Carrying out aluminothermic welding of rails - technology, necessary equipment, nuances

The process of welding rails can be carried out in several ways: electric arc, gas press, aluminothermic welding.
The latter technology is the most popular and is considered very effective, although the contact method is also often used. Aluminothermic welding of rails involves the use of a special device - an igniter. Due to a single-portion charge, the temperature regime increases (up to 1000°C), which is suitable for melting all types of rails. Welding rail joints is a difficult process that requires maximum concentration and care. Since the web material contains a lot of carbon, it is considered a difficult-to-weld metal. By making mistakes in creating a rail weld, you can get cracks in the joint. We will discuss the essence of this type of welding in this article.

What is aluminothermic welding

The essence of aluminothermic technology is based on the reduction of iron from oxides when interacting with aluminum oxides. The reaction is accompanied by the release of heat sufficient to melt the metal. For aluminothermite technology, a mixture (thermite) is used, consisting of 23% crushed aluminum and 77% iron scale. To ensure that the characteristics of the weld are close to the parameters of the rail material, alloying elements and steel particles are added to the powder.

The mixture is poured into a crucible placed above the joint. To begin the reduction reaction, the powder is heated to 1000⁰C with an igniter with a single-shot charge. After ignition, as a result of the chemical interaction of the ingredients, the temperature of the mixture rises to 2400⁰C, the reduced iron melts, flowing down, filling the joint gap. To increase the reliability of the connection, aluminothermic welding is completed by sealing the seam with a special press.

How to weld rails

When carrying out installation and repair work on sections of the railway track, as well as in similar conditions associated with laying rail lines, special welding technologies are used.

Features of rail welding technologies are expressed in increased requirements for the operational reliability of connections, as well as their resistance to mechanical loads.

Basic methods

Welding of rail joints belongs to the category of particularly important activities, the organization and implementation of which is impossible without the use of equipment and modern welding mechanisms.

The main types of welding technologies used in the installation and repair of rails are:

• electric contact welding;
• electric arc method;
• thermite treatment (aluminothermic welding of rails);
• modern gas-press welding.

Each of these methods has certain advantages and disadvantages. To become more fully acquainted with them, we will consider each of the listed welding methods in more detail.

Electric contact method

The electric contact approach to connecting rail joints is based on their strong heating and subsequent melting through an electric arc, which is formed by a significant low-voltage current.

To implement the method, special machine complexes are used that operate in automatic mode (MSGR-500, MS-5002 or K-190, for example).

Before welding, the rails to be processed are laid either directly on the tracks, or with a slight offset inside the branch or outside the track (at a distance of about 260 centimeters from its axis).

In this case, the welding mechanism itself moves along the restored thread, that is, it is a self-propelled rail welding station.

During its operation, replaceable contact heads of various types are used, providing the necessary welding modes (continuous reflow or intermittent heating of contacts).

Electric arc method

Non-contact arc welding is one of the most common methods used for mating rail joints.

According to this approach, the rails are first laid with a small gap, after which their ends are welded with metal electrodes melted through an arc discharge. This type of non-contact welding does not require the application of excess sedimentary pressure and is implemented using alternating or direct currents supplied from a mobile welding station.

The most effective way to perform arc welding of rails is the so-called “bathtub” method, according to which pre-cut rails across the longitudinal axis are laid strictly along the track line with a slight elevation and a gap of approximately 14-16 millimeters.

A working electrode is inserted between the ends of the rail blanks laid in this way, followed by passing a current of about 300-350 amperes through it.

As a result of this effect, the molten mass spreads evenly over the gap and completely fills it. To prevent it from flowing out, the gap between the rails is closed with special blocking barriers. Upon completion of welding, the resulting seams are ground over the entire joint area.

Thermite treatment

Aluminothermic technology has been tested by time. The use of thermite welding of rails is based on a reducing reaction that occurs when the base (aluminum) comes into contact with another component - iron oxide.

The resulting metal (reduced iron) at operating temperatures of about 2000 degrees is poured into a special fire-resistant form that matches the geometry of the rails being welded.

This reaction is accompanied by the release of a significant amount of thermal energy.

Welding rails using the thermite method began a very long time ago (from the mid-19th century), but since then this type of welding, due to the use of aluminum, began to be called aluminothermic.

It is important to note that the described chemical reaction after igniting a special high-temperature fuel (thermite) lasts only a few seconds.

In addition to the two components considered (iron and aluminum oxide), alloying additives and small steel particles are introduced into the working welded mixture, which slightly slow down or dampen the ongoing process. Additives are necessary to ensure that the steel in the welding zone acquires the required qualities and parameters characteristic of most rail products.

When considering the features of this type of welding process, it should be noted that upon completion of the reaction, the total chemical mass is divided into two fractions: liquid metal and light slag, which floats to the top of the mold.

Termitan technology allows the following types of track products to be connected to each other:

• surface-hardened rail blanks;
• volumetrically hardened joining parts of rails,
• rails that have not undergone special heat treatment in any combinations.

This type of welding ensures compliance with the requirements of the basic standards for high-speed rail lines, in terms of compliance with welding technology standards.

Gas press method

This welding technology is based on joining metal rail joints at relatively low temperatures (notably below the melting point), but at a sufficiently high pressure.

The main advantages of the gas pressing method include the homogeneity of the material structure in the welding zone, as well as the high strength of the resulting joint.

Thanks to the listed advantages, this method can effectively “cook” even very heavy and large railway products. Before welding, the ends of such rails are tightly joined to one another, after which they are cut simultaneously using a special tool (a rail cutting machine with a circular saw or a mechanical hacksaw).

As a result of preparatory operations, the required tightness of the end parts of the rails with high purity of the metal interface is ensured.

In addition, immediately before welding, the ends are treated with dichloroethane or carbon tetrachloride. At the stage of preparing the rails for welding, their ends are heated to the required temperature using special combination burners that ensure sufficient temperature is obtained.

After thorough heating, the ends of the rails are clamped using a specially designed hydraulic press and continue to heat up to 1200 degrees.

During the welding process, the torch bodies are slightly displaced relative to the joint being processed (they make small oscillatory movements). The frequency of such periodic movements, as a rule, does not exceed 50 vibrations per minute.

Simultaneously with these movements of the gas burner, the rails are compressed by a hydraulic press with a force of 10 to 13 tons, the exact value of which is determined by special calculations. As a result of this treatment, the welded metal at the joint is deposited by approximately 20 millimeters.

To implement the described technological chain, special gas pressing equipment (universal machines) is used.

Upon completion of the entire complex of gas welding operations, the finished joint is thoroughly cleaned of slag, and then returned to its normal appearance (they say that it is “normalized”).

So, the considered key methods for welding rail joints are applied in accordance with the technical requirements and conditions for carrying out repair and restoration activities.

Of all the approaches, aluminothermic welding stands out, as it best meets modern requirements for non-contact restoration of rails or laying railway lines. It is the thermite method that is most often used in the construction and repair of modern transport highways.

Advantages and disadvantages

The popularity of the technology is explained by the fact that aluminothermic welding has the following advantages compared to other methods:

1. High speed of work. The full cycle of creating a connection takes no more than half an hour. The team, using the division of labor method, can apply 10-12 stitches in an hour. This is possible if one carries out the preparation and moves on to the next joint, the other carries out welding, the third processes the connection
2. There is no connection to stationary power sources, which allows the use of aluminothermic technology in field conditions.
3. Expensive materials and complex equipment are not required, therefore the cost of maintaining railway tracks is reduced. Everything you need is available in building materials stores.
4. The wear of wheels of cars and locomotives is reduced.
5. The smooth running of trains is improved.
6. To master aluminothermic technology, it is enough to carry out 2 - 3 welds.

Advantages and disadvantages

This method is used quite often. This is because it has advantages that welders appreciate. Among them are the following:

• Fast operating speed. All stages take no more than half an hour. With a lot of experience, up to ten seams can be processed in 60 minutes.
• Do not use stationary electrical power. This allows you to move the device from place to place.
• All parts of the structure can be found in one place. They are sold in specialized stores.
• To obtain an even connection, you do not need to have a professional education. You need to try your hand at welding several times.
• If you do not forget about the rules of work and adhere to all technologies, you can get a lasting final result.
• The entire process does not pose a danger to workers if high-quality equipment and materials are used.
• Welding work can be carried out both at home, in workshops, and in the open field.

The use of aluminothermic welding involves some difficulties. They can be like this:

• The powder can easily catch fire, so you need to be extremely careful with it.
• Preparatory work is carried out carefully and in detail so as not to miss important nuances.
• The equipment you will work with must be in working order. Check its serviceability before starting welding.
• If drops of liquid get into a hot pan, then the entire mixture will splash. Don't let this happen.

Equipment and materials used

To carry out aluminothermic welding of rails you need to prepare:

• a barrel with thermite, closed with a plug;
• shape in accordance with the rail profile;
• if a weighed mixture is used instead of a barrel, a crucible made of refractory material will be required;
• seam compression press;
• grinding machine;
• hammer, chisel;
• a metal spatula for removing burning slag;
• gas burner for preheating.

Supplies you will need:

• finely dispersed thermite with granules no more than 0.5 mm;
• heat-resistant paste for sealing cracks and crevices;
• propane and oxygen for the burner in cylinders.

How to make thermite yourself

To make your own thermite mixture, you will need rust and aluminum powder. You can get rust at home; if it is wet, you should dry it using a stove. Then you should turn your rust into powder and heat it in a metal vessel. Making thermite yourself requires little effort and expense. Aluminum powder (you can buy it or extract it yourself using a file from a whole piece of aluminum) must be added to the iron oxide. The proportion of rust to aluminum will be 8 to 3.

Aluminothermic rail welding process

Before starting work, you need to make sure that there is enough thermite to completely fill the joint, otherwise you will have to redo everything. The technology of aluminothermic welding of railway joints is carried out in stages in a strict sequence.

Preparatory work

On sections of rails located near the joint, the fastenings to the sleepers are removed, and the distant ones are loosened. This is necessary so that they do not interfere with installation and to relieve metal tension. The areas near the joint are heated with a burner and cleaned of rust. Then the rails are set so that there is a distance of 25 mm between the ends.

The distance between the ends of the rails is 25mm

Rail alignment

Next, the gaskets are removed from all fastenings, replacing them with wedges on both sides. In order for the connection to withstand the load from passing trains without deformation, the rails must be straight horizontally and vertically. Pre-installation is done by eye. Then, knocking out the wedges with a hammer, achieve the required position. For control, a meter-long metal ruler is applied to the surfaces of the rails.

Sequence of work:

Preparatory stage

A significant role in how to weld a rail seam is played by preparation, which involves the following actions: near the joint, the rail fastenings must be loosened, and those located at the joint area must be completely removed. Next, using a burner, the rails are heated and also cleaned of rust. At the next stage, using wedges, they must be straightened both horizontally and vertically. The rails must be installed maintaining a gap of 2.5 mm, then a cutter is brought to them.

Alignment

Initially, you need to get rid of the gaskets of all fasteners, wedges are installed in their places, using a hammer they need to be adjusted to the required condition. Then you need to check how accurately the rails are laid; to do this, use a meter ruler.

Installation

At this stage, it is important to select the desired sealed form, then the burner must be placed in the middle in the area where the seam is expected to appear. You also need to seal the gaps; to do this, the mold must be pressed as tightly as possible against the seam; a spatula is also used here, which ensures a clean edge between the seam and the sand.

Heating and welding

Pre-heating of the rails is carried out with a burner at the following parameters: propane pressure – 1.5 bar, oxygen pressure – 5 bar. The flame is supplied for 1.5 - 2 minutes. When heating is complete, the burner is removed, the core is inserted and the crucible is turned. Then, at the site of the intended joint, a special barrel filled with charge is installed. In order to ignite the charge, a high-temperature fuse is used, then the liquid metal is placed in a mold and held for 4-5 minutes.

Sanding and quality control

At the end of the procedure, the joint is processed along the edges to a smooth state: while it is hot, the sagging is cut off.

Next, using abrasive tools, you need to sand the seam. To check the quality you need a ruler and a feeler gauge. It's important to keep things as straightforward as possible. The quality of the seam is checked by static bending, everything is done using measuring tools.

Aluminothermy

To weld rails using the aluminothermic method, aluminum powder and RedO iron oxide are used. Make up a thermochemical equation if the formation of 1 kg of iron produces 6340 kJ of heat. [p.35]

Aluminothermy produces ferrovanadium, ferroniobium and ferrotantalum. Pure vanadium metal can be reduced by calcium reduction in a steel bomb. The resulting particles of vanadium metal, after washing, are fused into an ingot in a vacuum furnace. The metal obtained in this way contains up to 99.9% vanadium and has good ductility. Niobium and tantalum can be obtained by thermal decomposition of pentaiodides or pentachlorides at 2000 C or reduction with metallic sodium or potassium. [p.371]

This property of A1 is the basis for the method of reducing metals (Cr, Mn, W, V, etc.) from their oxides - the aluminothermy method discovered by N. N. Beketov (1859). [p.277]

Relatively pure chromium is obtained by aluminothermy [p.373]

Aluminum metal is used primarily for the production of alloys. Aluminum alloys are less resistant to corrosion due to the occurrence of galvanic microelements in places where impurities are included. Aluminum is used to produce cables, foil, mirrors, and silver paint. The ability of aluminum to reduce metals from oxides at high temperatures served as the basis for the method of aluminothermy, i.e., the reduction of refractory metals, such as chromium or manganese, from their oxides [p.152]

In terms of content in the earth's crust, chromium is a fairly common element. The main chromium ore is chromium iron ore PeO-CrOz. Elemental chromium can be obtained by aluminothermy from chromium (III) oxide [p.273]

Experiment 9.3. Carrying out aluminothermy [p.145]

Aluminothermy is the name given to reactions occurring between metal oxides and aluminum, with the formation of the corresponding free metal, aluminum oxide. Aluminothermy can produce only those metals whose heat of formation of oxides is less than the heat of formation of aluminum oxide. Metals such as Cr, Mn, T are obtained in industry using this method [p.334]