Influence of corrosion on the behaviour of wheelsets in service

Transcription

1 Influence of corrosion on the behaviour of wheelsets in service P. FERAUD 1, J. J. VIET 2, M. DIENER 3, M. GRAB 4, 1 SNCF: Agence d Essai Ferroviaire 21, avenue Salvador Allende - F Vitry-sur-Seine 2 SNCF: Direction des Audits de Sécurité 20, rue de Rome F Paris 3 ETH Zürich : Laboratorium für Nanometallurgie, Wolfgang-Pauli-Strasse 10 CH-8093 Zürich 4 DB : Deutsche Bahn, DB Systemtechnik (TZF 25) Pionierstrabe 10 D Minden Running gears of a railway vehicle are theoretically designed for an unlimited lifetime. However, they can be subject to corrosion caused by railway conditions such as humidity, transportation of corrosive goods and ballast impacts. Failure can be prevented by design regulations, protection quality imposed during fabrication and operating inspections. The goal of this investigation is to understand and estimate the effect of corrosion on the lifetime of exposed elements such as wheels, axles and springs. Within the UIC regulation framework, several studies have been performed to estimate the effect of corrosion on the initiation and propagation of fatigue cracks. First of all, a bibliographical review, an analysis of the operating feedback from various international sources and the failure analysis of wheels, leaf springs and coil springs has been done. It has been shown that, despite the application of protection standards, generalized corrosion may still occur during the life of the gear. The main result of this study for wheel sets and springs is that there is no important coupling between corrosion and mechanical fatigue damage. This is probably due to the range of frequency of the mechanical load applied to these gears. If larger than the corrosion kinetic, these frequencies prevent a significant coupling. Therefore it appears that the corrosion accounts only partly for the stress concentration caused by notch effect subsequent of the apparition of corrosion pits. From these observations, an experimental program consisting of fatigue tests has been designed. The specimens have been exposed to corrosive saline fog (only the case of generalized corrosion is considered here), and then subjected to mechanical fatigue. In order to define two exposure times T1 and T2 corresponding to two levels of damage, the evolution of the loss of mass per surface unit versus the exposure time has been obtained by performing preliminary tests on A1N steel. The chosen times are T1=300 hours, corresponding to weak damage, and T2=1000 hours, corresponding to strong damage. The tests have been performed on axle steels A1N and A4T, wheel steel R7 and spring steel 51CV4 Corroded specimens subjected to different stress conditions (torsion, plane and rotating bending) have been tested for estimating the abatements of endurance limit. In the case of A1N steel and rotating bending load, if we disregard the fatigue limit decrease due to the loss of mass which is significant for the specimens but negligible for full scale axles, the abatement stabilizes at 25% for any corrosion level. At this point, we only consider cracks induced by pitting corrosion. Because of their geometry, these pits create a stress site agglomerate initiating fatigue cracks. Although the lack of influence of the exposure time on abatement can be explained by the limiting factor of the pitting corrosion process it self preventing the formation of large pits. Under four-points bending load, the abatement of fatigue limit at time T1 is around 30% for R7 steel and 35% for 51CV4 steel. We then studied, for the same materials and for two times T1 and T2, the effect of corrosion on fatigue crack propagation (Paris law) under positive and negative load ratios (R=0.4 ; R=0.2 ; R=0 ; R=-0.5 and R=-1). Because of the independence of corrosion and fatigue, we only focus on the influence of the presence of an oxide layer on the crack surfaces. No significant influence of corrosion has been observed for any of the test conditions. For A1N and R7 steels, the

2 fracture toughness has been measured on CT specimens subjected to stress corrosion during times T1 and T2. No significant difference with non-corroded steel has been observed either. All these results suggest the necessity to propose new recommendations for design, manufacture and maintenance. For example: For machined wheels, in order to account for mechanical behaviour under corrosive conditions, it is possible to apply the load level specified in UIC leaflet for an unrefined forging wheel in highly exposed areas (rim-plate linking area). In fact, the difference of fatigue limit between machined specimens and specimens taken from the surface of an unrefined wheel is similar to the difference of fatigue limit between corroded and non-corroded material determined here (UIC RP19). This new approach can be applied in both cases: numerical or experimental verification of the mechanical behaviour. For unrefined forging wheels, the fatigue limit remains unchanged. The only difference concerns the standard deviation which is much more important for a corroded wheel (σ=20mpa) than for a healthy wheel (σ=10mpa). This result can be of great interest for a probabilistic approach of the in-service behaviour of running gears. The EN standard defines four levels of protection against corrosion for axles: from class 1 while exposed to atmospheric corrosion and mechanical impacts to class 4 while exposed to atmospheric corrosion with calculated stresses below 60% of the threshold stress corresponding to an unprotected state. The results obtained suggest a fatigue limit for a corroded axle of 75% of the threshold stress, regardless of the corrosion time. Recommendations for class 4 (unprotected) are then conservative; thus it is not necessary to modify the actual specifications. For leaf springs, it is necessary to account for an abatement of 35% of the fatigue limit. This value has to be defined more precisely to take into account the influence of the shotpeening applied to these organs. Concerning maintenance, in the explored experimental conditions (mechanical load frequency sufficiently high to have no coupling between corrosion and fatigue) it seems possible to keep the same inspection intervals. A next step of this study could be the determination of the fatigue limit abatement in the case of localized corrosion which can appear when the protection is locally damaged.

3 1. OBJECT 1.1. Context Running gears of a railway vehicle are theoretically designed for an unlimited lifetime. However, they are subjected to corrosion caused by railway conditions such as humidity, transportation of corrosive goods and ballast impacts. Failure can be prevented by design regulations, protection quality imposed during fabrication and operating inspections Aim The goal of this investigation is to understand and estimate the effect of corrosion on the lifetime of exposed elements such as wheels, axles and springs. Within the UIC regulation framework, several studies have been performed to estimate the corrosion effect on the initiation and propagation of fatigue cracks Operating feedback Failure analysis results Failure analyses performed on a broken leaf springs (Fig.1) and broken helicoidal springs (Fig.2), cracked wheel (Fig.3-4), has brought the following points into sharper focus: For the three components: the occurrence of failure through the propagation of fatigue cracks [1]; In the case of springs, cracks initiate on corrosion pits; Corrosion crater Fig.1: Leaf spring Fig.2: Helicoidal spring In the case of the wheel, cracks, located at the plate-rim junction, also seem to initiate on corrosion pits. Moreover, numerical simulations (in accordance with UIC leaflet) have shown that the stress level was relatively high (see 1.4) and metallurgical exams highlighted the presence of a bainite in the crack initiation zone that could generate residual stresses. Corrosion zone Fig.3: Wheel Fig.4: Corrosion

4 No hydrogen-related embittement phenomenon is observed in fatigue propagation zones (no secondary crack at right angles with the cracking plane). This result shows that there is no strong coupling between fatigue damage mechanisms and corrosion damage mechanisms. These different observations would tend to demonstrate that the corrosion/fatigue coupling phenomenon is relatively negligible for the components examined. This result is probably linked to the stress frequency spectrum of these components (for example at 100 km/h, the mechanical load frequency of a wheel is at around 10 Hz), which is far too high to induce a strong coupling phenomenon. Corrosion therefore acts here primarily as a stress concentrator by creating pits that facilitate the initiation of fatigue cracks. [2] The experimental procedure described in the following chapter is therefore based on a sequential approach and not on a coupled approach: 1st stage: corrosion-induced damage to the material free of mechanical stress; 2 nd stage: damage induced by mechanical fatigue in the room condition Calculations We performed numerical simulations in accordance with the UIC leaflet, on the cracked German wheel of type DR presented in 1.3 and on an ORE (Fig.5-6) French wheel in order to verify the UIC dimensioning. Fig.5: Von Mises stress - wheel DR UIC case n 2 Fig.6: Von Mises stress - wheel ORE UIC case n 2

5 Fig.7: Haigh diagram - wheel DR UIC result - threshold 145 MPa Fig.8: Haigh diagram - wheel ORE - UIC result - threshold 145 MPa Zone of corrosion 145MPa 0MPa 0MPa 0MPa 0MPa 145MPa 145MPa 145MPa Fig.9: Detail of UIC result for one section - wheel DR Fig.10: Detail of UIC result for one section - wheel ORE Calculations show that, in the case of a DR wheel, the UIC criterion is largely exceeded (Fig.7) contrary to the ORE type that remains under the 145 MPa limit (Fig.8). It is worth noticing that, for the DR type, the stress distribution on a wheel cross section shows that hot spots are localized at the plate-hub junction. In this zone, the stress level is beyond the UIC requirement (Fig.9). In the case of an ORE wheel, no point exceeds the UIC threshold (Fig.10). In contrast, the fatigue crack initiation point is localized at the external face of the rim-plate contact area. This difference in positioning between the hot spot and the crack can be explained by the presence of corrosion in this zone (Fig.3-4).

6 2. EXPERIMENTAL PROCEDURE 2.1. Determination of corrosion times Salt-spray exposure tests were carried out on specimens to determine the equation of loss-ofmass as a function of exposure time. The salt-spray test is described in Standard ISO 9227 and the test performed is the saline-solution test (or NSS test). Six A1N specimens, identical in size and pre-weighed, were salt-spray exposed: each specimen was removed from the chamber after a different exposure time (ranging from 0 to 1000h). The specimens were then weighed after having been chemically ridden of its oxides. We obtain in this way the curve giving the loss of mass per surface unit Δm ΔS time t whose polynomial approximation can be formulated as follows (1) (Fig 11): as a function of Δm ΔS = t t (1) with Δm ΔS expressed in g/m² and t in hours. Loss of mass/unit area relatively to time loss of mass / UA (g/m²) 2500, , , ,00 500,00 0,00 y = x x R 2 = T1=300h T2=1000h 0,00 200,00 400,00 600,00 800, , ,00 t (h) Fig.11: loss of mass per surface unit versus corrosion time exposure In order that a wide range of corrosion levels may be scanned, exposure times T1 = 300h (moderate damage level) and T2 = 1000h (extreme damage level) have been selected for the next study phase Fatigue test Four different materials have been studied: R7 steel for wheels, A1N and A4T steels for axles and 51CV4 steel for springs. R7 fatigue specimens were removed from a machined new loco wheel. A4T and A1N specimens were taken from two new axles according to EN standard. The 51CV4 fatigue specimens were taken from two new leaf-springs. After machining, the specimens were salt-spray exposed during times T1 and T2, except for 15 51CV4 specimens used for the determination of the endurance limit of non-corroded material. The total number of specimens available for the fatigue tests is:

7 Axle: (A1N) - 13 notched aeronautical specimens at time T1-15 smooth aeronautical specimens at time T1-15 smooth aeronautical specimens at time T2 (A4T) - 15 smooth aeronautical specimens at time T1-9 smooth aeronautical specimens at time T2 Wheel (R7): - 16 torsion specimens at time T1-13 four-points bending specimens at time T1 Spring (51CV4): - 15 four-points bending non corroded specimens - 15 four-points bending specimens at time T1 All the fatigue tests were carried out and analyzed according to the staircase method described in NF A standard. The load-ratio cycles is obtained for a 50% non-failure probability. σ min R = is 1 for all tests. The fatigue limit at 10 7 σ max The level of abatement is obtained by comparing the endurance level as determined on corroded specimens σ ex with that of sound material σ ref. However the resulting abatement takes into account two phenomena: - The local increase of stress linked to the geometry of corrosion pits (notch effect); - The increase of the effectively applied stress resulting from the cross-section loss in the specimens. The impact of cross-section loss (which could be significant on specimens but remains negligible on full scale pieces) can be overcome by taking into account the effective residual cross-section of the specimen after corrosion. The latter can be calculated from equation (1), and this gives the corrected endurance limit σ excor obtained on corroded specimens. The abatement linked solely to the surface degradations of specimens (pits) can then be calculated easily. Finally, two abatements are defined, as follows: (2-3) Total abatement A t σ σ ref ex = (2) Abatement resulting solely from surface degradations: σ ref A ds σ σ ref excor = (3) σ ref 2.3. Crack propagation tests The crack propagation tests were carried out in accordance with NF A standard. The specimens are of type CCT. All the specimens were taken from new wheel, axle and springs. A protocol designed to maintain the soundness of the cracking zones was applied on each specimen. It involves placing a scotch at the extreme point of the visible crack before each saltspray exposure (Fig.12).

8 Corrosion Notch Spared zones Fig.12: Example of specimen after corrosion. The specimens are salt-spray exposed, except for those used in calculating the crack growth rate in sound material. They are removed from the chamber after times T1 or T2. The crack monitoring tests are carried out under 5 different load ratios R (-1 / -0.5 / 0 / 0.2 / 0.4). The value of maximal load σ max is kept constant (σ max =100MPa) and the load frequency is 25 Hz. For each specimen there is a corresponding material (A1N, R7, 51CV4), with its load ratio R (-1.02 / -0.5 / 0 / 0.2 / 0.4) and its corrosion time (T1, T2). Starting from the premise that the corrosion and cracking are not coupled together, and to ensure that the crack surfaces are subjected to a constant corrosion level, each test is stopped three times, with the specimen salt-spray exposed during an exposure time T1 or T2 (Fig.13). σ Salt-spray exposure (Time T1 or T2) σ Paris law: da dn = CΔ m K eff Fig.13: Principle of crack growth test protocol

9 The different salt-spray exposures are clearly detectable on the failure surface observed at the end of the test (Fig.14): First corrosion exposure over time T1 or T2, Zone 1 (notch with pre-cracking) Second exposure over time T1 or T2, Zone 2 (5mm crack) Third exposure over time T1 or T2, Zone 3 (10mm crack) Zone 4 (15mm crack) Failure Zone 1 Zone 2 Zone 3 Zone 4 Failure Fig.14: Fracture surface at the end of a test for an exposure time of 300h The aim is to cover the crack surfaces with an oxide layer and to evaluate its influence with respect to the crack propagation. The results obtained are presented in the form of Paris law (4) and compared with that the non-corroded material one: da / dn = C ΔK (4) 2.4. Fracture toughness test The fracture toughness tests are carried out according to the NF EN ISO standard. The tests were restricted to R7 and A1N steels. The R7 specimens were taken from the plate of the same wheel as the CCT specimens, with the notch pointed in the X-Y direction as per NF EN ISO standard. The AIN specimens were taken from the shaft of a new axle with the notch pointed in the Z X direction. In order to simulate real stress-corrosion conditions, the specimens are subjected to 4.2 kn load during exposure for R7 specimens and at 10 kn load for AIN specimens (Fig.15). Two specimens per exposure time and per material were tested. Fig.15: specimens corroded

10 3. RESULTS 3.1. Fatigue test The results are given in the table below: [3] Tab1: Results of fatigue test Material Test σ ref (MPa) A1N Notched aeronautical T1 Smooth aeronautical T1 Smooth aeronautical T σ ex (MPa) Total abatement (%) Abatement resulting from surface defects only (%) / A4T Smooth aeronautical T1 Smooth aeronautical T R7 Torsion 4-point bending T CV4 4-point bending T To study the influence of the corrosion level on the fatigue limit of axles, changes in the total abatement and abatement induced by surface degradations only were charted by reference to the salt-spray exposure time for A1N and A4T steels (Fig.16). The behaviour observed is identical for the two materials. 45% Rotative bending - smooth 40% 25% abatement 35% 30% 25% 20% 15% 10% 5% total abatement (A1N) surface defects alone (A1N) total abatement (A4T) surface defects alone (A4T) abatement due to section reduction abatement due to surface defects 0% exposure time (h) Fig.16: Influence of exposure time on abatement for axles This representation, obtained from the corrosion kinetics, introduces a distinction between the abatement ascribable to cross-section loss and that ascribable to the formation of corrosion pits, which stabilizes at around 25%. This stabilization phenomenon results from a levelling effect of surface defects through corrosion and only becomes apparent in cases of general corrosion). Cross-section loss has a non-negligible impact on total abatement for long-exposure times. This phenomenon is linked to the size of the specimens used, which is not of the same magnitude as the size of a full scale axle, and which are therefore much more sensitive to the loss of mass.

11 Fig.17 actually shows that for diameters exceeding 30 mm, the incremental stress impact linked to the reduction in cross-section is negligible. Stress Diameter (mm) Exposure time Fig.17: Influence of exposure time and diameter of specimen on the stress increase induced by loss of mass This result means that, for a full-scale axle, the total abatement of the endurance limit will solely be linked to the formation of surface defects, and will therefore stabilize at around 25% (for the testing conditions explored) Crack propagation tests The results are presented in the Paris diagram. They comprise a trend line (black-colored) representing the behavior of the sound material, and two trend lines corresponding to the test results obtained for exposure times T1 and T2. A color code is used to distinguish the three crack propagation phases (Fig.18). [4] Paris law A1N T0, T1 and T2 for R=0 da/dn (mm/cycle) 0, ,0001 A1N T0 A1N T1 1 A1N T1 2 A1N T1 3 AIN T2 1 A1N T2 2 A1N T2 3 Paris A1N T1 Paris A1N T0 Paris A1N T2 A1N sain y = 2E-10x 4,0447 0,00001 ΔK (MPa.(m)-1) A1N T1 y = 2E-10x 4,0039 A1N T2 y = 3E-10x 3,9869 Fig.18: Example of results for A1N specimens at T1 and T2 R=0.

12 It is seen from an analysis of the overall results obtained that corrosion has no major influence on the fatigue crack growth rate, for the components examined and in the context of the test protocol (decoupled approach). Indeed the differences recorded are consistent with the natural scatter inherent in this type of test (this being all the more significant in our case as the tests were not performed on a continuous basis, with three interruptions recorded for each specimen) Fracture toughness tests The results are expressed in terms of K Q and K max, as the conditions for obtaining a valid critical stress intensity factor K IC cannot be met for the materials examined. Tab2: The results obtained for steel R7. Tab3: The results obtained for steel A1N R7 T0 (MPa m) T1 (MPa m) T2 (MPa m) A1N T0 (MPa m) T1 (MPa m) T2 (MPa m) K Q K max K Q 53 K max K Q K max K Q K max / No clear trend is identifiable. The results obtained are consistent with the scatter inherent in this type of test. It is seen from an analysis of the overall results that corrosion has no significant influence on the fatigue crack growth rate, nor on the fracture toughness value of the material for the components examined, and in the context of the test protocol adopted (sufficiently-high load frequency compliant with the scenario where corrosion-induced damage and crack-induced damage are not coupled together) Axles 4. RECOMMENDATIONS The EN standard defines four levels of protection against corrosion for axles: from class 1 while exposed to atmospheric corrosion and mechanical impacts to class 4 while exposed to atmospheric corrosion with calculated stresses below 60% of the threshold stress corresponding to an unprotected state. The results obtained suggest a fatigue limit for a corroded axle of 75% of the limit of a non-corroded one, regardless of the corrosion time. In the case of axles without protection (Class 4 type) subject only to atmospheric corrosion, the value of 60% of limit stress specified in EN could therefore be increased to 75% subject to validation by a full-scale test campaign. In the absence of corrosion effect on crack growth rate at a sufficiently high frequency decoupling fatigue and corrosion, there is no particular recommendation to follow in the design, manufacture or maintenance.

13 4.2. Springs Despite of the fact that a protection against corrosion is specified in the UIC leaflet, the operating feedback shows a generalized corrosion in the fastening zone of springs (Fig.19). Fig.19: Spring corroded For leaf springs, it is necessary to account for an abatement of 35% of the fatigue limit. This value has to be defined more precisely to take into account the influence of the shotpeening applied to these organs Wheels Despite of the fact that a protection is applied during manufacture, in service, wheels are subjected to generalized corrosion preferentially localized at the rim-plate junction (Fig.20) Fig.20: Wheel corroded For machined wheels, in order to account for mechanical behaviour under corrosive conditions, it is possible to apply the load level specified in UIC leaflet for an unrefined forging wheel in highly exposed areas (rim-plate linking area). In fact, the difference of fatigue limit between machined specimens and specimens taken from the surface of an unrefined wheel (UIC RP19) is similar to the difference of fatigue limit between corroded and non-corroded specimens. This new approach can be applied in both cases: numerical or experimental verification of the mechanical behaviour. For unrefined forging wheels, the fatigue limit remains unchanged. The only difference concerns the standard deviation which is much more important for a corroded wheel (s=20mpa) than for a healthy wheel (s=10mpa). This result can be of great interest for a probabilistic approach of the in-service behaviour of running gears. 5. CONCLUSIONS AND PROSPECTS The main hypothesis of the work presented here concerns the non coupling between corrosion damage and mechanical fatigue damage for running gears. This assumption has been adopted regarding existing results in the literature (in particular the influence of load frequency) and the operating feedback. Within this framework, fatigue limits abatements of corroded wheels, axles

14 and springs have been determined and it has been shown that corrosion has negligible effect on fatigue crack growth rate. The next steps of this study will consist in the realization of real fatigue-corrosion tests to estimate the frequency level for which coupling between corrosion and fatigue becomes important and to validate hypothesis adopted here. The determination of the fatigue limit abatement in the case of localized corrosion, which can appear when the protection is locally damaged, has to be established too. 6. REFERENCES [1] FERAUD, P.:UIC study on running-gear corrosion Batch 2 Task 2.2 DOC / S [2] LIEURADE, H.P.: Essais de fatigue-corrosion Doc. M 135. [3] FERAUD, P.: Influence of corrosion on fatigue limit of UIC project B252-1: running-gear stresses Task 7 DOC / S [4] FERAUD, P.: Influence of corrosion on fatigue-crack propagation of UIC project B252-1 : running-gear loads task 8, batch 8.2 DOC / S000200