Welding a Rail Joint
The purpose of welding is to join rail ends together by the application of heat and thus eliminate the evil effects of rail joints. There are four welding methods used on railways.
(a) Gas pressure welding
(b) Electric arc or metal arc welding
(c) Flash butt welding
(d) Thermit welding
The detailed descriptions of these methods are given below.
A.Gas Pressure Welding
In this type of welding, the necessary heat is produced by the combination of oxygen and acetylene gases. The rail ends to be welded are brought together and heat is applied through a burner connected to oxygen and acetylene cylinders by means of regulators and tubes. A temperature of about 1200 o C is achieved. At this temperature, the metal of the rail ends melts, resulting in the fusion and welding together of the ends.
The rails to be welded are clamped at the wall by applying a pressure of 40 t pressure, heated to a temperature of about 1200 o C to 1400 o C, and butted with an upset pressure of about 20 t. Then the joint is again heated to a temperature of 850 o C and allowed to cool naturally. It has been seen that this method of welding is cheaper as compared to flash butt welding. The quality of this welding joint is also claimed to be quite good. There are both stationary and mobile units available for gas pressure welding.
The process, though simple, has not yet been adopted on a large scale by Indian Railways. The main reason behind this is its limited output and the difficult and irregular availability of gas. India has only one plant that offers gas pressure welding, which is located at Bandel on the ER (Eastern Railways) and the progress in this plant has been nominal.
B.Electric or Metal Arc Welding
In this method, heat is generated by passing an electric current across a gap between two conductors. A metal electrode is energized by a voltage source and then brought close to another metal object, thereby producing an arc of electric current between the two objects. A lot of heat is generated by this electric arc, causing the two rail ends to fuse or weld. This type of welding can be done using any of the following methods.
(a) Insert plate technique
(b) Scheron process
(c) Enclosed space technique
Indian Railways has recently started welding rail joints using the metal arc process on a trial basis and the performance so far has been satisfactory.
C. Flash Butt Welding
In flash butt welding, heat is generated by the electric resistance method. The ends of the two rails to be welded are firmly clamped into the jaws of a welding machine. One of the jaws is stationary, while the other one is moveable and as such the gap between the two rail ends can be adjusted. It is not necessary to specially prepare the rail ends, though these can be preheated with an oxy-acetylene torch, if necessary. The rail ends are brought so close together that they almost touch each other. An electric current of 35 kA is passed between the interfaces of the two rails, developing a voltage of 5 V. The rails are subjected to a predetermined number of preheats (15 for 52-kg rails and 13 for 90 R rails) before they are welded. A lot of flashing (sparking) occurs and considerable heat is generated by the passage of electrical current between the rail ends. The rail ends are automatically moved to and fro by the machine till the temperature rises to a fusion limit in the range of 1000 o C to 1500 o C. At this juncture, the rail ends are pressed together with an upset pressure of about 37 t and final flashing takes place joining the two rail ends together. The process is so well regulated that any steel that might have been oxidized during the preheating phases gets completely eliminated. The total time taken for welding a joint is 150-200 sec and the loss in rail length is about 20 mm for each weld. In the case of 90 R rails, the total welding time is 161 sec, which includes a burn-off period of 20 sec, on-preheat time of 65 sec (13 × 5 sec), off-preheat time of 36 sec (12 × 3 sec), and final flashing time of 40 sec.
High-quality welded joints are produced by the flash butt welding method. The cost of a welded joint using this method is also quite low compared to other methods of welding. The method, however, can be adopted most economically and efficiently only in a workshop, for which capital investment is required.
The flash butt welding method is the standard method of welding of rails on Indian Railways. Most railways have one or more flash butt welding plants where rails are welded together. The existing plants for Indian Railways are listed in Table 16.1.
Table 16.1 List of welding plants for Indian Railways
The stepwise procedure for the flash butt welding of rails is as follows.
Pre-straightening of rails The rails are straightened before they are welded in order to ensure that the welded rail has a good alignment.
End cleaning The ends of the rails are cleaned for a length of 150-225 mm using electric or pneumatic grinders.
Adjustment of rail ends The rail ends are then brought together in the flash butt welding machine and longitudinally and vertically aligned by suitably adjusting the machine.
Welding The rail ends are then welded in the flash butt welding machine. Most machines on Indian Railways are those manufactured by A.I. Welders, Inverness, Scotland. The important characteristics of a typical machine manufactured by A.I. Welders are presented in Table 16.2.
Stripping As soon as the rails are welded, they are made to pass through a stripping machine, where all the extra metal, called upset metal, is chipped off.
Table 16.2 Characteristics of a welding machine
Hot chipping In case there is no stripping machine available, the extra material on the rail head is chipped off manually using pneumatic chisels while the metal is still hot.
Spray cooling After the hot metal is chipped off, the rails are cooled by spray cooling.
Profiling The rails are then correctly profiled.
Post straightening The rails are straightened in the post straightening machine, which removes both horizontal and vertical kinks, if any, so as to ensure perfect alignment in both directions.
Ultrasonic inspection The rails as well as the welds are examined to ensure that there are no flaws in them. This is particularly important for second-hand rails.
Examination and inspection The rail ends are finally examined and inspected with regard to specified tolerances so that the welded surface has a good finish.
C.1 Output and Cost
The average time taken for welding a joint is about 6 min. for 52-kg rails and 5.5 min. for 90 R rails, and about 70-90 joints can be welded per 8-h shift. The flash butt welding plant at Meerut (Northern Railways) welds about 160 joints per day by working in double shifts. The approximate cost comes to about Rs 350 per weld including overheads, depreciation charges, etc.
C.2 Welding Recorder
The quality of the welding can be checked using a 'welding recorder', which automatically records all the parameters that control the quality of a weld. The following parameters are recorded by this device.
(a) Primary amperage
(c) Butting pressure
(d) Loss of length
A graphical study of the records of these parameters helps in judging the quality of the welding, after which the desired action can be taken if any one of these parameters is found to be improperly regulated. A few of these welding recorders have recently been purchased by Indian Railways and are being used in flash butt welding plants.
C.3 Automatic Flash Butt Welding Machine
Indian Railways has procured a few of the latest superior quality Mark IV type of flash butt welding machines (APHF-60). The new design of this machine permits the welding of rails of sections up to 60 kg/m or above made up of medium manganese and of the wear resistant type. Most of the operations in this machine are automatic. These machines are capable of aligning and de-twisting the rail end to facilitate the formation of high-quality welded joints. The technical characteristics of this machine are given in Table 16.3.
Table 16.3 Characteristics of APHF-60
The new automatic flash butt welding machine has many supplementary machines like th grinding machine, pre- and post-straightening machines, the short blasting or brushing machine for end cleaning, generators, etc. The cost of the ensemble of supplementary machines is about Rs 90 million whereas the cost of the main welding machine is about Rs 40 million. The new welding machine is able to perform most of the operations automatically and the time taken for welding a 52-kg rail is approximately 70 sec. The average output of the machine is 20 welds per hour.
C.4 Manual for Flash Butt Welding of Rails
The code of practice for the flash butt welding of rails has been standardized by Indian Railways from time to time. The latest instructions in this regard are contained in the Manual for Flash Butt Welding of Rails 1994. The manual describes the type and suitability of the rails to be welded and the general procedure to be followed, and enlists the tolerances for the finished joints as well as the acceptance tests the joints must undergo to ensure quality control.
Tolerances for flash butt welded joints
Each completed flash butt welded joint should be checked for its straightness, alignment, and finish using 1-m and 10-cm-long straight edges. The permissible tolerances are given in Table 16.4. These tolerances also apply to thermit welded joints barring the web zone, where the tolerance specified is +10 mm and -0.0 mm.
Table 16.4 Limits of tolerances
Testing of rail joints
A rail joint should be tested for its strength and hardness before it is considered acceptable for use in railways. The following tests are prescribed on Indian Railways.
Transverse test One joint should be tested using the transverse test daily before work starts in all flash butt welding depots where there is no provision of welding recorders. In depots where recorders have been provided, one in every 1000 joints should be tested using the transverse test.
In the transverse test, a 1.5-m-long test piece with a weld in the centre is taken and placed on two cylindrical supports that have a diameter of 30 to 50 mm and are placed 1 m apart. When pressure is applied in the form of a load at the centre of the test piece, it should show the minimum recommended deflection without any sign of cracking (Fig. 16.6).
Metallurgical test A macro graphic examination of the flash butt weld is done after every 5000 welds. This test checks the presence of any porosity due to cracks, slag inclusion, or other welding defects.
Hardness test A hardness test may also be carried out for the welded head affected zone. The Brinell hardness number (BHN) should be between 210 and 250, presuming that the BHN of the parent rail is 230.
Ultrasonic flaw detection Every joint is USFD (ultrasonic flaw detection) tested using normal 45 o /37 o , 70 o , and 80 o probes to cover the head, web, and foot.
Fatigue test Testing is done for two stress ranges with a 20% reversal. +27.5 kg/mm2 to -5.5 kg/mm 2 (range 33 kg/mm2)
+25 kg/mm2 to -5 kg/mm 2 (range 30 kg/mm2)
Welding of second-hand rails
Second-hand rails can be welded conveniently in flash butt welding depots after being cropped for use on branch lines. European countries implement the welding of second-hand rails on a large scale in order to economize. The aspects that require particular attention in the welding of second-hand rails are as follows:
(a) Checking of the dimensions of old rails as per specifications
(b) Matching the old rails
(c) Sawing the rail ends
(d) Planing the rail head
(e) Permissible wear of the rails to be welded
(f) Marking the gauge side
(g) Ultrasonic inspection of the rails
DD. Thermit Welding of Rails
This is the only form of site welding which is being adopted universally. The method was first developed by Gold Schmidt of Germany towards the end of the nineteenth century. A code of practice for welding rail joints using the alumino-thermic process has been developed by Indian Railways. The code defines the method of welding and the precautions and steps to be taken before, during, and after welding for the production of satisfactory weld joints.
D.1 General Principles
The principle behind this process is that when a mixture of finely divided aluminium and iron oxide, called thermit mixture, is ignited, a chemical reaction takes place which results in the evolution of heat and the production of iron and aluminium oxide:
Fe2O3 + 2Al = 2Al2O3 + 2Fe + heat
In this reaction, 159 g of iron oxide combines with 54 g of aluminium to give 102 g of aluminium oxide, 112 g of iron, and 182 kcal of heat. The reaction is exothermic and it takes about 15-25 sec to achieve a temperature of about 2450 o C.
The released iron is in the molten state and welds the rail ends, which are kept enveloped in molten boxes. The aluminium oxide, being lighter however, floats on top and forms the slag.
D.2 Different Types of Thermit Welding
There are two types of alumino-thermic welding processes sanctioned for the welding of rails on Indian Railways. These are conventional welding and SKV welding. SKV is the short form of the German phrase 'Schweiss-Verfahran mit Kurz vorwarmung' meaning the short preheat welding method. The technique is therefore also termed SPW (short preheat welding). The Railway Board, as a matter of general policy, has decided that the SKV welding technique should be introduced as soon as possible on the Indian Railways. Table 16.5 gives the details of these two types of thermit welding.
D.3 Thermit Welding Operations
Thermit welding involves the following operations.
(a) A special type of moulding mixture is used to create moulds of the rail in halves. For green moulds this moulding mixture is essentially high silica sand mixed with bentonite sieved to the required gradation so that it is coarse enough to permit ventilation. The sand should neither be too dry not too wet. It is mixed with dextrin (a form of molasses) to make it as pliable as desired. The moulds are clamped at the rail joint in such a way that there is adequate peripheral clearance around the rail profile. Normally, green sand moulds are used for conventional thermit welding and prefabricated carbon dioxide sand moulds are used for SKV welding.
(b) After fixing and luting the moulds, the rail ends are heated with a blue flame so as to attain a temperature of 950 o C to 1000 o C for the conventional process and 600 o C for the short preheating process. In case of conventional welding, heating should be continued till the rail ends have turned yellowish red or orange, which can be checked visually through a coloured glass.
An opening is provided in the mould through which heat is supplied by the means of burners that use any one of the following fuels:
(i) air and petrol
(ii) oxygen and cooking gas (LPG-liquefied petroleum gas)
(iii) oxygen and propane.
The time taken for preheating is about 30-45 minutes for conventional welding and 10-12 minutes for short preheating (SKV) welding.
(c) A special type of crucible lined with magnetite is fixed near the rail joint in such a way that, when required, it can be swung round and brought exactly over the joint. A hole is provided in the bottom of the crucible which is plugged with a closing pin and has asbestos wool sprinkled over it to protect it from the molten steel. Powdered slag is then strewn over the asbestos wool so that it lies undisturbed. (Fig. 16.7).
(d) The thermit mixture is then placed inside the crucible. About 4-7 kg of the mixture is required for conventional thermit welding and about 9.0-15 kg of it is required for SKV welding.
(e) As soon as preheating is completed, the thermit mixture is ignited using special igniters made up of barium peroxide and aluminium. A violent reaction takes place in the crucible that leads to the evolution of heat, and the thermit mixture turns into a molten bath. The slag, being lighter, floats to the top and the molten iron remains at the bottom. The reaction takes place for about 15-25 sec and an extra margin of about 5 sec is kept for the separation of the slag. A temperature of about 2540 o C is reached during the process.
(f) The crucible is then swung round and the closing pin is taped up. Molten iron flows down and fills the peripheral area around the mould. The crucible is then swung further and the slag flows out.
(g) The molten thermit steel fuses around the preheated surface of the rail ends and a homogeneous weld is made.
(h) The moulds are removed after about 5 min. When demoulding, only the head, and not the foot and the web, should be exposed. The excess metal is chipped off from top of the rail and the gauge face while it is still red hot.
D.4. Post-welding Operations
The following operations are carried out after the welding of the rail ends is complete.
(a) The rail ends are cooled for 3-4 min.; controlled cooling is required for alloy steel rail joints.
(b) In order to ascertain that the rail profile is correct, the finish of the welded joint is achieved either by the use of hand files or portable grinders.
(c) The welded joint is now ready. The sleepers are shifted to their original positions and properly packed. At least 30 min. should have elapsed since the pouring of the metal before the first train is allowed to pass on the welded joint.
(d) The USFD testing of new welds made by thermit welding should be completed within 30 days of executing the welds.
The thermit process is a very convenient form of welding for times when work needs to be carried out work at the site. No extra power is required and there is enough heat generated during the chemical reaction. The welded joint, however, is found to be weak in strength as compared to the flash butt welded joint. The conventional process has generally been abandoned on Indian Railways to pave way for the short preheat process.
D.5 Short Preheat Thermit Welding Technique
The short preheat welding (SPW or SKV) method has recently been developed by Indian Railways for medium manganese, wear resistant, and special alloy rails. With this technique, it is possible to reduce the total time taken for welding and chipping by about 30 minutes.
The main feature of this technique is that only a length of 3-5 mm at each end of the rails to be welded is heated to a temperature of 6000 o C, as against the heating of the entire cross section of the rail to 1000 o C over a length of 10-15 mm in the conventional method of thermit welding. The large quantity of heat necessary for heating the rail ends is supplied by the use of a large quantity of thermit mixture.
D.6 Conventional Welding Versus SKV Welding
The salient features of conventional thermit welding in the case of 52-kg medium manganese rails versus those of short preheat welding are listed in Table 16.6.
Table 16.6 Important features of conventional and SKV welding
D.7 Precautions During Thermit Welding
In order to ensure the quality of thermit welded joints, the following precautions should be taken.
Follow prescribed procedure Thermit welding of joints should be carried out strictly as per the prescribed norms. The horizontal and vertical alignment of two rail ends require special attention at the joint. In particular, case should be taken to see that the rail ends are square and that their alignment is perfect.
Equipment in good order All the relative equipment and gadgets should be in working order and be available at the site. The important welding equipment and gadgets are rail thermometer, rail tensor, stop watch, 10-cm straight edge, feeler gauge, leather glove, blue goggles, wire brush, slag container, spatula, and first aid box.
Qualified welder Thermit welding should be done only by a qualified welder who holds a valid competency certificate.
Effective supervision Thermit welding should be done only under the supervision of a qualified PWI/PWM (permanent way inspector/permanent way mistry) with a valid competency certificate.
End cropping Second-hand rails should not be welded before their ends have been cropped. The rail ends should be cropped vertically and thoroughly cleaned with kerosene oil with the help of a brush.
Proper gaps In order to get good results, proper gaps should be ensured between the two rails to be welded. The standard gaps recommended are the following:
Conventional welding 11 ± 1 mm
SPW or SKV welding 24 ± 1 mm
50-mm welding 50 ± 2 mm
Adequate block When the conventional method is used for the thermit welding of rails on a running line, the work should normally not be completed in a time block of less than 75 min. In the case of SKV/SPW welds, the same work should not be done in a block of less than 50 minutes.
Use of rail tensor A rail tensor must be used for maintaining the correct gap when thermit welding rails in a decreasing range of temperature and also when repair welding on LWR/CWR (long welded rail/continuous welded rail) tracks. In the case of repair welding, 100 m on either side of the weld should be destressed in order to get good results.
Work to be done on cess In the field, thermit welding should be done on the cess as far as possible to ensure the quality of the welded joints. Luting should be done after ensuring that the moisture content is minimum so as to improve the quality of the weld. In the case of cess welding, rails should be supported by about 10 wooden blocks under each rail seat.
Adequate pressure Welding should normally be done at a pressure of 100-110 psi. The time taken for preheating should normally be about 10-12 minutes.
Use of wooden planks The portion of the rail to be welded should be kept on wooden planks to ensure that moisture does not enter these portions.
Finishing of joint After welding, the joint should be given a proper finish on both the gauge as well as the non-gauge side and any extra collar should be removed in order to enable SFD testing.
Joggled fish plate After thermit welding of LWR, the joint should be joggle fish plated and supported on wooden blocks till it has cleared the USFD test.
D.8 Testing of Thermit Welded Joints
A rail joint should be tested for its strength and hardness before it can be accepted for use on the railways. To this end, the following tests are prescribed on Indian Railways.
Reaction test The characteristic reaction of the thermit mixture when it is placed in a standard crucible is scrutinized to ensure that it conforms to the specified standards. The alumino-thermit steel is extracted out of the melted metal and its chemical composition is determined. The aluminium content should be between 0.3% and 0.7%. The reaction test should be carried out on the mixture for every 250 portions or part thereof.
Hardness test The Brinell hardness test is carried out in welded zones, in heat-affected zones, on the parent metal of the rail, and at the top and sides of the head of the test weld using a 3000-kg load and 10-mm-diameter ball for 10 sec. The average Brinell hardness number (BHN) for welded and heat-affected zones as well as for the parent metal of different rail sections should be as given in Table 16.7.
Table 16.7 Value of BHN for different rail sections
Transverse breaking load test The test weld is positioned on cylindrical or semi-cylindrical supports of diameter 30-50 mm at a distance of 1 m from centre to centre, with the weld placed at the centre of the span and loaded in such a manner that the foot of the rail is in tension. The load is gradually increased till a rupture occurs in the weld. The test weld should withstand the minimum deflection that has been specified for all the different sections and types of rails.
One out of every 100 welded joints should be picked up at random and be subjected to both the hardness and transverse tests. For 90 UTS rails weighting 50-60 kg/m, the minimum breaking load is 80 t with a minimum deflection of 15 mm at the centre. The tolerances for the various dimensions of thermit welded joints are the same as specified for flash butt welded joints.
The important features of flash butt welding and thermit welding are compared in Table 16.8.
Table 16.8 Comparison of flash butt and thermit welding