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SKS technology (crane rail welding)

SKS technology is intended for welding of steel crane rails of special profiles of CR70, CR80, CR100, CR120 and CR140 types, manufactured according to GOST 4121, and of foreign-made profiles of types A45-A150 manufactured according to DIN 536, or types MRS and CR and those used for lifting crane track laying. The profile of crane rails significantly differs from the profile of railroad or other kinds of rails. Compared to others, crane rails have a massive railhead and web (especially width-wise) and proportionate sizes of the base (width- and depth-wise), and smaller ratio between the height and width of the base. These critical distinctions are due to the specifics of crane rail operation and the nature of effective loads, as well as due to design specifics of crane rails of different types. All these factors affect the physical processes of welding and the technical parameters of joints; that is why the special SKS technology was developed and is being applied for the quality crane rail welding. 

Welded joints of crane rails produced through the SKS Thermit© welding technology significantly exceed the quality and durability of joints, produced as a result of traditional electric welding. At that, the SKS welding process is 2.5 times faster, and there are no such flaws in the thermite metal joint as pores, slag inclusions, or lack of fusion. The thermite joint does not require any additional maintenance after welding and polishing along the perimeter of the railhead on top and on the sides. Its service life equals the rail service life.  

Welding under the SKS technology consists of the following steps:

1. Preparation of open joints and rails for welding 

In the open joint area, the rails raise at the height of at least 55 mm from the base and are cleared of dirt and rust. The rail ends are cut with a rail saw, abrasive cutting-off disc in a way that the gap is equal to 24-26 mm. In case of complete replacement of rails or in case of construction of new crane tracks rails are laid on wooden blocks in one line. The required welding joints are achieved through shifting. It is also allowed to cut rails with the use of a burning torch, propane and oxygen mixture in accordance with a template.

2. Alignment of rail ends 

The alignment of rail ends is carried out on the base with the use of grip wedges or special lifting jacks. 

The rails are aligned vertically in a way that the rail bases are laying in the same plane, and horizontally in a way that adjoining sides of the rail base and head were in line.

3. Casting molds setup 

A special retaining clamp is fixated on the rail, as well as a tray for sand and molds. 

Casting molds consist of three parts: magnesite plate and two half molds. The plate is placed under the rail base and is attached to them from below, while half molds in metal mold frames are fixated at the joint on the both sides of the rail with the use of mounting brackets.

The molds are then compacted with special sand along the rail outline, and slag cups are hung on the sides.

4. Preparation of crucible and thermite portion 

A standard disposable euro-crucible is used for welding of the majority of crane rails (or a euro-crucible increased in height for welding of heavy rails of CR140 type and similar), as well as special thermite portions corresponding to the rail profile and the grade of steel that it is made of. 

Crucible is fully ready. Thermite portion is added to it.

5. Rail pre-heating 

Prior to thermite reaction, rail ends are pre-heated up until the temperature of 900-1000 °С. This step is carried out with the use of propane and oxygen mixture and a special burner. Pre-heating time depends on the rail profile and might make up from 10 to 35 minutes. 

At that, the heat rate is controlled by the welder visually based on the metal emission color of the rail ends. Pre-heating should be uniform across the rails section.

6. Thermite reaction molds casting 

Immediately after the pre-heating, the burner is removed from the mold. A crucible is placed on top, while the thermite mixture is ignited with the use of a special match.

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. 

While pouring the molten thermite steel into the clinch, the rail ends are melted through up to 3 mm, the thermite steel of the joint is crystallized, and rails are thus welded.

7. Burr removal 

After the casting, steel is resting in the mold for 4.5...5.5 minutes until crystallization is completed. 

After the specified time, the mold frames, and the retaining clamp are removed, the remaining parts of molds on top and on sides are also removed, while burr is cut along the roll surface of railhead and its side with the use of a weld trimmer and a set of special knives selected for the particular crane rail profile.

8. Joint polishing 

Joint polishing along the roll surface and sides of the railhead is carried out with the use of a polishing machine in two stages:

  • full rough polishing on hot metal, which is performed right after the burr removal;
  • fine polishing, which is performed after the joint cooled off to 200 °С and below. While performing the fine polishing, the smoothness of rail is provided on the roll surface within the specified allowance.

If necessary, the welded joint on the side of rail base is also cleared and polished.

9. Open joint quality control 

After the fine polishing and complete cooling off of the joint, visual quality control of the welded joint is performed. 

Basic control method is capillary with the use of an agent and penetrant set. 

If relevant procedure and technical facilities are available, an ultrasonic quality control method with the use of a portable inspection equipment with a set of sensors can be used.

According to SKS technology, the welding of the crane rail joints can be performed on lifting crane tracks of any type---ground, placed on wooden or reinforced concrete cross sleepers, or placed at height, on solid reinforced concrete or steel bearing elements: in the open air or in shops, as well as on construction sites prior to rail laying works. 

With regard to the maintenance of lifting equipment and bearing elements, one of the key reasons of its excessive wear and early breakage is the presence of non-welded joints in the crane track. In these conditions, increased dynamic loads such as vibration and shakes are generated in the non-welded joints upon the crane movement. That is why many foreign manufacturers of lifting equipment give a crane warranty only under the mandatory condition of crane rails welding on the track that is used. 

The suggested technology of crane rail Thermit© welding---the SKS process---is an optimal choice, which helps create a jointless crane track and eliminate the issues related to increased vibrodynamic loads. Only in this case it is guaranteed that you will get quality and reliable welded joints, which will serve up until the rail replacement, thus significantly reducing the maintenance expenses for crane tracks.

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