Diagnosis of the microstructural and mechanical properties of over century-old steel railway bridge components

Journal Pre-proofs Diagnosis of the microstructural and mechanical properties of over centuryold steel railway bridge components Kowal Maciej, Szala Mirosław PII: DOI: Reference:

S1350-6307(19)31150-1 https://doi.org/10.1016/j.engfailanal.2020.104447 EFA 104447

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Engineering Failure Analysis

Received Date: Revised Date: Accepted Date:

7 August 2019 12 November 2019 11 February 2020

Please cite this article as: Maciej, K., Mirosław, S., Diagnosis of the microstructural and mechanical properties of over century-old steel railway bridge components, Engineering Failure Analysis (2020), doi: https://doi.org/ 10.1016/j.engfailanal.2020.104447

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Diagnosis of the microstructural and mechanical properties of over century-old steel railway bridge components Kowal Maciej, PHD Eng* Lublin University of Technology Faculty of Civil Engineering and Architecture Nadbystrzycka 40, 20-816 Lublin, Poland E-mail: [email protected] *corresponding author

Szala Mirosław, PHD Eng Lublin University of Technology Faculty of Mechanical Engineering Nadbystrzycka 36, 20-816 Lublin, Poland E-mail: [email protected]

Abstract The mechanical and microstructural properties as well as durability of more than century-old steel railway bridges that are still widely in use, among others in Poland, have not been well recognized. Ignoring the effect of material and mechanical properties on the operation of an old-steel bridge may eventually lead to the structure’s failure. This study investigates the reliability of a 100-year-old bridge made of wrought (puddled) iron. The aim of this study is to identify the microstructural and mechanical properties of over century-old steel bridge components in relation to the requirements for modern mild steel grades. The paper presents an analysis of the material and mechanical behaviour of a truss railway bridge that had been in operation for over a century. Laboratory tests were performed on samples taken from the component parts of a dismantled railway bridge, i.e. stringers, crossbars, lower belts, stringer bracings and wind bracing. Fatigue, fracture, hardness and tensile tests are performed to estimate the mechanical properties of the investigated steel bridge component parts. Spectroscopy is used to examine the chemical composition of every element. Light optical microscopy and scanning electron microscopy are employed to examine the microstructure of the steel component parts and its non-uniformities. Scanning electron microscopy (SEM) and energy dispersive X-ray spectroscopy (EDS) are used to obtain data about the fracture mechanism and the chemical composition of non-metallic inclusions. Microstructural and mechanical results are discussed in relation to modern mild steel grades S355 and S235, which are widely used structural ferrous alloys. Based on the chemical composition analysis, the possibilities of increasing the strength of the bridge by welding methods and the application of fibre-reinforced polymer (FRP) composite materials are discussed. Obtained results add to the body of knowledge about the maintenance, reinforcement and demolition of bridge structures, which is crucial for preventing failure of over 100-year-old steel structures. This research is an introduction to the study on reinforcing old steel structures by FRP composite materials. Keywords: railway bridge, old steel, structure, mechanical testing, fractography, microstructure

Highlights     

Material properties of an over 120-year-old steel railway bridge are investigated. The mechanical properties of old steel, i.e. fatigue strength, hardness and fracture toughness, are thoroughly examined. Fractography is used to determine impact failure in a wide temperature range from -30ºC to +20ºC. The results demonstrate that the presence of non-metallic inclusions in the steel microstructure affects its fracture toughness failure mechanism. The mechanical and chemical properties of old steel are compared in relation to modern steel grades.

1. Introduction The problem of functional age of steel railway facilities is widespread all over the world [1–5]. Under the Eurocode 3 design standards for steel structures, every steel structure must be designed and manufactured in a way that ensures its compliance with the EC1993-1 standard requirements regarding load-bearing capacity, serviceability and durability for the intended period of use. The EC1990 standard also establishes that approximate design periods of use for bridges (Category 5) are set to 100 years. Steel structures should have sufficient toughness to withstand various operating temperatures. Standards relating to steel products generally require that the impact energy of the Charpy V-notch test – at a given test temperature – is not lower than 27J (according to PN-EN 10025). With thicknesses of up to 30 mm, Eurocode 3 requires that the temperature is 20ºC for steel bridges. However, this requirement applies to new welded structures designed from modern structural steels with high impact parameters, taking into account the material's resistance to brittle fracture throughout the expected service life. Hence, the question arises how we should treat structures that have reached the age of over 50 or even 100 years. According to Hołowaty et al. [4], over 75% of railway bridges in Poland are more than 50 years old, and almost 45% of them have been in operation for over 100 years, which is already beyond the currently assumed durability. Under this statistics, the share of steel railway facilities amounts to 42%. As noted by Siriwardane in [6], most railway bridges in the world are near the end of their design lives, and many of them have exceeded 100 years of age. The assessment of properties of old steel components is an up-to-date problem. The literature of the subject reports studies on bridge component parts made of wrought (puddled) iron or cast steel [4,5,7–9]. Also, assembly elements such as rivets [1,2] are examined. In order to ensure high safety levels in old riveted metallic bridges, railway authorities have to invest heavily in the maintenance and retrofitting of these structures. Residual life calculations for existing and operating bridges should particularly take into account their fatigue properties and progressive damage mechanisms [2]. Unfortunately, there are very few publications describing the effect of temperature changes on the fracture toughness of old steel components. Material and mechanical properties of old structures are often unknown. Furthermore, there are very few studies comparing the properties of over century-old steel in relation to modern steel grades. This problem is crucial when selecting the reinforcement method to ensure the reliability of the structure. The development of a renovation or modernization strategy for an old bridge structure intended for a long-term use requires identification of a construction material. In the case of steel structures, it is necessary to identify the chemical composition of a structure, to estimate its mechanical properties and current design strength, and, finally, to determine its load-bearing capacity. In the case of steel structures, the option of using welding for reinforcement in a conventional manner should be specified [10]. An alternative reinforcement method is adhesive joining of steel-based components with composite elements [11–13]. This idea seems beneficial when renovating over century-old bridges that sometimes should be left in use as a living monument of the by-gone engineering infrastructure. Moreover, the decision about demolishing old structures and replacing them with new ones should be preceded by detailed investigation of these structures. Unfortunately, it is taken for granted that if a structure has reached the required useful life, i.e. it is more than 100 years old, it should be taken out of service. This is justified by the structure’s incapability to achieve the required in situ speed and load capacity, and the presence of a significant degree of corrosion associated with the lack of profitability of renovation. Moreover, the maintenance and handling of old bridges pose a serious problem. To sum up, accurate identification of the properties of materials used in old structures is crucial for proper assessment of their suitability for repair, reinforcement or modernization, and, ultimately, preventing the steel structure from failure. The aim of this study was to identify the microstructural and mechanical properties of over centuryold steel bridge components in relation to the requirements for modern mild steel grades. This study can be a useful contribution to the body of knowledge about the material and mechanical properties of over 100-yearold railway bridge component parts.

2. Over century-old railway bridge 2.1. History of the object The object of this study was a 120-year-old riveted steel railway bridge, located in Poland, on the Łuków-Lublin line No. 30 at 5+858 km. The bridge is shown in Figure 1. The railway line No. 30 ŁukówLublin Północny is located in Lublin Province, Poland. It was built in the years 1894-1898. The priority of the line was rapid movement of the troops in case of war. The bridge structure was erected approx. at the end of 1896, as part of the Łuków-Lublin railroad. The purpose of the railroad was to create a supply delivery route for the Russian armed forces resisting German and Austrian attacks along the line of Narew and Vistula Rivers (Poland) [8]. In the 21st century the railroad was closed for regular passenger traffic for several years. The line

was closed to passenger traffic after more than 100 years of operation (2002), primarily due to economic reasons. The closure was prompted by a low demand for transport services. The train journey from Łuków to Lublin was more than twice as long as the journey by bus. Another reason for the closure was the poor condition of the railway in the northern part of the line, the renovation of which was not profitable. Not to mention the fact that – starting from the early 21st century – freight trains are rarely used on the line. Passenger traffic was restored on the line in 2013, in the form of rail buses. First, the services were restored on the Lublin-Lubartów route, then the trains' relationship was extended to Parczew. In September 2016, the manager of the PKP Polish Railway Lines signed a contract for the modernization of the ŁukówParczew section. This was necessary to restore the functioning of the modernized railway line No. 7 on the Warsaw-Otwock-Dęblin-Lublin section. Once the modernization of line No. 30 is complete, passenger trains will run at the speed limit of 120km/h and freight trains – at 80km/h. To achieve these parameters, the engineering structures has to be modernized by replacing them with new structures. The bridge was demolished in 2015 because of its usability loss resulting from insufficient maintenance that led to severe corrosion damage of the bridge members. Prior to its demolition, an investigation showed that – in light of current requirements such as PN-EN 1991-2:2007 – the bridge had an insufficient bearing capacity to enable increasing the speed of trains on the railway tracks. As a result, a new bridge was erected in the place of the old one.

Figure 1 View of the bridge at the time of demolition

2.2.

Technical characterization of the object

The object was a single-span steel structure based on a simply-supported beam design. The dimensions of the structure are given in Figure 2.

Figure 2 Schematic drawing of the bridge

The main girders of the object were two trusses of a theoretical span of 22.36 m, with the railroad tracks placed on the beams of the bottom belt of the truss. The pratt truss had parallel belts. The crossbars were placed at approx. 2.8m. The trusses were 3.05m in height. The spacing between the main girders was 5.26m. Two stringers with a 1.86m axial spacing were attached to the crossbars, stiffened with a vertical bracing in

the middle of the truss span and horizontal bracings on the plane of the top and bottom belts. The main trusses were supported on bridgeheads, using fixed and sliding steel bearings. The bridge was made of low-carbon puddled steel, which was very common for structures of that time [14], and its structural components were joined with rivets. The bridge component parts had different dimensions, as shown in Figure 2. The stringers, crossbar and belts have a thickness of 11 mm, while the wind bracings and stringer bracing were thinner, with the dimensions of 8-9 mm. From the metallurgical point of view, it can be claimed that differences in component part dimensions can lead to differences in mechanical properties of the over 100-year-old steel bridge components due to element solidification segregation and steelmaking parameters, resulting in a higher content of inclusions in rimmed steel than is required for modern structural steel grades, i.e. S235 or S355. These material factors have impact on the mechanical properties of steel structure components and consequently – on the failure probability of this steel bridge. This is crucial due to the fact that many old steel objects are still in operation. On the other hand, there are very few studies investigating the material and mechanical properties of over 100-year-old steel components, especially with relation to modern steel grades. Thus, as it was mentioned above, this study identifies the microstructural and mechanical properties of over century-old steel bridge components in relation to the requirements for modern mild steel grades. 3. Material and methods 3.1. Preparation of test samples Test specimens were obtained from steel component parts from several elements of the bridge span, i.e. elements of the bottom belts of the main girders, crossbars, stringers and bracing. The sampling locations of the elements subjected to the tests discussed herein are shown in Figure 3. Schematic drawings of the joints and cross-sectional views are presented in Figure 4. Samples were taken from every element type shown in Figure 3 by oxycutting, then they were sampled with water jet and, finally, machined to specific dimensions, Figure 4. The nomenclature of the samples is given in Table 1. The samples were located across (bottom belts, crossbars, stringers) and along (bracing elements), see Figure 4. Impact strength, tensile strength, fatigue strength and hardness tests were prepared for 158 (see Fig. 5), 43 (see Fig.6), 29 (see Fig. 7) and 5 samples, respectively.

Figure 3 Bridge structure, as well as the numeration and location of the bridge components used for sample preparation

(a)

(b) Figure 4 Fragmented bridge structure from Figure 3 (a), dimensions and design of water-jet cut specimens (b) Table 1. Description of specimens, each component number corresponds to the numbers given in Figure 3 and Figure 4 Compo nent number

Structural element type

Compon ent thicknes s [mm]

Number of prepared samples /Number of tested samples Impact resistance (see Fig. 5)

Ultimate strength (see Fig. 6)

Fatigue strength (see Fig. 7)

Hardness

Chemical composition

Microstructu ral investigation s

Bottom belt 11 15 4/1WS 1 1 1 Bottom belt 11 16 4/1WS,1WZ 3/1WZ/2WS 1 1 Wind bracings 9 18 4 6/2WZ/1WS 1 Wind bracings 9 17 4/1WS,1WZ 6/2WZ/1WS 1 1 1 Crossbar 11 6 3/1WZ 2/1WS 1 Crossbar 11 13 4/1WZ 5/2WZ/1WS 1 1 1 Crossbar 11 16 4/1WS,1WZ 4 1 1 Stringer 11 22 4/1WS -/1WS 1 1 1 Stringer 11 Stringer bracing 8 18 5/1WS,1WZ 6/1WZ,1WS 1 1 Stringer bracing 9 17 5/1WS,1WZ 5/1WS 1 1 Total number of samples 158 43 37 Total count of tested samples: 108 7WS, 7WZ 8WZ, 9WS 5 7 10 WS – WS type samples for tensile strength analysis (see Fig. 6); WZ – WZ type samples for fatigue strength analysis (see Fig. 7). Explanations for ultimate strength: 4/1WS, 1WZ – 4 samples made for ultimate strength test; ultimate strength was tested on 1 WS sample and 1 WZ sample. Explanations for fatigue strength: 3/1WZ, 2WS – 3 samples made for fatigue strength test; fatigue strength was tested on 1 WZ sample and 2 WS samples 1 2 3 4 5 6 7 8 9 10 11

3.2.

Methods

The study presents the results of tests examining the chemical composition, microstructure and mechanical properties such as impact resistance, ultimate strength, fatigue strength and hardness of specimens fabricated from selected components of a railroad bridge. The chemical composition and mechanical properties of the over 100-year-old steel bridge are compared with those of modern steel grades employed in bridge construction, given in Table 2. Table 2. Nominal chemical composition and mechanical properties of steel used in bridge construction, based on literature [4] and technical data provided in EN-10027-1 [15,16]

ReH [MPa]

Steel grade St3M St3S S355 S235 Puddled steel Cast steel

Static mechanical properties Hardness Rm [MPa] ReH / Rm

At [%]

min.240

370÷470

0.57

-

min. 25

215÷235

375÷460

0.54

-

23÷26

355

470÷630

0.64

155HV10*

22

235

360÷510

0.54

128HV10*

24

220÷230

330÷400

0.62

-

10÷25

370÷450

0.56

-

18÷25

220÷240 * Measured by Vickers method.

3.2.1. Chemical composition and microstructure The chemical composition and microstructure of every analysed component part of the bridge were examined. The examination of the chemical composition of seven types of components, i.e. plates of the bottom belt of the main truss girder (denoted as 1 and 2, according to Table 1), crossbars (5, 6, 7), wind bracing elements (3, 4), stringers (8), and stringer bracings (10, 11), was made using the Polyvac 2000 emission spectrometer, according to the procedure given in the PN-H-04045:1997 standard. The chemical composition results were compared with those of steel grades usually employed in bridge construction, given in Table 2. Microstructure was examined in the cross-section of the metallographic specimens using the Nikon MA200 metallographic microscope. The preparation of metallographic specimens involves the use of coupons that are obtained from every structural element (listed in Table 1) and mounted in epoxy resin. The specimens were ground with abrasive papers (granularity range from 100 to 1200) and polished with diamond suspension. Finally, the metallographic samples were etched with Nital (3-5% nitric acid with ethanol) and examined under a light optical microscope (LOM) using the bright light technique. The average grain size and phase content were calculated by quantitative metallography methods, using the ImagePro software. A macrographic examination was performed to identify the content of sulphur (Baumann method). The Baumann method involves contact printing using silver salts and sulphuric acid, according to the ISO 4968:1979. In addition, a reference sample made from modern structural steel grade S235 was examined. In addition to that, weldability of the steel components was investigated by assessing their resistance to hot and cold cracking, according to standard formulas provided by the International Institute of Welding and those reported in the literature of the subject [4,10,17]. Based on the conducted chemical composition examination, the metallurgical and structural weldability of steel was estimated. The following coefficients were specified [4,10,17]:  carbon equivalent value, Ce (1)  hot cracking resistance, HSC (2)  carbon equivalent for cold cracking, Ce` (3)  heat-affected zone (HAZ) hardness, HVmax` (4)  hardness range after welding, HVmax (5)  ratio of the manganese to sulphur content, Mn/S. 𝐶𝑒 = 𝐶 +

(

𝑀𝑛 𝐶𝑟 + 𝑀𝑜 + 𝑉 𝑁𝑖 + 𝐶𝑢 + + < 0.41, 6 5 15

𝐻𝑆𝐶 = 100 𝑆 + 𝑃 + 𝐶′𝑒 = 𝐶 +

(1)

)

𝑆𝑖 𝐶 𝑁𝑖 + + < 4,0., (2) 25 100 3𝑀𝑛 + 𝐶𝑟 + 𝑀𝑜 + 𝑉

𝑀𝑛 𝑃 𝑀𝑜 𝑁𝑖 𝐶𝑢 𝐶𝑟 + 𝑉 + + + + + + 0.0024𝑡 < 0.4, (3) 2 4 15 13 5 6 𝐻𝑉𝑚𝑎𝑥` = 1200𝐶′𝑒 ― 200 < 300𝐻𝑉, (4)

𝐻𝑉𝑚𝑎𝑥 = 90 + 1050𝐶 + 47𝑆𝑖 + 75𝑀𝑛 + 30𝑁𝑖 + 31𝐶𝑟 < 350𝐻𝑉, (5)

3.2.2. Impact resistance, fatigue, yield strength, ultimate strength and hardness Impact strength tests were performed with the Charpy method. V-notch samples were prepared and analysed in compliance with the PN-EN ISO 148-1:2010 standard. An impact test hammer, Zwick/Roell Hit50, was used, and the investigation was conducted for 108 (out of 158) samples. Specifically, 11, 25, 23, 18, 14 and 17 samples were tested at the temperature of - 30ºC, - 20ºC, -10ºC; 0ºC; +10ºC; +20ºC, respectively. The results of KV50 fracture tests obtained for the 108 Charpy samples in the temperature range T = -30 to +20oC are given in Table 5. The KV50 fracture operation was determined for six groups of samples (1524 samples per group). To properly prepare samples from sheet metal with a thickness of up to 10mm, 10x10x55 mm Charpy samples were fabricated, having a 2 mm V-notch, according to the PN-EN 14045-1: 1994, and the thicknesses of 5.0mm and 7.5mm instead of the typical 10.0mm. The results of the KV50/5.0 and KV50/7.5 fracture tests led to determination of the impact strength of the tested samples KCV50/5.0 and KCV50/7.5, i.e. the quotient of the fracture operation and the surface area of the tested samples at V-notch. According to the PN-EN 1993-1-10 standard, the value of fracture work of the tested samples amounting to KV=27J is taken as the critical value required by Eurocode 3 for steel, with A=0.80 cm2 considered as the minimum value in new structures. The impact strength of modern steel required at -20oC is KCV = 33.75J/cm2. The results should therefore be considered in relation to the impact strength of the KCV50/5.0 (A=0.40cm2) and KCV50/7.5 (A=0.60cm2) samples. Additionally, the mechanism of fracture was analysed with a scanning electron microscope (SEM, PhenomWord ProX, Phenom World), using the backscattered electron (BSE) mode and the EDS detector. The chemical composition in micro-areas was investigated by the energy-dispersive X-ray method (EDS). Samples for tensile strength tests were prepared in compliance with the guidelines specified in the PNEN ISO 6892-1:2010 standard (Figure 6). Samples for fatigue strength tests were prepared in compliance with the guidelines under the ASTM International E468-11 standard (Figure 7). The strength tests were conducted on a high-force load frame tensile testing machine, MTS 319.25. Results obtained for the over century-old steel were compared with the properties of modern structural steel grades. First, ultimate static strength of the samples was determined. Then, on the basis of the obtained ReH and Rm, fatigue tests were carried out in various stress ranges with a reference to ReH. The lower limit of the stress range was assumed to be approximately 15% of the ReH (load in the truss bridge components, due to deadweight obtained by static calculations). Initially, ReH was rigidly assumed to be 296MPa (for WS-type samples) and 320MPa (WZ-type samples). The samples were loaded in the range of 0.15÷1.185 (WS-type samples) and 0.155÷1.10/1.15/1.30/1.40 (WZ-type samples) of the assumed ReH. The introduction of the real value ReH obtained in the laboratory static strength tests for a group of samples caused a change in the actual range of the lower limit stress range (0.121÷0.166 ReH) and the upper limit stress range (0.889÷1.173ReH). Fatigue tests were conducted on MTS 319.25 with a load frequency of 20Hz. The static mechanical properties of the over 100-year-old steel bridge were compared with those of steel grades usually employed in bridge construction, presented in Table 2. Hardness of the steel component parts was measured on the ground flat surface of the coupons. Vickers hardness tests were performed in accordance with the PN-EN ISO 6507-1:2007 standard, using the Zwick/Roell ZHU187,5 hardness tester with a load force of HV10. In order to ensure the accuracy of statistical analysis, at least 9 indentations were made on each sample to obtain the average value. For the determined HV10 values, tensile strength (RmV) was determined. Moreover, according to the PN-H-04357:1993 standard, the ratio α=ReV/RmV equal to  was applied for every tested old steel bridge component.

Figure 5. Samples prepared for impact tests

Figure 6. WS-type samples for tensile strength analysis (a) and an exemplary set of samples (b)

Figure 7. WZ-type samples for fatigue strength analysis (a) and an exemplary set of samples (b)

4. Results and discussion 4.1. Results of the chemical composition and microstructural analyses

The chemical composition test results listed in Table 3 point to very high discrepancies between carbon contents of individual samples. Compared to modern mild steel, the small percentage of carbon, higher contents of phosphorus and sulphur, and a limited silicon content prove that the old steel component parts were fabricated at the end of the nineteenth century. According to Cremona et al. [1], wrought iron was used between 1850 and approximately 1900, whereas old steel – from approximately 1890 to 1925. In other words, the investigated steel component parts may be classified as wrought (puddled) iron. The chemical composition of the tested steel is typical of steel bridges built at the end of the 19th century in Poland, as described by Hołowaty and Wichtowski [4,9,18] At that time, steel refining processes allowed to tap effervescing steel containing large amounts of slags and inclusions. The steel under study exhibits a relatively low carbon content, and the amount of manganese is lower than 1% and that of silicone is below 0.012%. Rimmed steels are usually tapped either without the addition of deoxidizers (such as Si) in the furnace or only small amounts of deoxidizers are added to the molten steel in the ladle. The presence of oxygen facilitates gas evolution by reacting carbon, which results in a brisk evolution of carbon oxide, leading to a reduced carbon content [19]. Table 3. Comparison of steel chemical composition analyses with the nominal values reported in literature [4] [15] [16] Results of investigated steel chemical composition, wt% Component (nr) C Mn Si 0.059 0.420 0.001 Bottom belt (1,2) 0.063 0.458 0.007 Wind bracing (4) 0.037 0.540 0.010 Crossbar (5,6,7) 0.031 0.560 0.009 Stringer (8) 0.039 0.520 0.004 Stringers bracing (10,11) 0.046 0.046 0.500 Average 0.0008 0.0008 0.0138 SD Max 0.031 0.031 0.42 Min 0.063 0.063 0.56 Chemical composition according to literature data 0.04-0.30 traces-0.33 0.10-0.33 Puddled steel [4] 0.03-0.35 0.04-0.75 traces-0.18 Cast steel [4] max.0.17 max.1,40 S235JR [15] max.0.17 max.1,40 S235J2 [15] max.0.20 min.0.40 0.12÷0.30 St3M [16]

P 0.037 0.039 0.017 0.036 0.022 0.006 0.0001 0.001 0.01 0.02-0.46 0.004-0.16 max.0.035 max.0.025 max.0.05

S 0.035 0.049 0.034 0.034 0.037 0.030 0.0004 0.017 0.039

Cr 0.008 0.010 0.010 0.012 0.009 0.038 0.0002 0.034 0.049

Cu 0.040 0.086 0.056 0.051 0.073 0.010 0.0000 0.008 0.012

Ni 0.018 0.023 0.023 0.021 0.030 0.061 0.0013 0.04 0.086

Al. -

0.10-0.60 0.004-0.09 0.007-0.014 0.11-0.14 0.03-0.04 0.01-0.02 max.0.035 max.0.55 max.0.025 max.0.55 max.0.05 max.0.30 max.0.30 max.0.02

Figure 8 presents selected results of the metallographic examination. The microstructure of the examined components consists of ferrite (bright grains, F) and perlite (darker areas, P) arranged along the ferrite grain boundaries, Figure 8a. Moreover, the presence of cementite (C) precipitates, widely known as

tertiary cementite and non-metallic inclusions (IN) mainly relating to sulphur, phosphorus and silica bearing phases (see Figure 8b), is identified, which is considered to be responsible for the embrittlement of low carbon steels [20]. A qualitative microstructural analysis demonstrates that the perlite content ranges 10 ± 3% and the average grain size (expressed as the equivalent diameter) equals 25 ± 3 μm, see Figure 8b and Figure 8c. The measured grain size seems similar to that of the microstructures presented in [5] by Lesiuk et al. The investigated steel component parts of the bridge exhibit a higher average grain size and a lower content of perlite than modern mild steel grade S235, see Figure 8d, where the measured average grain size equals 12 ± 3 μm and contains 18% of perlite. In other words, in comparison to modern mild steel (S235), the microstructure of the investigated component parts is characterized by coarser ferrite grains and the presence of longitudinal non-metallic inclusions. It is known that coarse grains and second phases (inclusions) can decrease the mechanical properties of ferrous alloys such as fracture toughness, which is discussed in further sections of this paper.

Figure 8. Microstructure of steel bridge components (a-c) and reference modern steel S235 (d), LOM.

It is reported in the literature of the subject that irrespective of chemical composition, special attention must be paid to toughness or weldability of old steel structures [1,4,10,21]. In particular, attention must be paid to the role of hot cracking resistance and cold cracking resistance, neither of which can be neglected. Based on the results of a spectrometric chemical analysis of steel, weldability indicators were determined, see Table 4. Table 4. Results of steel weldability assessment (according to the literature [4,10,17])

Sample No.

Calculated weldability coefficients Ce

HSC

Ce`

HVmax`

HVmax

Mn/S

Bottom belt (1,2)

0.134

7.26

0.177

12.0

184.2

12.0

Wind bracing (4)

0.196

5.14

0.221

65.0

253.9

11.7

Crossbar (5,6,7)

0.134

5.18

0.165

*

170.8

15.8

Stringer (8,9)

0.132

7.07

0.173

7.0

165.9

16.4

Stringer bracing (10,11)

0.134

5.97

0.160

*

171.3

14.0

Acceptance limit

≤0.410

<4.00

<0.400

<300 HV

<350 HV

≅22

* - overestimated.

Except for hot cracking resistance, the calculated values of the weldability indicators are lower than the critical values reported in literature. The computed Ce (carbon equivalent) ranges from 0.132% do 0.196%, and in each case it is below 0.34%, which indicates that the material is readily weldable. The bridge steel was found to be resistant to cold crack formation during welding. It showed a tendency to form hot cracks, as both HSC > 4% and Ce > 0.15% as well as the Mn/S quotient are located in the hot cracking zone. On the basis of

the carbon content and Ce calculations, it can be stated that the over century-old steel component parts of the bridge have sufficient overall weldability, which agrees with the literature data reported for other old railway structures [4]. In addition to that, the chemical composition and mechanical properties discussed in further sections of this paper indicate that this steel can be classified as Group 1.1 steel under the classification system of ISO/TR 15608. Hence, it does not require special treatment during welding. It seems that the bridge structure can easily be reinforced by increasing the cross-section of the component parts by welding. However, it should be stressed that the literature recommends – in relation to a cast steel bridge from 1875 – that the base metal should be preheated to eliminate carburizing and low-hydrogen fillers should be used [10]. 4.2. Results of impact strength testing Impact strength tests were conducted on samples collected from the bottom belt of the main truss girder, crossbars, stringers and bracing elements. The impact strength tests were conducted for six temperatures ranging from -30ºC to +20ºC. The results of fracture toughness (KV50 impact tests of 108 Charpy samples) are given in Table 5, and the results of fractographic SEM observations are given in Figure 9. The results demonstrate that the analysed old steel shows a relatively high fracture toughness in comparison to the requirements for modern steel grades under the EN 10025 [15] standard, i.e. 27J/cm2. The results of the bridge under study were then compared with the results obtained for an almost twin object located on the same railway line No. 30 at 25 km + 414 km, which was built in the same period (1896) [8]. Out of the 108 tested samples, 30 were not destroyed under the initial energy of 50J. Of the remaining 78 samples, a lower impact strength than the required KCV=33.75J/cm2 at -20oC was obtained for 11 individual results (2 at 0ºC, 3 at -10ºC, 4 at -20ºC and 2 at -30ºC). With the exception of one element (wind bracing no. 4 KCV50/7.5=28.4J/cm2), the average results obtained for other elements show higher impact values than those required by the current standards at -20oC: KCV50/5.0=(62.0; 58.8) J/cm2 and KCV50/7.5=(42.1, 56.8, 45.8) J/cm2. This can be attributed to the high discrepancy between the impact strength results obtained for individual bridge elements at a given temperature (Table 5). It is worth noting that the samples taken from the lower belt (denoted as 1 and 2) show significant differences in impact strength. The samples from component part no. 1 have an average impact strength of KCV50/7.5 = 42.1 J/cm2 at -20ºC, which complies with the current requirements for bridge steel. The samples taken from component no. 2 have not been damaged at all. It is noteworthy that the damage of as many as 21 samples (87.5%) has the form of scrap with delamination. Only three samples of steel (two at -20ºC and one at -10ºC) were damaged with total scrap. The results indicate that the significant difference between the mechanical properties of individual steel component parts may result from the presence of non-metallic inclusions identified in the microstructure by LOM and SEM microscopy (see Figure 8 and Figure 9), leading to the typical discrepancy in mechanical properties. Similar results were reported for over hundred-year-old wrought iron by Cremona et al [1].

Table 5. Impact strength test results for different temperatures.

Element Bottom belt (1) Bottom belt (2) Average Wind bracings (3) Wind bracings (4) Average Crossbar (5) Crossbar (6) Crossbar (7) Average Stringer (8)

So [cm2]

20oC

Impact strength KCV [J/cm2] 10oC 0oC -10oC -20oC -30oC

0.60

ND 56.3

74.2 52.3

0.40

ND 56.3 ND

63.2

66.7 ND

ND

28.6 101.6 103.1 ND ND

76.9 73.8 61.0 60.4 60.9 66.6 55.0 67.1

102.3 45.4 58.8 64.9 54.3 58.1 56.3 52.9 56.0

0.40

0.60

0.60

0.60

20.7 47.4 17.8 ND

66.7 51.8 63.4 72.2 ND 60.5 62.0 58.1 67.8

62.9 61.0 54.4 Average ND ND 89.2 Stringer 94.5 ND 118.3 bracings (10) 0.40 85.4 123.1 ND Stringer ND 112.4 bracings (11) 90.0 117.8 103.8 Average ND – no destruction with fracture work of 50J

11.5 45.8 53.2 ND ND 36.8 109.1 16.8 37.4 ND ND 8.6 42.9 48.2 62.6 56.0 48.8 ND 53.9 49.8 55.8 54.2 53.3 62.3 ND ND ND 62.3

51.3 22.6 52.5 ND ND 42.1 61.8 62.3 ND 34.9 ND 21.9 45.2 49.1 62.5 62.5 57.7 52.3 56.8 58.4 51.1 28.0 45.8 23.2 75.3 82.8 46.0 ND 66.8 58.8

ND ND

44.8 67.6 47.8 53.4 46.2 41.6 26.6 38.1 ND

64.2 6.8 35.5

The results of scanning electron microscopy (SEM) observations of the samples fractured at different temperatures (given in Figure 9) allow us to point to the evolving nature of failure affected by the fracture temperature. The examined fracture mechanism develops from ductile fracture (at 20ºC, Figure 9a) to brittle fracture (cleavage) observed at negative temperatures (-30ºC, see Figure 9e). In the fracture observed at 20ºC (Figure 9a) and 10ºC (Figure 9b) a large number of shallow dimples is visible, non-metallic inclusion e.g. marked by arrows in a spot 2 area in Figure 9b, with second-phase particles found in the majority of small dimples. The size and arrangement of inclusions are similar to those of the second-phase particles in the metallographic microstructure. Mostly sulphur and manganese inclusions were identified by the SEM-EDS method, which is marked as spot 1 in Figure 9b, and the dimple filament contains gases such as nitrogen and oxygen. Thus, the impact of inclusion on the fracture mechanism is clearly visible in the investigated over 100year-old steel (Figure 9c), which consists in cracking/ fracture initiation at non-metallic inclusions. The ductility is low perpendicular to the rolling plane, which results in lamellar tearing. Although the brittle fracture mode dominates when the temperature is decreased, one can observe the presence of ductile fracture regions characterised by shallow dimples adjacent to the grain boundaries. Menzemer et al. reported [22] similar phenomena for 8320-alloy steel samples dynamically deformed at -70ºC. The remnants of locally ductile failure was identified as shallow dimples and microscopic voids along the grain boundaries. Although it is known in literature that the fracture test temperature has impact on the fracture mode of steel samples [22,23], there are very few studies describing the fracture testing of over century-old steel at

various temperatures. For this reason, the influence of decreasing impact testing temperature on the fracture mechanism was studied in this work. With decreasing the test temperature from 20ºC to -30ºC, ductile-brittle fracture develops, from the cracking mode along the grain boundaries (intergranular fracture) to the cleavage fracture mode propagating through the grains (transgranular) without any remnants of intergranular rupture. What dominates is a river-like pattern that relies on cracking initiated at the grain boundary. In addition, the SEM analysis of fracture has revealed that the grain size is comparable to the results obtained by quantitative image analysis of the LOM microstructure. Moreover, the presence of microstructure non-homogeneities such as sulphides and oxides is probably the main cause of the anisotropy of mechanical properties and thus the scattering of the experimental results. Additionally, the fracture results of the over century-old steel are analysed by the method proposed by Macek [24,25]. Finally, it should be emphasized that the fracture toughness of the investigated over 100-year-old steel bridge components is comparable to the nominal values required for modern low carbon steel.

a)

b)

c)

d)

e)

f) Figure 9. Fracture evolution in the steel bridge components with decreasing the temperature from +20 to -30ͦC (SEM-EDS, 500x and 2000x)

4.3. Results of fatigue tests Table 6 lists the fatigue strength results of the samples subjected to loads in the range of 0.055 to 1.173 ReH. In the first stage of the fatigue strength tests, sample WS.02.1 was initially loaded in the range of 0.44÷0.79ReH (ReH=310MPa) for calibration purposes. After 1474903 load cycles, the test was stopped. The sample showed no signs of damage. The sample was then loaded in the range of 0.148-1.135ReH. Then the sample was subjected to 13719 fatigue cycles. Table 6. Fatigue test results with reference to the measured ReH (given in increasing order of cycles)

Component number 5 2 10 6 4 3 2 3 8 11 6 4 3 10 6

Specific Spec. No. WS.05.2 WS.02.1 WS.10.2 WS.06.2 WS.04.1 WS.03.3 WZ.02.1 WZ.03.6 WS.08.3 WS.11.4 WZ.06.3 WZ.04.2 WZ.03.4 WZ.10.2 WZ.06.1

No. of cycles 1025 13719 71185 89381 106558 187219 277776 291269 316246 338512 525362 806250 1318898 1679255 2050000

ReH [MPa] 340 309 309 308 343 300 309 300 279 299 308 343 300 309 308

ReH range 0.135÷1.032 0.148÷1.135 0.148÷1.135 0.149÷1.139 0.133÷1.023 0.153÷1.169 0.129÷0.994 0.055÷1.127 0.166÷1.093 0.153÷1.173 0.121÷0.929 0.128÷0.943 0.141÷1.077 0.131÷0.961 0.121÷0.889

 range [MPa] 46÷351 46÷351 46÷351 46÷351 46÷351 46÷351 40÷307 17÷338 46÷305 46÷351 37÷286 44÷323 42÷323 40÷297 37÷274

An assessment of the fatigue test results is neither simple nor unambiguous. The fatigue strength test was carried out in order to recognize this parameter in over century-old steel which had previously been subjected to unknown loads. According to the ASTM E468-11standard, the material should transfer 1x107 fatigue cycles so that fatigue strength can be determined within a given load range. The test was discontinued due to obtaining over 2x106 fatigue cycles (EN1993-2:2006). This value is required for the whole steel bridge structure. It is worth noting that 2x106 fatigue cycles were obtained for the load range of 0.12÷0.89ReH. The upper limit is therefore only slightly higher than the required value of steel design strength ReD=ReH/=ReH/1.15=0.87ReH. It could therefore be inferred that – in an extreme case – the structure would withstand the required number of fatigue cycles. There is no data about the actual payloads which the bridge had been subjected to until the end of its use. It is also unknown whether the range of moving loads had been

similar to or lower than the designed one and how many of these load cycles the bridge structure had experienced. Analysing Table 6, it can be seen that there is no repeatability of fatigue strength results. The upper limit of the load range points to ambiguity regarding the number of fatigue cycles withstood by the samples cut from various component parts of the bridge. This may be caused by both the aging of the object and the microstructural uniformity of steel. On the basis of fatigue test results, however, it is possible to state that – without reinforcement – the structure would not have been able to transfer a greater static load or increase the upper limit of the amplitude of fatigue loads. 4.4. Results of tensile strength and hardness testing The results of tensile strength tests are summarised in Table 7, the stress-strain curves are plotted in Figure 12 and Figure 13, and hardness results are given in Table 8. Table 7. Mechanical properties of steel samples.

Component no. 1 2 3 4 5 6 7 8 10 11 Average Min Max SD CV Specific value 95%

Rm [MPa] 380 368 409 407 381 382 355 373 388 375 382 354.5 409 16.62 0.044 354.2

ReL [MPa] 242 277 281 293 275 262 244 248 278 262 266 242.1 292.95 17.32 0.065 237.5

ReH [MPa] 265 309 300 343 340 308 296 279 309 299 305 264.7 343.15 23.94 0.079 265.3

ReH/Rm 0.70 0.84 0.73 0.84 0.89 0.81 0.83 0.75 0.80 0.80 0.80 0.70 0.89 0.058 0.073 0.702

max [mm/mm] 0.221 0.245 0.203 0.216 0.198 0.216 0.230 0.259 0.212 0.254 0.225 0.198 0.259 0.021 0.093 0.191

The obtained parameters of the tested steel given in Table 7 are close to the standard values of St3M steel, see Table 2. It can be observed that, according to the PN-89/H-84023/04 standard applied in bridge construction in Poland (Re≥240MPa, Rm≥370MPa and A5≥25%), the results listed in Table 2 and their nominal values are consistent with those proposed by the UIC (Union Internationale des Chemins), which are as follows: ReH=220MPa, Rm=320-380MPa [26]. Moreover, the plotting diagrams for the samples are comparable to the typical stress-strain curves for modern steels of grade S235. The  graphs show a characteristic plastic shelf with measured values of the upper yield point ReH (Figure 10, Figure 11 - from 265 to 343MPa) and the lower yield point ReL (between 242 and 293MPa).

Figure 10. Stress-strain curves, static strength samples (WS type).

Figure 11. Stress-strain curves, fatigue strength samples (WZ type).

The hardness of the investigated steel (Table 8) is in a lower range of hardness measured for modern structural steel grades S235 (128 HV) and S355 (155 HV), see Table 2 and literature [17]. It can be claimed that the scatter of hardness results is caused by variations in the chemical composition and grain size of modern and over century-old steels. It is widely known that both finer grain and high carbon content lead to increased hardness and mechanical properties. On the basis of the measured hardness of the steel components, their mechanical properties can be calculated with the use of the PN-H-04357 standard. Generally, the calculated values are in agreement with the in situ experimental values Re and Rm listed in Table 7. The discrepancies between the results of Rm and Re obtained by the hardness tests and static strength method can be observed in Table 8. The hardness results of the belt (V01) and stringer bracing (V10) are overestimated. The tensile strength is overestimated by 6% to 21%, and the yield strength – by 6% to 9%. Regarding the samples of other component parts (V04-wind bracing, V06-crossbar, V08-stringer), these values are underestimated: the tensile strength is underestimated by 16% to 30%, and the yield strength – by 4% to 13%. Table 8. Hardness of tested components measured by Vickers method and the estimated mechanical properties. Parameter

Measured, tensile test

Measured, hardness test

Component

Hardness, HV10

ReH [MPa]

Calculated based on hardness and tensile tests RmV ReV [MPa] [MPa]

No.

Average

SD

CV

SV 95%

Rm [MPa]

1

126.0

9.15

0.073

110.9

380

265

403

4

120.7

11.75

0.097

101.4

407

343

6

106.5

5.40

0.051

97.6

382

308

8

108.3

2.02

0.019

105.0

373

10

150.9

11.77

0.078

131.5

388

Comparison of measured and calculated results ReH/ReV

Rm/RmV

282

0.94

0.94

387

271

1.28

1.05

339

237

1.30

1.13

279

345

242

1.16

1.08

309

482

337

0.91

0.80

ReV – yield strength estimated based on Vickers hardness RmV – ultimate tensile strength estimated based on Vickers hardness

5. Discussion The decision about the demolition of the over century-old bridge was made based on visual inspection, after the assessment of the bearing capacity. However, no detailed investigation of the component material properties was made. Excessive corrosion of a large amount of rivets and individual component parts was identified. The structure was replaced due to its failure to meet the requirements for railway loads being projected. According to the review of the periodic five-year visual inspection of the facility, the technical condition was set at level 1 (pre-damage), a scale of 0 (damage) to 5 (very good), in accordance with the guidelines of Polish railways – Id-16. This technical condition was determined due to the lack of rail fenders, lack of the fire and anti-derailment sheets, complete railway sleepers degradation, general corrosion of truss bars, elements deformation of the lower and upper trusses belts. The bearing capacity on the site was limited to 100 kN/axle and the speed of movement to 30 km/h. According to the assumptions for the modernization of railway line No. 30, the bridge structures were to carry LM71 loads (according to EN 1991-2) with the coefficient  =1.21 (302.5kN/axle) at a speed of v 120 km/h. However, no static and dynamic trial load tests of old bridge were carried out. The object's bearing capacity was estimated on the basis of visual technical condition inspection, geometric inventory and static calculations only. To ensure the highest possible operating time of the historical steel structure, the mechanical and material properties of steel should have been

recognized prior to the decision about demolition. Based on microstructural and mechanical investigations discussed in the present study, it can be claimed that the structure should not have been demolished. The microscopic results demonstrate that the analysed steel has a ferritic microstructure with coarser grains than modern mild steel grade S235. Moreover, perlite and non-metallic inclusions can be observed in the metallographic microstructure. Given its chemical composition, it can be claimed that in the case of over 100-year-old steel, the refining process produces more non-metallic impurities than is the case with modern steel grades such as S235JR. This is proven by a high content of alloy elements such as P and S. Also, the nonmetallic inclusions were identified by metallographic observations. The SEM fractographic analysis indicates that these material non-uniformities particularly affect the scattering of fracture toughness results and the lamellar tearing mechanism of fracture. Additionally, the impact strength tests of steel performed at the temperature ranging from -30ºC to 20ºC demonstrate that the impact strength of the samples is unexpectedly high in relation to the results reported in [8]. However, the fracture mode is strongly affected by the temperature, and its behaviour evolves with decreasing the temperature, from the semi-ductile to brittle fracture mode. The obtained parameters of the tested steel are close to the standard values of St3M steel according to the PN-89/H-84023/04, and the plots resemble typical graphs for tensile testing of contemporary structural steel grade S235 (acc. to EN 10025). The chemical composition, microstructure and properties of the over hundred-year-old component parts of the bridge are comparable, which may indicate that the material came from the same ironworks and was smelt at the same period of time. The laboratory tests show that – in terms of its material properties – the investigated steel resembles the modern steel grade S235. This means that the bridge did not necessarily have to be demolished if the payload was not to be increased. A comprehensive approach to assessment of the bearing capacity of a given structure based on material tests, reinforcement analysis and durability prediction could prevent this structure from any reinforcement, as was the case with the Yedikule railway bridge [27]. The above analyses show that, pursuant to current guidelines, the investigated old steel would be suitable for use in construction today. Due to the fact that many historical railway and industrial structures have been in use for longer than a century, they require either regular maintenance and repairs or changes resulting from their adaptation to new purposes. It is therefore essential to continue research on this type of structures and to develop methods for their renovation in order to adapt them to the binding design guidelines. If it is technically and economically viable, the reconstruction of historical bridges adapted to current load capacity requirements should consist in partial or global reinforcement of these structures rather than in replacing them with new ones. Additionally, due to high discrepancies in material properties of historical structures, as was the case in this study, every change made to such a structure should be preceded by material analyses in order to determine its chemical and physical properties. It is necessary to provide the designer with detailed information about all parameters of a given steel grade. This will allow to use the existing material more efficiently and to choose a suitable method for repairing or strengthening the structure. A diagnosis may reveal that the strengthening or repair of the structure is at the limit of cost-effectiveness. Nevertheless, if it is technically possible to preserve historical structures, such solution should be considered. The choice of a reinforcement method should, of course, be adapted to the desired use as well as to technical and technological capabilities. The application of a reinforcement method depends on the usage requirements, such as the load class of an object and the permissible speed of a rolling stock on the line, and – in the case of steel structures – fatigue strength (PN-EN 1991-2:2007). If the usage requirements are satisfied or they do not have to be significantly improved, the choice of a repair or strengthening method “only” depends on the designer's experience, the selection of a repair technique and, consequently, on the social and economic costs of the proposed idea. In addition to the classical methods of reinforcing riveted objects such as increasing the cross section of the element by welding or fixing it with prestressed screws, it is possible to strengthen a bridge by adhesive joining of composite materials [3,28]. The new idea of reinforcing structures by composite material adhesive joining is a promising alternative to the currently used methods for reinforcing steel structures [29]. Summing up, it can be recommended that the material and mechanical properties of old steel components should be examined before considering steel structure reinforcement. The decision about demolition of the over100-year-old railway-bridge based on visual inspection should have been supported by a detailed material investigation, and then it could have been changed into a decision about rehabilitation of the railway bridge. This study is an introduction to the problem of strengthening old steel structures by FRP (fibrereinforced polymer) composite materials [11–13] and the results of mechanical properties will be utilized in FEM modelling of the behaviour of old bridge structures.

6. Conclusions This study thoroughly investigated the microstructural and mechanical properties of over 120-yearold steel bridge components such as steel stringer bars, bracings and belts. The results obtained for the over century-old steel components were compared with the properties of modern structural steel. It has been found that the steel bridge meets the requirements for modern railway structures. The structure should not have been demolished, and the results of mechanical properties indicate that all investigated structural pieces, i.e. steel stringer bars, bracings and belts, could have been accepted for operation. Based on the results of material and mechanical properties, the following conclusions can be drawn: 1. The mechanical properties of the investigated bridge component parts, i.e. fatigue strength, fracture toughness and hardness, are in range of the structural steel (i.e. S235) used for building the modern bridge structures. 2. The steel used for the bridge contains a smaller carbon content (C average 0.02xxx%) while the contents of sulphur and phosphorus (S average 0.027%, P average 0.034%) are higher than those in modern coarse-grained ferritic microstructures with perlite, and the presence of the non-metallic inclusions as well as tertiary cementite is confirmed. The railway bridge under study was built from an effervescing low-carbon ferrous alloy, i.e. wrought (puddled) iron. 3. The fracture mechanism in the V-notch Charpy samples is influenced by the presence of steel inclusions, and evolves with decreasing the temperature, from ductile (for testing at 20ºC) to brittle intergranular fracture (samples tested at -30ºC). Inclusions have impact on the presence of lamellar tearing. Nonetheless, the sufficient impact strength of the investigated steel (higher than the required 27J/cm2) was confirmed for all testing temperatures (20, 10, 0, -10, -20, and -30ºC). 4. New data about the over century-old steel material was obtained from fatigue strength testing. The findings show that there is a discrepancy between the fatigue strength results, which may result from the aging of the object or the microstructural heterogeneity of the steel produced during the bridge construction period. 5. The ultimate tensile strength of the investigated steel components is close to the standard values of St3M steel (PN-89/H-84023), recently applied in bridge construction in Poland (Re≥240MPa, Rm≥370MPa and A5≥25%), and these values are consistent with those proposed by the UIC, i.e. ReH=220MPa, Rm=320-380MPa. Moreover, the obtained plotting diagrams resemble the typical stressstrain curves for modern steel grade S235. 6. The results demonstrate that the investigated steel bridge components can be reinforced. However, instead of using classical methods for reinforcing riveted objects, like increasing the cross section of the element by welding or fixing it with prestressed screws, alternative methods that do not significantly affect the object mass, such as adhesive joining of composite materials, should be taken into consideration. Acknowledgements The research was financed in the framework of statutory measures of the Department of Roads and Bridges of the Faculty of Civil Engineering and Architecture at the Lublin University of Technology (S50/B/2015; S-50/B/2016; S-50/B/2017; S-50/B/2018).

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Diagnosis of the microstructural and mechanical properties of over century-old steel railway bridge components Table 1. Description of specimens, each component number corresponds to the numbers given in Figure 3 and Figure 4 Compo nent number

Structural element type

Compon ent thicknes s [mm]

Number of prepared samples /Number of tested samples Impact resistance (see Fig. 5)

Ultimate strength (see Fig. 6)

Fatigue strength (see Fig. 7)

Hardness

Chemical composition

Microstructu ral investigation s

Bottom belt 11 15 4/1WS 1 1 1 Bottom belt 11 16 4/1WS,1WZ 3/1WZ/2WS 1 1 Wind bracings 9 18 4 6/2WZ/1WS 1 Wind bracings 9 17 4/1WS,1WZ 6/2WZ/1WS 1 1 1 Crossbar 11 6 3/1WZ 2/1WS 1 Crossbar 11 13 4/1WZ 5/2WZ/1WS 1 1 1 Crossbar 11 16 4/1WS,1WZ 4 1 1 Stringer 11 22 4/1WS -/1WS 1 1 1 Stringer 11 Stringer bracing 8 18 5/1WS,1WZ 6/1WZ,1WS 1 1 Stringer bracing 9 17 5/1WS,1WZ 5/1WS 1 1 Total number of samples 158 43 37 Total count of tested samples: 108 7WS, 7WZ 8WZ, 9WS 5 7 10 WS – WS type samples for tensile strength analysis (see Fig. 6); WZ – WZ type samples for fatigue strength analysis (see Fig. 7). Explanations for ultimate strength: 4/1WS, 1WZ – 4 samples made for ultimate strength test; ultimate strength was tested on 1 WS sample and 1 WZ sample. Explanations for fatigue strength: 3/1WZ, 2WS – 3 samples made for fatigue strength test; fatigue strength was tested on 1 WZ sample and 2 WS samples 1 2 3 4 5 6 7 8 9 10 11

Diagnosis of the microstructural and mechanical properties of over century-old steel railway bridge components Table 2. Nominal chemical composition and mechanical properties of steel used in bridge construction, based on literature [4] and technical data provided in EN-10027-1 [15,16]

Steel grade St3M St3S S355 S235 Puddled steel Cast steel

ReH [MPa]

Static mechanical properties Hardness Rm [MPa] ReH / Rm

At [%]

min.240

370÷470

0.57

-

min. 25

215÷235

375÷460

0.54

-

23÷26

355

470÷630

0.64

155HV10*

22

235

360÷510

0.54

128HV10*

24

220÷230

330÷400

0.62

-

10÷25

220÷240

370÷450

0.56

-

18÷25

* Measured by Vickers method.

Diagnosis of the microstructural and mechanical properties of over century-old steel railway bridge components Table 3. Comparison of steel chemical composition analyses with the nominal values reported in literature [4] [15] [16] Results of investigated steel chemical composition, wt% Component (nr) C Mn Si 0.059 0.420 0.001 Bottom belt (1,2) 0.063 0.458 0.007 Wind bracing (4) 0.037 0.540 0.010 Crossbar (5,6,7) 0.031 0.560 0.009 Stringer (8) Stringers bracing (10,11) 0.039 0.520 0.004 Average 0.046 0.046 0.500 SD 0.0008 0.0008 0.0138 Max 0.031 0.031 0.42 Min 0.063 0.063 0.56 Chemical composition according to literature data 0.04-0.30 traces-0.33 0.10-0.33 Puddled steel [4] 0.03-0.35 0.04-0.75 traces-0.18 Cast steel [4] max.0.17 max.1,40 S235JR [15] max.0.17 max.1,40 S235J2 [15] max.0.20 min.0.40 0.12÷0.30 St3M [16]

P 0.037 0.039 0.017 0.036 0.022 0.006 0.0001 0.001 0.01 0.02-0.46 0.004-0.16 max.0.035 max.0.025 max.0.05

S 0.035 0.049 0.034 0.034 0.037 0.030 0.0004 0.017 0.039

Cr 0.008 0.010 0.010 0.012 0.009 0.038 0.0002 0.034 0.049

Cu 0.040 0.086 0.056 0.051 0.073 0.010 0.0000 0.008 0.012

Ni 0.018 0.023 0.023 0.021 0.030 0.061 0.0013 0.04 0.086

Al. -

0.10-0.60 0.004-0.09 0.007-0.014 0.11-0.14 0.03-0.04 0.01-0.02 max.0.035 max.0.55 max.0.025 max.0.55 max.0.05 max.0.30 max.0.30 max.0.02

Diagnosis of the microstructural and mechanical properties of over century-old steel railway bridge components Table 4. Results of steel weldability assessment (according to the literature [4,10,17])

Sample No.

Calculated weldability coefficients Ce

HSC

Ce`

HVmax`

HVmax

Mn/S

Bottom belt (1,2)

0.134

7.26

0.177

12.0

184.2

12.0

Wind bracing (4)

0.196

5.14

0.221

65.0

253.9

11.7

Crossbar (5,6,7)

0.134

5.18

0.165

*

170.8

15.8

Stringer (8,9)

0.132

7.07

0.173

7.0

165.9

16.4

Stringer bracing (10,11)

0.134

5.97

0.160

*

171.3

14.0

Acceptance limit

≤0.410

<4.00

<0.400

<300 HV

<350 HV

≅22

* - overestimated.

Diagnosis of the microstructural and mechanical properties of over century-old steel railway bridge components Table 5. Impact strength test results for different temperatures. Element Bottom belt (1) Bottom belt (2) Average Wind bracings (3) Wind bracings (4) Average Crossbar (5) Crossbar (6) Crossbar (7) Average Stringer (8)

So [cm2]

20oC

Impact strength KCV [J/cm2] 10oC 0oC -10oC -20oC -30oC

0.60

ND 56.3

74.2 52.3

0.40

ND 56.3 ND

63.2

66.7 ND

ND

28.6 101.6 103.1 ND ND

76.9 73.8 61.0 60.4 60.9 66.6 55.0 67.1

102.3 45.4 58.8 64.9 54.3 58.1 56.3 52.9 56.0

0.40

0.60

0.60

0.60

20.7 47.4 17.8 ND

66.7 51.8 63.4 72.2 ND 60.5 62.0 58.1 67.8

62.9 61.0 54.4 Average ND ND 89.2 Stringer 94.5 ND 118.3 bracings (10) 0.40 85.4 123.1 ND Stringer ND 112.4 bracings (11) 90.0 117.8 103.8 Average ND – no destruction with fracture work of 50J

11.5 45.8 53.2 ND ND 36.8 109.1 16.8 37.4 ND ND 8.6 42.9 48.2 62.6 56.0 48.8 ND 53.9 49.8 55.8 54.2 53.3 62.3 ND ND ND 62.3

51.3 22.6 52.5 ND ND 42.1 61.8 62.3 ND 34.9 ND 21.9 45.2 49.1 62.5 62.5 57.7 52.3 56.8 58.4 51.1 28.0 45.8 23.2 75.3 82.8 46.0 ND 66.8 58.8

ND ND

44.8 67.6 47.8 53.4 46.2 41.6 26.6 38.1 ND

64.2 6.8 35.5

Diagnosis of the microstructural and mechanical properties of over century-old steel railway bridge components Table 6. Fatigue test results with reference to the measured ReH (given in increasing order of cycles)

Component number 5 2 10 6 4 3 2 3 8 11 6 4 3 10 6

Specific Spec. No. WS.05.2 WS.02.1 WS.10.2 WS.06.2 WS.04.1 WS.03.3 WZ.02.1 WZ.03.6 WS.08.3 WS.11.4 WZ.06.3 WZ.04.2 WZ.03.4 WZ.10.2 WZ.06.1

No. of cycles 1025 13719 71185 89381 106558 187219 277776 291269 316246 338512 525362 806250 1318898 1679255 2050000

ReH [MPa] 340 309 309 308 343 300 309 300 279 299 308 343 300 309 308

ReH range 0.135÷1.032 0.148÷1.135 0.148÷1.135 0.149÷1.139 0.133÷1.023 0.153÷1.169 0.129÷0.994 0.055÷1.127 0.166÷1.093 0.153÷1.173 0.121÷0.929 0.128÷0.943 0.141÷1.077 0.131÷0.961 0.121÷0.889

 range [MPa] 46÷351 46÷351 46÷351 46÷351 46÷351 46÷351 40÷307 17÷338 46÷305 46÷351 37÷286 44÷323 42÷323 40÷297 37÷274

Diagnosis of the microstructural and mechanical properties of over century-old steel railway bridge components Table 7. Mechanical properties of steel samples.

Component no. 1 2 3 4 5 6 7 8 10 11 Average Min Max SD CV Specific value 95%

Rm [MPa] 380 368 409 407 381 382 355 373 388 375 382 354.5 409 16.62 0.044 354.2

ReL [MPa] 242 277 281 293 275 262 244 248 278 262 266 242.1 292.95 17.32 0.065 237.5

ReH [MPa] 265 309 300 343 340 308 296 279 309 299 305 264.7 343.15 23.94 0.079 265.3

ReH/Rm 0.70 0.84 0.73 0.84 0.89 0.81 0.83 0.75 0.80 0.80 0.80 0.70 0.89 0.058 0.073 0.702

max [mm/mm] 0.221 0.245 0.203 0.216 0.198 0.216 0.230 0.259 0.212 0.254 0.225 0.198 0.259 0.021 0.093 0.191

Diagnosis of the microstructural and mechanical properties of over century-old steel railway bridge components Table 8 . Hardness of tested components measured by Vickers method and the estimated mechanical properties Parameter

Measured, tensile test

Measured, hardness test

Component

Hardness, HV10

ReH [MPa]

Calculated based on hardness and tensile tests RmV ReV [MPa] [MPa]

SD

CV

SV 95%

Rm [MPa]

126.0

9.15

0.073

110.9

380

265

403

120.7

11.75

0.097

101.4

407

343

387

6

106.5

5.40

0.051

97.6

382

308

8

108.3

2.02

0.019

105.0

373

279

10

150.9

11.77

0.078

131.5

388

309

No.

Average

1 4

ReV – yield strength estimated based on Vickers hardness RmV – ultimate tensile strength estimated based on Vickers hardness

Comparison of measured and calculated results ReH/ReV

Rm/RmV

282

0.94

0.94

271

1.28

1.05

339

237

1.30

1.13

345

242

1.16

1.08

482

337

0.91

0.80

Highlights Material properties of an over 120-year-old steel railway bridge are investigated. The mechanical properties of old steel, i.e. fatigue strength, hardness and fracture toughness, are thoroughly examined. Fractography is used to determine impact failure in a wide temperature range from -30ºC to +20ºC. The results demonstrate that the presence of non-metallic inclusions in the steel microstruc-ture affects its fracture toughness failure mechanism. The mechanical and chemical properties of old steel are compared in relation to modern steel grades.