Thermit (Aluminothermic) welding method for rail joints
This is a description of in situ welding of rail joints using Thermit welding or aluminothermic welding. In this process, the highly exothermic reaction between aluminium and iron oxides results in the production of molten steel which is poured into a mould around the gap to be welded. The superheated molten metal causes the rails to melt at the edges of the gap to be welded, and it is also the filler metal, so that the material from the rails coalesces with and joins the added molten steel as it solidifies to form a weld. Thermit is the trade name for one of the granular mixtures of aluminium metal and powdered ferric oxide that is used (also known generically as thermite). Ignition of the Thermit is usually carried out by lighting a magnesium ribbon or sparkler. More information below.
Procedures for aluminothermic welding:
- The rails are cut square and the gap to be welded is prepared within prescribed limits1. (If the rail ends are cut skewed, the gap will be non-uniform and the fusion of the rails will be asymmetric.)
- The cut faces are cleaned with kerosene oil and a wire brush to remove rust, dust, or greasy material, etc. (Otherwise, this material may get fused with the weld material and this may render the weld defective.)
- A 1m-long steel straightedge is used to align the running edge2 of the rail head. The rail ends are 'peaked' to accommodate contraction during solidification and cooling of the 'Thermit' steel. If 'rising' of the rails is not done, the joint will sag due to differential cooling of the rail head (where more material is available and hence the cooling is slower) and rail foot after cooling. A sagged joint gives bad riding and becomes a maintenance problem. Such a joint will be subject to larger stresses due to the dynamic augment. For lateral and vertical alignment, wedges are used.
- Stands for crucible and torch are fixed on the railhead, at appropriate locations, on opposite sides of the welding gap and position and the height of the torch stand is checked and adjusted by placing the preheating burner or welding torch on it which is then removed and set aside for later use.
- A set of prefabricated moulds of the appropriate rail section is selected and examined for suitability3. The rail profile of the mould is checked4 by placing the mould against the side of the rail to be welded. If required, small adjustments to the mould profile are made by rubbing the mould gently against the sides of the rail. Then the moulds are placed in the mould shoe (i.e., clamp), seating it properly using luting sand. The placement of the mould should be central over the gap as otherwise while pouring the molten metal, one rail end will get more heat than the other and the fusion of the metal at the other rail end may not be complete. The recess, if any, between the mould and the rail profile is sealed with luting sand. A slag bowl is attached to the mould shoe to collect the overflowing slag and molten metal during the pouring.
- The magnesite lines crucible is housed at the correct height and alignment on the swiveling crucible stand. A closing pin is placed at the bottom over the opening. This pin's head is covered by about 5g of asbestos powder, so that it does not melt in contact with the molten metal and 'auto tapping' takes place.
- The crucible is swung away from the rail and the 'portion' (self-igniting mixture which yields the molten metal) is poured into the crucible, heaped in a conical shape.
- Using LPG (commercial use cylinders) and oxygen (or petrol and compressed air, an older technique, but still in use), the preheating burner or welding torch is lit and the flame is tuned. This torch is placed in its stand which is fixed over the gap, and the flame is directed into the mould through the central opening. The flame heats the rail ends and this is done for a specified time for each rail section and the pre-heating gases employed.
- As the preheating is completed, the Thermit reaction is initiated by igniting a sparkler and putting it into the crucible. The reaction is allowed a specified time and the slag is allowed to be separated from the molten metal. Thereafter, the closing pin is tapped from the outside, thus discharging the metal into the top central cavity of the mould. Thereafter, the crucible and torch stands are removed.
- The excess Thermit steel over the head of the rail (head riser) is removed after solidification (but when the metal is still red hot) by either manual chiseling or using hydraulic weld trimmers.
- The remaining refractory material is removed and the steel vent risers attached to the collar of the foot of the weld are snapped off.
- The wedges, etc., are removed, any fastenings that were removed, are re-fixed and the railhead is ground manually or using grinding machines.
|Time for each activity|
|Nominal Gap||25mm gap||50mm gap||75mm gap|
|Portion weight — 52kg section (kg)||10.8||13.5||22.0|
|Vertical alignment, either side of 1m straightedge||1.0-1.25mm high||1.25-1.50mm high||2.5-3.0mm high|
|Lateral alignment (gauge side), at end of 1m straightedge||0-0||0-0||0-0|
|Heating time with petrol and compressed air at 100-110 psi, 7-7.7kg/cm2 (minutes)||10-12||18-20||20-25|
|Heating time with LPG at 2.0-2.5kg/cm2 and O2 at 7-8kg/cm2 (minutes)||2.0-2.5||2.5-3.0||3.0-4.0|
|Reaction time (seconds)||20±3||20±3||25±5|
|Mould waiting time (minutes)||4-5||6-7||10-12|
|Chipping time — manual (minutes)||4||5-6||8-9|
|Chipping time — weld trimmer (minutes)||0.5-1||0.5-1||0.5-2|
|Train passing time after pouring (minutes)||30||30||30|
|Vertical tolerance for finished weld||±0.4mm at centre of 10cm straightedge||±0.4mm at centre of 10cm straightedge||±0.4mm at centre of 10cm straightedge|
|Lateral tolerance for finished weld||0-0.3mm at centre of 10cm straightedge||0-0.3mm at centre of 10cm straightedge||0-0.3mm at centre of 10cm straightedge|
Such welding is done in a typical traffic block of 55 minutes. After welding, the weld is fishplated using special joggled fishplates and clamps till the weld is tested with ultrasonic rail testing machines and found 'good'. The collar of the weld is painted with anti-corrosive paint as the bulge has been known to collect dripping waste from the open discharge toilets of trains and develop corrosion at the junction of the web of rail and the collar.
1. ±1 mm for 25mm, 50mm, or 75mm gap welding
2. Top and gauge face where the wheel tyre and flange makes contact.
3. It shouldn't be cracked, worn, etc.
4. Sometimes, wear of the rail head leaves gaps between the mould and the rail profile, which are to be sealed with luting sand.
Thermit reaction details Aluminium reacts with iron oxides, particularly ferric oxide, in highly exothermic reactions, reducing the iron oxides to free iron, and forming a slag of aluminium oxide.
3Fe3O4+ 8Al = 4Al2O3+ 9Fe (3088°C, 719.3kCal↑)
3FeO + 2Al ⇒ Al2O3 + 3Fe (2500°C, 187.1kCal↑)
Fe2O3 + 2Al ⇒ Al2O3 + 2Fe (2960°C, 181.5kCal↑)
The various iron oxides are used in appropriate proportions so as to get the correct resultant quantity and temperature of molten steel. Approximately equal quantities of molten steel and liquid aluminium oxide are separated at about 2400°C, after a few seconds of the exothermic reaction. The iron obtained from such a reaction is soft and unusable as a weld metal for joining rails. To produce an alloy of the correct composition, alloys like ferro-manganese are added to the mixture along with pieces of mild steel, both as small particles, to allow rapid dissolution in the molten iron, to control the temperature and to increase the 'metal recovery'. Complete slag separation in a short time and better fluidity of the molten metal is achieved by adding compounds like calcium carbonate and fluorspar, etc.
Pre-heating the rail ends (to about 1000°C) is required to help the poured molten metal in washing away the surface oxidation on the rail ends, as otherwise, the molten metal may chill and solidify immediately on coming in contact with cold rail ends, without washing off the surface oxidation.
The procedure of 'controlled localized reaction' to keep the thermit mixture ignition under control was invented by Dr Goldschmidt and hence the process is sometimes also known as the Goldschmidt process. Individually patented processes have led to different trade names such as 'Thermit', 'Boutte', 'Argothem', etc.