Influences of preheating parameters on the quality of weld by thermite rail welding

The major goal of this study is to enhance the mechanical and metallurgical characteristics of rail steel grade R260 joined by thermite welding under various preheating conditions, including preheating time and gas pressure. Mainly two conditions, referred to as the Normal Condition and Improved Condition, are carried out for experiments. Prior to welding, the Normal Condition was preheated using liquefied petroleum gas (LPG) and oxygen gas pressures of 1 bar and 4.5 bar for 3 min, and the Improved Condition was preheated using liquefied petroleum gas and oxygen gas pressures of 1.2 bar and 4.5 bar for 6 min and 30 s. To investigate the mechanical and physical properties, micro-Vickers hardness tests, tensile tests and slow bending tests were also carried out. Welded metal in Normal Condition has many defects, including gas holes and shrinkage cavities. When comparing the Normal Condition to the Improved Condition, the Improved Condition demonstrates significantly more bending load and deflection. Specifically, the thermite welded rail sample of Improved Condition demonstrated a remarkable ability to endure bending loads of 108 tonnes and a deflection of 16 mm, and this sample remained unbroken until it exceeded 50% of the standardized deflection limit (10 mm). In addition, the average hardness values for the Improved Condition of the weld metal zone and the heat-affected zone were 331 HV and 289 HV, respectively. The Normal Condition produced an unsatisfactory fracture surface after slow bending test. This was caused by weld defects at the thermite weld due to inappropriate preheating.


Introduction
Train travel is the most affordable means of transportation in many countries today.As a result, it is imperative to consider expanding the railway.The most common method of joining rail in the past was bolting, but bolted railways were costly to repair and prone to certain types of failure [1].Continuous welded rail (CWR) has been in use as a replacement for the conventional way of connecting rail sections using bolts [2].The significant benefits of CWR tracks are improved railway safety, particularly for high-speed lines, and cost savings [3,4].Rails are joined using various welding processes for the building of new railway lines as well as the replacement of existing or damaged rails in the field.
Currently, the most common welding techniques for joining rails are flash-butt welding and aluminothermic (thermite) welding [5].However, other techniques, such as gas pressure welding and enclosed arc welding, are also utilized [6].Flash-butt welding is a form of resistance welding that can be done using mobile welding equipment or in a stationary plant [7].On the other hand, thermite welding is a casting technique that is most frequently employed in the field to join rails.Before welding, the rails are set with a gap between two rail ends, then this area is enclosed in a sand mold.After being ignited, the molten steel generated by the exothermic reaction of the aluminothermic mixture fills the space and completes the weld [8].The rails are joined using molten steel produced by a chemical reaction between iron oxide and aluminum.Several important steps must be completed during the thermite welding process, including preheating, tapping, pouring, solidifying, shearing, and cooling [9,10].
Preheating is the first process before welding and one of the most crucial operations for the thermite rail welding process.Prior to welding, a burner or torch can utilize various fuels to preheat the rail ends and sand mold for thermite welding.Several gas mixtures (especially Oxygen and Propane or Liquefied Petroleum Gas (LPG)) can be employed for preheating [11].Furthermore, all preheating methods in thermite welding of rail steel can be performed without restriction in every gas combination [12].Preheating is essential to eliminate moisture from the rail ends and sand mold before pouring the liquid metal from the crucible into the mold cavity.It also helps maintain a slow cooling rate.Otherwise, the molten metal may rapidly cool and solidify when it encounters cold rail ends and the surface oxidation of rail ends cannot be eliminated as well [13].In addition, preheating is one of the most important processes to get a defect-free weld metal.The potential causes of defects formation in thermite weld are inadequate preheating, climatic condition, operational mistakes, the existence of moisture in the mold, the crucible, or the thermite mixture.Most of the rail welding defects are generated by inadequate preheating [14].
The running rail is embrittled at welding joints, which is the primary reason for thermite welded rail failures.As a result, welding joints are critical areas.Thermite welding may also produce a few inclusions or minor defects that might serve as possible crack initiators and facilitate the spread of failure [15].Failures due to defects in rail thermite welds can occur when welding parameters are not properly controlled, which damages the welded rail joints.One of the keys focuses of track safety is to prevent rail failure [16].Therefore, in the thermite welding process, it is necessary to manage some variables to get rail welding of a high enough quality [17].Investigating the preheating parameters utilized for thermite welding is fundamental.Y Chen et al [18] investigated the effect of preheating time on the solidification of thermite weld metal after pouring molten metal into the mold.The rate of weld metal solidification decreases with increasing preheating time.It provides a wider weld deposit.Therefore, the preheating process influences the quality of weld metal in thermite process.The preheating process is mostly determined by two important variables: preheating duration and gas pressure used in the preheating torch.It is necessary to adjust and find the best parameters of preheating in thermite welding.Due to the increased humidity and more variable weather conditions in the tropical region, the contractors' thermite welding parameters may not be correct, leading to problems with the old rail welds.Improving the quality of thermite rail welds was the motivation for this study, which aimed to examine the effects of preheating parameters on metallurgical and mechanical properties of thermite welded rail.

Welding procedures
The rail steel grade used in this research is R260 with rail profile EN54E1.In this research, standardized Thermit ® Australia's welding kit and a reusable crucible were employed.For composition of thermite mixtures by the supplier company are presented in table 1. Figure 1(a) depicts a schematic of two rails welded by the thermite welding process before metallography and mechanical testing.The gap between the rail ends was 27 mm, and the distance between the preheating torch and rail top surface is 45 mm (see figure 2).
Throughout the study, the previous preheating parameters given by the local contractor are referred to as 'Normal Condition,' whereas the new preheating parameters obtained from our experiment's results are referred to as 'Improved Condition.'The Normal Condition was pre-heated for 3 min by an LPG-Oxygen torch under a pressure of 1 and 4.5 bar, whereas the Improved Condition was preheated for 6 min and 30 s by an LPG-Oxygen torch with a pressure of 1.2 and 4.5 bar.Then the thermite mixture was put into a crucible and ignited.After the molten metal had solidified in the mold for 3 min, the mold was removed, and excess welded metal was eliminated by a hydraulic trimming machine.When the specimens were cooled down, the weld metal was ground to get an actual shape and size of the rail profile by grinding machine.With the use of optical emission spectroscopy (OES), the chemical composition of rail steel R260 and both weld metals under various welding conditions is examined.The chemical composition of the parent rail and different weld metals (Normal Condition and Improved Condition) are presented in table 2.

Metallographic characterization
The metallographic sample was taken from the longitudinal of the thermite rail weld from the center of the weld to the base rail, which ran through the weld zone, fusion line, heat-affected zone (HAZ), and unaffected parent rail, as seen in figure 1(c).The specimen for macrostructure was polished and etched with 4% Nital [19].Optical microscopy was carried out utilizing a Carl Zeiss Axio Scope.A1.The Quanta 400 FEI scanning electron microscope (SEM) with Oxford energy dispersive x-ray spectroscopy (EDS) was utilized to examine the microstructure at high magnification and elemental analysis of metallic inclusions.The metallographic samples for each condition were cut from the longitudinal cross-section of the rail head and they were examined at weld metal, different regions of HAZ and Base rail.The samples were polished and etched with 2% Nital for 8 s according to ASTM E407 [20].

Hardness testing
Figure 1(c) illustrates the application of the micro-Vickers hardness test, which was conducted from the center of the weld metal to one side of the heat-affected zone (HAZ) and base metal.This was due to the longer length of the specimen [21].The testing specifications consisted of subjecting the area below 5 mm on the top rail head to a load of 200 g for 10 s.The measurements were taken at each interval of 0.5 mm using an MMT-X7B model instrument manufactured by Matsuzawa, Japan.

Tensile test
The Tensile tests were conducted using a Hounsfield model H100KS universal testing machine, England, with a constant cross-head speed of 1.8 mm min −1 .Specimens have been prepared from the longitudinal section of the thermite rail weld, specifically from the rail head and foot as seen in figure 1(d).The tensile testing specimens, which had a reduced diameter of 6 mm and a gauge length of 120 mm as shown in figure 1(e), were tested following ASTM E8/E8M-11 [22].The engineering stress-strain curves for each sample have been generated.

Slow bending test
Slow bending test of joints was performed according to EN 14730-1 [19], a 3-point bend test using a rail static bend testing machine (TE-2000kN, China).The test specimens were run through the loading rate at 5 kN s −1 , bend to qualifying criterion load and deflection.The thermite welded rail joint must withstand a minimum load of 80 tonnes with a minimum deflection of 10 mm for the upper side bend.Figure 3 shows the detail dimensions and setups of slow bending test with a photo of the actual equipment.

Preheating thermal cycles
The preheating temperature was recorded using thermocouples in the rail head and rail foot.The record of preheating is plotted in figure 4. The experiment was conducted with LPG at 1 bar and 4.5 bar oxygen for preheating under Normal Conditions.The preheating gas pressure of 1.2 bar LPG and 4.5 bar oxygen was used for the Improved Condition.The preheating time for the Normal Condition was 3 min (180 s), whereas the preheating time for the Improved Condition was 6.5 min (390 s).After the preheating process for Normal Condition (using LPG at 1 bar and Oxygen at 4.5 bar for 180 s) was done, the temperature of the rail head was 398 °C and the temperature of the rail foot was 277 °C.After preheating for Improved Condition, the rail head and rail foot reached temperatures of 635 °C and 704 °C respectively.This was achieved using LPG at a pressure of 1.2 bar and Oxygen at a pressure of 4.5 bar for 390 s.Furthermore, the American Welding Society recommends a preheating temperature range of 600 °C to 1000 °C for thermite welding [9].
A shorter preheating period leads to a lower temperature.Additionally, the preheating torch's increased pressure resulted in a higher temperature at the rail foot.The primary objective of examining the thermal cycles of thermite welding was to identify the highest temperature achieved during the specific preheating time.No post-preheating interval is required during actual welding operations.Once the preheating is complete, the thermite powder in the crucible is fired, allowing the exothermic reaction to take place.The resulting molten metal is then poured from the exothermic reaction as weld metal into the preheated mold cavity.

As-welded condition
Figures 5 and 6 show the as-welded appearances of Normal and Improved Conditions.Both conditions have the running surface (top view), rail web (side view), and rail foot (bottom view).As-welded conditions were obtained after the samples cooled to room temperature and the welded region was trimmed to the ideal rail profile (EN54E1).There has been a visual inspection.In figure 5, the rail head side and running surface of Normal Condition have blowhole defects.The gas formed when luting sand encounters liquid steel and fills spaces between the mold and rail surface may be the cause of the blowhole defects.This leads to surface defects [23].

Macrostructure
The etched macrographs for different conditions are shown in figure 7. The longitudinal thermite rail weld across the weld zone, heat affected zone (HAZ) and base metal can be seen in the macrostructure.The HAZ and the weld metal size varied under different welding conditions.Improved Condition gave the result of a broader HAZ and Weld Metal.Table 3 shows the width of the different areas on the macrograph.Measurements were obtained from the middle of the weld metal.The size of the weld metal is influenced by different preheating times [24].The width of the weld metal and HAZ increases when preheating time is longer.Moreover, preheating increases weld penetration (melt-back of the base rail) and widens the rail head in the metal [25].figure 8 displays an illustration representing the extent of rail penetration upon thermite welding.Once the cross-sectional sample has been etched, the weld defects in Normal Condition are visible, whereas no defects are observed in Improved Condition.

Optical microscopy
Figure 9 depicts the microstructure of R260 base rail steel, which is one of the most frequently employed grades of rail steel.Alternating layers of ferrite and cementite in a lamellar structure, having a fully pearlitic microstructure.Carbon steels having a pearlitic microstructure are commonly used for railways [26].In a softer ferrite matrix, oriented cementite lamellae make up the lamellar microstructure known as pearlite [27].This microstructure offers superior wear resistance and strength for railway applications [28].Figure 12 shows the microstructure of weld metal near the fusion line of thermite weld.Both the fusion of the parent metal and the initiation of the solidification of the molten metal take place in this region.According to Yang Liu [29], this area was termed the mixed melted zone.Additionally, this area was referred to as the solidified zone by Lesage et al [30].Its grain size is smaller than that of the weld center.Compared to the heat-affected zone, the grain size is bigger.Proeutectoid ferrite phases along the grain boundaries and some intragranular ferrite  phases were obtained in both conditions.Many weld defects appeared in Normal Condition.Figure 12(a) points out the apparent porosities that have been identified.It should be noted that weld defects can be observed along the weld metal (from the center of the weld to the fusion line) in Normal Condition.Weld defects typically cause weld metal quality to deteriorate.The presence of defects in the weld metal promotes the development of fractures and the propagation of cracks when stress or load is exerted on this area, potentially leading to an unexpected or catastrophic failure.
Figure 13 shows the microstructure of the fusion line and heat-affected zone of thermite welding for Normal and Improved Conditions.The fusion line creates a boundary between the weld metal's end and the beginning of coarse grain heat affected zone (CGHAZ).In both conditions, the fusion line is divided into two distinct microstructures.The pearlite phase with ferrite on the grain boundaries is apparent in the microstructure of HAZ areas.The ferrite is seen as the white phase.The lamellar pearlite structure is difficult to observe at lower magnifications.This means that pearlite is generally the dark phase under the lower magnification optical   micrograph.The heat-affected zone of thermite welding can be divided into three areas.They are the coarse grain heat affected zone (CGHAZ), fine grain heat affected zone (FGHAZ), and intercritical heat affected zone (ICHAZ) [31].The base metal near the fusion line was subjected to annealing by the welding process and its heat generated the new grains with a size of around 45 μm in the CGHAZ area.Due to partial annealing, the area in the middle of the thermite HAZ had a finer grain that was around 30 μm in size.The microstructure of HAZ close to the unaffected base rail displays the spheroidized structure in the finest grain size of around 10 μm because this area was affected by the least heat from the thermite welding process under eutectoid temperature.As a result, the spheroidization of cementite occurs in this region instead of the lamellar pearlite structure [15].
There is no obvious difference in the phases found in the microstructure of the weld metal and its HAZ for both conditions.Additionally, no porosity was found in either condition's HAZ.

Scanning electron microscopy
A scanning electron microscope equipped with an SE (Secondary Electron) detector and BSE (Backscattered Electron) detector was utilized to examine the microstructure in detail.Figure 14 shows Normal Condition's scanning electron microscopy images with different detector modes.Some ferrite phases can be seen in the pearlite microstructure.In SEM images, ferrite appears in dark due to the contrast of SEM [28].The only difference between the two SEM images is the detectors; elsewhere, the settings were identical.Due to the optical contrast in the Secondary Electron (SE) image, the ferrite phases appear to be weld defects.In the Backscattered Electron Detector (BSED) image, the weld defects are obvious and clear.Gas pores and shrinkage cavities are found.As molten metal solidifies during welding, gases become trapped and form gas pores.The trapped gas during pouring or the chemical reaction byproducts like slag during the thermite reaction is the two factors that cause the formation of these gas pores.Lack of filler metal to make up for shrinkage as the cast metal solidifies causes the defects.Additionally, shrinkage defects in thermite welds are experienced due to additional contaminants or dissolved gases during solidification [29].Figure 15 shows the SEM image of weld metal microstructure in Improved Condition.The microstructure and phases that emerged in SEM pictures of the Improved Condition were similar to those in the Normal Condition.The result is a pearlite microstructure with some ferrite phases.On the other hand, the SEM image of the Improved Condition shows nearly no defects.The apparent gas pores are reduced in the Improved Condition.
Energy dispersive x-ray spectroscopy (EDS) was used to analyze the elemental composition of welding defects.To achieve accuracy, the weld defects from the weld center and the weld metal near the fusion line of Normal Condition were studied.Due to the high number of weld defects including microporosities and shrinkage voids in the weld metal of Normal Condition, only the weld metal of that condition was analyzed using EDS.The weld metal at the weld center was mapped using EDS, as shown in figure 16.There are shrinkage voids or cavities seen in the SEM picture.The presence of weld defects like shrinkage cavities, which have a considerable impact on the weld quality, is a significant problem during thermite weld service.Higher concentrations of aluminum, oxygen, manganese, and sulfur have been detected, according to mapping analysis using energy dispersive x-ray spectroscopy (EDS).Aluminum oxide (Al 2 O 3 ) and secondary inclusions that are  always present during thermite welding may have a negative impact on the ductility and toughness of the weld metal.Secondary inclusions, which are considerably smaller than aluminum oxide inclusions, were created when the slag solidified.These secondary inclusions were droplets of liquid slag in the liquid weld metal that made their way into the weld gap [32].
EDS analysis was also performed on weld defects close to the fusion line of the Normal Condition.The results of the elemental analysis matched those near the fusion line and the results can be seen in figure 17.Aluminum oxide (Al 2 O 3 ) and manganese sulfide (MnS) were found as the predominant forms of inclusions in the examined samples according to the elemental analysis from the SEM/EDS system.During the pouring operation, alumina (Al 2 O 3 ) normally develops as slag on top of the molten metal following the thermite reaction.Aluminum oxide inclusions develop at the welding zone and the fusion line as a result of inadequate preheating.At the thermite weld metal, the aluminum oxide particles are dispersed.This factor may play a crucial role in the significant decrease in ductility seen at the weld metal of Normal Condition.It needs to be observed that the Al 2 O 3 oxide particle dispersion affects the crack formation and leads to the catastrophic brittle fracture at the thermite weld [29].

Hardness testing
The hardness values of the Normal and Improved Conditions are shown in figure 18.A micro-Vickers hardness tester was used to record the hardness distribution along the weld metal (WM), heat-affected zone (HAZ), and base metal (BM) of two different thermite welding conditions.Figure 18(a) exhibits the hardness profiles of the Normal Condition.The hardness results of the Improved Condition are shown in figure 18(b).From the weld center to the parent rail, the hardness was assessed.The railhead was the main source of evaluation for the hardness profiles because the hardness distribution was measured 5 mm below the running surface.
In figure 19, the hardness comparison between Normal and Improved Conditions.The Improved Condition was found to have lower HAZ hardness and higher weld metal hardness.In Improved Condition, the hardness near the fusion line, so-called the partially melted area, is lower than that in the weld zone.The weld metal hardness is reduced towards the fusion line in Improved Condition.The hardness of weld metal under Normal Condition, however, is almost constant from the weld center to the fusion line.For the Normal Condition, the average hardness results for CGHAZ, FGHAZ, and ICHAZ are 331 HV, 292 HV, and 302 HV, respectively.Based on the results for the Improved Condition, the average hardness of CGHAZ is 304 HV, FGHAZ is 280 HV, and ICHAZ is 267 HV.In the Improved Condition, the hardness in the HAZ is noticeably decreased.The HAZ temperature of Normal Condition was lower because the preheating cycle was shorter.A lower preheating temperature results in a faster cooling rate.Higher hardness is produced in the HAZ region through faster cooling rates.
Figure 20 compares the average hardness of the Normal and Improved Conditions.Both weld metals are generally comparable in average hardness (around 330 HV).In this study, the parent rail is the same in all   conditions.As a result, the base metal's average hardness (around 313 HV) is consistent in both cases.In the HAZ, the noticeable variation in average hardness is apparent.The average hardness of Normal Condition and Improved Condition in the HAZ area is approximately 323 HV and 289 HV, respectively.The optimum difference between the two conditions is around 35 HV.However, the Improved Condition's reduced average hardness at HAZ (289 HV) is comparable to that of pearlitic rail steel (about 310 HV).The hardness level usage of   more than 260 HV is permissible in railroad applications.When the preheating temperature is lower, the cooling rate is also faster.In the HAZ region, accelerated cooling rates result in increased hardness.

Tensile test
Table 4 provides a concise overview of the tensile test outcomes for both the rail and welds.It includes data on the ultimate tensile strength and elongation.In the Normal Condition, the welded metal exhibits lower tensile strength and ductility compared to the Improved Condition and the base rail R260. Figure 21 shows the stressstrain curves, which are consistent with figures 11 and 14 in showing that the weld metal of Normal Condition is brittle and has lower tensile strength because there are many weld defects, including gas pores, shrinkage cavities, and inclusions in the micrograph of Normal Condition.On the contrary, under the Improved Condition, the weld metal exhibited minimal weld defects, and its tensile properties were improved due to the appropriate preheating parameters.The thermite weld in Improved Condition exhibits an ultimate tensile strength of 860 MPa in the rail head specimen and 798 MPa in the rail foot specimen.Additionally, the elongation of Improved Condition is 6.26% in the rail head specimen and 5.04% in the rail foot specimen.The base rail R260, meanwhile, possesses an ultimate tensile strength of 987 MPa and an elongation of 13.27%.Accordingly, it can be observed that the ultimate tensile strength of Normal Condition is nearly half of the standard rail tensile strength.

Slow bending test
Slow bending tests were performed to investigate the quality of the thermite welds.Slow bend testing is required to examine welds' mechanical and physical properties to regulate rail welds' quality [33].Thermite welded joints must pass bending tests with a minimum bending test requirement of 80 tonnes and a minimal deflection under loading of 10 mm.The results of the slow bending test for the Normal and Improved Conditions are shown in figure 22. Table 5 also contains a summary of the experiments performed under different conditions.The tested sample for Normal Condition broke under the load of 30 tonnes with just a 3 mm deflection.In Normal Conditions, strength needs to be approximately tripled, or around 50 tonnes difference, in order to attain the  minimum load required to meet the standard.In contrast, the sample under Improved Condition remained unbroken until a load of 108 tonnes, causing a deflection of 16 mm.For Improved Condition, the test sample passed the minimum standard and the deflection exceeded 50% of the standard.Testing was stopped because the sample wasn't broken until it reached above the standard load and deflection.The Improved Condition sample could endure a bending load that was almost four times and a deflection that was more than five times greater than under Normal Conditions. Figure 23 displays the slow bending tested samples of Normal Condition (broken) and Improved Condition (not broken).Additionally, the figure shows the fracture surface of a broken rail under Normal Condition.The fracture surface evaluation of the Normal Condition revealed that the defective welds were characterized by an excessive presence of porosities that closely resembled a 'honeycomb' structure within the weld.These defects are mostly caused by inadequate preheating and the presence of moisture in the mold [14].Since the tested rail for Improved Condition was unbroken, there is no fractured surface for Improved Condition.

Discussions
This research presents and discusses the influence of preheating parameters on weld quality.Many preliminary tests have been carried out to get the proper parameters for Improved Condition.To get the desired result, it was crucial to adjust the pressure of the preheating torch and preheating duration.Different pressures were regulated for LPG.The thermocouples were used to record the temperature while the gas pressure was changed in order to determine the ideal preheating period.When the target temperature was obtained at the rail ends before welding, the preheating time and gas pressure were determined for Improved Condition.The temperature of the preheating profile is modified by the increasing gas pressure.The temperature of the rail head was greater than the temperature of the rail foot in the low-pressure setting (Normal Condition-LPG 1 bar and Oxygen 4.5 bar).The temperature of the rail foot was greater than the rail head in the high-pressure setting (Improved Condition-LPG 1.2 bar and Oxygen 4.5 bar).
After preheating the rail ends, there is a slight variation in temperature between the rail head and foot.The difference arises due to variations in the length of the flame that depends on the intensity of gas pressure during the preheating process.Figure 4(a) indicates that the lowest temperature was observed at the rail foot due to the reduced pressure of LPG used under Normal Condition.When casting into the weld joint, the solid metal quickly starts to grow in the center of the rail foot, which starts the solidification process.This quick solidification might result in poor adhesion.As a result, the solidified metal becomes more fragile near the middle of the rail foot [34].Inside the weld had many porosities and blisters.Therefore, the welded rail broke when the bending load was applied to it.Mutton and Alvarez [35] found that most straight-break failures were caused by foot centerline shrinkage defects, which led to a vertical fracture at the weld's centerline.The most likely reason for the weld defect inside the thermite weld was inadequate preheating temperature, according to the analysis of weld failures in previous study.Increasing the pressure of the preheating torch results in a greater  temperature at the rail foot.The Improved Condition led to the maximum temperature on the rail foot, perhaps it also prevents the aforementioned failure.The substance used in mold is another source of porosity.The sand mold is typically used in thermite welding and sand normally contains moisture.When the liquid metal solidifies, moisture from sand that was not thoroughly removed before welding causes gas bubbles [36].When the molten metal from the thermite reaction encounters moisture present in the sand mold, the heat quickly converts the moisture into steam.Since the conversion of moisture to steam significantly increases its volume, this rapid expansion can force the molten metal to move in unpredictable ways, leading to the formation of gas bubbles within the weld as the steam tries to escape.Moisture can also react with molten metal to produce hydrogen gas.This gas can get trapped within the solidifying metal, leading to porosity.Therefore, a proper preheating procedure is crucial for thermite welding.Chen et al [37] also discussed that the longer preheating time can avoid several welding defects such as shrinkage cavity and microporosity in thermite weld.Longer preheating time resulted in higher preheating temperatures.According to Yang Liu [38], there will be fewer residual stresses as a result of a greater preheating temperature.
The welding quality is also impacted by environmental factors like humidity.Humidity is one of the variables to be considered during early tests to determine the ideal parameters for Improved Condition.In tropical areas, after rain and during the rainy season, 80%-90% of the humidity is present.Thermite welding requires a humidity level of between 50 and 60%, according to the findings of the experiments.Under the same settings and procedures, humidity levels over 70% resulted in decreased bending strength.Also, high humidity levels cause the fatigue strengths of welded metal to decrease [39].Thermite welding is a field welding technique; therefore, it is challenging to manage weather conditions.As a result, selecting the appropriate parameters and techniques for preheating has a significant impact on the quality of thermite weld metal.
Both the Normal Condition and the Improved Condition have comparable microstructures and hardness.Pearlite with proeutectoid ferrite represents the microstructure of weld metal under both Normal and Improved Conditions.The presence of microporosities in the weld metal distinguishes the two different conditions.The micrographs display the different amount of microporosities found in the weld metal.Increasing amounts of microporosities in the weld metal are present under Normal Conditions.Weld metal becomes brittle due to the presence of many porosities and inclusions [40].Despite having the same microstructure in both conditions, porosity has an evident negative impact on the bending strength of rail welded joints.
The amount of weld defects in the weld metal is the noticeable difference between Normal and Improved Conditions.Compared to the Improved Condition, the Normal Condition had more gas pores and inclusions in the weld metal.As a result, the bending strength was decreased in Normal Condition, and the fracture surface displayed many blisters along the rail profile.The quality of the welded joint was diminished by the presence of weld defects in the weld metal.Therefore, the micro porosities and inclusions in the weld metal led to lower mechanical strength and failure.Mold moisture is the primary source of formation defects in weld metal.When the mold and rail end surfaces are not adequately preheated, the high humidity environment, particularly in tropical climates, maintains the moisture in the mold.In order to reduce the effects of humidity surrounding the welding operation and its influence on moisture in the sand mold and rail ends, a greater preheating temperature is required prior to welding.This issue can be resolved by using a preheating torch with a higher gas pressure and a longer preheating time.

Conclusions
The following findings are taken from the current study: 1.In the macrostructure, the Improved Condition contained wider weld metal and HAZ than the Normal Condition.The results show that preheating period affects both HAZ and weld metal size.
2. The pearlite microstructure was apparent in the weld metal and HAZ in both conditions.When compared to Normal Condition, Improved Condition reveals significantly less defects in the metal weld.
3. Weld metal hardness was higher than that of HAZ in the Improved Condition.In terms of average hardness, both weld metals are typically equivalent, around 330 HV.
4. During a slow bending test, the Normal Condition sample broke at 30 tonnes with a 3 mm deflection, but the Improved Condition sample remained unbroken until 108 tonnes with a 16 mm deflection.The fractured surface of the Normal Condition showed various porosities and blisters from railhead to rail foot.
5. In the tropical climate region, according to this study, thermite welding is appropriate due to the higher gas pressure of the preheating torch and the longer preheating interval.

Figure 1 .
Figure 1.Schematic of the full-sized thermite welded rail sample for (a) slow bending test, (b) cutting unnecessary base rails, (c) extraction specimen for metallography and hardness test, (d) location of tensile specimens from thermite weld, (e) detail dimension of tensile specimen [Unit of measurement: millimeter (mm)].
Figure 6  demonstrates the absence of any obvious defects in the sample after welding, showing Improved Condition.

Figure 2 .
Figure 2. Preheating torch to rail top surface and the gap between two rail ends.

Figure 3 .
Figure 3. Slow bending test (a) Schematic of testing dimensions and setups (b) Testing machine [Unit of measurement: millimeter (mm)].

Figure 10
Figure 10 compares the microstructure of the weld metals under different conditions.Pearlite microstructure dominates the weld metal microstructure.Some ferrite phases randomly appeared in the weld metal.White color phases in the optical micrograph are ferrite, while dark color phases are pearlite.The weld metal microstructure of Normal Condition has many microporosities, whereas the weld metal microstructure of Improved Condition contains little porosity.Both micrographs were obtained at the center of weld metal.The phases observed in both micrographs exhibit similarities, although the amount of porosity differs.The number of porosities is different between Normal Condition and Improved Condition, as shown by the image analysis results in figure 11.The image analysis was done for the presence of porosities in the weld metal.Figure12shows the microstructure of weld metal near the fusion line of thermite weld.Both the fusion of the parent metal and the initiation of the solidification of the molten metal take place in this region.According to Yang Liu[29], this area was termed the mixed melted zone.Additionally, this area was referred to as the solidified zone by Lesage et al[30].Its grain size is smaller than that of the weld center.Compared to the heat-affected zone, the grain size is bigger.Proeutectoid ferrite phases along the grain boundaries and some intragranular ferrite

Figure 10 .
Figure 10.Microstructure of weld metal at the center of weld (a) Normal Condition (b) Improved Condition.

Figure 12 .
Figure 12.Microstructure of weld metal near the fusion line (a) Normal Condition (b) Improved Condition.

Figure 14 .
Figure 14.SEM images of Normal Condition with different detector modes (a) Secondary electron mode, (b) Backscatter electron mode.

Figure 15 .
Figure 15.SEM images of Improved Condition with different detector modes (a) Secondary electron mode, (b) Backscatter electron mode.

Figure 16 .Figure 17 .
Figure 16.SEM-EDS result of weld defects in the weld centre of Normal Condition (a) SEM image, (b) elemental mapping and (c) elemental spectrum.

Figure 19 .
Figure 19.Comparison hardness profiles of Normal Condition and Improved Condition.

Figure 20 .
Figure 20.Average hardness comparison between different conditions.

Figure 22 .
Figure 22.Slow Bending test result of (a) Normal Condition (b) Improved Condition.

Figure 23 .
Figure 23.Slow bending tested samples of different conditions and the fracture surface of a broken rail under Normal Condition.

Table 2 .
Chemical Composition of parent rail steel R260 and different weld metals (wt%).

Table 3 .
The width of the different areas on the macrograph.
VariablesThe width of the different areas (mm)Weld metal (WM)Heat-Affected zone (HAZ)

Table 4 .
Tensile test results for different conditions of thermite welds and base rail R260.

Table 5 .
Slow bending test result of Normal and Improved Condition.