Thursday, March 29, 2018

Welding of Rails

Welding and Its Necessity

Joining of two rails ends by the application of heat is called “WELDING”

A fish plated joint has been a necessary evil in the track. To over come the various problem posed by the fish plated joint such as battering, hogging, rail end
failure, noise pollution, distortion of track geometry etc and to reduce the maintenance cost welding is best solution.

Types of Welding:-

1. Gas Pressure Welding (Oxy-Acetylene Welding)
2. Electric Arc Welding
3. Thermit Welding
4. Flash Butt Welding

Alumina-Thermit Welding:-

General Principle:- When a mixture of finely divided aluminum and iron oxide (called thermit portion) is ignited, a chemical reaction takes place with evolution on heat producing iron and aluminum oxide (slag).

• 3Fe3O4+8Al=4Al2O3+9Fe (3088OC,719Kcal)
• 3FeO+2Al=Al2O3+3Fe (2500OC, 187Kcal)
• Fe2O3+2Al=Al2O3+2Fe (2960OC, 181Kcal)
(159 gm)+(54 gm) = (102 gm)+(112)gm

The reaction is exothermic and takes 20±3 seconds to achieve a temperature of about 2500ºC. The iron fuses with preheated rail ends and forms a continuous fusion welded joint and aluminum oxide being lighter floats on top and forms the slag. The strength of weld is about 56% of strength of parent rail.

Types of A.T. Welding

1. Conventional Welding
In this method the process involves use of green sand mould hand operated compressor is used for side heating. It takes 45 minutes for preheating. This method is not in use now.

2. S.K.V Welding / SPW (Short Pre-heat Welding, the translation of German phrase Schweissverfahren Mit Kurz Vorwärmung)

 
Detailed Procedure of A.T.Welding

1. Preliminary Work

a) The fastenings of at least 5 sleepers on either side of weld purposed are  removed.
b) The rail ends are cleaned with K-oil and wire brush in 150mm length.
c) The burr if any is removed from rail ends.
d) The gap between rail ends is kept 25 ± 1mm.
e) Both the rail ends are aligned horizontally and vertically with the help of 1m
straight edge. The tolerances are as given below-

In Lateral Direction ±0.5mm.
In Vertical Direction ±1mm.

f) In order to give allowance for differential cooling the rail ends are raised by 1.5mm to 3mm for 72UTS rail and 1.2mm for 90 UTS rail.

2. Pre-heating of Crucible:-

Crucible is a metallic pot lined with refractory sand (magnisite / crushed alumina
slag). The crucible is cleaned first and then preheated at temperature 600ºC to wipe out all moisture.

3. Fixation of Mould:-

Pre-Fabricated mould made of high silica sand (confirming to IS-1987) mixed with sodium silicate. This mixture is hardened by CO2 therefore these mould are
also called CO2 mould or Dry Mould.

The mould is placed into prefabricated mould shoe of M.S. sheet attached with rails profile. It is ensured that the centre of mould should coincide with centre of
gap. Both halves of mould are held with mould pressure clamp.

4. Luting of Mould:-

The surface of mould coming in contact with rail profile is sealed with luting sand to avoid any miss run of heat and weld metal.

5. Pre-Heating of Rail Ends:-

The vaporizer (burner) is kept 40mm above rail top. The pre-heating is done to a
temperature of 600°C ± 20°C. This temperature can be measured with the help of
pyrometer.


6. Loading of Crucible:-

The crucible is placed on crucible stand provided fork and ring opposite to vaporizer stand. The position of crucible is such that the tap hole and pouring gate of mould are in one line and with a vertical distance of about 50 mm. The thimble is used to ensure faster pouring of liquid metal


7. Plugging of Crucible:-

A closing pin is provided in tap hole at the bottom of crucible. Tap hole is covered by a layer of asbestos powder to avoid any contact of molten metal with closing pin. Asbestos powder being light is covered with slag powder to avoid displacement of asbestos powder. The thermit portion is now unloaded in crucible. The igniter (sparkler) made of aluminum and barium peroxide is placed
at the top of portion in crucible.

8. Ignition of Portion and Tapping:- 

The portion is ignited using igniter and crucible cap. The crucible cap is replaced. Now exothermic reaction takes place upto following duration.


After reactions subsides about 3 seconds are given for separation of slag from weld metal which may be checked by coloured glass. The molten steel fills the gap between 2 rail ends.

Demoulding and Stripping of Extra material:-

Demoulding is done on solidification of molten metal after 4-6 minutes if trimming is to be done by weld trimmer and 3.5-4 minutes if trimming is to be done by chiseling. Care is taken that there should not be any under cutting only 1- 1.5 mm extra metal is left for grinding. The runner and riser protruding from the foot of the rail shall be tilted away to facilitate flange clearance the train shall be allowed only after 30 minutes of pouring of weld metal.

10. Grinding:- 

The grinding is done in two steps

• Rough Grinding:- Rough grinding is done on the same day by using rail profile weld grinder.
• Final Grinding: - Final grinding must be done within 24 hrs of welding. After grinding the finished weld should be brought within following tolerances 
• Vertical alignment: Variation not more than +1.0 mm, -0 mm measured at the end of one metre straight edge.
• Lateral alignment: Variation not more than ± 0.5 mm measured at centre of one metre straight edge.
• Finishing of top surface: + 0.4 mm, -0 mm measured at the end of 10 cm straight edge.
• Head finishing on sides: ± 0.3 mm over gauge side of the rail head measured at the centre of 10 cm straight edge.

11. Marking:-

Each weld is serially numbered in a kilometer indicating month, year, agency, welders code and weld number. An aluminum strip of 30 x 100 mm is pasted on the web of rail with a epoxy adhesive at 300 mm away from the joint.

12. Painting of Thermit Welds:-

The weld is painted 100mm on either side to protect from corrosion. Subsequent painting is done every year in corrosion prone area and once in 4 years in other areas






Precautions to be taken for Good Weld Joints

1. Trained welding supervisor
2. Trained welder.
3. Supervise not more than two welding team (With in 50 m).
4. Portion matching type & chemistry.
5. End squarness & verticality .
6. Perfect alignment .
7. Perfect cleaning of ends.
8. Stop watch.
9. Pressure in the tanks.
10. Correct gap at the head, web & foot.
11. Correct preheating time.
12. Cleaning of nozzle of burner.
13. Correct tapping & mould waiting time.
14. Gloves and coloured glasses.
15. Don’t use damp mould.
16. Lateral wear ≤ 6mm.
17. No weld should lie within 4 m of any other welded/fish plated joint.
18. Don’t use moist portion.
19. Approved agency, supplier.
20. No welding is done when it is raining.
21. Rail surface on either side to be covered by rail guards.
22. A thermit weld done in situ shall be joggled fish plated with two clamps till
tested as good by USFD. (CS.no 99)

WIDE GAP WELDING:-

In railways has recently adopted the new technique of welding to remove defective weld and quick restoration of traffic. The Defective weld is removed by creating a gap of 50mm or 75mm and executing only 1 weld of 50mm or 75mm respectively.

• 50mm wide gap welding
Defective weld is cut creating a gap of 50±1 mm. Rail tensor is applied to maintain the gap during welding. The remaining procedure of welding is same as 25mm gap SKV welding.

• 75mm wide gap welding
Defective weld is cut creating a gap of 75±1 mm. Rail tensor is applied to maintain the gap during welding. The remaining procedure of welding is same as 25mm gap SKV welding.

Weight of Portion --------- 22 Kg
Pressure ---- LPG --------- 2-4.5 Kg / cm²
Oxygen ---------7-8 Kg / cm²


Advantage of Wider Gap Welding

• Insertion of rail closure is eliminated.
• Quick restoration of traffic can be done.
• Technique is easier and cheaper in cost.
• Welding is done by competent welding team.
• Superior quality of welding is obtained

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