The welding methods described above have been further improved in the recent past by employing new techniques and equipment. These techniques and equipment are described below in detail.
Automatic welding recorder The invention of a welding recorder, for controlling the quality of welding in the flash butt welding method is a recent development. The recorder is able to not only identify the defects in the flash butt welded joint, but also indicate the reasons for the same. A very sophisticated recording system has recently been developed by the German Railways with the help of Siemens. This system gives visible and audible indications whenever any parameter controlling the quality of the weld transgresses the predetermined limits. The recorder helps considerably in exercising proper control on the quality of welding.
Thermit welding without pre-heating A new method of thermit welding rails that does not involve preheating is currently being tested on German Railways. The necessary heat is produced by much larger quantities of a specially manufactured alumino-thermit mixture. This method also involves the use of special prefabricated moulds made from pure quartz sand. This method of welding requires considerably less amount of time due to the prior solidification of the material in the mould. A similar method is also being tried on Indian Railways on an experimental basis.
Flame cutting and welding technique A new flame cutting and welding technique is under trial on Northern Railways. It is proposed that this technique be implemented to get the desirable gap in a welded track or to repair welding where site conditions so warrant. The time taken in flame cutting is only 2 min. as against the 15-20 min. that it takes for cutting by hacksaw.
Welding with 50-mm gap A new technique of welding joints by introducing 50-mm gaps has recently been standardized on Indian Railways. In this technique, a gap of 50 mm is created by cutting the defective joint. The gap is held in position with the help of a tensor. This gap is fitted with a wider mould and heated for 20 min. The welding mixture is taken in much larger quantities and ignited, and after the reaction is complete, the same is poured into the mould. Demoulding is carried out after about 5 min. and a joint with a perfect tolerance is created.
Thermit welded joint with 50% decrease in height of collar At present, experiments are being conducted on Northern Railways to produce thermit welds with a reduction of 50% in the height of the collar. This will help in reducing the joggle in the joggled fish plate. The bottom collar is also likely to be designed as a plate rather than a curve. This will facilitate testing by ultrasonic rail flaw detection (USFD) equipment.
Mobile flash butt welding machines In order to produce good quality welded joints, Indian Railways has recently procured a few K335-type mobile flash welding machines manufactured by Messers Plasser and Theureri as shown in Fig. 16.8.
Fig. 16.8 Mobile flash butt welding equipment
The new machine is self-propelled with a separate electric generator and can weld about 60 joints per day under field conditions. The approximate cost of welding comes to about Rs 500 per joint using this machine.
Rail joints are used to join rails in series and are the weakest links in the track. Fish-plated joints are particularly weak. The continuous hammering at the joints results in the battering and wear of rail ends. Welded rail joints are the most suitable as they satisfy almost all the requirements of an ideal rail joint. There are several
methods of welding rails but the flash butt and thermit welding methods are normally preferred. Flash butt welding is cheaper than thermit welding and also gives better results. Recent developments in welding techniques have made it possible to produce high-quality welded joints.
1. List the requirements of an ideal rail joint. What are the problems that arise due to rail joints? Enumerate the different types of rail joints.
2. Why is the welding rails necessary? Obtain an expression for the force required to prevent the expansion of rails due to temperature variations.
3. Name the different methods of welding rails. Describe any one method.
4. What are the different methods of welding rails? What are their advantages and disadvantages? Discuss any one method in detail and elaborate on the tests conducted to check the quality of the weld.
5. How are rails welded using the thermit welding method? What are its relative advantages/disadvantages vis-a-vis flash butt welding method?
6. What are the specifications laid down on Indian Railways for flash butt welding of rails? What are the tolerances prescribed for flash butt welded joints? Briefly describe the various tests required to be conducted before a welded joint is accepted for use on a railway track.
7. What are the recent developments in welding techniques on Indian Railways?
8. Describe in detail the various steps involved in flash butt welding. What are the various tests for determining whether the welds are of the right quality?
9. What is the principle of thermit welding? Describe the various steps involved in the thermit welding of a rail joint.
10. Write short notes on (a) SKV welding, (b) suspended joint, (c) welding of second-hand rails, (d) automatic welding recorder.