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Influence of welding on microstructure and strength of rail steel
Construction and Building Materials 243 (2020) 118220
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Construction and Building Materials journal homepage: www.elsevier.com/locate/conbuildmat
Influence of welding on microstructure and strength of rail steel Yasin Sarikavak a,b,⇑, Osman Selim Turkbas c,d, Can Cogun e a
Mechanical Engineering Department, Ankara Yıldırım Beyazıt University, Ankara 06010, Turkey Railway Research and Technology Centre, Turkish State Railways, Ankara 06105, Turkey c Mechanical Engineering Department, Gazi University, Ankara 06570, Turkey d Mechanical Engineering Department, Near East University, Nicosia, Northern Cyprus, Mersin 10, Turkey e Mechatronics Engineering Department, Cankaya University, Ankara 06815, Turkey b
h i g h l i g h t s Significant changes have been investigated in grain size as it ranges 2–8 by ASTM-E112. Three different zones have been identified with their hardness and tensile properties. Tensile strength gradually decreases in Zone 2 and parent metals respectively. FE model has been developed and validated under various loads for stress distributions. Stress concentrations have observed between Zone 2 and parent metal.
a r t i c l e
i n f o
Article history: Received 5 December 2018 Received in revised form 29 December 2019 Accepted 18 January 2020
Keywords: Welded rail steel Grain size Tensile strength Finite element analysis Stress distribution
a b s t r a c t In this study, metallurgical and mechanical aspects of cold flash butt welded rail joints were investigated. The relationship between the material characteristics and the distance to the fusion line were evaluated. Significant changes observed in grain size as it ranges 2–8 according to ASTM E 112. Hardness tests conducted and the average results of the rail head, web, and foot were recorded. According to the results, three different zones were identified including the fusion line, recrystallization zone, and transition areas. Tensile properties and strength characteristics have been investigated for the specified zones separately. Obtained experimental values were used as input parameters for the developed finite element model (FEM) to calculate stress distribution. The welded rail was subjected to realistic wheel load and stress distributions in weld were evaluated. The developed model has also been validated by comparing the theoretically obtained stress values with the experimental ones under various loading conditions. Ó 2020 Elsevier Ltd. All rights reserved.
1. Introduction Research organisations, railway operators, and manufacturers conducted several studies to improve the technological level of the rail materials, joint types, and railway systems since the beginning of railways. The railways provide safe system of transport and many areas in railways experiencing considerable increasing in performance [1]. Due to the advantageous of maintenance and material integrity in railway lines, bolted rail joints were replaced with welded joints. Welded rails have lower maintenance costs, failure frequency, as well as the lower environmental impact [2]. In recent years, rail materials have higher carbon contents and
⇑ Corresponding author at: Mechanical Engineering Department, Yıldırım Beyazıt University, 06010 Ankara, Turkey. E-mail addresses: [email protected], [email protected] (Y. Sarikavak). https://doi.org/10.1016/j.conbuildmat.2020.118220 0950-0618/Ó 2020 Elsevier Ltd. All rights reserved.
larger cross-sections. This improves the mechanical resistance; however, it hardens to weld the rail. The difficulty in the weld assessment is principally due to the differences in the shape and the material property distributions in the localized regions of the welded joint [3]. Recently operational forces due to speed and axle load increased and cracking of the rail is still one of the major concerns. Cracks may either exist in structures or may initiate during operational conditions. The initiation process often occurs with stress concentration either in microscopic or macroscopic scales [4]. Therefore generally mechanical and microstructural properties of steels and various welding methods were studied in the literature. Ilic et al. [5] studied on hardness characteristics, mechanical properties, microstructure and fracture mechanism of thermite welded rail joints. Improved mechanical strength and elongation observed at the finer ferrite and pearlite microstructure. Meric et al. [6] studied the mechanical and metallurgical properties of welded rails via
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the thermite process. Maximum hardness observed at the interface between HAZ and weld metal as 109 HB. Chen et al. [7] studied the weld defect formation in thermite welds. The influence of welding parameters on weld defects in thermite welds was simulated with a developed heat transfer model. A case study on the fracture of welded U71Mn rail was conducted by Yu et al. [8]. Fracture surface morphology, microstructure, and micro hardness were analysed and the values around the fracture surface observed in normal level. Free solidification microstructure, which extends from outside to inside at the rail head, was observed as the crack source of the fatigue failure. Josefson and Ringsberg [9] studied life prediction in welded rails. Respectively, the assessment of uncertainties in life prediction of fatigue crack initiation and propagation in the rail head and web were conducted. Mansouri and Monshi [10] studied microstructure and residual stress variations in the weld zone of flash butt welded railroads. During cooling of the weld zone, the contraction in the web is higher than the head and base of the rail according to the current in copper electrodes. Zerbst et al. [11] investigate the damage tolerance behaviour of railway rails. The research study is a review of crack propagation and fracture of rails. It includes loading conditions and residual stresses from manufacturing and welding in the field. Wang et al. [12] studied the effects of microstructure on spalling damage in 20Mn2SiCrMo bainitic wing rails. Film like martensite and austenite (M/A) constituents have better performance to decrease crack propagation. Ziemian et al. [13] investigated the varying process parameters on microstructure, hardness and tensile properties of structural steels. The results showed that hardness was increased in the weld zone for all specimens. Franklin et al. [14] modelled the rail steel microstructure and its effect on crack initiation. Mahto et al. [15] investigated the force, temperature, mechanical properties and microstructural characterizations in friction stir lap welding of dissimilar materials. Microstructure and property relationship for friction stir welding were also studied in the literature [16]. Wang et al. [17] conducted finite element simulation on the ultra-fine microstructure of welded steel. The simulation gives out the grain size distribution in heat affected zone due to the temperature gradient. Kuroda et al. [18] studied flash butt resistance welding for duplex stainless steels. The cross-sectional microstructure of the weld bond region was observed using microscopy. Tensile strength and impact energy increased with increasing finegrained metal. Tawfik et al. [19] conducted an experimental and numerical investigation of tensile residual stresses in flash butt welds by rapid post weld heat treatment. A thermo mechanical finite element model including phase transformation characteristics of the rail material has been used to predict residual stress distribution. An experimental measurement of post-weld cooling rates using infrared thermography was used to validate the model. Rasanen and Martikainen [20] conducted an experimental metallurgical study on flash weld defects in welded joints. Athukorala et al. [21] studied on the characterization of head-hardened rail steels in terms of microstructure and cyclic plasticity response for Australian Standard AS1085.1 rail steel. The most common welding techniques in railway lines consist of thermite welding and flash butt welding processes [17–20]. Flash butt weld can be processed both in factory and mobile on the field. The metallurgical and mechanical properties of the welded joints are important to understand the roots of failures. (i) In the most of the published studies; rail steels, thermite welding processes and other welding techniques such as friction stir welding which are not practically applicable for joining rail materials were investigated in terms of microstructure, crack formation and strength. The process parameters of thermite welding are not compatible with flash butt welding process. Flash butt welded joints are critical sections in railways in terms of varieties in microstructure and mechanical properties. These differences
represent discontinuities in the structure under harsh environmental conditions such as wheel load and stresses. (ii) The majority of the failures on rails occur at the welded joints according to stress concentrations at the discontinuities. In published studies authors focused on some specific points such as heat transfers, residual stresses, phase transformations or microstructure for modelling the system. However each difference in microstructure and strength characteristics should be considered independently. For accurately modelling the welded rail these differences in each section should be evaluated individually and carefully. Based on this, the aim of the present work was to analyse the cold flash butt-welded rail joints in terms of microstructure and strength to accurately model the stress distribution under various loading conditions. The relationship between the material characteristics and the distance to the fusion line in the welded joint were evaluated. Hardness tests, microstructural analysis were conducted to differentiate the different zones in the welded joint. Grain boundaries and grain sizes were also evaluated for each zone and the results were compared with the hardness parameters. Then the zones, which have dissimilar properties, were considered as a graded material structure and it is divided into zones to individually evaluate the mechanical properties. Mechanical properties of each zone are clarified with the tensile test and findings were discussed. The obtained results were used as an input data for the suggested finite element model. The developed model calculates the stress distribution and the weakest section in the welded rail joint under various loading conditions that represents the wheel load of the rolling stock. The model has also been validated by comparing the theoretically obtained stress values with the experimental ones by using strain gages attached to the weld collar, under various wheel loads.
2. Material and methods Flash butt welding is one of the resistance welding processes. UIC 60 R 260 profile rail used for the experimental work that is 172 mm in height and 150 mm width in rail foot. The parent rails are positioned end to end and clamped with a device. The heavy current should pass through the end of the parent rail. Rail cold flash welding process and post weld applications conducted on the field with a mobile flash butt welding device. The device applies 20 kA current and 380 V voltages that results with the melting temperatures at rail ends. The process takes 3 min in total for each weld and total shortening in rail is about 30–44 mm in two rail ends. Following the flash butt welding process ultrasonic inspection conducted for investigating any internal discontinuity. No discontinuities were detected for the specimens. Spectrolab metal spectrometer was used for the chemical analysis of rails. Chemical compositions of UIC 60 R260 standard rail (limits) and measured results with a metal spectrometer for the test rail are given in Table 1. In order to study microstructure, specimens were sectioned in the longitudinal direction and prepared for the tests. Hardness test in the Vickers scale (HV 30) (Q-Ness device) conducted for the sections in rail head, web and food separately. After hardness test macro etching process applied to the other welded rail to validate the transition zones. The microstructural examination was carried out to investigate the change in grain size according to the distance to the centre of the fusion line. According to the results, different zones were determined. Tensile test specimens were prepared from the specified zones of welded rail. Main mechanical properties including modulus of the elasticity, yield strength, maximum tensile strength, and % elongation evaluated for each zone. The
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Y. Sarikavak et al. / Construction and Building Materials 243 (2020) 118220 Table 1 Chemical compositions of the test rail and standard rail. Steel sample grade (UIC 60)
R 260
Test Rail EN 13674-1
104% ppm. Max. by mass
By mass %
C
Si
Mn
P
Cr
Al
V
N
Oa
Hb
0.72 0.60/0.82
0.23 0.13/0.60
1.02 0.65/1.25
0.019 0.030
0.04 0.15 max.
0.002 0.004 max.
0.0005 0.030 max.
0.009 0.010max.
– 20
– 2.5
obtained data were used as an input for the finite element model for the stress analysis. To investigate the stress distribution, the welded rail was sectioned 1100 mm in length. Four strain gages of MicroMeasurements were installed 20 mm below and over the neutral axis, head and foot at the weld collar. IMC CRONOSflex 2000G data logger with a frequency of 100 Hz was used for data collection. Collected strain values were converted to the stress by using the software simultaneously. As explained in Section 3.3, the 211.7 GPa experimentally obtained average elasticity modulus of weld collar were used for the stress calculations. The strain was mainly calculated according to the change in length and resistance as shown in Eq. (1) [22].
e¼
dL dR=R ¼ L k
ð1Þ
where; e is the strain; dL is the change in length (mm); L is the original length (m); dR is the change in resistance (X); R is the resistance of strain gauge (X); and k is the gauge factor refers to the ratio of relative length change to change in resistance. By the applied force, obtained strain is very small due to the change in resistance. Therefore Quarter Bridge used to convert these changes into voltage changes. Thus, the voltage calculated according to the Eq. (2). The Eq. (3) gives the strain value, dependent to gauge factor and voltage parameters [23].
Ua ¼ Ue
e¼
4U a kU e
dR Ue ¼ ½ke 4R 4
ð2Þ
ð3Þ
where; Ua is the measurement voltage (mV); and Ue is the excitation voltage (V). The bonded quarter bridge single axis strain gages have a resistance value of 120 X with 6 mm measuring base. The factor k is equal to 2.155 ± 0.5. Strain, calculated according to Eq. (4) [22,23].
hlmi
e
m
¼
4ð1000Þ U a mV k Ue V
ð4Þ
Equipped welded rail installed on 500 kN capacity four-point bending servo-hydraulic test machine. Experimental setup and locations of the strain gages can be seen in Fig. 1. The upper span of four-point bending test machine is 154 mm and the lower span is 1020 mm [24]. The same design used for the finite element calculations as well. The bending stress for the FEM analysis was calculated according to the Eq. (5) [25].
r¼
Mc I
ð5Þ
where; r is the bending stress (MPa); M is the bending moment (Nmm); c is the vertical distance of the point of interest from the welded rails’ neutral axis (mm); and I is the moment of inertia of the rail cross sectional area (mm4). Bending moment is dependent to applied load and the span of the four point bending equipment and given in Eq. (6).
M¼
PL 2
ð6Þ
where; P is the applied load (kN); and L is the distance between the first upper and lower span of the roller (mm). There are many theorems used for understanding the influence of mean stresses in the structures. In this study, Soderberg theorem which is dependent to yield strength is used as given in Eq. (7) [25,26].
ra rm þ ¼1 re ry
ð7Þ
where; ra is the alternating stress (MPa); rm is the mean stress (MPa); re is the endurance limit (MPa); and ry is the yield stress (MPa). Mean and alternating stresses are dependent to minimum and maximum stress values across the cross section of the rail profile.
Fig. 1. Servohydraulic bending test set up and the location of installed strain gages.
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3. Results and discussion 3.1. Hardness test
Fig. 2. Microhardness values (HV30) across the weld collar.
The welded rail specimens were sectioned from the longitudinal vertical axis and the sectioned surface was grinded for hardness measurements. Hardness tests (HV30) were conducted 4 mm below the running surface of the rail head with 2 mm spacing [24]. Measurements repeated for rail head, web and foot separately with 10 mm spacing. Tests were conducted along a line 200 mm in length that centres the weld collar. Average measured results of welded rails can be seen in Fig. 2. 300 HV measured around fusion line for the experiments conducted with 2 mm spacing below the running surface of the rail. It was decreased to 250 HV at a distance of 20 mm from the fusion line. Average hardness values in the web are higher than the foot and head of the rail specimens. Some research studies in the literature show significant decreases of hardness at the transition
Fig. 3. (a) Microstructure of the centre of fusion line (b) Recrystallization zone (Zone 1) (c) Recrystallization region between Zone1 and Zone 2 (d) Coarse microstructure at the recrystallization region between Zone1 and Zone 2 (e) Zone 1 and Zone 2 transition region (f) Main structure.
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zones [27]. The decrease in hardness was also observed for rail head, web, and foot in the fusion line. The highest decrease in the fusion line was investigated at the rail head. The zone 40 mm in total centring the fusion line is called Zone 1. For the rail head around 40 mm distance to Zone 1 minor decreases in hardness were also observed with a difference of 10–20 HV in total.
5
side. The whole structure in Fig. 3(e) is pearlite. Retained austenite in microstructure plays a significant role on rolling contact fatigue crack propagation [29]. Some inclusions were also observed in all regions, which may affect the rails’ strength performance. The pearlitic main structure of Zone 2 and the parent metals can be observed in Fig. 3(f). The grain size according to ASTM E 112 was calculated as 5–6.
3.2. Microstructural analysis Macro etching process applied to the grinded specimen surface with 5% HNO3 and C2H6O solution. Solution was kept for 1.5 min on the surface and the fusion line, Zone 1, Zone 2 (adjacent to Zone 1) and the transition lines to the parent metal were observed after the etching process. A tiny difference for a length 20 mm in total centring the fusion line was observed after the etching process. Due to the dimensional differences such as thickness, wider boundary zones were observed at the rail head, foot, and web respectively. The polished surface of flash butt welded rail specimen was etched with 5% HNO3 and C2H6O for 7 s and the etched surface was observed with invert type microscope with 100X for microstructure analysis and grain size determination (Fig. 3) [28]. The relationship between the material characteristics and the distance to the fusion line in the welded joint were investigated. Fig. 3(a) shows the microstructure of the fusion line. Around 0.10% decarburized region was observed in 2 mm zone at the centre of the fusion line. According to ASTM E 112, grain size was calculated as 2–3. In figure brown zones refers the pearlitic and white zones refers to the ferrite structure in the fusion line. Fig. 3(b) shows the microstructure of the recrystallization region that is named as Zone 1. The main structure is totally perlite and some tiny ferrites can be seen on grain boundaries. It was observed that grain growth was less than the fusion line. According to ASTM E 112, grain size was calculated as 5–6. The microstructure of transition region between Zone 1 and Zone 2 can be seen in Fig. 3(c). Finer grain boundaries were observed with 8 in grain size according to ASTM E 112 at the areas closer to Zone 1. In Fig. 3(d) coarser microstructure was observed as the grain size is 4–5 according to ASTM E 112 and the microstructure was totally pearlite. As Fig. 3 (e) shows recrystallization zone is at the left-hand side of the picture and retained austenite phase can be seen at the right-hand
3.3. Mechanical properties Tensile test was applied with a mechanical extensometer to the welded rail joints. Locations and codes of cylindrical M10 test specimens are shown in Fig. 4. Stress-strain diagram of parent metals, Zone 1 and Zone 2 can be seen in Fig. 5. Table 2 shows the main strength values of each specimen where; mE is the modulus of elasticity in (GPa), Rp0.2 is the 0.2% offset yield strength in (MPa), Rm is stress at maximum force in (MPa), Fm is the maximum force in (kN). Strength tests showed that average elasticity modulus values are 204.2 GPa; 198.7 GPa and 211.7 GPa; yield strengths are 460.3 MPa; 491.2 MPa and 659.5 MPa and stress at maximum force is 909.6 MPa; 923.1 MPa and 1011.3 MPa for the parent metal, Zone
Fig. 5. Stress-strain diagrams of test specimens.
Fig. 4. Specimens extracted from the welded rail, locations and codes for tension test.
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Table 2 The results of the tension test. Specimen
mE (GPa)
Rp0.2 (MPa)
Rm (MPa)
Fm (kN)
Parent Metal – L – A Parent Metal – L – B Parent Metal – R – A Parent Metal – R – B ZONE 2 – L – A ZONE 2 – L – B ZONE 2 – R – A ZONE 2 – R – B ZONE 1 – A ZONE 1 – B
203.5 211.1 212.8 189.5 222.9 213.2 167.2 191.5 200.9 222.5
506.9 504.8 498.8 330.9 458.0 501.1 518.4 487.4 704.9 614.1
926.2 926.2 888.2 897.9 888.8 935.4 936.9 931.3 1027.1 995.6
18.2 18.2 17.4 17.6 17.4 18.4 18.4 18.3 20,2 19.5
2 and Zone 1 respectively. Zone 1 has the highest strength results. Tensile strength is higher around 11% in Zone 1 than parent metals; however, % strain is lower than the parent metals and Zone 2. 3.4. Strain measurement and stress analysis The maximum static axle load is 140 kN for high-speed trains under operation in Turkish State Railways. Loading applied in the range 10 kN-70 kN with 5 s pause at each load to the strain gage equipped welded rail. According to measured strain results, the calculated stress can be seen in Fig. 6. For 70 kN wheel load measured stress values at the weld collar are 36 MPa in rail head and foot (SG1; SG4) and 8 MPa below and above 20 mm of the neutral axis (SG2; SG3). Strain and stress data collected for the increased loads until 400 kN. 3.5. Finite element analysis (FEA) and loading Finite element method is an effective tool for prediction of stress, strain and remaining life for the welded joints as long as the material behaviour can be accurately modelled. Therefore designed UIC 60 R260 profile rail including Zone 1 and Zone 2 was imported to the finite element analysis software. Obtained mechanical properties in Section 3.3 were entered into the model separately. Welding induced residual stresses were neglected. For meshing, tetrahedron shape element was used in finite element calculations. The element size was taken as 7 mm for the Zone 1 and Zone 2 to obtain accurate results. The meshed design consists of 70,490 nodes and 39,235 elements. After meshing, boundary conditions were entered into the model. The high-speed trains
Fig. 6. Calculated stress for 10–70 kN loading against time via raw strain measurement.
under operation in Turkish State Railways have 70 kN static wheel load. The defined load distributed on two actuators contacting with rail head. Supports located at rail foot are fixed to the ground. The experimentally measured and calculated stress values with the finite element method on weld collar can be seen in Fig. 7. Theoretically obtained stress values are consistent with the experimental results. As shown in Fig. 7, theoretically calculated stress for 70 kN static wheel load is ±8.2 MPa and measured experimental value is ±8 MPa at rail web which is symmetrical to neutral axis (SG2, SG3). Calculated stress for rail head (SG1) is 37.7 MPa and the experimental result is 36 MPa. For rail foot (SG4), the calculated stress is 22.5 MPa and the obtained values for experimental work is 36 MPa at 70 kN wheel load. Theoretical findings are lower than the experimental ones for rail foot. With increasing wheel load, the gap between theoretical and the experimental values are getting higher particularly for rail foot. For rail head and rail web experimental and theoretical results are very consistent. Theoretical results at rail head (SG1) and rail web (SG2) for 400 kN, wheel load is 216 MPa and 54 MPa. The experimental results are 200 MPa and 45 MPa respectively. All analysis indicates that the experimental stress results are in good agreement with the finite element model results, especially for the loads lower than 300 kN. The finite element calculations were repeated for fatigue analysis of welded rail for the specified boundary conditions. Stress life and Soderberg theorem were used for the fatigue analysis. Soderberg theorem for the calculation of mean stresses is suitable for the materials in low ductility [26]. Equivalent alternating stress distribution can be seen in Fig. 8. The highest stress values were observed at the transition areas between Zone 2 and parent metals at the running surface of rail head (54.5 MPa). In Zone 1, the obtained stress is 19 MPa. It is nearly equal to the transition areas between Zone 1 and Zone 2 in rail head. The stress values are getting higher (26 MPa) at the areas closer to the Zone 2 in parent metals. The stress values gradually decrease until the neutral axis at the rail web and gradually increases until the surface at rail foot. The highest stress in rail foot is 11 MPa at a location in weld collar. The analysis indicates that stress values decrease when moving outward from the centre of the weld collar. The fatigue analysis shows that the welded joint reaches the infinite life (>106 cycles) under the 70 kN wheel load. The calculated endurance limit for the wheel load is 96.25 kN. The welded rail for this system can be considered reliable for the wheel loads lower than 96.25 kN. FEM results showed that when the load is increased twice (140 kN), the fatigue life was calculated as 1.54x105 cycle. The stress concentrations were observed at the transition areas of parent metal and Zone 2 in the x-axis across the rail
Fig. 7. Measured and calculated (FEA) stress for various wheel loads on weld collar.
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Fig. 8. The equivalent alternating stress distribution and the deformation in rail section.
head (see Fig. 8). Therefore, these stresses may lead surface discontinuities on the welded joint under repeated service loads.
Funding The author(s) received no financial support for the research, authorship, and/or publication of this article.
4. Conclusion In this work microstructure, tensile strength and stress distribution of flash butt welded rail steel were studied to clarify the metallurgical and mechanical aspects of the welded joints. The main conclusions from this study are as follows. 1. Hardness tests showed considerable decreases in the transition zones, which are around 20 mm in length symmetrical to the fusion line. Also, a significant decrease observed at the fusion line and it gradually increases at the adjacent points. 2. In Zone 1, the maximum hardness values are 300, 318 and 301 (HV30) for rail head, web and foot respectively. Hardness in rail web is higher than head and foot in all specified zones. The decrease in profile thickness in the rail web is a factor that affects the measured difference. 3. Due to the geometric differences in rail profile, Zone 2 lays out 30–40 mm in length that is symmetrical to the fusion line. Maximum length in Zone 2 is observed at the rail head. 4. Grain size ranges 2–8 according to ASTM E 112. Grain size in the fusion line calculated as 2–3 according to ASTM E 112 with 2 mm length in total. At the recrystallization zone, grain size is 5–6 according to ASTM E 112. Finer grain boundaries observed at the transition zone between Zone 1 and Zone 2 as 8 according to ASTM E 112. 5. The microstructure and the mechanical properties of welded rail vary with the distance to the fusion line. The differences in microstructure affect the strength parameters. Decreasing the number of differences in micro level shall lead improvements in welded rail performance. Finer grain boundaries lead the highest tensile and yield strength. The strength gradually decreases in Zone 2 and parent metals respectively. The lowest elasticity modulus observed at Zone 2 and the highest was observed in Zone 1. 6. The developed finite element model for stress distribution is in good agreement with the experimental results. Stress concentrations were observed at the transition areas, where coarser microstructures were obtained (5–6 grain size according to ASTM E 112). The highest stresses occur at the running surface of rail head between Zone 2 and parent metal.
CRediT authorship contribution statement Yasin Sarikavak: Conceptualization, Methodology, Software, Validation, Investigation, Writing - original draft. Osman Selim Turkbas: Methodology, Investigation, Writing - review & editing. Can Cogun: Methodology, Supervision. Declaration of Competing Interest The authors declare that they have no known competing financial interests or personal relationships that could have appeared to influence the work reported in this paper. References [1] S. Iwnicki, Future trends in railway engineering, Proc. Inst. Mech. Eng., Part C: J. Mech. Eng. Sci. 223 (2009) 2743–2750, https://doi.org/10.1243/ 09544062JMES1545. [2] P.J. Webster, G. Mills, X.D. Wang, W.P. Kang, T.M. Holden, Residual stresses in alumino-thermic welded rails, J. Strain Anal. Eng. Des. 32 (1997) 389–400, https://doi.org/10.1243/0309324971513508. [3] T.H. Hyde, W. Sun, A.A. Becker, J.A. Williams, Life prediction of repaired welds in a pressurised CrMoV pipe with incorporation of initial damage, Int. J. Press. Vessels Pip. 81 (2004) 1–12, https://doi.org/10.1016/j.ijpvp.2003.12.015. [4] L.J. Fellows, D. Nowell, D.A. Hills, On the initiation of fretting fatigue cracks, Wear 205 (1997) 120–129, https://doi.org/10.1016/S0043-1648(96)07302-4. [5] N. Ilic´, M.T. Jovanovic´, M. Todorovic´, M. Trtanj, P. Šaponjic´, Microstructural and mechanical characterization of postweld heat-treated thermite weld in rails, Mater. Charact. 43 (1999) 243–250, https://doi.org/10.1016/S1044-5803(99) 00006-6. [6] C. Meric, E. Atik, S. Sahin, Mechanical and metallurgical properties of welding zone in rail welded via thermite process, Sci. Technol. Weld. Joining 7 (2002) 172–176, https://doi.org/10.1179/136217102225004211. [7] Y.-R. Chen, F.V. Lawrence, C.P.L. Barkan, J.A. Dantzig, Weld defect formation in rail thermite welds, J. Rail Rapid Transit. 220 (2006) 373–384, https://doi.org/ 10.1243/0954409JRRT44. [8] X. Yu, L. Feng, S. Qin, Y. Zhang, Y. He, Fracture analysis of U71Mn rail flash-butt welding joint, Case Stud. Eng. Fail. Anal. 4 (2015) 20–25, https://doi.org/ 10.1016/j.csefa.2015.05.001. [9] B.L. Josefson, J.W. Ringsberg, Assessment of uncertainties in life prediction of fatigue crack initiation and propagation in welded rails, Int. J. Fatigue 31 (2009) 1413–1421, https://doi.org/10.1016/j.ijfatigue.2009.03.024. [10] H. Mansouri, A. Monshi, Microstructure and residual stress variations in weld zone of flash-butt welded railroads, Sci. Technol. Weld. Joining 9 (2004) 237– 245, https://doi.org/10.1179/136217104225012201.
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[11] U. Zerbst, R. Lundén, K.O. Edel, R.A. Smith, Introduction to the damage tolerance behaviour of railway rails – a review, Eng. Fract. Mech. 76 (2009) 2563–2601, https://doi.org/10.1016/j.engfracmech.2009.09.003. [12] K. Wang, Z. Tan, C. Cheng, B. Gao, G. Gao, R.D.K. Misra, B. Bai, Effect of microstructure on the spalling damage in a 20Mn2SiCrMo bainitic rail, Eng. Fail. Anal. 70 (2016) 343–350, https://doi.org/10.1016/j. engfailanal.2016.09.011. [13] C.W. Ziemian, M.M. Sharma, D.E. Whaley, Effects of flashing and upset sequences on microstructure, hardness, and tensile properties of welded structural steel joints, Mater. Des. 33 (2012) 175–184, https://doi.org/10.1016/ j.matdes.2011.07.026. [14] F.J. Franklin, J.E. Garnham, D.I. Fletcher, C.L. Davis, A. Kapoor, Modelling rail steel microstructure and its effect on crack initiation, Wear 265 (2008) 1332– 1341, https://doi.org/10.1016/j.wear.2008.03.027. [15] R.P. Mahto, R. Kumar, S.K. Pal, S.K. Panda, A comprehensive study on force, temperature, mechanical properties and micro-structural characterizations in friction stir lap welding of dissimilar materials (AA6061-T6 & AISI304), J. Manuf. Processes 31 (2018) 624–639, https://doi.org/10.1016/j. jmapro.2017.12.017. [16] S. Mironov, Y.S. Sato, S. Yoneyama, H. Kokawa, H.T. Fujii, S. Hirano, Microstructure and tensile behavior of friction-stir welded TRIP steel, Mater. Sci. Eng., A 717 (2018) 26–33, https://doi.org/10.1016/j.msea.2018.01.053. [17] W. Wang, Y. Shi, Y. Lei, Z. Tian, FEM simulation on microstructure of DC flash butt welding for an ultra-fine grain steel, J. Mater. Process. Technol. 161 (2005) 497–503, https://doi.org/10.1016/j.jmatprotec.2004.07.098. [18] T. Kuroda, K. Ikeuchi, H. Ikeda, Flash butt resistance welding for duplex stainless steels, Vacuum 80 (2006) 1331–1335, https://doi.org/10.1016/ j.vacuum.2006.01.068. [19] D. Tawfik, P.J. Mutton, W.K. Chiu, Experimental and numerical investigations: alleviating tensile residual stresses in flash-butt welds by localised rapid post-
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weld heat treatment, J. Mater. Process. Technol. 196 (2008) 279–291, https:// doi.org/10.1016/j.jmatprotec.2007.05.055. A.-N. Rasanen, J. Martikainen, Experimental review of metallurgical flash weld defects in welded joints: essence of flat spots, penetrators and oxide inclusions, Sci. Technol. Weld. Joining 16 (2011) 471–476, https://doi.org/ 10.1179/1362171811Y.0000000025. A.C. Athukorala, D.V. De Pellegrin, K.I. Kourousis, Characterisation of headhardened rail steel in terms of cyclic plasticity response and microstructure for improved material modelling, Wear 366–367 (2016) 416–424, https://doi.org/ 10.1016/j.wear.2016.03.024. A.S. Khan, X. Wang, Strain Measurements and Stress Analysis, Prentice Hall, 2001. IMC Meßsysteme GmbH, Instruction Book – IMC Cronos PL/SL CompactFlex, Version 4.0, R13 ed., Berlin, 2013. EN 14587-2, Railway applications – Track – Flash butt welding of rails - Part 2: New R220, R260, R260Mn and R350HT grade rails by mobile welding machines at sites other than a fixed plant, 2010. R.G. Budynas, J.K. Nisbett, Shigley’s Mechanical Engineering Design, 9th Ed., Mc Graw Hill, 2011. R. Browell, A. Hancq, Calculating and displaying fatigue results, ANSYS Inc. (2006). P. Mutton, J. Cookson, C. Qiu, D. Welsby, Microstructural characterisation of rolling contact fatigue damage in flashbutt welds, Wear 366–367 (2016) 368– 377, https://doi.org/10.1016/J.WEAR.2016.03.020. ASTM E 112, Standard Test Methods for Determining Average Grain Size, West Conshohocken, PA, 2013. X. Gui, K. Wang, G. Gao, R.D.K. Misra, Z. Tan, B. Bai, Rolling contact fatigue of bainitic rail steels: The significance of microstructure, Mater. Sci. Eng., A 657 (2016) 82–85, https://doi.org/10.1016/j.msea.2016.01.052.
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Construction and Building Materials journal homepage: www.elsevier.com/locate/conbuildmat
Influence of welding on microstructure and strength of rail steel Yasin Sarikavak a,b,⇑, Osman Selim Turkbas c,d, Can Cogun e a
Mechanical Engineering Department, Ankara Yıldırım Beyazıt University, Ankara 06010, Turkey Railway Research and Technology Centre, Turkish State Railways, Ankara 06105, Turkey c Mechanical Engineering Department, Gazi University, Ankara 06570, Turkey d Mechanical Engineering Department, Near East University, Nicosia, Northern Cyprus, Mersin 10, Turkey e Mechatronics Engineering Department, Cankaya University, Ankara 06815, Turkey b
h i g h l i g h t s Significant changes have been investigated in grain size as it ranges 2–8 by ASTM-E112. Three different zones have been identified with their hardness and tensile properties. Tensile strength gradually decreases in Zone 2 and parent metals respectively. FE model has been developed and validated under various loads for stress distributions. Stress concentrations have observed between Zone 2 and parent metal.
a r t i c l e
i n f o
Article history: Received 5 December 2018 Received in revised form 29 December 2019 Accepted 18 January 2020
Keywords: Welded rail steel Grain size Tensile strength Finite element analysis Stress distribution
a b s t r a c t In this study, metallurgical and mechanical aspects of cold flash butt welded rail joints were investigated. The relationship between the material characteristics and the distance to the fusion line were evaluated. Significant changes observed in grain size as it ranges 2–8 according to ASTM E 112. Hardness tests conducted and the average results of the rail head, web, and foot were recorded. According to the results, three different zones were identified including the fusion line, recrystallization zone, and transition areas. Tensile properties and strength characteristics have been investigated for the specified zones separately. Obtained experimental values were used as input parameters for the developed finite element model (FEM) to calculate stress distribution. The welded rail was subjected to realistic wheel load and stress distributions in weld were evaluated. The developed model has also been validated by comparing the theoretically obtained stress values with the experimental ones under various loading conditions. Ó 2020 Elsevier Ltd. All rights reserved.
1. Introduction Research organisations, railway operators, and manufacturers conducted several studies to improve the technological level of the rail materials, joint types, and railway systems since the beginning of railways. The railways provide safe system of transport and many areas in railways experiencing considerable increasing in performance [1]. Due to the advantageous of maintenance and material integrity in railway lines, bolted rail joints were replaced with welded joints. Welded rails have lower maintenance costs, failure frequency, as well as the lower environmental impact [2]. In recent years, rail materials have higher carbon contents and
⇑ Corresponding author at: Mechanical Engineering Department, Yıldırım Beyazıt University, 06010 Ankara, Turkey. E-mail addresses: [email protected], [email protected] (Y. Sarikavak). https://doi.org/10.1016/j.conbuildmat.2020.118220 0950-0618/Ó 2020 Elsevier Ltd. All rights reserved.
larger cross-sections. This improves the mechanical resistance; however, it hardens to weld the rail. The difficulty in the weld assessment is principally due to the differences in the shape and the material property distributions in the localized regions of the welded joint [3]. Recently operational forces due to speed and axle load increased and cracking of the rail is still one of the major concerns. Cracks may either exist in structures or may initiate during operational conditions. The initiation process often occurs with stress concentration either in microscopic or macroscopic scales [4]. Therefore generally mechanical and microstructural properties of steels and various welding methods were studied in the literature. Ilic et al. [5] studied on hardness characteristics, mechanical properties, microstructure and fracture mechanism of thermite welded rail joints. Improved mechanical strength and elongation observed at the finer ferrite and pearlite microstructure. Meric et al. [6] studied the mechanical and metallurgical properties of welded rails via
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the thermite process. Maximum hardness observed at the interface between HAZ and weld metal as 109 HB. Chen et al. [7] studied the weld defect formation in thermite welds. The influence of welding parameters on weld defects in thermite welds was simulated with a developed heat transfer model. A case study on the fracture of welded U71Mn rail was conducted by Yu et al. [8]. Fracture surface morphology, microstructure, and micro hardness were analysed and the values around the fracture surface observed in normal level. Free solidification microstructure, which extends from outside to inside at the rail head, was observed as the crack source of the fatigue failure. Josefson and Ringsberg [9] studied life prediction in welded rails. Respectively, the assessment of uncertainties in life prediction of fatigue crack initiation and propagation in the rail head and web were conducted. Mansouri and Monshi [10] studied microstructure and residual stress variations in the weld zone of flash butt welded railroads. During cooling of the weld zone, the contraction in the web is higher than the head and base of the rail according to the current in copper electrodes. Zerbst et al. [11] investigate the damage tolerance behaviour of railway rails. The research study is a review of crack propagation and fracture of rails. It includes loading conditions and residual stresses from manufacturing and welding in the field. Wang et al. [12] studied the effects of microstructure on spalling damage in 20Mn2SiCrMo bainitic wing rails. Film like martensite and austenite (M/A) constituents have better performance to decrease crack propagation. Ziemian et al. [13] investigated the varying process parameters on microstructure, hardness and tensile properties of structural steels. The results showed that hardness was increased in the weld zone for all specimens. Franklin et al. [14] modelled the rail steel microstructure and its effect on crack initiation. Mahto et al. [15] investigated the force, temperature, mechanical properties and microstructural characterizations in friction stir lap welding of dissimilar materials. Microstructure and property relationship for friction stir welding were also studied in the literature [16]. Wang et al. [17] conducted finite element simulation on the ultra-fine microstructure of welded steel. The simulation gives out the grain size distribution in heat affected zone due to the temperature gradient. Kuroda et al. [18] studied flash butt resistance welding for duplex stainless steels. The cross-sectional microstructure of the weld bond region was observed using microscopy. Tensile strength and impact energy increased with increasing finegrained metal. Tawfik et al. [19] conducted an experimental and numerical investigation of tensile residual stresses in flash butt welds by rapid post weld heat treatment. A thermo mechanical finite element model including phase transformation characteristics of the rail material has been used to predict residual stress distribution. An experimental measurement of post-weld cooling rates using infrared thermography was used to validate the model. Rasanen and Martikainen [20] conducted an experimental metallurgical study on flash weld defects in welded joints. Athukorala et al. [21] studied on the characterization of head-hardened rail steels in terms of microstructure and cyclic plasticity response for Australian Standard AS1085.1 rail steel. The most common welding techniques in railway lines consist of thermite welding and flash butt welding processes [17–20]. Flash butt weld can be processed both in factory and mobile on the field. The metallurgical and mechanical properties of the welded joints are important to understand the roots of failures. (i) In the most of the published studies; rail steels, thermite welding processes and other welding techniques such as friction stir welding which are not practically applicable for joining rail materials were investigated in terms of microstructure, crack formation and strength. The process parameters of thermite welding are not compatible with flash butt welding process. Flash butt welded joints are critical sections in railways in terms of varieties in microstructure and mechanical properties. These differences
represent discontinuities in the structure under harsh environmental conditions such as wheel load and stresses. (ii) The majority of the failures on rails occur at the welded joints according to stress concentrations at the discontinuities. In published studies authors focused on some specific points such as heat transfers, residual stresses, phase transformations or microstructure for modelling the system. However each difference in microstructure and strength characteristics should be considered independently. For accurately modelling the welded rail these differences in each section should be evaluated individually and carefully. Based on this, the aim of the present work was to analyse the cold flash butt-welded rail joints in terms of microstructure and strength to accurately model the stress distribution under various loading conditions. The relationship between the material characteristics and the distance to the fusion line in the welded joint were evaluated. Hardness tests, microstructural analysis were conducted to differentiate the different zones in the welded joint. Grain boundaries and grain sizes were also evaluated for each zone and the results were compared with the hardness parameters. Then the zones, which have dissimilar properties, were considered as a graded material structure and it is divided into zones to individually evaluate the mechanical properties. Mechanical properties of each zone are clarified with the tensile test and findings were discussed. The obtained results were used as an input data for the suggested finite element model. The developed model calculates the stress distribution and the weakest section in the welded rail joint under various loading conditions that represents the wheel load of the rolling stock. The model has also been validated by comparing the theoretically obtained stress values with the experimental ones by using strain gages attached to the weld collar, under various wheel loads.
2. Material and methods Flash butt welding is one of the resistance welding processes. UIC 60 R 260 profile rail used for the experimental work that is 172 mm in height and 150 mm width in rail foot. The parent rails are positioned end to end and clamped with a device. The heavy current should pass through the end of the parent rail. Rail cold flash welding process and post weld applications conducted on the field with a mobile flash butt welding device. The device applies 20 kA current and 380 V voltages that results with the melting temperatures at rail ends. The process takes 3 min in total for each weld and total shortening in rail is about 30–44 mm in two rail ends. Following the flash butt welding process ultrasonic inspection conducted for investigating any internal discontinuity. No discontinuities were detected for the specimens. Spectrolab metal spectrometer was used for the chemical analysis of rails. Chemical compositions of UIC 60 R260 standard rail (limits) and measured results with a metal spectrometer for the test rail are given in Table 1. In order to study microstructure, specimens were sectioned in the longitudinal direction and prepared for the tests. Hardness test in the Vickers scale (HV 30) (Q-Ness device) conducted for the sections in rail head, web and food separately. After hardness test macro etching process applied to the other welded rail to validate the transition zones. The microstructural examination was carried out to investigate the change in grain size according to the distance to the centre of the fusion line. According to the results, different zones were determined. Tensile test specimens were prepared from the specified zones of welded rail. Main mechanical properties including modulus of the elasticity, yield strength, maximum tensile strength, and % elongation evaluated for each zone. The
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Y. Sarikavak et al. / Construction and Building Materials 243 (2020) 118220 Table 1 Chemical compositions of the test rail and standard rail. Steel sample grade (UIC 60)
R 260
Test Rail EN 13674-1
104% ppm. Max. by mass
By mass %
C
Si
Mn
P
Cr
Al
V
N
Oa
Hb
0.72 0.60/0.82
0.23 0.13/0.60
1.02 0.65/1.25
0.019 0.030
0.04 0.15 max.
0.002 0.004 max.
0.0005 0.030 max.
0.009 0.010max.
– 20
– 2.5
obtained data were used as an input for the finite element model for the stress analysis. To investigate the stress distribution, the welded rail was sectioned 1100 mm in length. Four strain gages of MicroMeasurements were installed 20 mm below and over the neutral axis, head and foot at the weld collar. IMC CRONOSflex 2000G data logger with a frequency of 100 Hz was used for data collection. Collected strain values were converted to the stress by using the software simultaneously. As explained in Section 3.3, the 211.7 GPa experimentally obtained average elasticity modulus of weld collar were used for the stress calculations. The strain was mainly calculated according to the change in length and resistance as shown in Eq. (1) [22].
e¼
dL dR=R ¼ L k
ð1Þ
where; e is the strain; dL is the change in length (mm); L is the original length (m); dR is the change in resistance (X); R is the resistance of strain gauge (X); and k is the gauge factor refers to the ratio of relative length change to change in resistance. By the applied force, obtained strain is very small due to the change in resistance. Therefore Quarter Bridge used to convert these changes into voltage changes. Thus, the voltage calculated according to the Eq. (2). The Eq. (3) gives the strain value, dependent to gauge factor and voltage parameters [23].
Ua ¼ Ue
e¼
4U a kU e
dR Ue ¼ ½ke 4R 4
ð2Þ
ð3Þ
where; Ua is the measurement voltage (mV); and Ue is the excitation voltage (V). The bonded quarter bridge single axis strain gages have a resistance value of 120 X with 6 mm measuring base. The factor k is equal to 2.155 ± 0.5. Strain, calculated according to Eq. (4) [22,23].
hlmi
e
m
¼
4ð1000Þ U a mV k Ue V
ð4Þ
Equipped welded rail installed on 500 kN capacity four-point bending servo-hydraulic test machine. Experimental setup and locations of the strain gages can be seen in Fig. 1. The upper span of four-point bending test machine is 154 mm and the lower span is 1020 mm [24]. The same design used for the finite element calculations as well. The bending stress for the FEM analysis was calculated according to the Eq. (5) [25].
r¼
Mc I
ð5Þ
where; r is the bending stress (MPa); M is the bending moment (Nmm); c is the vertical distance of the point of interest from the welded rails’ neutral axis (mm); and I is the moment of inertia of the rail cross sectional area (mm4). Bending moment is dependent to applied load and the span of the four point bending equipment and given in Eq. (6).
M¼
PL 2
ð6Þ
where; P is the applied load (kN); and L is the distance between the first upper and lower span of the roller (mm). There are many theorems used for understanding the influence of mean stresses in the structures. In this study, Soderberg theorem which is dependent to yield strength is used as given in Eq. (7) [25,26].
ra rm þ ¼1 re ry
ð7Þ
where; ra is the alternating stress (MPa); rm is the mean stress (MPa); re is the endurance limit (MPa); and ry is the yield stress (MPa). Mean and alternating stresses are dependent to minimum and maximum stress values across the cross section of the rail profile.
Fig. 1. Servohydraulic bending test set up and the location of installed strain gages.
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3. Results and discussion 3.1. Hardness test
Fig. 2. Microhardness values (HV30) across the weld collar.
The welded rail specimens were sectioned from the longitudinal vertical axis and the sectioned surface was grinded for hardness measurements. Hardness tests (HV30) were conducted 4 mm below the running surface of the rail head with 2 mm spacing [24]. Measurements repeated for rail head, web and foot separately with 10 mm spacing. Tests were conducted along a line 200 mm in length that centres the weld collar. Average measured results of welded rails can be seen in Fig. 2. 300 HV measured around fusion line for the experiments conducted with 2 mm spacing below the running surface of the rail. It was decreased to 250 HV at a distance of 20 mm from the fusion line. Average hardness values in the web are higher than the foot and head of the rail specimens. Some research studies in the literature show significant decreases of hardness at the transition
Fig. 3. (a) Microstructure of the centre of fusion line (b) Recrystallization zone (Zone 1) (c) Recrystallization region between Zone1 and Zone 2 (d) Coarse microstructure at the recrystallization region between Zone1 and Zone 2 (e) Zone 1 and Zone 2 transition region (f) Main structure.
Y. Sarikavak et al. / Construction and Building Materials 243 (2020) 118220
zones [27]. The decrease in hardness was also observed for rail head, web, and foot in the fusion line. The highest decrease in the fusion line was investigated at the rail head. The zone 40 mm in total centring the fusion line is called Zone 1. For the rail head around 40 mm distance to Zone 1 minor decreases in hardness were also observed with a difference of 10–20 HV in total.
5
side. The whole structure in Fig. 3(e) is pearlite. Retained austenite in microstructure plays a significant role on rolling contact fatigue crack propagation [29]. Some inclusions were also observed in all regions, which may affect the rails’ strength performance. The pearlitic main structure of Zone 2 and the parent metals can be observed in Fig. 3(f). The grain size according to ASTM E 112 was calculated as 5–6.
3.2. Microstructural analysis Macro etching process applied to the grinded specimen surface with 5% HNO3 and C2H6O solution. Solution was kept for 1.5 min on the surface and the fusion line, Zone 1, Zone 2 (adjacent to Zone 1) and the transition lines to the parent metal were observed after the etching process. A tiny difference for a length 20 mm in total centring the fusion line was observed after the etching process. Due to the dimensional differences such as thickness, wider boundary zones were observed at the rail head, foot, and web respectively. The polished surface of flash butt welded rail specimen was etched with 5% HNO3 and C2H6O for 7 s and the etched surface was observed with invert type microscope with 100X for microstructure analysis and grain size determination (Fig. 3) [28]. The relationship between the material characteristics and the distance to the fusion line in the welded joint were investigated. Fig. 3(a) shows the microstructure of the fusion line. Around 0.10% decarburized region was observed in 2 mm zone at the centre of the fusion line. According to ASTM E 112, grain size was calculated as 2–3. In figure brown zones refers the pearlitic and white zones refers to the ferrite structure in the fusion line. Fig. 3(b) shows the microstructure of the recrystallization region that is named as Zone 1. The main structure is totally perlite and some tiny ferrites can be seen on grain boundaries. It was observed that grain growth was less than the fusion line. According to ASTM E 112, grain size was calculated as 5–6. The microstructure of transition region between Zone 1 and Zone 2 can be seen in Fig. 3(c). Finer grain boundaries were observed with 8 in grain size according to ASTM E 112 at the areas closer to Zone 1. In Fig. 3(d) coarser microstructure was observed as the grain size is 4–5 according to ASTM E 112 and the microstructure was totally pearlite. As Fig. 3 (e) shows recrystallization zone is at the left-hand side of the picture and retained austenite phase can be seen at the right-hand
3.3. Mechanical properties Tensile test was applied with a mechanical extensometer to the welded rail joints. Locations and codes of cylindrical M10 test specimens are shown in Fig. 4. Stress-strain diagram of parent metals, Zone 1 and Zone 2 can be seen in Fig. 5. Table 2 shows the main strength values of each specimen where; mE is the modulus of elasticity in (GPa), Rp0.2 is the 0.2% offset yield strength in (MPa), Rm is stress at maximum force in (MPa), Fm is the maximum force in (kN). Strength tests showed that average elasticity modulus values are 204.2 GPa; 198.7 GPa and 211.7 GPa; yield strengths are 460.3 MPa; 491.2 MPa and 659.5 MPa and stress at maximum force is 909.6 MPa; 923.1 MPa and 1011.3 MPa for the parent metal, Zone
Fig. 5. Stress-strain diagrams of test specimens.
Fig. 4. Specimens extracted from the welded rail, locations and codes for tension test.
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Table 2 The results of the tension test. Specimen
mE (GPa)
Rp0.2 (MPa)
Rm (MPa)
Fm (kN)
Parent Metal – L – A Parent Metal – L – B Parent Metal – R – A Parent Metal – R – B ZONE 2 – L – A ZONE 2 – L – B ZONE 2 – R – A ZONE 2 – R – B ZONE 1 – A ZONE 1 – B
203.5 211.1 212.8 189.5 222.9 213.2 167.2 191.5 200.9 222.5
506.9 504.8 498.8 330.9 458.0 501.1 518.4 487.4 704.9 614.1
926.2 926.2 888.2 897.9 888.8 935.4 936.9 931.3 1027.1 995.6
18.2 18.2 17.4 17.6 17.4 18.4 18.4 18.3 20,2 19.5
2 and Zone 1 respectively. Zone 1 has the highest strength results. Tensile strength is higher around 11% in Zone 1 than parent metals; however, % strain is lower than the parent metals and Zone 2. 3.4. Strain measurement and stress analysis The maximum static axle load is 140 kN for high-speed trains under operation in Turkish State Railways. Loading applied in the range 10 kN-70 kN with 5 s pause at each load to the strain gage equipped welded rail. According to measured strain results, the calculated stress can be seen in Fig. 6. For 70 kN wheel load measured stress values at the weld collar are 36 MPa in rail head and foot (SG1; SG4) and 8 MPa below and above 20 mm of the neutral axis (SG2; SG3). Strain and stress data collected for the increased loads until 400 kN. 3.5. Finite element analysis (FEA) and loading Finite element method is an effective tool for prediction of stress, strain and remaining life for the welded joints as long as the material behaviour can be accurately modelled. Therefore designed UIC 60 R260 profile rail including Zone 1 and Zone 2 was imported to the finite element analysis software. Obtained mechanical properties in Section 3.3 were entered into the model separately. Welding induced residual stresses were neglected. For meshing, tetrahedron shape element was used in finite element calculations. The element size was taken as 7 mm for the Zone 1 and Zone 2 to obtain accurate results. The meshed design consists of 70,490 nodes and 39,235 elements. After meshing, boundary conditions were entered into the model. The high-speed trains
Fig. 6. Calculated stress for 10–70 kN loading against time via raw strain measurement.
under operation in Turkish State Railways have 70 kN static wheel load. The defined load distributed on two actuators contacting with rail head. Supports located at rail foot are fixed to the ground. The experimentally measured and calculated stress values with the finite element method on weld collar can be seen in Fig. 7. Theoretically obtained stress values are consistent with the experimental results. As shown in Fig. 7, theoretically calculated stress for 70 kN static wheel load is ±8.2 MPa and measured experimental value is ±8 MPa at rail web which is symmetrical to neutral axis (SG2, SG3). Calculated stress for rail head (SG1) is 37.7 MPa and the experimental result is 36 MPa. For rail foot (SG4), the calculated stress is 22.5 MPa and the obtained values for experimental work is 36 MPa at 70 kN wheel load. Theoretical findings are lower than the experimental ones for rail foot. With increasing wheel load, the gap between theoretical and the experimental values are getting higher particularly for rail foot. For rail head and rail web experimental and theoretical results are very consistent. Theoretical results at rail head (SG1) and rail web (SG2) for 400 kN, wheel load is 216 MPa and 54 MPa. The experimental results are 200 MPa and 45 MPa respectively. All analysis indicates that the experimental stress results are in good agreement with the finite element model results, especially for the loads lower than 300 kN. The finite element calculations were repeated for fatigue analysis of welded rail for the specified boundary conditions. Stress life and Soderberg theorem were used for the fatigue analysis. Soderberg theorem for the calculation of mean stresses is suitable for the materials in low ductility [26]. Equivalent alternating stress distribution can be seen in Fig. 8. The highest stress values were observed at the transition areas between Zone 2 and parent metals at the running surface of rail head (54.5 MPa). In Zone 1, the obtained stress is 19 MPa. It is nearly equal to the transition areas between Zone 1 and Zone 2 in rail head. The stress values are getting higher (26 MPa) at the areas closer to the Zone 2 in parent metals. The stress values gradually decrease until the neutral axis at the rail web and gradually increases until the surface at rail foot. The highest stress in rail foot is 11 MPa at a location in weld collar. The analysis indicates that stress values decrease when moving outward from the centre of the weld collar. The fatigue analysis shows that the welded joint reaches the infinite life (>106 cycles) under the 70 kN wheel load. The calculated endurance limit for the wheel load is 96.25 kN. The welded rail for this system can be considered reliable for the wheel loads lower than 96.25 kN. FEM results showed that when the load is increased twice (140 kN), the fatigue life was calculated as 1.54x105 cycle. The stress concentrations were observed at the transition areas of parent metal and Zone 2 in the x-axis across the rail
Fig. 7. Measured and calculated (FEA) stress for various wheel loads on weld collar.
Y. Sarikavak et al. / Construction and Building Materials 243 (2020) 118220
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Fig. 8. The equivalent alternating stress distribution and the deformation in rail section.
head (see Fig. 8). Therefore, these stresses may lead surface discontinuities on the welded joint under repeated service loads.
Funding The author(s) received no financial support for the research, authorship, and/or publication of this article.
4. Conclusion In this work microstructure, tensile strength and stress distribution of flash butt welded rail steel were studied to clarify the metallurgical and mechanical aspects of the welded joints. The main conclusions from this study are as follows. 1. Hardness tests showed considerable decreases in the transition zones, which are around 20 mm in length symmetrical to the fusion line. Also, a significant decrease observed at the fusion line and it gradually increases at the adjacent points. 2. In Zone 1, the maximum hardness values are 300, 318 and 301 (HV30) for rail head, web and foot respectively. Hardness in rail web is higher than head and foot in all specified zones. The decrease in profile thickness in the rail web is a factor that affects the measured difference. 3. Due to the geometric differences in rail profile, Zone 2 lays out 30–40 mm in length that is symmetrical to the fusion line. Maximum length in Zone 2 is observed at the rail head. 4. Grain size ranges 2–8 according to ASTM E 112. Grain size in the fusion line calculated as 2–3 according to ASTM E 112 with 2 mm length in total. At the recrystallization zone, grain size is 5–6 according to ASTM E 112. Finer grain boundaries observed at the transition zone between Zone 1 and Zone 2 as 8 according to ASTM E 112. 5. The microstructure and the mechanical properties of welded rail vary with the distance to the fusion line. The differences in microstructure affect the strength parameters. Decreasing the number of differences in micro level shall lead improvements in welded rail performance. Finer grain boundaries lead the highest tensile and yield strength. The strength gradually decreases in Zone 2 and parent metals respectively. The lowest elasticity modulus observed at Zone 2 and the highest was observed in Zone 1. 6. The developed finite element model for stress distribution is in good agreement with the experimental results. Stress concentrations were observed at the transition areas, where coarser microstructures were obtained (5–6 grain size according to ASTM E 112). The highest stresses occur at the running surface of rail head between Zone 2 and parent metal.
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