THERMITE RAIL WELDING: HISTORY, PROCESS DEVELOPMENTS, CURRENT PRACTICES AND OUTLOOK FOR THE 21st CENTURYMetallurgical Engineer Conrail Technical Services Laboratory ABSTRACT This paper provides a broad overview of thermite rail welding as information for those who use thermite welding or those who have an interest in the process.
A short history of thermite welding is included and the process itself is described with emphasis on practical and technical advantages and disadvantages.
Some of the numerous efforts performed by manufacturers and other researchers to improve thermite welding are reviewed and the results are discussed with respect to final weld properties.
Throughout the paper emphasis is placed upon the improvements in thermite weld quality that have been achieved over the years.
Finally, current railroad industry practices for use of the thermite weld are described along with an outlook for the process in the 21st Century.
The author provides a list of challenges for railroads and thermite weld manufacturers to consider in order to help insure that thermite weld performance will remain strong in the coming years.
INTRODUCTION AND HISTORICAL BACKGROUNDAlthough today considered to be standard operating practice, the development of thermite welding technology to successfully join railroad rails in the field can, if the proper definition is chosen, be considered a miracle for the railroad industry.
The authors of Websters Ninth Collegiate Dictionary (Ref.1) provide one definition of a miracle as, An extremely outstanding or unusual event, thing, or accomplishment, and certainly the development of rail welding fits this definition.
A less enthusiastic observer might classify thermite rail welding as an innovation, or perhaps as only significant, incremental technological improvement, but rail welding clearly has been and is very important for railroads around the world.
Thermite welding made possible the installation of continuous welded rail and with this came many well established associated benefits.
Reduced bolt hole rail failures and bolted joint maintenance, increased rail life, better track circuit reliability, reduced equipment wear, a better ride quality and reduced track maintenance costs (Ref.2) are among the benefits that can be directly attributed to thermite welding.
In 1893 Hans Goldschmidt of Germany began to experiment with aluminothermic reactions (highly exothermic processes involving reactions of metallic oxides with aluminum powders) for the production of high purity chromium and manganese (Ref.
This work led to a patent application for the Thermit process in 1895 and sales of chromium quickly increased.
Due to the large amount of heat released by exothermic chemical reactions and the versatility of the thermit process, other applications were quickly found and Goldschmidt started a corporation in 1897.
By the end of the 19th Century, the thermit process had been successfully used to make repairs to large cast and forged steel parts, compression welding using the heat of reaction products had been performed and the first rails were joined.
When the change was made from horse power to electric power for European street railways in the 1890s, the higher speeds and loads of the new equipment resulted in unsatisfactory rail joint performance (Ref.3).
In 1899, the first rail weld was installed in Essen, Germany, where the Goldschmidt Corporation had its headquarters.
Railways quickly saw the advantages of using a fast, relatively simple, field repair method and usage of the process spread throughout Germany.
In 1904 the Goldschmidt Thermit Company was founded in New York and extensive use on street railways in the United States followed.As is the case today, railway engineers in the early part of the 20th Century tried to find the best methods and practices for various processes, including the welding of rails.
A detailed investigation was carried out in the United States by the Committee On Welded Rail Joints which was composed of members from the American Bureau of Welding and the American Electric Railway Engineering Association (Ref.4).
This group had the cooperation of the National Bureau of Standards.
The goals of the work were to improve and standardize the making of welded rail joints.
The committees final report, published in 1932, contains voluminous results from tensile, impact, drop, and bend tests along with results of various weld process parameter tests for rail weld joints produced over the course of a decade.
A substantial amount of baseline data was created and knowledge on the subject of rail welding was vastly expanded and improved.
Steam railroads around the world began to see the benefits of rail welding.
From 1924 to 1930 the German State Railway tested thermite welded rail sections of various lengths and the Krefeld Railway in Germany made 7,000 meters of continuously welded rail (Ref.
Thermite welding also played an important role in the reconstruction of the German railway network after World War II.
In the United States, the Central of Georgia Railroad used welded rail for tunnel trackage in 1930 and the Delaware and Hudson Railroad is credited with the first open-track installation of thermite rail welds in 1933 (Ref.3, 5).
By 1980, it was estimated that continuous welded rail installations represented more than 80,000 miles of main track in the United States (Ref.
Although not all of these welds were made with the thermite process, the aluminothermic method certainly paved the way for rail welding.
Today there are three major thermite weld manufacturers active in North America.
THE THERMITE PROCESS - GENERAL DESCRIPTIONThe chemical reactions associated with the thermite process are highly exothermic and therefore release tremendous amounts of heat which can be used for welding.
The reactions used for todays rail welding processes are between fine aluminum and iron oxide powders which are ignited in a crucible.
The most commonly used reactions are as follows (Ref.6): Eqn.1
Fe + 2Al
2Fe + Al + 181.5 kcal Eqn.
3Fe + 8Al
9Fe + 4Al + 719.3 kcal
The theoretical temperature created by the second reaction is about 5,600(Ref.7), but heat losses due to nonreacting alloy additions, radiation from the crucible, etc., bring the melt temperature down to 3,500F (Ref.6).
When the exothermic reaction in the crucible is completed, about 20 to 25 seconds is required for separation of the slag from the molten steel.
After the slag (mostly aluminum oxide) has floated to the top of the crucible, molten steel is released from the bottom of the crucible.
Liquid steel pours down into the hardened sand mold which has been packed with luting sand and special paste around the two rail ends to be joined.The rail ends, which have been preheated with gas torches, are partially melted by the liquid steel as the mold fills and the weld then is allowed to cool and solidify.When the weld is solid the molds, head riser and base risers are removed and the rail head is finish ground to the proper contour for train traffic.
The thermite rail welding process is therefore essentially a portable foundry which can be brought to almost any location in the field.
Process advantages include portability, relative ease and speed of installation, the flexibility of the process to weld almost any rail sections together, and cost effectiveness.
The main process disadvantages are the many process steps that can be altered by the welder and/or the environmental conditions and can result in poorer weld performance.
These variables include such things as rail end alignment, rail section size mismatch, the rail end gap, quality of the mold packing job, duration of preheat, preheat of the crucible and diverting plug, moisture in the air, time duration until molds are removed, excessive rail motion during weld solidification, etc.
Publications produced by thermite weld manufacturers far more completely describe the detailed steps that are necessary to create a proper thermite weld that will survive in track (Ref.8, 9).Historically it is known that thermite welds contain substantial microporosity and inclusions and this is a contributing cause of their poor ductility and impact toughness (Ref.10).
Myers et al.
(Ref.10) published values for thermite weld sample tensile test percent elongation (1% to 5.6%), tensile test percent reduction in area (1.8% to 3.5%) and charpy impact toughness (2 ft-lb at room temperature).
Since the weld is actually a casting, large columnar grains are found in the microstructure and this is also a major cause of thermite welds being so brittle and tending to fracture in a cleavage mode (Ref.11).
Hauser (Ref.2) stated that the most common reasons for thermite weld failure in service are due to porosity, voids and inclusions in the weld metal, or gouges and local areas transformed to martensite during post weld finish grinding.
PROCESS IMPROVEMENTS AND RESULTSThe increasing axle loads and tonnages experienced by North American railroads have increased the demands placed upon all track components, including thermite welds.
Thermite rail welds have historically been a weak link in continuous welded rail due to their cast microstructure.
However, manufacturers have not been idle and many process improvements have been made over the years and during the last decade in particular
Section 20720 Thermite Rail Welding - Caltrain.com
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Thermite Welding - Vermont Agency Of Transportation
THERMITE WELDING **From Castleton-West Rutland/Castleton AC STP 2705(1)/2908(1)/2909(1) xx. DESCRIPTION. ... Ends of rails to be welded shall be … (caltrain.com)
Welding For The Railroad Industy
ribbon rails, which are then transported by a rail train to the location where they will be installed. ... Aluminum plays a key role in thermite welding, ... (caltrain.com)
The next section of this paper provides an overview of some of those improvements.Weld Hardness and Grain Size ImprovementsIn order to reduce the amount of rail fatigue and wear, rail steels have become significantly cleaner and harder in recent years, particularly with the introduction of new head hardened rail.
Fully pearlitic head hardened rail can be in the 360 BHN to 380 BHN range (Ref.
12), and another rail manufacturer reports hardness values over 400 BHN.
Wear rates for head hardened rail were seen to be far superior to wear rates for standard rail in tests at the Association of American Railroads (AAR) Facility for Accelerated Service Testing (FAST) (Ref.13).
Although there was little correlation between thermite weld batter rate and hardness in the same recent FAST work, higher thermite weld hardness should intuitively perform better.
As a result of customer requests for higher hardness welds to match harder rails, thermite weld manufacturers have produced charges with different chemistries which resulted in harder welds.
In the 1980s, the hardness of most weld charges matched that of standard rail at 285 BHN.
Today several thermite weld charge hardnesses (fully pearlitic) are available in the marketplace as follows: Standard Hardness (305 BHN
20) High Strength Weld - hardness level 1 (340 BHN
20) High Strength Weld - hardness level 2 (370 have also refined the grain size of their welds.
In general, smaller grains possess better strength, ductility and fracture toughness
properties and are preferred over large, cast columnar grains.
Data for older thermite welds reveals an ASTM grain size (weld zone) of approximately 1.5 (Ref.
Data for newer thermite welds (weld zone) show that this grain size has been substantially improved to greater than ASTM 4 (Ref.15).
This has been largely achieved with improvements in charge chemistry through the addition of alloying Crucible ImprovementsThe crucible is the component in which the initial chemical reaction, melting of the charge and slag separation takes place.
As such it must be able to withstand very high temperatures over a number of welds.
A major improvement in the not too distant past was the development of the self tapping crucible thimble.
The thimble automatically releases the molten steel when the proper temperature is reached (Ref.16).
Before the automatic thimble, the molten steel release was a manual effort and thus consistency of slag separation time, hence final weld product, was less than perfect.
Another important development in crucibles came when the one-shot crucible entered the marketplace.
This process variation reduced the necessary pieces of hardware from 13 to 6 and uses a new crucible for each weld.
Advantages of this process include less equipment weight, no need to preheat the crucible and theoretically should reduce the number and/or size of inclusions from the crucible in the weld since the crucible is not used again (Ref.
Development of lighter weight crucibles and associated equipment continues to be important as railroads strive to make equipment safer and more ergonomically friendly for employees.
Mold ImprovementsSince the thermite weld is a casting the weld surface is not expected to be perfect.
However, a smoother surface has fewer and less severe discontinuities which will act as stress concentrations in service.
Such discontinuities can include sand, hot tears, porosity, etc.
Molds produced today for thermite welding are lightweight and made of hardened sand to uniform dimensions.
This is an improvement over early thermite welding molds which were heavy, cumbersome and required more extensive labor efforts such as sand banking (Ref.4).
Todays thermite weld molds are composed of improved refractory materials and have better resistance to heat.Some molds are coated with a zircon wash.
Both of these improvements lead to a better weld surface finish (Ref.18) and less final grinding is needed (Ref.
Also, a significant amount of research work, including finite element analyses, has been performed to find the weld collar design which has the lowest levels of longitudinal and vertical stress (Ref.19).
Manufacturers now offer a variety of compromise molds in order to reduce the amount of rail base mismatch when rails of different section sizes are joined.
Minimization of the stress concentration at the base can reduce the chance for base initiated fatigue failures.
Improvements In Manufacturing Practices The major thermite weld manufacturers have parent firms with a headquarters in Europe which means that the ISO 9000 quality process has been a part of their corporate culture for some time.
This focus has clearly helped to improve their operations in the United States.
Manufacturing standards have been tightened and equipment has been modernized at plants that produce weld charges.
Charge components are now carefully measured to insure that the proper mixture is obtained and all manufacturing information is fully traceable from identification tags on the charge portion bags.
Quality efforts have included standardization of manufacturing practices along with routine slow bend tests of welds made with each production run of a weld charge.
Improvements In TrainingThermite weld manufacturers are well aware of the fact that their products are normally viewed in the light of how many failures occur in service on the railroad.
They are also aware that the railroad thermite welder determines the ultimate fate of the weld when it is installed.
If the weld is installed properly, it stands a much better chance of performing well in service.
However, improper installation will lead to premature sudden failure or longer term fatigue cracking.
In order to combat weld failures due to improper practices, weld charge manufacturers have produced significantly better training publications in recent years (Ref.8, 9).
They also have stepped up their field hands on training efforts in order to insure that railroad welders are aware of best practices.
Representatives of manufacturers work with railroad welding gangs in the field across North America and provide important instruction and advice on thermite welding.
RESEARCH REVIEW-RESULTS AND DISCUSSIONAs mentioned before, much has been written about the poor ductility and toughness properties of thermite welds relative to the rails they join (Ref.2, 10, 11, 20).
A variety of methods to improve properties have therefore been studied by manufacturers and other researchers over the years.
These methods generally seek to reduce porosity and inclusions and/or reduce the final as cast grain size of the weld metal.
If a cost effective method can be found to produce a fine equiaxed grain structure with a minimum of internal discontinuities, properties will improve and service life will likely increase.
Other research work, including efforts to detect defects as the weld is produced, mathematical fatigue modeling and variations of the thermite weld process are also discussed.
Although not every bit of past thermite weld research work is covered, the reader is provided with a working knowledge of what has been done in the field of thermite welding as an overview.
Recent testing of one manufacturers currently produced product shows that the fracture toughness of thermite weld steel is quite close to that of rail steels (Ref.
The reported toughness values are quite similar to results previously published by Australian researchers (Ref.
Earlier test data provided by a researcher in Canada showed that the fracture toughness of a thermite weld was substantially lower (Ref.22).
This highlights the thermite weld charge improvements that have been made by manufacturers in recent years.One method that has been attempted in order to improve properties by reducing the number of discontinuities present in the final weld is the squeeze welding process (Ref.
This process uses the normal thermite welding procedure but adds a mechanical forging step while the weld is still liquid.
Much of the liquid steel is expelled and a narrower weld joint with fewer inclusions and areas of porosity remains.
Tensile tests showed that the ductility (% elongation, % reduction in area) of the weld metal in the base was substantially improved over a thermite weld made with the normal procedure (Ref.24).
Rotating beam fatigue tests showed that squeeze weld samples had better properties than conventional thermite welds.
However, problems with the method include maintaining rail alignment during the forging sequence, removal of excess metal with the shear after welding and poor slow bend test results.
More extensive testing will be necessary in order to develop parameters for creation of consistent welds and to develop equipment that is practical and useable for railroad field welders.It is a well known fact that grain refinements are possible by applying vibrations to solidifying castings.
A cast structure with many more smaller, equiaxed grains will have better properties than one with larger columnar grains.
One study was conducted to investigate the effect of sonic power on the solidification structure of thermite welds joining construction reinforcing bar sections (Ref.
This work showed that sonic power has a strong effect and that the grain size in the thermite weld metal is generally refined by such vibrations and becomes equiaxed.
However, vibrations imparted during the last stages of solidification can cause hot tears in the weld center.
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