Home > Thermite Welding
We all know that railroad tracks come in certain lengths, such as 12.5 meters, 25 meters, and extended versions of 50 to 100 meters. No matter how long the rails are, they need to be connected end-to-end to form a continuous railroad. This can be achieved by rail joints (connecting rods) or by welding methods. At this moment, we focus on welded rails.
There are four common methods of welding rails. They are flash butt welding, gas pressure welding, closed arc welding, and hot fusion welding. What are the differences between these methods? We have summarized their applications under different construction conditions for your comparison.
| Construction condition | thermite welding | enclosed-arc welding | flash-butt welding | gas-pressure welding |
| Fixed venue | Yes | No | Applicable | Applicable |
| Fixed venue | Not for massive use | For massive use | For massive use | |
| On railway line | Applicable | No | No | No |
| Specially shaped rail end | Applicable | No | No | No |
| During short driving intervals | Applicable | No | No | No |
| No stop (Tramway only) | Applicable | No | No | No |
All in all, railroad thermal welding is considered to be the most efficient method of welding railway tracks. Due to its high efficiency, reliable quality and adaptability, railway thermal welding has become the most popular method in railroad construction. The application of hot welding is very wide. It is used to weld rails in railroad construction. Seamless rails can be made by hot melt welding technology.
Railway hot melt welding is a welding method that uses heat generated by a chemical reaction as a heat source. To weld, the ends of two rails are pre-fixed in a mold and then a mixture of aluminum and iron oxide powders (called thermite) is heated in a crucible. The exothermic reaction produces liquid metal (iron) and slag, which is injected into the mold. The liquid metal flows into the joint and forms the weld metal, while the slag floats on the surface. To adjust the temperature of the melt and the chemical composition of the weld metal, appropriate additives and alloys are usually added to the thermite.
Check the end of the rail. Mainly check the size of the end rail to be welded , and confirm that the end rail is free from damage, cracks and distortion. If there is damage or crack, it must be removed, and the distortion be straightened. When welding old rails, if there is a low joint or damage at the end of the rail head, the part must be cut off. It is recommended to perform flaw detection within 200mm of the welding face If conditions permit.
--Use wire brush or wire wheel to clean the ends of the rail. To cleaning the end face and the horizontal of the rail. The length of the vertical cleaning is not less than 200mm, and the surface deposits such as rust and oil are removed.
--If there is metal edge on the rail to be welded, use a grinding wheel to polish at least 50mm from the rail end, level with the rail surface.
--If there is a track bed, the ramp from the bottom of gap will be slotted at least 100 mm to provide space for the sand mould installation.
--Place a rail temperature gauge on the side of the backlight of the rail. If the rail temperature is lower than 15 ℃, the rail shall be dried and heated within 1 m on both sides of the rail to remove the moisture on the rail surface.
--The rail to be welded is padded, and the support pad distance at the joint as close as possible under the condition of easy operation.
--See the smoothness of the joint from a little distance
--Adjust the size of the rail gap to 26~30mm. Place the steel ruler on the side of the two rails bottom, make sure that the bottom of the two rails are level first. Then put the one-meter straight ruler against the side of the two rail heads, so that the two sections of steel rails are tightly attached to the straight ruler.
--Adjust the amount of camber. Place a meter ruler on the top of the rail, center line, and the weld gap, the height of each end is 1.5~2mm. It must be measured with a sharp-point feeler gauge.
--Check that the rail gap, sharp point, horizontal alignment, and unequal tilt adjustments are made after the first adjustment is completed, until adjustment is no longer needed.
--Mark the both side of welded rail in the fourth rail sleeper bottom and the rail bottom plate, to ensure that the weld size is not shifted.
Need to pay attention to the welding process:
> If the rail end has a low collapse, the low collapse depth is greater than 2mm, length is greater than 20mm, it must be cut before welding.
>If the rail head width is different, the rail waist and rail bottom should be aligned first, gap should be filled with paper card.
>If the height difference between the two ends of the rail joint is less than 3mm, it is recommended to align the rail head, and the height difference is eliminated at the bottom of the rail; if the height difference between the two ends of the rail joint is 3~8mm, it is recommended that the difference in the rail height be half at the bottom , half at the rail head.
--The sand mould must be inspected before installation as it is fragile, exclude whether there is defective, damp, crack, make sure the vent, gate and riser are unobstructed.
--To fix the sand mold by the mould clamp in the end face of the rail to be welded, check whether the mould bottom plate is aligned with the center of the weld gap, and ensure that the mould clamp is clamped without deviation.
--After the bottom mold is installed, check if any changes of rail on the straightness. If there is any deviation, please re-adjusted.
--Tear off the cardboard cover on the sand mould top to prevent objects falling into the sand mould.
--The sealing paste should be sealed twice. The first time sealed to the outer groove tightly. The second time is sealed again at the intersection of the rail and the sand mould.
--Put a proper amount of dry sand in the slag tray, place it at the specified position, and connect the slag tray and sand mould with the sealing material.
During the sand mould installation and sealing process, pay attention to the following 4 points:
>Do not screw the sand clamp too tight, otherwise mould will easily crack.
>The center of the bottom plate is aligned with the center of the weld and cannot be offset.
>The sealing material should in appropriate amount. If too much, is not easy to dry during the preheating process, it will cause welding defects. If it is too little, it will easily cause leakage of molten steel.
>When the weld position is on the curve, place the slag tray on the inside of the curve to prevent slag overflowing.
--Check if there is any damage of crucible before use, whether there is floating sand inside.
--Check if the package of welding portion is damage, and no agglomeration, reconfirm whether the welding portion is match to the rail. Gently rotate the welding portion into the crucible, the welding portion appear conical in the crucible, put high temperature ignitor on the portion top and waiting for welding.
--Cover the crucible with the cap to prevent contamination.
--It should be noted that if there is water in the rail, slag tray or crucible during the preparation, it must be dried first before use.
--Before preheating, adjust the distance 47~53mm between the bottom of heating nozzle and the top surface of the rail. At the same time, the center line of preheating device base is aligned with the center line of the rail, preheating nozzle is vertically downward in the center of the weld gap.
--Open the gas cylinder, adjust the gas pressure, ignite the preheating device and adjust the flame, the oxygen valve opens to the end, flame core length is 20~25mm.
--Place the preheating device on the preheating device bracket, record the preheating time.
--Pay attention to the position of the preheating device during the preheating process, to ensure that the flames on the sides of the sand mold are unobstructed and the height is the same. In the windless weather, the flame height from the risers on both sides is 45~55mm.
--By adjusting the mixing ratio of oxygen and combustible gas in the preheating device, Uniform preheating of the rail end face, when the entire end surface of the rail is brightly red when the naked eye is observed, the preheating device is taken out and the heating is stopped. According to the rail type and the quality of the air source, the preheating time can be adjusted. Usually, the preheating time of 60kg/m rail is 5~6min, 75kg/m rail is 6~8min.
--After the preheating is finished, the gas valve is alternately closed to prevent tempering.
--Prepare the plugging rod during the preheating process
Pay attention to the following 2 points during the preheating process:
>The distance between the gas bottle and the oxygen cylinder is not less than 5m, and the distance between the gas cylinder and the flame is not less than 10m.
>During the welding process, it is forbidden to stamp on the air pipe to ensure safe and smooth of gas. Wire cable, air pipe should be away from flame and high temperature operation area.
--After the preheating is completed, remove the preheating device. Put in the diverter plug.
--Place the filled crucible on the sand mould, insert the high-temperature match into the welding portion at a 45-degree angle. Insert the depth to 1/3 (about 25 mm) ignition the welding portion.
--After the portion is ignited, cover the crucible and take the plug standing aside waiting for pouring. The time from the end of preheating to the ignition of the portion should not exceed 30 seconds.
--From flame appearance to shrinking is the reaction time, 8 to 15 seconds usually. When the molten steel appears at the bottom of the crucible, it is the pouring time, after reaction is completed to produce the molten steel called calm time , usually 6 to 18 seconds.
--1 minute to remove the crucible after the end of the pouring , and the slag should be removed after the slag is solidified.
--Record the reaction time and calm time after ignition.
Pay attention to the following 4 points during the pouring process:
>Before ignition, the flammable materials should be evacuated from the work area, free flow personnel are strictly prohibited to close the area. Do not use a preheating device to ignite the portion directly.
>Do not moving rails during welding.
>Crucible and sand mould after pouring shall not be directly touched by hand to prevent burns.
>During the joint cooling, one shot crucible, sand mould and reaction slag should be placed outside of working area to prevent accidents during the operation process.
--6'30" after the pouring, remove the sand clamp and sealing material on both sides of the weld. Place the rail shear machine on the rail after cleaning, shearing the extra metal in 8’30’’ after pouring completed. Finally, place the extra metal into the abandoned crucible. The large risers on both sides are swayed apart by the riser separation sleeve, and the small riser is bent so that it does not affect hot grinding in the subsequent step.
--The demoulding & shearing time can be adjusted according to the actual situation.
Pay attention to the following 3 points in demoulding & shearing
>Early removal will affect the quality of welded joint. In contrast, late demoulding will make the shearing process more difficult.
>If the rail temperature is lower than 15℃, apply a heat shield for 10 minutes.
> 15 minutes after pouring, the rail aligning device can be removed.
--After the shearing is completed, it can be hot polished immediately. Grinding force can not be too large. After the hot grinding is completed, arching value should be greater than the specified value of 1-1.5 mm to prevent low joints after cooling.
--Cold-grinding will start after the joint temperature is cooled to ambient temperature, during the grinding process, the flatness should be checked several times, to prevent the flatness of the rail head exceeding the standard.
--cleaning the waste, scrap steel, sand mould etc. after the welding, buried or took to the designated place when leaving. It is strictly forbidden to discard in the turnout or along the railway, it is easily causes accidents.
--If the rail temperature is lower than 15℃, the welding head should be kept warm for 10 minutes after welding.
--After the welding is finished, when the joint temperature is higher than 300 °C, it is strictly forbidden to release the train.
--Fill in the welding record truthfully.
| Item | Data | Remark |
| Gap | 26~30mm | - |
| Sharp point | 1.5~2mm | Press one end and increase the height at the other end by 3~4mm. |
| Propane pressure | 0.08~0.10MPa | - |
| Oxygen pressure | 0.25~0.30Mpa | - |
| Flame length | 20~25mm | The oxygen valve must be opened to the end. |
| Preheater height | 47~53mm | - |
| Preheating time | 60kg rail：5~6mins； | According to the temperature can be adjusted appropriately. |
| 75kg rail：6~8mins； | ||
| Time between preheating to ignition | ≤30 seconds | - |
| Crucible remove time | 1 mins after pouring | - |
| Demoulding time | 6’30’’ after pouring | According to the temperature can be adjusted appropriately. |
| Shearing time | 8’30’’ after pouring | According to the temperature can be adjusted appropriately. |
| Remove rail aligning equipment | 15 mins after pouring | - |
| Welding head Insulation time | 10 mins | Take this action when the rail temperature is below 15 degrees. |
| 1 | Welding materials | Thermite Powder | 1Bag |
| Sand mould | 1PR | ||
| One shot cruicble | 1PC | ||
| Luting sand | 1Bag | ||
| High temperature ingitor | 1PC | ||
| 2 | Alignment tools | Rail Aligner | 1PR |
| Spanner | 2PCS | ||
| 1 meter Aligner Ruler | 1PC | ||
| Feeler Gauge | 1PC | ||
| Verticality checker (Square ruler or L ruler) | 1PC | ||
| Steel wedge | ＞10PCS | ||
| 3 | Special preheating unit | preheat gun | 1PC |
| 15m gas pipe with joint(blue orange) | 1 UNIT | ||
| compressed gas regulator for propane | 1PC | ||
| compressed gas regulator for OXY | 1PC | ||
| propane tempering plug | 1PC | ||
| Oxygen tempering plug | 1PC | ||
| stop watch | 1PC | ||
| oxygen | 1 Bottle | ||
| propane | 1 Bottle | ||
| 4 | SPecial material | Side sand mould template | 1PR |
| Bottom sand mould template | 1PC | ||
| Mould clamp | 1PC | ||
| Slag tray | 1PC | ||
| Crucible Cap | 1PC | ||
| 5 | Supporting machines | Generator | 1SET |
| Steel wire | 1SET | ||
| Hydraulic rail shear machine | 1SET | ||
| Rail cutting machine | 1SET | ||
| Rail profile grinding machine | 1SET | ||
| Angle grinder | 1SET | ||
| Straight grinder | 1SET | ||
| 6 | Other supporting tools | Rail temperature gauge | 1PC |
| infrared thermometer | 1SET | ||
| Welding joint insulation cover | 2PCS | ||
| Sunglasses | 1PR | ||
| Riser seperation casting | 1PC | ||
| 7 | personnel requirement | Welding personnel | No less than 3 person |
| Inspector | 1 person |
Formula for calculation molten steel temperature in thermit portion
Initial temperature of reactant: the temperature of liquid steel increase by 0.6℃ for every 1℃ increase in initial temperature of reactant.
Proportion of additives: When the amount of additives increase by 1%, the temperature of molten steel decrease by 25.7℃.
Influence of FeO proportion :The temperature of molten steel decrease when the proportion of FeO in reactant ferric oxide increase.
Thermit reaction speed control method
Research on sand mould
Adjustment of width design of sand mould bottom plate
Adjust size of gap
The welding joint is 20mm and 22mm, the preheating temperature of rail bottom is low, high- temperature thermite hot molten steel cool rapidly at the bottom of the rail. And the aluminum hot molten steel at the bottom of the rail fails to melt the rail base material and solidifies, an unwelded joint is formed.
When the welding joint is 24mm~30mm, the end face of the steel rail to be welded can be heated to dark red by adjusting the preheating time, which can meet the requirements of the welding preheating temperature.
When the welded rail seam is 32~40mm, because there is a lot of thermite molten steel in the weld, during solidification and shrinkage, the alumite molten steel in the center solidifies too late, and the effective feeding of the riser cannot be obtained. In addition, the temperature of the molten steel is high, The amount of solidification shrinkage is large, so microscopic shrinkage is formed in the center of the weld.
Production of Thermit portion
Production of sand mould
The range of thermit welding rail slot is 23~30mm. When the rail slot exceeds 30mm, a 6m or 12m long steel rail must be equipped to weld the two joints.
The aluminothermic welding technology of wide rail slot rail can be welded directly when the rail slot is within a certain range.
Rail thermit welding technology has simple equipment and flexible operation mode. It is especially suitable for in-situ welding for repairing the broken rail and welding of seamless rail turnout across sections. It is currently widely used in China and railway welding abroad.
China started research on rail aluminothermic welding technology since 1952, The ZTK-1 rail aluminothermic welding product developed in 2006, it has passed the technical inspection by the Ministry of Railway Transportation Bureau. In 2007, a study on improving the performance of rail aluminothermic welding joints was carried out. The product testing performance reached the level of similar products to other brands, some of the performances exceeded. At present, the product has been used in domestic high-speed, heavy-duty lines and welding in plateau alpine climates.
In the year of 2010, the large-scale production workshops with 2500㎡and its production equipment was carried out for the supporting construction. A total of 12 lines for sand mould production, crucible automated equipment were put into use, as well as thermit portion automated production line was established, which greatly guaranteed the stability of thermit welding portion.
| Testing items | Germany | France | China |
| Band test | 1375kN | 1466kN | 1435kN |
| Fatigue test | qualified | qualified | qualified |
| tensile strength | 785MPa | 805MPa | 865MPa |
| Microstructure | Pearlite | Pearlite | Pearlite |
| Description | Germany | France | China |
| Gap size | 27~30mm | 25~28mm | 26~30mm |
| Oxygen pressure | 0.45MPa | 0.49MPa | 0.30MPa |
| Propane pressure | 0.12MPa | 0.07MPa | 0.10MPa |
| Preheating time | 3.5-4.5min | 5min | 5-6min |
| Demould time | 4’30” | 5’ | 6’ |
| Shearing time | 6’30” | 7’40” | 8’30” |