Aluminothermic Weld Research Articles

Investigation of Thermal Conditions of the Aluminothermic Welding and their Influence on the Structure and Properties of Metal Rails

Rail welded joints are integral part of continuous welded rail. However, they often do not have sufficient reliability during the operation. The article is devoted to the assessment of temperature influence effect on the mechanical properties and structure of weld metal and welded rails. The temperature distribution across the rail section in welded zone during the cooling process of aluminothermic rail welding is obtained using simulation by LVMFlow. The results of the study of hardness and structure of metal rail aluminotermitic welded joints are given. It is shown that the hardness of rail welded joints increases from 24 HRC to 38 HRC in the fusion zone of the weld metal and rail metal. It is due to the harmful effects of overheating of the metal during the welding process. The hardness is confirmed by microstructural analysis. Microstructural analysis showed the differences in the grains sizes of metal welded zone and heat affected zone. The structure of welded metal is acicular dendritic. Owing to a difference between structures of the welded joint zones the probability of occurrence of cracks on the boundary of fusion weld and metal is increased.

Longitudinal rail weld geometry control and assessment criteria

Abstract This study covers the parametric variations of vehicle and track characteristics to inform on the requirements for an optimum and yet pragmatic control of longitudinal rail weld profile. A statistical study has been carried out using a large set of measured weld profiles (arbitrary mix of flash butt welds and aluminothermic welds) in order to establish relationships between degradation mechanisms and the longitudinal geometry of the finished weld. The potential benefits which would be expected from improved controls over the welded profile, with respect to not only rail running surface damage but also susceptibility to rail breaks and ballast degradation, are demonstrated. Finally, recommendations for the review of the geometric controls of finished weld geometry in the relevant Euro Norms are made.

MODERN WELDING METHODS OF THE RAILS OF NEW GENERATION

Nowadays, the railways in the Russian Federation are moving to the path of seamless construction. The development of technologies to ensure the possibility of continuous welded rail track with the desired quality characteristics is the actual direction now. One of the key technologies for the construction, repair and maintenance of a seamless path is welding of rails, for which various methods have been developed and are used, such as: electric-contact, gas-press, induction, laser welding, friction welding, aluminothermic, electric arc electrodes, in the environment of protective gases, electroslag, powder wires, etc. The article describes the advantages and disadvantages of the main types of welding rails that are most widespread today: resistance flash welding and aluminothermic welding. It is shown that as the primary method of rails welding in Russia, electric-contact welding is used. However, in connection with the inability to weld rails in the zones of turnouts by contact machines, the use of aluminothermic welding of rails started on the railways of Russia. It has been shown that the most important factors that determine the quality and reliability of welded rails are metallurgical and welding technologies, as well as subsequent processing methods that reduce the impact of high temperature effects. The technological features of these methods, advantages and disadvantages of modern methods of solving problems in welding, as well as economic costs were considered taking into account operational properties of joints. Experience has shown that during the exploitation of welded joints made by aluminothermic method on the railway and at testing on the experimental ring of JSC All-Russian Scientific Research Institute of Railway Transport, their quality is slightly inferior of electric-contact welding. Special attention is paid to the peculiarities of welding technology for differentially thermally strengthened rails. The analysis of existing methods of rails welding allows working out the main directions of research to solve this problem.

Use of Waste From Aluminothermic Welding of Railroad Tracks in Structural Materials Science

The article discusses the possibility of using mineral waste from the aluminothermical rail welding in the form of waste refractory forms, and slag in construction materials in the preparation of ceramic, refractory and cement containing materials. The article provides optimal compositions using crushed and ground mineral waste and physic-mechanical studies of the obtained construction materials. For the study of the waste phase composition and structure of synthesized materials powder x-ray diffraction, differential thermal analysis and electron microscopy were used.

A Model of Optimization and Control the Thermite Kit for Aluminothermic Welding

Thermite welding (TW) is now widely used all over the world to weld, maintenance and modernization railway and tram rails [1]. The main materials from the thermite composition (TC), results from manufacturing scraps, which can be retrieved all over the country. Exothermic welding (EW) which is based on the exothermal reaction between iron oxides (FeO, Fe2O3 and Fe3O4) and aluminium powder, takes place at temperatures up to 3500 °C and during casting rail ends are melted in order to be welded.In was studied the thermal effects of the aluminothermic reduction reactions of the iron oxides, which were dosed in different percentages. The aluminothermic reaction efficiency is given by strictness in the ratio of the TC selection and mainly in the iron oxides types. Their correct dosage makes possible the control the exothermic reaction (ER) effect. Also it is revealed the effect of particle size from thermite powder (TP) on the thermite dynamics reactions. Is also presented a model of optimization and control the thermite kits (TK) for aluminothermic welding (AW). Finally, the TK composition can be calculated from "QUARK1" Diagram, as function of the heat amount necessary for the welding.

Qualifying the technology of the aluminothermic welding of tramway rails on the basis of quality assurance system requirements in welding engineering

Qualifying the technology of the aluminothermic welding of tramway rails on the basis of quality assurance system requirements in welding engineering

Improving service properties of welded joints produced by aluminothermic welding

The results of metallographic studies of welded joints in rails produced by aluminothermic welding are presented. The results of hardness measurements of these welded joints show that the mean hardness level of the metal of the welded joint is 240–290 HV. According to the TU 0921-127-01124323-2005 standards, the hardness values do not fully satisfy the required minimum values.

THE INVESTIGATION OF MECHANICAL PROPERTIES OF WELDING QUALITY OF R65 TYPE RAIL / R65 TIPO BĖGIO TERMITINIO VIRINTINĖS JUNGTIES MECHANINIŲ SAVYBIŲ TYRIMAS

This article is about the aluminothermic welding quality ofrail joints, using different parameters. The thermit portion andpreheat time estimated for quality of welded joints as well asthe mechanical characteristics of aluminothermic welds in railshave been examined. Santrauka Šiame straipsnyje nagrinėjama termitiniu būdu ir skirtingais parametrais suvirintų bėgių kokybė. Nustatyta termitinio suvirinimo mišinio kiekio ir pakaitinimo laiko įtaka virintinių jungčių kokybei, ištirtos termitiniu būdu suvirintų bėgių siūlių mechaninės savybės.

METALLOGRAPHIC INVESTIGATION INTO THE ALUMINOTHERMIC WELDING OF RAIL JOINT R65 / R65 TIPO BĖGIO TERMITINIO SUVIRINIMO JUNGTIES METALOGRAFINIS TYRIMAS

The article describes inherent defects to aluminothermic welds, the origin of reasons and the ways of avoidance. The paper presents visual surface and fracture testing of rail joint R65 applying the method of aluminothermic welded joints. Santrauka Straipsnyje nagrinėjami galimi termitiniu būdu suvirintų jungčių defektai, jų atsiradimo priežastys ir nurodomi būdai, kaip jų išvengti. Atlikti R65 tipo bėgių termitiniu būdu suvirintų jungčių vizualiniai paviršiaus ir lūžio tyrimai.

Finite element modeling of the aluminothermic welding with internal defects and experimental analysis

There are several hundreds aluminothermic welds and tenth of new welds being made daily on the Moroccan rail network. Although the aluminothermic welding technique is well proven, it is a critical safety component of the rail infrastructure. The consequences of a single failure could result in the derailment. The observations suggest that these defects are mainly of two types: defects of adhesion (or bonding defects) and porosity defects (or multiple blisters defects). Each of these defects may constitute a privileged site of stress concentration. Depending on the level of these stresses, potential damage process may be developed and put off the rail. It therefore seems necessary to model the mechanical behavior of these welds with these defects in order to classify them by their criticality. In this study, we therefore modeled with the same load and with the same boundary conditions, the weld seam for each type of defect. The numerical and experimental results show that the defects of adhesion tend to concentrate the most constraints.

The effect of geometric features on multi-axial fatigue behaviour of aluminothermic rail welds

Aluminothermic rail welds typically exhibit a variable and often inferior performance when compared with flash butt welds. Fatigue failure as a result of surface defects or harsh geometric features in the presence of high axle loads and tractive forces gives rise to the so-called straight break and horizontal split web fractures. In this study, the reduction of fatigue performance due to geometric irregularities is investigated using a multi-axial high cycle fatigue criterion based on the critical plane concept. A thermo-structural finite element model of a track containing an aluminothermic weld is linked with a developed computer code to perform fatigue crack initiation analysis. Two geometrically different aluminothermic welds, one of which is widely used in Australian heavy haul railways, are investigated in terms of fatigue probability, to facilitate a better insight into the effect of weld collar shape and geometry at the edge of the weld collar on fatigue behaviour. The results confirm that the amount of fatigue damage is critically dependent on the geometric features of the collar edge, especially at the underhead radius which undergoes severe fatigue loading under some operational conditions.

NEWSDESK: Towards the automatic volumetric inspection of aluminothermic rail welds; NDT Systems celebrates 40th year with move

NEWSDESK: Towards the automatic volumetric inspection of aluminothermic rail welds; NDT Systems celebrates 40th year with move

Multi-axial fatigue analysis of aluminothermic rail welds under high axle load conditions

Fatigue crack initiation analysis is performed at the edge of the weld collar of an aluminothermic weld, in order to examine one of the failure modes that occur in this weld type under high axle load conditions. Since the stress state is multi-axial and the stress history involves rotating principal stresses, a shear based multi-axial fatigue criterion based on the concept of critical plane is exploited. It is believed that the material distress as a result of wheel–rail contact, bending, residual and seasonally-dependent thermal stresses, as well as the geometrical features associated with the weld collar, would directly influence the location of fatigue crack initiation and the severity of damage. The fatigue assessment is performed using thermo-structural finite element analysis followed by the Dang Van multi-axial fatigue criterion implemented as a customized computer code. The method is applied to investigate the effect of operational parameters including the conditions of vehicle hunting or curving, residual stress, seasonally-dependent variations in rail temperature, and deficiencies in track support conditions in the vicinity of aluminothermic welds. The finite element simulation is validated through strain gauge measurements, and the results of fatigue analysis are compared with the locations of fatigue cracking and failure modes that occur in-service.

Improving Fatigue Performance of Alumino-Thermic Rail Welds

Rail transport development offers economic and ecological interests. Nevertheless, it requires heavy investments in rolling material and infrastructure. To be competitive, this transportation means must rely on safe and reliable infrastructure, which requires optimization of all implemented techniques and structure. Rail thermite (or alumino-thermic) welding is widely used within the railway industry for in-track welding during re-rail and defect replacement. The process provides numerous advantages against other welding technology commonly used. Obviously, future demands on train traffic are heavier axle loads, higher train speeds and increased traffic density. Thus, a new enhanced weld should be developed to prevent accidents due to fracture of welds and to lower maintenance costs. In order to improve such assembly process, a detailed metallurgical study coupled to a thermomechanical modeling of the phenomena involved in the rail thermite welding process is carried out. Obtained data enables us to develop a new improved alumino-thermic weld (type A). This joint is made by modifying the routinely specified procedure (type B) used in a railway rail by a standard gap alumino-thermic weld. Joints of type A and B are tested and compared. Based on experimental temperature measurements, a finite element analysis is used to calculate the thermal residual stresses induced. Besides, experimental investigation was carried out in order to validate the numerical model. Hence, X-Ray diffraction has been used to map the residual stress field that is generated in welded rail of types A and B. In the vicinity of the weld, the residual stress patterns depend on the thermal conditions during welding. Their effect on fatigue crack growth in rail welds is studied. In the web region, both longitudinal and vertical components of residual stresses are tensile, which increases the susceptibility of that region to crack initiation and propagation from internal material defects. Indeed, weld fracture in track initiates at the web fillet. Thus, to be closer to real issue, fatigue tests specimens has been defined within the split-web area. Fatigue tests was performed on the defined specimens, welded by conventional and improved processes and obtained results adjudicates on the new advances.

Occurrence Conditions and Preventive Methods for Solidification Cracks in Alumino-thermic Welds

Although improvements in ultrasonic inspection methods have brought about a reduction in the incidence of alumino-thermic weld failure, some failures still occur in the early stages after installation of weld as a result of solidification cracks. It is considered that such cracks arise from rail movement in the longitudinal direction just before the weld metal solidifies. However, the details of the related occurrence conditions are not yet clear. In this study, the occurrence conditions of solidification cracking and preventive methods for weld failures caused by such cracking were discussed based on simulative testing of solidification cracks. As a result, the occurrence conditions were clarified, and preventive methods and detective methods for solidification cracks were suggested.

Improving fatigue performance of rail thermite welds

Rail transport development offers economic and ecological interests. Nevertheless, it requires heavy investments in rolling material and infrastructure. To be competitive, this transportation means must rely on safe and reliable infrastructure, which requires optimization of all implemented techniques and structure. Rail thermite (or aluminothermic) welding is widely used within the railway industry for in-track welding during re-rail and defect replacement. The process provides numerous advantages against other welding technology commonly used. Obviously, future demands on train traffic are heavier axle loads, higher train speeds and increased traffic density. Thus, a new enhanced weld should be developed to prevent accidents due to fracture of welds and to lower maintenance costs. In order to improve such assembly process, a detailed metallurgical study coupled to a thermomechanical modelling of the phenomena involved in the thermite welding process is carried out. Obtained data enables us to develop a new improved thermite weld (type A). This joint is made by modifying the routinely specified procedure (type B) used in a railway rail by a standard gap alumino-thermic weld. Joints of type A and B are tested and compared. Based on experimental temperature measurements, a finite element analysis is used to calculate the thermal residual stresses induced. In the vicinity of the weld, the residual stress patterns depend on the thermal conditions during welding as it also shown by litterature [1, 2]. In parallel, X-Ray diffraction has been used to map the residual stress field that is generated in welded rail of types A and B. Their effect on fatigue crack growth in rail welds is studied. An experimental study based on fatigue tests of rails welded by conventional and improved processes adjudicates on the new advances and results will be shown.

Assessing the Economic Life of Rail Using a Stochastic Analysis of Failures

In this article, a model is developed to evaluate the economic life of rail using a stochastic analysis of rail failures. The occurrence of rail defects and failures is analysed and issues related to rail maintenance are addressed. The hazard rate of alumino-thermic weld failures is predicted for continuously welded rail, where the expected number of welds is increasing in tonnage as a result of replacement. Imperfect inspections are modelled for predicting the expected number of failures by employing a filtered non-homogeneous Poisson process. The impact of maintenance on failure occurrence and whole life-cycle cost (LCC) is analysed and integrated within the model. The application and capability of the proposed model and sensitivity of parameters are illustrated through the use of a practical example. It is shown that an optimal economic life of rail can always be found, as the repair cost and the risk cost of accident are increasing with tonnage that passed through it. In addition, the inspection frequency has a significant impact on rail LCC. The results also suggest that the proposed model is also valid for optimizing the interval between rail inspections.

Carriles para alta velocidad. Comportamiento en fatiga

In recent years, passanger train speed and freight train load have increased to enhance efficiency of rail road transportation. These trends have increased the severity of rail service conditions, calling for rails with greager wear resistance, strength and fatigue behavior. In the United States and Europe, track site weld rails are made entirely by alumino thermic process. This work describe the results of experimental study conducted on bending fatigue strength of plain rails and alumino thermic welded rails with preheating procedures (oxipropane and air-induced propane) approved by railways authorities. Compliance with the required fatigue strength shall be ascertained by 4 point pulsating bending test in accordance with European standard by alumino thermic weld in rails. The Locati method, based in the empirical Miner's law about the cumulative damage on a fatigue tested material, allows, once known the Wohler curve of the welding process in use to settle the fatigue tensile limit at 50% with only one test. The values obtained at 2.10 cycles for plain rails (S f = 353 MPa), oxipropane preheated aluminothermic weld rails (S f = 225 MPa), and propane-air induced aluminothermic weld rails (S f = 210 MPa) are very similar to those resulting from test method stated in the European Standard. From our point of view and due to its ease, speediness and savings, this is the most suitable test to check the quality and compare the aluminothermic processes in use.

Failure modes in aluminothermic rail welds under high axle load conditions

Aluminothermic welding of rails is widely used within the railway industry for in-track welding during re-rail and defect replacement. The process provides flexibility and low capital cost, but suffers from variable quality in finished welds, due to the inherent limitations of the processes used, and their operator dependency. The service performance of such welds can be considered in terms of the integrity of the joint, i.e. its ability to support the service loads without fatigue damage or fracture, and the batter behaviour of the running surface, which in turn influences the extent of impact loading. Under high axle load conditions, these performance requirements are unforgiving in terms of weld quality and material characteristics. In response to recent failures in aluminothermic welds, and in recognition that such welds represent one of the main risks for a catastrophic derailment, and a major limitation to further increases in axle loads, a program was undertaken to both remedy the current problem with premature failures, and to develop improved rail welding and track maintenance practices which would meet the performance demands of higher axle loads.

Guided wave testing of rail

Guided wave inspection has a number of advantages over conventional ultrasonic inspection. For instance, guided waves can propagate over many tens of metres in rail, they can fully penetrate alumino-thermic welds and theyare very sensitive to cracks in the transverse-vertical plane that may cause catastrophic failure. With financial and technical support from Network Rail, the authors are developing a guided wave inspection system for rail, which is described in this paper. Experimental and finite element results showing the interaction of various guided wave modes with a wide variety of defect geometries are presented. This study has enabled characteristic mode conversion signatures for various defects and features to be obtained and these signatures will provide the basis for an automatic feature extraction algorithm. Results from laboratory tests and site trials are presented that demonstrate the ability of the system to detect a range of defects in free rail, aluminio-thermic welds and in rail at a level crossing.