coating and 3) with a micro-zinc phosphate (ZP) coating.
2.
MATERIALS AND METHODS
2.1 Materials The conrod fasteners investigated in this paper are M9 bolts with a pitch of 1.25mm and a length of 45mm (Fig. 1). All the samples belong to the same batch. Tab. 1.a shows the chemical composition of the adopted steel 32CrB4, a high strength low alloy steel defined in ISO-4967 [18]. The purity of the steel (i.e. absence of nonmetallic inclusions) is very high: the purity grade K2 according to ISO-4967 [19] is lower than 3. A thermal treatment is applied to the samples that implies 80 minutes into the A3 zone (austenizing). After quenching and tempering heat treatment, the average value of the ultimate tensile strength (UTS) is equal to 1250 MPa, and the core hardness is around 40 HRC. Hence, this bolt has a 12.9 grade according to ISO 898-1 [7]. Tab. 1.b summarizes the details of the mechanical properties. This bolt is mounted in internal combustion engine connecting rods for automotive applications and it is a “critical fastener”. As critical fastener, manufacturers have to guarantee the absence of any embrittlement risk. Fig. 2 shows the magnifications collected by means of an optical microscope before any coating process, which confirm the absence of surface decarburization (Figs. 2.A and 2.C), the absence of surface phosphorus diffusion or delta-ferrite (Figs. 2.A and 2.C), martensitic tempered steel without inclusions (Fig. 2.B), the absence of surface defects on head fillet (Fig. 2.A) and absence of thread forming laps (Fig. 2.C).
2.2 Manufacturing process Fig. 3 shows a flowchart with the steps followed during the manufacturing process of the conrod bolts. The steel wire is obtained by rolling a steel billet, produced with a continuous casting process.
Before any cold forging operation, annealing is a heat treatment to be performed in order to obtain the required mechanical properties, i.e. has been applied to all the three manufacturing processes. A very important step is the preparation of the wire before cold forging: after rolling and annealing processes, a layer of steel oxide is generated on wire surface. This oxide layer should be removed before the following thermo-mechanical operations. A chemical cleaning (acid pickling) or a mechanical cleaning (dry sandblasting with sharp sand) can be adopted alternatively. In critical applications, as the present case, sandblasting is preferred [10]. Once the surface is free of any oxide residuals, the following step is to apply a lubricating coating to the whole wire surface, that is necessary for to the next cold forging operation. After the sandblasting, we selected a polymer-wire coating to lubricate the wire surface. Then, the bolt is shaped through cold forging operations, consisting of wire drawing and several forging steps. As described in [20], hardness distribution after cold forging is not constant. Therefore, the ultimate tensile strength (UTS) of the bolt is not a parameter under control during the manufacturing process. In order to govern this mechanical property of the final product, a quenching and tempering treatment is required [21]. Thread rolling operation performed after heat treatment is fundamental to improve the fatigue performance of the bolts, due to the compressive residual stresses induced by the treatment [22]. Non-coated (NC) bolts are collected after the thread rolling; on the other hand, bolts with two different coatings are considered. Bolt surface is prepared for the phosphate coating following specific procedures: acid pickling for Mn-phosphate (MP) coated bolts, and sandblasting for Zn-phosphate (ZP) coated bolts. The acid adopted to clean MP bolt surface before phosphate coating is a 30% solution of HCl at 10%, H 3PO4 at 5%, and H2SO4 at 20% in water. Acid bath temperature of 30°C was considered with exposition time of 10min. Sandblasting not only removes the oxides generated during quenching treatment from the ZP bolt surface, but also increases surface micro-hardness [23]. ZP is defined as “micro” coating because the average dimension of the phosphate crystals in this application is 4 µm. Conversely, MP crystals have higher dimensions. A comparison analysis between MP and ZP (in terms of crystal morphology, corrosion and protection characteristics) can be found in [24,25]; moreover, the reference [26] gives some details on the phosphating coating process.
2.3 Measurement of hydrogen content Hydrogen embrittlement is a phenomenon consisting in a complex interaction between solute hydrogen atoms
and the micro-structure of the material. Consequently, the effects of hydrogen on the mechanical properties of steels depend on many parameters: chemical composition, microstructure (phases, constituents, precipitates and inclusions) and macrostructure (banding and segregations) of the steel, and the hydrogen charging conditions (source of hydrogen, temperature, surface conditions, stress/strain conditions during charging [27,28]) and the loading conditions (stress gradients). From the microstructural viewpoint, absorption and diffusion of hydrogen in steel depend on its crystal lattice, the presence and distribution of the phases and the reticular defects [29]. The flow of hydrogen inside the material is influenced by the presence of traps [30] that can be classified according to their type of interaction and activation energies. Depending on the temperature and time required to release the hydrogen from the steel we may distinguish: 1.
Reversible traps (e.g. voids, grain boundaries, dislocations [31]) which require low activation energy; hydrogen trapped in these traps is the so-called diffusible hydrogen. In this case the hydrogen can be released from steel at temperatures below 200°C (also at ambient temperature for an exposition time above 24h [28]). The diffusible hydrogen is then able to move into the lattice and towards highly stressed regions of the component, such as notches and crack tips, thus inducing embrittlement [32].
2.
Irreversible traps (e.g. fine Cr, V or Mo carbides in steel [12,33], MnS or Fe3C [34,35]) require high activation energy; the hydrogen trapped here is called non-diffusible hydrogen. As shown in [32], also after long-term backing at 190°C this hydrogen is not released: temperatures higher than 200°C are required. Irreversible traps can reduce hydrogen diffusivity and steel vulnerability to embrittlement [36,37].
Among the different experimental methods to measure of hydrogen embrittlement in steels, we selected two techniques: 1) the glycerin method, allowing for the measurement of the diffusible hydrogen content, and 2) the hot extraction method by LECO machine, allowing for the measurement of the total hydrogen content, i.e. diffusible and trapped. To measure the diffusible hydrogen, all the bolts are collected immediately after production, stored in dry ice for 2h (for the transport from the production site to the analysis laboratory), and then stored in liquid nitrogen before the measurement with the glycerin method. Test is performed by dipping the sample into a hot glycerin bath at 200°C, and by measuring the released hydrogen in the gas form.
On the other side, regarding the hot extraction method, tests for the determination of total hydrogen content are performed after a rest time of at least 2 weeks at ambient temperature by means of a LECO ONH-836-Series machine.
2.4 Fatigue tests Fatigue tests are carried out on NC, MP and ZP bolts at least within 2 weeks after the production. Uniaxial fatigue tests are performed according to ISO 3800 [38] at constant mean stress, using threaded nuts with hardness 42 HRC. The used hydraulic fatigue testing machine is the MTS 810, with the maximum capacity of ±250kN. Fatigue tests are performed with the following settings: - mean load 37.9 kN, i.e. mean stress 865 MPa, corresponding to 77% of minimum yield stress of the bolt’s material; - test frequency: 100Hz; - maximum number of cycles to consider runout: 5 000 000. The fatigue limit is obtained testing by 13 bolts for each type, i.e. NC, MP and ZP. In addition to these 13 specimens, for the estimation of Wohler’s curve 8 NC, and 10 MP and 10 ZP coated bolts are tested. Test nuts used are in accordance with ISO 3800; new nuts are used for each test. Fracture surfaces are then investigated by means of a scanning electron microscope (SEM).
3.
RESULTS
3.1 Hydrogen content Tab. 2 shows the results of the measured diffusible and total hydrogen content. Although the glycerin method can be affected by some measurement errors and the sensitivity is low [39,40], it is possible to state that in all specimens the diffusible hydrogen measured is negligible. The amount is, in fact, lower than 0.1ppm in all specimens. This diffusible hydrogen is the most dangerous, because it can move through the lattice of the steel and concentrate at the most stressed regions. On the other hand, the measured total hydrogen is not negligible, in the range between 2.5-2.9 ppm, but similar in all samples. This means that most of the hydrogen embedded into the samples is in irreversible traps, while a very little quantity is in the lattice and can diffuse. Therefore, the results in Tab. 2 show that the final coating operation does not influence the diffusible hydrogen and its quantity remains negligible.
3.2 Fatigue tests Tests are performed in accordance with [38], for the determination of Wohler’s curve (Fig. 4) and the fatigue limit with the stair case approach (Tab. 3). Wohler’s mean lines are obtained by interpolating experimental data with the exponential law:
in the bi-logarithmic plot of Fig. 4.
From the results of Fig. 4 and Tab. 3 it is possible to assess that the differences between non-coated bolts NC and bolts with micro-Zn phosphate coating ZP deposited after sandblasting are negligible. Conversely, the fatigue limit of bolts coated with manganese coating MP (whose surface was prepared with acid pickling) is averagely 13% lower than the NC samples. Moreover, the slope of Wohler’s curve in the finite life region of MP bolts, i.e. B coefficient in Tab.3, is 41% higher than the corresponding NC one. This confirms lower fatigue resistance of MP bolts compared with NC and ZP bolts.
3.3 Fractography Fig. 5 shows the different failure zones of the bolts, as reported in Tab. 3. All the tested bolts, belonging to the same set, failed systematically in these ways. Fig.6 shows the SEM analyses of the fracture surface performed on a NC bolt, subjected to a stress amplitude of σa = 139MPa and failed at Nf = 282669 cycles. Three different zones are visible: a)
the fatigue crack propagation zone that is highlighted by a red line in Fig. 6a. Point C (yellow square in Fig.6b) is a possible initiation zone at the external surface. Fig.6c is an enlargement of the initiation zone and Fig.6e shows a magnification of the morphology of the crack propagation zone;
b) the transition between the fatigue crack growth and the ductile fracture zone (Fig.6d); c)
the final instantaneous fracture zone that is ductile and characterized by dimples (Fig.6f).
Fig. 7 shows the SEM images of the fracture surface of a ZP bolt, subjected to a stress amplitude of σa = 160 MPa and failed at Nf= 206785 cycles. The fatigue crack propagation zone is highlighted by a red line (Fig.7a) and it is similar to the previous one; the initiation is always at the external surface (point B in Fig. 7a, and magnification in Fig. 7b). In Fig.7c the morphology of the fatigue propagation zone is similar to the previous one (Fig. 6e) with evident fatigue striations. Fig. 7d shows the final ductile fracture zone with dimples. Fig. 8 shows the SEM analyses of the fracture surface of MP bolt, subjected to a stress amplitude of σa = 89 MPa and failed at Nf = 636069 cycles. The same fracture zones of the previous samples can be evidenced: a red line in Fig. 8a highlights the fatigue crack propagation zone; the crack initiation is at the external surface (point
B in Fig. 8a). The morphology of the crack propagation, shown in Figs. 8b and 8c, is similar to the previous samples (Fig. 6e and Fig. 7c) and the final instantaneous fracture zone (Fig. 7d) is ductile characterized by dimples. By comparing all the performed analyses, we can conclude that the fracture surfaces for the different bolts are characterized by similar fatigue propagation morphologies. The crack initiation is located always at the external surfaces. No intergranular crack propagation can be identified and the presence of dimples is evident in the final fracture zone. None of the analyzed fracture surfaces shows the typical hydrogen embrittlement characteristics, visible for instance in [14,27,41–43]. We can therefore state that failures are not caused by hydrogen embrittlement. Another important aspect is related to the extension of the fatigue propagation region with respect to the final fracture zone. We can state, in fact, that these extensions are similar for all the samples, even if the applied stress amplitudes and the durations are different. We could suppose that the effect of the different treatments is accounted by different nucleation phases.
4.
DISCUSSION
The difference in failure locations is an important aspect to be considered: all NC bolts failed at the head shoulder fillet (under head failure mode), while all MP and ZP bolts failed systematically at the first engaged thread. In order to provide an insight into this behavior, we will focus on the following factors that can influence fatigue strength and life, especially locally at the surface: 1) the stress concentration factor Kt, function of bolt geometry, and in particular of the fillet radius, which is the same for all the samples; 2) the presence of residual stresses; 3) the different surface qualities of the bolts obtained from the manufacturing process. According to [44], the stress concentration factor at the head shoulder fillet (Kth) is smaller than that of the first engaged thread root Ktt.. The respective values are: Kth= 2.3 and Ktt= 3.8. This difference, that is about 65%, is mainly due to the non-uniform distribution of the applied load among the threads. Indeed, for standard nuts, it results typically that the first engaged thread has to bear about 30-35% of the total load [44]. Besides, according to [22,23] thread rolling after heat treatments generates a compressive field of residual stresses, leading to beneficial effects on fatigue performances. Thread rolling is applied only at the threads, and not at the fillet below bolt head. Therefore, for this particular geometry, we can assume that NC bolts fail at the
head shoulder fillet because the effect of residual stresses on the threads prevails on the stress concentration related to Ktt. ZP and MP samples fail at thread root probably because the compressive residual stresses are reduced during the coating processes. On the other hand, attention should be addressed to the surface quality to explain the differences in fatigue performance of MP and ZP bolts, having the same geometry and residual stresses due to thread rolling. Indeed, even if they experience the same failure mode at the first engaged thread, their fatigue strength and life are different. Fig. 9 shows a magnification with optical microscope to support the discussion on the surface finishing at the thread root of MP and ZP bolts that are, respectively, treated with acid pickling and sandblasting. Comparing Fig. 9.a and Fig. 9.b, we can state that the MP bolts show a more irregular surface with small acid corrosion of maximum depth of 10µm, with round shape. ZP bolts do not exhibit pitting on bolt surface, as the maximum depth of the surface roughness is lower than 2µm (see detail in Fig. 9.b). According to the literature, when dealing with crack propagation from defects, one of the main parameters is the square root area introduced by [45]. The considered square root area,
, corresponds to the area of a defect
projected in the direction of the maximum tensile stress. It is a geometrical parameter proportional to the maximum stress intensity factor and it controls the fatigue strength of materials containing small defects, because it is related to the threshold of crack propagation from defects. Referring to the surface defects found in MP bolts (Fig. 9.a), we can state that they are characterized by , which has almost no influence on the fatigue strength of metals [46], even if we suppose its location at the thread root. Indeed, according to this paper, the minimum considered value of
is around 25
μm to appreciate a variation in the rotating fatigue limit of at least 10%. Thus, the decrease in the fatigue limit of MP with respect to ZP bolts remains a non-trivial question. Some works showed that sandblasting not only improves surface quality but also induces partially and locally the compressive residual stresses [23,47], leading to beneficial effects in terms of fatigue resistance. However, it is opinion of the authors that sandblasting has a limited effect on residual stresses, because its input energy is much smaller than the requirements for shot peening. Moreover, studies on a different application proved that residual stresses are relaxed due to corrosive phenomena [48–50]. The partial relaxation of residual stresses due to acid pickling can be one of the reasons inducing MP bolts to have the lowest fatigue resistance among the investigated processes.
As a conclusion to this discussion, we can hypothesize that the decrease in fatigue limit between ZP and MP bolts could be a mixed phenomenon attributed to small improvements related to sandblasting operation in ZP bolts on the one hand, and on the other to small worsening related to the surface finishing with pit formation and to partial stress relaxation due to acid pickling of MP bolts. Anyway, the authors underline that more research is required in this direction to better understand the phenomenon.
CONCLUSIONS In this paper different manufacturing processes of high strength steel bolts have been investigated. Particular attention has been paid to the final surface preparation (acid pickling or sandblasting required before final coating) and to the coating process. The different investigated bolts are: i) NC bolts, thread rolled after heat treatment but not coated; ii) ZP bolts, coated with micro-zinc phosphate after sandblasting; and iii) MP bolts coated with manganese phosphate after acid pickling. These coating processes are correlated with hydrogen content, surface quality and fatigue resistance of the final product. The following conclusions can be drawn: - The investigated processes did not underline the risk of hydrogen embrittlement. Indeed, the measured of diffusible hydrogen was negligible in all tested samples. Moreover, the fractography images collected after fatigue testing confirmed that the failures are not related to hydrogen embrittlement in any of the series. - Fatigue tests on the three bolt types evidenced a different fatigue life, both in terms of Wohler’s slope and of fatigue limit between the series. Despite MP bolts are produced according to OEMs requirements, they had lower fatigue resistance (-13% with respect to NC samples). Failure modes were also different: NC bolts failed at the head fillet, while ZP and MP bolts failed at the first engaged thread. - Fractography analysis suggested that cracks propagated from bolt surfaces in all investigated samples. - Focusing on NC samples, we could hypothesize that the failure at the head, where stress concentration factor is smaller than the first engaged thread, is due to thread rolling. Indeed, this mechanical treatment induces a compressive residual stress field on the threads but not at the bolt head. - Comparing MP and ZP bolts, both failing at the threads, we could identify different but not comprehensive explanations to the decrease of fatigue limit of MP series. Considering this bolt type, we could identify some micro-pitting due to acid pickling, which can also partially relax the residual stresses; on the other hand, for ZP bolts, sandblasting could have partially improved this field and helped to widen the range between fatigue limit of MP and ZP bolts. Probably, the experimental evidence of this variation in the fatigue performances is connected with a combination of all these phenomena.
ACKNOWLEDGEMENTS The authors gratefully acknowledge the support of E.S. Brugola for its continuous effort in research and development of fastening technologies. The authors also acknowledge Brugola OEB Industriale S.p.A. (Piazza Giovanni XXIII 36, 20851 Lissone, Italy) for the production of the bolts discussed in this paper. A special thanks goes to R. Cenni, G. Ambrosini and Fimeur Group S.r.l. (Via P. M. Curie 31, 25046 Cazzago S. Martino, Italy) for the support during the coating process described in this research. The authors thank Eng. Francesca Brunella from Politecnico di Milano, Chemistry, Material and Chemical Engineering Department ‘‘Giulio Natta’’ for the SEM analyses and discussions.
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Highlights · · · ·
Connecting rod bolts are critical fasteners Fatigue tests to highlight the effect of coatings (Mn-phosphate and micro Zn-phosphate) on fatigue performance Analysis of hydrogen content, correlated to the manufacturing process Fractographic investigation of crack initiation and propagation
Figure 1 – Geometry of the investigated bolt; dimensions in mm.
Figure 2 – A: bolt head fillet; B: martensitic tempered structure at the center of the bolt; C: bolt threads. These images are collected by an optical microscope after thread rolling process and confirm the absence of decarburization, delta-ferrite, inclusions, surface defects or thread forming laps before coating process.
Rolling Annealing
Sandblasting Polymer wire-coating Cold forging Quenching and tempering Thread rolling
Acid pickling
Sandblasting
Mn-phosphate bolt-coating
Zn-phosphate bolt-coating
MP bolts
ZP bolts
NC bolts
Figure 3 – Typical steps of the conrod bolt manufacturing process from rolling to final bolt coating. The three bolt types considered in the present paper are collected: after thread rolling (NC); after coating with Mn (MP), and after coating with Zn (ZP).
Figure 4 – Results of the fatigue tests: Wohler’s curve determined for NC (non-coated bolts), MP (bolts with acid cleaning and manganese phosphate coating), and ZP (sandblasted bolts coated with micro-zinc phosphate). Both axes are in logarithmic scale.
Figure 5 – Examples of failure mode in fatigue testing.
Figure 6 – SEM analysis of NC bolt failed during fatigue testing. A) and B) Fracture surface of bolt head. C) Possible location of crack initiation. D) Transition from fatigue growth and ductile crack propagation. E) Morphology of fatigue crack propagation. F) Final failure dimples. NC specimen with alternate stress σa = 139MPa, number of cycles to failure Nf = 282669 cycles.
Figure 7 – SEM analysis of ZP bolt failed during fatigue testing. A) Fracture surface (the red line indicate approximately the fatigue crack propagation). B) Detail of fatigue fracture surface. C) Morphology of fatigue crack propagation. D) Final failure dimples. ZP specimen with alternate stress σa = 160 MPa, number of cycles to failure Nf = 206785 cycles.
Figure 8 – SEM analysis of MP bolt failed during fatigue testing. A) Fracture surface. B) Possible location of crack initiation. C) Morphology of fatigue crack propagation. D) Final failure dimples. MP specimen with alternate stress σa =89 MPa, number of cycles to failure Nf = 636069cycles.
a. Surface cleaning: acid pickling 50 μm
b. Surface cleaning: sandblasting 50 μm
10 μm
Figure 9 – Thread roots of: a. MP bolts: measure of the corrosion pit. b. ZP bolts with magnification: no pitting.
Table 1 – a. Chemical composition (weigh %) and (b.) mechanical properties of the adopted steel.
a. C
S
P
Mn
Cr
Ni
Mo
B
Fe
0.33
0.008
0.013
0.81
1.09
0.026
0.012
0.0037
97.5
b. Average ultimate tensile strength, UTS (MPa)
1250
Average yield stress, YS (MPa)
1125
Property class of the fastener according to ISO 898-1
12.9
Average core hardness (HRC)
40
Steel microstructure
Tempered martensite
Purity grade according to ISO 4967
K2<3
Table 2 – Diffusible and total hydrogen content measured at different stages of the production process.
Sample
Diffusible H [ppm]
Total H [ppm]
NC
< 0.1
2.7
MP
< 0.1
2.9
ZP
< 0.1
2.5
Table 3 – Alternate fatigue limit measured on bolts at different stages of production process; confidence limits are calculated following ISO 12107:2012(E) standard. Wohler’s average coefficients are obtained by . interpolating the experimental data with the law:
Sample
Fatigue limit [MPa]
A [MPa]
B
Failure zone
NC
108±9
2886
-0.242
under the head
MP
94±2
686
-0.144
at the first engaged thread
ZP
118±4
1955
-0.209
at the first engaged thread
Conflict of Interest. Declaration of interest: none
S1350-6307(19)30689-2 https://doi.org/10.1016/j.engfailanal.2019.104330 EFA 104330
To appear in:
Engineering Failure Analysis
Received Date: Revised Date: Accepted Date:
17 May 2019 25 September 2019 11 November 2019
Please cite this article as: Acri, A., Beretta, S., Bolzoni, F., Colombo, C., Vergani, L.M., Influence of manufacturing process on fatigue resistance of high strength steel bolts for connecting rods, Engineering Failure Analysis (2019), doi: https://doi.org/10.1016/j.engfailanal.2019.104330
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© 2019 Published by Elsevier Ltd.
Influence of manufacturing process on fatigue resistance of high strength steel bolts for connecting rods
A. Acri1, S. Beretta2, F. Bolzoni2, C. Colombo3, L.M. Vergani3 1
2
Brugola OEB Industriale S.p.A., Piazza Giovanni XXIII 36, 20851 Lissone (MB), Italy [email protected]
Politecnico di Milano, Chemistry, Material and Chemical Engineering Department ‘‘Giulio Natta’’, Via Mancinelli 7, 20131 Milan, Italy [email protected], [email protected] 3
Politecnico di Milano, Department of Mechanical Engineering, Via La Masa 1, 20156 Milan, Italy [email protected], [email protected]
Abstract: The connecting rod (or conrod) is an important component of an internal combustion engine. An eventual failure of the bolts clamping the big end of conrods can cause failure of the engine; for this reason, they are addressed as “critical fasteners”. Bolts manufacturers must guarantee steel purity and avoid any surface defects, thread forming laps, surface decarburization, surface phosphate diffusion or hydrogen embrittlement risk in order to increase their reliability. This paper is focused on coating treatments of bolts and on their effect on fatigue life. An experimental testing campaign has been performed to investigate the effect of Mn-phosphate coating and micro Zn- phosphate coating on fatigue performances of the bolts, compared with the not coated ones. Particular attention is paid to the surface preparation of the bolt (i.e. by acid pickling or sandblasting) before the final phosphate coating operation. Diffusible and total hydrogen has been measured on all the bolts to verify the possible influence of hydrogen embrittlement.
KEYWORDS: Conrod bolt; high strength steel; coating; fatigue; hydrogen embrittlement.
1.
INTRODUCTION
Internal combustion engine connecting rod (or conrod) is a critical component with high volume production. It connects the piston to the rotating crankshaft, transmitting the thrust generated from combustion, and it is subjected to complex loading. It undergoes high cyclic loads of the order of one billion cycles, which range from high compressive loads due to combustion, to high tensile loads due to inertia. Therefore, durability of this component is of critical importance. There are structural and fatigue analyses of connecting rods available in the literature [1–3]. Although these studies show that, from a design point of view, the weak components of a conrod are not the bolts, still under certain circumstances a fatigue failure may occur to on the bolts, leading to the failure of conrod and engine [4]. Typical phenomena investigated in the literature, which can result in bolt failure, are: -
forming laps [4] or surface cracks [5];
-
surface decarburization [6,7];
-
surface phosphorus diffusion, occurring during heat treatment when the steel is coated with phosphate prior to cold heading [8,9];
-
hydrogen embrittlement (HE) [10–14] and stress corrosion cracking (SCC) [15,16].
Avoiding these phenomena during bolt production process will allow the compliance of the bolts with international standards and Original Equipment Manufacturers (OEMs) demands. Due to these requirements and the critical safety importance of connecting rod integrity, the bolts clamping the big end of a connecting rod are addressed as “critical fasteners”. The manufacturing process plays a basic role with respect to the reliability of the bolts. In the present paper we focus on the treatments needed to coat the bolts. Bolts coating aims at obtaining adequate tribological properties required during engine assembly and working conditions. As illustrated in [17], a phosphate coating with lubricating oil is a good combination which guarantees long-lasting adequate friction properties. Before coating the bolts, it is necessary to remove the surface oxides that can be found after the quenching and tempering treatment. Chemical or mechanical cleaning (i.e. respectively acid pickling or sandblasting) can be used. In the present paper we focus on two different coatings for the bolts: manganese phosphate (MP) coating after chemical cleaning (i.e. acid pickling) of the bolt surface, and micro-zinc phosphate (ZP) coating after sandblasting cleaning of the bolt surface. Hence, the main difference between these two considered treatments is the surface cleaning process before the final bolt phosphate coating operation.
Our aim consists in investigating the effect of these two alternative coating treatments on bolts’ fatigue life and failure modes. Moreover, a further specific goal of the research is to assess if the chemical cleaning and/or the coating operations of bolt surface can cause the presence of hydrogen in concentration sufficient to induce an embrittling effect. Therefore, we performed an extensive experimental campaign on conrod bolts made of high strength steel. Three series of bolts, having the same geometry, were subjected to different manufacturing processes to obtain the series: 1) without coating, 2) with a manganese phosphate (MP) coating and 3) with a micro-zinc phosphate (ZP) coating.
2.
MATERIALS AND METHODS
2.1 Materials The conrod fasteners investigated in this paper are M9 bolts with a pitch of 1.25mm and a length of 45mm (Fig. 1). All the samples belong to the same batch. Tab. 1.a shows the chemical composition of the adopted steel 32CrB4, a high strength low alloy steel defined in ISO-4967 [18]. The purity of the steel (i.e. absence of nonmetallic inclusions) is very high: the purity grade K2 according to ISO-4967 [19] is lower than 3. A thermal treatment is applied to the samples that implies 80 minutes into the A3 zone (austenizing). After quenching and tempering heat treatment, the average value of the ultimate tensile strength (UTS) is equal to 1250 MPa, and the core hardness is around 40 HRC. Hence, this bolt has a 12.9 grade according to ISO 898-1 [7]. Tab. 1.b summarizes the details of the mechanical properties. This bolt is mounted in internal combustion engine connecting rods for automotive applications and it is a “critical fastener”. As critical fastener, manufacturers have to guarantee the absence of any embrittlement risk. Fig. 2 shows the magnifications collected by means of an optical microscope before any coating process, which confirm the absence of surface decarburization (Figs. 2.A and 2.C), the absence of surface phosphorus diffusion or delta-ferrite (Figs. 2.A and 2.C), martensitic tempered steel without inclusions (Fig. 2.B), the absence of surface defects on head fillet (Fig. 2.A) and absence of thread forming laps (Fig. 2.C).
2.2 Manufacturing process Fig. 3 shows a flowchart with the steps followed during the manufacturing process of the conrod bolts. The steel wire is obtained by rolling a steel billet, produced with a continuous casting process.
Before any cold forging operation, annealing is a heat treatment to be performed in order to obtain the required mechanical properties, i.e. has been applied to all the three manufacturing processes. A very important step is the preparation of the wire before cold forging: after rolling and annealing processes, a layer of steel oxide is generated on wire surface. This oxide layer should be removed before the following thermo-mechanical operations. A chemical cleaning (acid pickling) or a mechanical cleaning (dry sandblasting with sharp sand) can be adopted alternatively. In critical applications, as the present case, sandblasting is preferred [10]. Once the surface is free of any oxide residuals, the following step is to apply a lubricating coating to the whole wire surface, that is necessary for to the next cold forging operation. After the sandblasting, we selected a polymer-wire coating to lubricate the wire surface. Then, the bolt is shaped through cold forging operations, consisting of wire drawing and several forging steps. As described in [20], hardness distribution after cold forging is not constant. Therefore, the ultimate tensile strength (UTS) of the bolt is not a parameter under control during the manufacturing process. In order to govern this mechanical property of the final product, a quenching and tempering treatment is required [21]. Thread rolling operation performed after heat treatment is fundamental to improve the fatigue performance of the bolts, due to the compressive residual stresses induced by the treatment [22]. Non-coated (NC) bolts are collected after the thread rolling; on the other hand, bolts with two different coatings are considered. Bolt surface is prepared for the phosphate coating following specific procedures: acid pickling for Mn-phosphate (MP) coated bolts, and sandblasting for Zn-phosphate (ZP) coated bolts. The acid adopted to clean MP bolt surface before phosphate coating is a 30% solution of HCl at 10%, H 3PO4 at 5%, and H2SO4 at 20% in water. Acid bath temperature of 30°C was considered with exposition time of 10min. Sandblasting not only removes the oxides generated during quenching treatment from the ZP bolt surface, but also increases surface micro-hardness [23]. ZP is defined as “micro” coating because the average dimension of the phosphate crystals in this application is 4 µm. Conversely, MP crystals have higher dimensions. A comparison analysis between MP and ZP (in terms of crystal morphology, corrosion and protection characteristics) can be found in [24,25]; moreover, the reference [26] gives some details on the phosphating coating process.
2.3 Measurement of hydrogen content Hydrogen embrittlement is a phenomenon consisting in a complex interaction between solute hydrogen atoms
and the micro-structure of the material. Consequently, the effects of hydrogen on the mechanical properties of steels depend on many parameters: chemical composition, microstructure (phases, constituents, precipitates and inclusions) and macrostructure (banding and segregations) of the steel, and the hydrogen charging conditions (source of hydrogen, temperature, surface conditions, stress/strain conditions during charging [27,28]) and the loading conditions (stress gradients). From the microstructural viewpoint, absorption and diffusion of hydrogen in steel depend on its crystal lattice, the presence and distribution of the phases and the reticular defects [29]. The flow of hydrogen inside the material is influenced by the presence of traps [30] that can be classified according to their type of interaction and activation energies. Depending on the temperature and time required to release the hydrogen from the steel we may distinguish: 1.
Reversible traps (e.g. voids, grain boundaries, dislocations [31]) which require low activation energy; hydrogen trapped in these traps is the so-called diffusible hydrogen. In this case the hydrogen can be released from steel at temperatures below 200°C (also at ambient temperature for an exposition time above 24h [28]). The diffusible hydrogen is then able to move into the lattice and towards highly stressed regions of the component, such as notches and crack tips, thus inducing embrittlement [32].
2.
Irreversible traps (e.g. fine Cr, V or Mo carbides in steel [12,33], MnS or Fe3C [34,35]) require high activation energy; the hydrogen trapped here is called non-diffusible hydrogen. As shown in [32], also after long-term backing at 190°C this hydrogen is not released: temperatures higher than 200°C are required. Irreversible traps can reduce hydrogen diffusivity and steel vulnerability to embrittlement [36,37].
Among the different experimental methods to measure of hydrogen embrittlement in steels, we selected two techniques: 1) the glycerin method, allowing for the measurement of the diffusible hydrogen content, and 2) the hot extraction method by LECO machine, allowing for the measurement of the total hydrogen content, i.e. diffusible and trapped. To measure the diffusible hydrogen, all the bolts are collected immediately after production, stored in dry ice for 2h (for the transport from the production site to the analysis laboratory), and then stored in liquid nitrogen before the measurement with the glycerin method. Test is performed by dipping the sample into a hot glycerin bath at 200°C, and by measuring the released hydrogen in the gas form.
On the other side, regarding the hot extraction method, tests for the determination of total hydrogen content are performed after a rest time of at least 2 weeks at ambient temperature by means of a LECO ONH-836-Series machine.
2.4 Fatigue tests Fatigue tests are carried out on NC, MP and ZP bolts at least within 2 weeks after the production. Uniaxial fatigue tests are performed according to ISO 3800 [38] at constant mean stress, using threaded nuts with hardness 42 HRC. The used hydraulic fatigue testing machine is the MTS 810, with the maximum capacity of ±250kN. Fatigue tests are performed with the following settings: - mean load 37.9 kN, i.e. mean stress 865 MPa, corresponding to 77% of minimum yield stress of the bolt’s material; - test frequency: 100Hz; - maximum number of cycles to consider runout: 5 000 000. The fatigue limit is obtained testing by 13 bolts for each type, i.e. NC, MP and ZP. In addition to these 13 specimens, for the estimation of Wohler’s curve 8 NC, and 10 MP and 10 ZP coated bolts are tested. Test nuts used are in accordance with ISO 3800; new nuts are used for each test. Fracture surfaces are then investigated by means of a scanning electron microscope (SEM).
3.
RESULTS
3.1 Hydrogen content Tab. 2 shows the results of the measured diffusible and total hydrogen content. Although the glycerin method can be affected by some measurement errors and the sensitivity is low [39,40], it is possible to state that in all specimens the diffusible hydrogen measured is negligible. The amount is, in fact, lower than 0.1ppm in all specimens. This diffusible hydrogen is the most dangerous, because it can move through the lattice of the steel and concentrate at the most stressed regions. On the other hand, the measured total hydrogen is not negligible, in the range between 2.5-2.9 ppm, but similar in all samples. This means that most of the hydrogen embedded into the samples is in irreversible traps, while a very little quantity is in the lattice and can diffuse. Therefore, the results in Tab. 2 show that the final coating operation does not influence the diffusible hydrogen and its quantity remains negligible.
3.2 Fatigue tests Tests are performed in accordance with [38], for the determination of Wohler’s curve (Fig. 4) and the fatigue limit with the stair case approach (Tab. 3). Wohler’s mean lines are obtained by interpolating experimental data with the exponential law:
in the bi-logarithmic plot of Fig. 4.
From the results of Fig. 4 and Tab. 3 it is possible to assess that the differences between non-coated bolts NC and bolts with micro-Zn phosphate coating ZP deposited after sandblasting are negligible. Conversely, the fatigue limit of bolts coated with manganese coating MP (whose surface was prepared with acid pickling) is averagely 13% lower than the NC samples. Moreover, the slope of Wohler’s curve in the finite life region of MP bolts, i.e. B coefficient in Tab.3, is 41% higher than the corresponding NC one. This confirms lower fatigue resistance of MP bolts compared with NC and ZP bolts.
3.3 Fractography Fig. 5 shows the different failure zones of the bolts, as reported in Tab. 3. All the tested bolts, belonging to the same set, failed systematically in these ways. Fig.6 shows the SEM analyses of the fracture surface performed on a NC bolt, subjected to a stress amplitude of σa = 139MPa and failed at Nf = 282669 cycles. Three different zones are visible: a)
the fatigue crack propagation zone that is highlighted by a red line in Fig. 6a. Point C (yellow square in Fig.6b) is a possible initiation zone at the external surface. Fig.6c is an enlargement of the initiation zone and Fig.6e shows a magnification of the morphology of the crack propagation zone;
b) the transition between the fatigue crack growth and the ductile fracture zone (Fig.6d); c)
the final instantaneous fracture zone that is ductile and characterized by dimples (Fig.6f).
Fig. 7 shows the SEM images of the fracture surface of a ZP bolt, subjected to a stress amplitude of σa = 160 MPa and failed at Nf= 206785 cycles. The fatigue crack propagation zone is highlighted by a red line (Fig.7a) and it is similar to the previous one; the initiation is always at the external surface (point B in Fig. 7a, and magnification in Fig. 7b). In Fig.7c the morphology of the fatigue propagation zone is similar to the previous one (Fig. 6e) with evident fatigue striations. Fig. 7d shows the final ductile fracture zone with dimples. Fig. 8 shows the SEM analyses of the fracture surface of MP bolt, subjected to a stress amplitude of σa = 89 MPa and failed at Nf = 636069 cycles. The same fracture zones of the previous samples can be evidenced: a red line in Fig. 8a highlights the fatigue crack propagation zone; the crack initiation is at the external surface (point
B in Fig. 8a). The morphology of the crack propagation, shown in Figs. 8b and 8c, is similar to the previous samples (Fig. 6e and Fig. 7c) and the final instantaneous fracture zone (Fig. 7d) is ductile characterized by dimples. By comparing all the performed analyses, we can conclude that the fracture surfaces for the different bolts are characterized by similar fatigue propagation morphologies. The crack initiation is located always at the external surfaces. No intergranular crack propagation can be identified and the presence of dimples is evident in the final fracture zone. None of the analyzed fracture surfaces shows the typical hydrogen embrittlement characteristics, visible for instance in [14,27,41–43]. We can therefore state that failures are not caused by hydrogen embrittlement. Another important aspect is related to the extension of the fatigue propagation region with respect to the final fracture zone. We can state, in fact, that these extensions are similar for all the samples, even if the applied stress amplitudes and the durations are different. We could suppose that the effect of the different treatments is accounted by different nucleation phases.
4.
DISCUSSION
The difference in failure locations is an important aspect to be considered: all NC bolts failed at the head shoulder fillet (under head failure mode), while all MP and ZP bolts failed systematically at the first engaged thread. In order to provide an insight into this behavior, we will focus on the following factors that can influence fatigue strength and life, especially locally at the surface: 1) the stress concentration factor Kt, function of bolt geometry, and in particular of the fillet radius, which is the same for all the samples; 2) the presence of residual stresses; 3) the different surface qualities of the bolts obtained from the manufacturing process. According to [44], the stress concentration factor at the head shoulder fillet (Kth) is smaller than that of the first engaged thread root Ktt.. The respective values are: Kth= 2.3 and Ktt= 3.8. This difference, that is about 65%, is mainly due to the non-uniform distribution of the applied load among the threads. Indeed, for standard nuts, it results typically that the first engaged thread has to bear about 30-35% of the total load [44]. Besides, according to [22,23] thread rolling after heat treatments generates a compressive field of residual stresses, leading to beneficial effects on fatigue performances. Thread rolling is applied only at the threads, and not at the fillet below bolt head. Therefore, for this particular geometry, we can assume that NC bolts fail at the
head shoulder fillet because the effect of residual stresses on the threads prevails on the stress concentration related to Ktt. ZP and MP samples fail at thread root probably because the compressive residual stresses are reduced during the coating processes. On the other hand, attention should be addressed to the surface quality to explain the differences in fatigue performance of MP and ZP bolts, having the same geometry and residual stresses due to thread rolling. Indeed, even if they experience the same failure mode at the first engaged thread, their fatigue strength and life are different. Fig. 9 shows a magnification with optical microscope to support the discussion on the surface finishing at the thread root of MP and ZP bolts that are, respectively, treated with acid pickling and sandblasting. Comparing Fig. 9.a and Fig. 9.b, we can state that the MP bolts show a more irregular surface with small acid corrosion of maximum depth of 10µm, with round shape. ZP bolts do not exhibit pitting on bolt surface, as the maximum depth of the surface roughness is lower than 2µm (see detail in Fig. 9.b). According to the literature, when dealing with crack propagation from defects, one of the main parameters is the square root area introduced by [45]. The considered square root area,
, corresponds to the area of a defect
projected in the direction of the maximum tensile stress. It is a geometrical parameter proportional to the maximum stress intensity factor and it controls the fatigue strength of materials containing small defects, because it is related to the threshold of crack propagation from defects. Referring to the surface defects found in MP bolts (Fig. 9.a), we can state that they are characterized by , which has almost no influence on the fatigue strength of metals [46], even if we suppose its location at the thread root. Indeed, according to this paper, the minimum considered value of
is around 25
μm to appreciate a variation in the rotating fatigue limit of at least 10%. Thus, the decrease in the fatigue limit of MP with respect to ZP bolts remains a non-trivial question. Some works showed that sandblasting not only improves surface quality but also induces partially and locally the compressive residual stresses [23,47], leading to beneficial effects in terms of fatigue resistance. However, it is opinion of the authors that sandblasting has a limited effect on residual stresses, because its input energy is much smaller than the requirements for shot peening. Moreover, studies on a different application proved that residual stresses are relaxed due to corrosive phenomena [48–50]. The partial relaxation of residual stresses due to acid pickling can be one of the reasons inducing MP bolts to have the lowest fatigue resistance among the investigated processes.
As a conclusion to this discussion, we can hypothesize that the decrease in fatigue limit between ZP and MP bolts could be a mixed phenomenon attributed to small improvements related to sandblasting operation in ZP bolts on the one hand, and on the other to small worsening related to the surface finishing with pit formation and to partial stress relaxation due to acid pickling of MP bolts. Anyway, the authors underline that more research is required in this direction to better understand the phenomenon.
CONCLUSIONS In this paper different manufacturing processes of high strength steel bolts have been investigated. Particular attention has been paid to the final surface preparation (acid pickling or sandblasting required before final coating) and to the coating process. The different investigated bolts are: i) NC bolts, thread rolled after heat treatment but not coated; ii) ZP bolts, coated with micro-zinc phosphate after sandblasting; and iii) MP bolts coated with manganese phosphate after acid pickling. These coating processes are correlated with hydrogen content, surface quality and fatigue resistance of the final product. The following conclusions can be drawn: - The investigated processes did not underline the risk of hydrogen embrittlement. Indeed, the measured of diffusible hydrogen was negligible in all tested samples. Moreover, the fractography images collected after fatigue testing confirmed that the failures are not related to hydrogen embrittlement in any of the series. - Fatigue tests on the three bolt types evidenced a different fatigue life, both in terms of Wohler’s slope and of fatigue limit between the series. Despite MP bolts are produced according to OEMs requirements, they had lower fatigue resistance (-13% with respect to NC samples). Failure modes were also different: NC bolts failed at the head fillet, while ZP and MP bolts failed at the first engaged thread. - Fractography analysis suggested that cracks propagated from bolt surfaces in all investigated samples. - Focusing on NC samples, we could hypothesize that the failure at the head, where stress concentration factor is smaller than the first engaged thread, is due to thread rolling. Indeed, this mechanical treatment induces a compressive residual stress field on the threads but not at the bolt head. - Comparing MP and ZP bolts, both failing at the threads, we could identify different but not comprehensive explanations to the decrease of fatigue limit of MP series. Considering this bolt type, we could identify some micro-pitting due to acid pickling, which can also partially relax the residual stresses; on the other hand, for ZP bolts, sandblasting could have partially improved this field and helped to widen the range between fatigue limit of MP and ZP bolts. Probably, the experimental evidence of this variation in the fatigue performances is connected with a combination of all these phenomena.
ACKNOWLEDGEMENTS The authors gratefully acknowledge the support of E.S. Brugola for its continuous effort in research and development of fastening technologies. The authors also acknowledge Brugola OEB Industriale S.p.A. (Piazza Giovanni XXIII 36, 20851 Lissone, Italy) for the production of the bolts discussed in this paper. A special thanks goes to R. Cenni, G. Ambrosini and Fimeur Group S.r.l. (Via P. M. Curie 31, 25046 Cazzago S. Martino, Italy) for the support during the coating process described in this research. The authors thank Eng. Francesca Brunella from Politecnico di Milano, Chemistry, Material and Chemical Engineering Department ‘‘Giulio Natta’’ for the SEM analyses and discussions.
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Highlights · · · ·
Connecting rod bolts are critical fasteners Fatigue tests to highlight the effect of coatings (Mn-phosphate and micro Zn-phosphate) on fatigue performance Analysis of hydrogen content, correlated to the manufacturing process Fractographic investigation of crack initiation and propagation
Figure 1 – Geometry of the investigated bolt; dimensions in mm.
Figure 2 – A: bolt head fillet; B: martensitic tempered structure at the center of the bolt; C: bolt threads. These images are collected by an optical microscope after thread rolling process and confirm the absence of decarburization, delta-ferrite, inclusions, surface defects or thread forming laps before coating process.
Rolling Annealing
Sandblasting Polymer wire-coating Cold forging Quenching and tempering Thread rolling
Acid pickling
Sandblasting
Mn-phosphate bolt-coating
Zn-phosphate bolt-coating
MP bolts
ZP bolts
NC bolts
Figure 3 – Typical steps of the conrod bolt manufacturing process from rolling to final bolt coating. The three bolt types considered in the present paper are collected: after thread rolling (NC); after coating with Mn (MP), and after coating with Zn (ZP).
Figure 4 – Results of the fatigue tests: Wohler’s curve determined for NC (non-coated bolts), MP (bolts with acid cleaning and manganese phosphate coating), and ZP (sandblasted bolts coated with micro-zinc phosphate). Both axes are in logarithmic scale.
Figure 5 – Examples of failure mode in fatigue testing.
Figure 6 – SEM analysis of NC bolt failed during fatigue testing. A) and B) Fracture surface of bolt head. C) Possible location of crack initiation. D) Transition from fatigue growth and ductile crack propagation. E) Morphology of fatigue crack propagation. F) Final failure dimples. NC specimen with alternate stress σa = 139MPa, number of cycles to failure Nf = 282669 cycles.
Figure 7 – SEM analysis of ZP bolt failed during fatigue testing. A) Fracture surface (the red line indicate approximately the fatigue crack propagation). B) Detail of fatigue fracture surface. C) Morphology of fatigue crack propagation. D) Final failure dimples. ZP specimen with alternate stress σa = 160 MPa, number of cycles to failure Nf = 206785 cycles.
Figure 8 – SEM analysis of MP bolt failed during fatigue testing. A) Fracture surface. B) Possible location of crack initiation. C) Morphology of fatigue crack propagation. D) Final failure dimples. MP specimen with alternate stress σa =89 MPa, number of cycles to failure Nf = 636069cycles.
a. Surface cleaning: acid pickling 50 μm
b. Surface cleaning: sandblasting 50 μm
10 μm
Figure 9 – Thread roots of: a. MP bolts: measure of the corrosion pit. b. ZP bolts with magnification: no pitting.
Table 1 – a. Chemical composition (weigh %) and (b.) mechanical properties of the adopted steel.
a. C
S
P
Mn
Cr
Ni
Mo
B
Fe
0.33
0.008
0.013
0.81
1.09
0.026
0.012
0.0037
97.5
b. Average ultimate tensile strength, UTS (MPa)
1250
Average yield stress, YS (MPa)
1125
Property class of the fastener according to ISO 898-1
12.9
Average core hardness (HRC)
40
Steel microstructure
Tempered martensite
Purity grade according to ISO 4967
K2<3
Table 2 – Diffusible and total hydrogen content measured at different stages of the production process.
Sample
Diffusible H [ppm]
Total H [ppm]
NC
< 0.1
2.7
MP
< 0.1
2.9
ZP
< 0.1
2.5
Table 3 – Alternate fatigue limit measured on bolts at different stages of production process; confidence limits are calculated following ISO 12107:2012(E) standard. Wohler’s average coefficients are obtained by . interpolating the experimental data with the law:
Sample
Fatigue limit [MPa]
A [MPa]
B
Failure zone
NC
108±9
2886
-0.242
under the head
MP
94±2
686
-0.144
at the first engaged thread
ZP
118±4
1955
-0.209
at the first engaged thread
Conflict of Interest. Declaration of interest: none