Member of GOLDSCHMIDT Thermit Group

MKR technology (track conductor welding)

MKR technology is applied for Thermit© welding of underground steel track conductors, manufactured according to TR 14-2-788-88 or TR R 27.1-19305558-003:2009 and are considered the typical solution for most underground systems. 

Underground track conductor (or a third rail) is intended for transfer of electricity from railway substation to moving subway cars. This is why its profile is like no other. Track conductors are made of special electrically conductive steel and installed on tracks with brackets, head down. These construction features and rail application define the differences between the specially developed underground track conductor MKR Thermit© welding process and other technologies. 

From the point of location and work conditions, MKR process is versatile---track conductor welding is possible on both the active track (on rails lifted with brackets), and on tracks under construction (when rails are laying on the ground). Overall, the sequence of procedures under MKR process is similar to other processes, with the following features:

  • track conductor welding is performed on the working conductor, i.e. head down;
  • alignment and fixation of rail ends during welding is performed in a special versatile conductor;
  • special molds and thermite mixture are used for welding;
  • the mixture recipe ensures that the metal thermite joint qualities are as close to the qualities of a track conductor steel as possible;

The sequence of procedures of MKR process is as follows:

1. Preparation of open joints and rails for welding 

For works on active track: the protective case is removed, latches are removed from the joint, welded electrical connectors are cut, while rails in the joint are cleared of dirt and rust. 

The rail end is cut with a rail saw, abrasive cutting-off disc in a way that the gap is equal to 24-26 mm.

For works on tracks under construction: rail parts are laid head down along the even surface in a line.

2. Setup of a railway guard and alignment of rail ends 

The rail ends are clamped in a special railway guard, which allows automatically aligning the rails in two planes and securely fixating them for the period of welding works. 

The alignment of rails in the railway guard is carried out at the head (from below), which helps minimize further polishing.

3. Setup of casting molds and crucible 

A special retaining clamp is fixated on the rail, the casting molds are set up in the metal mold frames and fixated at the joint with the use of mounting brackets. 

Molds are then filled with fire sand mixture along the rail outline, while slag cups are hung along the mold frames. 

A disposable euro crucible is installed on a hinged arm above the molds.

4. Pre-heating

Rail end pre-heating up to the temperature of 900-1000 °С with the use of propane and oxygen mixture and a special burner prior to the thermite reaction. 

At that, the pre-heating time equals 6-8 minutes and depends on the rail-heating rate, which is controlled by the welder visually according to the metal emission color of the rail ends. Pre-heating should be uniform across the rails section.

5. Thermite reaction molds casting 

Immediately after the pre-heating, the burner is removed from the mold, and the thermite mixture is ignited with the use of a special lighter. 

After the thermite reaction (within 20-25 seconds), the crucible plug automatically opens up at the bottom of the crucible and the liquid thermite steel is poured into the mold. 

During casting of melted thermite steel into the joint, the rail ends are melted through and thus welded.

6. Burr removal 

After casting the steel is resting in the mold for 3.5 to 4 minutes after the crystallization process is completed. 

After the specified time is over, the mold frames and retaining clamp are removed, as well as the top of the mold, while burr is cut along the running surface of railhead and its side with the use of a weld trimmer and a set of special knives.

7. Joint polishing 

Joint polishing is performed with the use of a polishing machine in two stages:

  • on top of the track conductor base and on the sides---immediately after the burr removal;
  • along the sliding surface of the car’s current collecting device from below and on the railhead sides---after the joint cools off;

Where it’s necessary to lift the rail on a bracket at the joint, seam molding is additionally polished on both sides of the rail web.

The MKR Thermit© welding technology of underground track conductor is an alternative to the traditional method of electric contact welding, and as compared to such it has a couple of advantages, because it allows to effectively solve optimization issues for the whole number of construction and maintenance works while ensuring the equivalent technical result. The advantages of MKR Thermit© welding technology as compared to the electric contact welding are as follows:

 

  • it’s quite easy to organize conductor welding at the tracks under construction practically at any given time;
  • there is no need to plan a complicated and expensive delivery of long strings for existing tracks and be dependent on rail welding company which is usually stationed far away from the construction site;
  • it is possible to weld strings of any length and eliminate any existing bolt joints which helps reduce energy losses and increases the efficiency of traction energy system;
  • it is possible to weld joints at the existing track during repair works or replacement of certain parts of track conductor.


News

Успешно завершен цикл комплексных испытаний в АО «ВНИИЖТ» технологии НОРРА автоматической...

Read more

В период 15-16 февраля в электродепо Московское (ТЧ-3) Петербургского метрополитена были проведены...

Read more

После трудоустройства следует бесплатное обучение профессии Сварщик термитной сварки.

Read more