Thermit Welding - Old or New Technology?

By R.S. Johnson - Eur Eng - Sen Mem TWI
Director - Technical Sales, Thermit Welding (GB) Ltd

Aluminothermic welding is a unique joining method, which in many aspects is more akin to casting than welding. Currently, over 130,000 welds are installed in the UK railway infrastructure per year and it is estimated that there are in excess of 2.5 million in track.

The first recorded installation of aluminothermic welds was in Germany in 1895 and the first major installation was in Berlin Tramway in 1901. By 1904, Thermit welds were being installed in Leeds Tram and the first installation in the UK in mainline was on the Great Western in 1926.

Thermit fusion (left) & Thermit Pressure Shim (right) Welds

The aluminothermic welding method was used either as a full fusion weld, where the Thermit steel was cast into the sand moulds, or as the Thermit Pressure weld, where the Thermit steel was only used to heat the rail ends which were then forged together by mechanically applied pressure. A combined method, where the head is welded by pressure, but the web and foot retained the Thermit steel to produce the fusion weld and reinforcing weld collar, was widely deployed in America, and was later modified by introducing a 10mm mils steel shim into the joint in the head, to produce a Thermit Pressure Shim weld.

In 1944 a study of the Welding of rails included a summary of joints in service, reproduced as follows:

Country	Year	Welding method
		Thermit	Oxy Acetylene			Electric
			Seam	With plates	Pressure	Arc	Flash
Argentina	1943					86,774	45,000
Australia	1938	75,000	100			700	40,000
Czech	1935	12,500		30		92	54
France	1935	84,570					10,150
Germany	1938	430,000	30,000				480,000
G.B.	1944	2,200					29,600
Hungary	1935	25,577		24		6,800
India	1939	1,000
S.Africa	1935					21,000
U.S.A.	1942	6,556	3,128		1,861		7,559
Others	1935	2,014		569			56

Welded joints in track are subject to high dynamic loads throughout their service life - many welded joints in track are over 30 years old - and therefore have to withstand wear and fatigue, and yet have to be installed under very difficult site conditions. A correctly installed, reliable, welded joint in track relies on close partnership between the contributing businesses - those manufacturing the products and equipment, those using them, those training the specialized operators, and those responsible for setting and measuring standards - and development is usually pooled between those businesses.

New Developments

As part Thermit Welding (GB) ltd. has traditionally been committed to product development working with both the Railway Track Authorities and the principal contractors, and as part of the world wide Thermit^® Group is able to call on the resources of its Research and Development organization. Prior to privatization, development was principally focused on engineering and technical challenges - now development is targeted at improved reliability and reduced costs - however the following examples of recent or on-going developments illustrate how it is possible to offer both improved performance and control costs.

Fixed Height Preheating

The traditional prescribed method for preheating the joint for both the SkV-F and SkV-L welding processes rely on accurately positioning the preheater in order to avoid an uneven preheat. An additional problem occurs when welding worn or small section rails. The conventional preheating procedure presets the preheater at a fixed distance to the rail surface, irrespective of the rail depth. This results in the heat input into the rail foot varying dependent upon the rail depth, and with heavily worn rail the increased proximity of the flame to the base of the rail results in the moulds melting during preheating. This in turn produces a weld with a distorted or smooth surface to the weld collar, which may result in the weld being rejected by the Track Authority.

In order to avoid the need to compensate for rail profile by modifying the preheat parameters (e.g. reduce preheat time or pressures), Thermit Welding (GB) Ltd has developed a system in which the location of the preheater is fixed with respect to the position of the moulds. In addition to being simpler to prepare, the method automatically compensates for loss of head, or reduced rail depth rail profiles, as the position of the burner is set relative to the rail foot, and not the running surface. This system has been designated the FPH (fixed preheater) method.

The FPH system comprises of a modified SkV-F burner (either acetylene or propane), and a new burner holder, which attaches to the standard universal mounting. Using this system, the preheater will be central to the mould cavity, and vertical in the moulds.

Prior to it being adopted for use on Railtrack, a full approval programme was required comprising of metallurgical and mechanical testing on a range of new and worn rail profiles.

To check that the new system produced the same metallurgical features, weld fusion was measured and compared with standard data on the weld fusion widths in new rail generated on a number of batches of portions. This data was used to calculate the average for the normal preheating method and compared with the FPH alternative.

The approvals trial was extended to cover the SkV-L80 process, in addition to the standard SkV-F, and recently has been further extended to include the new SkV-T (Triple Riser) process. The system has recently been approved by Railtrack for both the SkV-F and L80 processes in new and worn rail and approval is currently imminent for the SkV-T process, applied to 60E1 rail.

Composite Welds - Joining BS110 to 98 FB Rail, and BS110 to 95R Bullhead

A large number of joints are produced between BS110A (or BS113A rail) and 98lb Flat Bottom rail, or 95R Bullhead rail. The abrupt change in rail section, coupled with the difficulties with installing such welds, and the relatively poor support across the joint, has resulted in these welds having an inherent unreliability and require replacement at regular intervals. To improve the performance the weld geometry was reassessed and a new weld designed based on the SkV-L80. In addition to offering improvements by designing a weld collar profile that minimised the transition in rail profile, the adoption of the wide gap system will enable existing welds to be replaced without the need to source lengths of rail. It was also recognised that the majority of the rail within which such welds would be installed, would be worn and corroded, and these features also were built into the mould design.

Tests on the two L80 variants have been undertaken under separate experiments with emphasis on the bend and fatigue performance of the new joint. In addition, recognising the need to join worn rail, the mould for the 110/98 rail combination was designed to weld for 3mm differential wear, in addition to up to 6mm of similar wear.

Fatigue tests were completed on the 110A/95R composite joint and the results are summarised below, where they are compared with the traditional composite joints. The 110A/98 composite joint was assessed initially by bend testing, again using the conventional joint as a comparison. Both types of joint were also subject to extensive metallurgical and ultrasonic assessment methods. The results on the fatigue and bend tests are shown below:

Fatigue tests: BS 113a/BR 95R Bull head Rail

Bend Tests: BS 113A/98FB Rail

Note the three 'Site' specimens were welds removed from track for comparison

Railtrack have issued a temporary approval of both types of welded joint to enable field trials to commence. A small number of welds have been installed in the North East of the UK and further sites have been identified for trial purposes. The trial is due to be completed within six months, after which performance will be reviewed and if necessary, appropriate modifications made to the two process applications.

SkV-T Triple Riser

The traditional SkV-F welding method has been used in the UK since 1977 and 25 years experience with the process, with in excess of 1,000,000 welds installed, has shown where improvements may be made. Using the resources of the Group's R & D organisation, with computer modelling of heat input, improvements have been made to the mould to maximise heat input, both during preheating and pouring. This has been principally achieved by changes to the positioning and the shape of the main vent risers.

The process is focused on extending the weld in the lower half of the rail profile to beyond the geometric edge of the collar re-inforcement - which remains at 40mm wide - in order to ensure complete fusion between the weld, rail, and collar.

Trials in the UK with the process was triggered by the need to offer a high quality, upgraded process for joining 60E1 grade 220 rail, which is currently being installed in both the East and West Coast Mainlines.

The adoption of the process required a full assessment procedure, which was applied initially with oxy acetylene preheating and subsequently with propane. The approval procedure required a test programme, which incorporated:

Measurements of the weld profiles for both processes are presented below:

For oxy-propane preheating:

This process - which is an adaptation of the SkV-F process - is to form the platform for a new product range, which will include changes to preheating (lower pressures), equipment and packaging.

Selective Head Alloying (High Performance Weld - HPW)

When welding wear resisting rails, it is necessary to develop Thermit portions, which produce a weld with equivalent mechanical properties to the parent rail. The traditional method is to amend the chemistry by adjustments to either the base Carbon or Manganese content (or both), or adding alloying elements such as Chromium. By selective alloying it is possible to vary the Chemistry down the weld such that the mechanical properties in the rail head can differ from those in the rail foot. In such a way a weld can be produced which has very high wear resistance, combined with a high toughness in the rail foot.

The HPW weld uses a central plug, which incorporates alloying compounds that melt into the Thermit Steel as the steel is poured into the rail head area of the moulds.

The above examples illustrate how even old technology can continue to develop to meet the changing demands of the Rail industry. A rail weld is in service for a considerable period - usually in excess of 20 years - and therefore must be installed correctly. While manufacturers continue to develop improved products and use new technology to ensure maximum quality, the final weld performance will ultimately be principally dependent upon the installation. Therefore process improvement must also extend to development of skills and competency - these being a combination of welding and foundry practice mixed with a solid grasp of Railway infrastructure construction and materials. The industry is working together to regularize the vocational training and competency assessment of Aluminothermic welding staff, and extending the knowledge and awareness of engineers and project managers with respect to the requirements for effective deployment of the aluminothermic welding method.