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Introduction to Copper Thermite Welding Bond1.

INTRODUCTION

Recent years have seen upgrading in railway signaling
systems aimed at safe and stabilized rail transportation.
One of the core components of railway signaling systems
is electric circuit called a track circuit using rails as electric conductor, which serves to detect existence of trains
when the rails are short circuited by the axle shaft of atrain.
An electric conductor directly jointed to the rail thereby
constituting track circuits is called a bond. Althoughbonds are mainly jointed to the rail by soldering conventionally, it has been found that jointed bonds frequently
fall off from their joint due to shock and vibration at thetime of train passage, resulting in malfunction of the track
circuit and thereby creating one of the major factors thatprevent stabilized transportation. Copper thermite welding
bond, with its superior mechanical strength and stabilized
work performance, has been favorably reevaluated as acountermeasure against the falling-off of bonds, and has
been employed by many domestic railroad companies
achieving many successful results. Outline of the copperthermite welding bond will be presented here.

2.

2.2 Use Environment
Since it is directly jointed to rails, the bond is exposed tointense vibration of the rail at the time of train passage.
Although intermittently, the vibration lasts for a long time,sometimes exceeding 300 G in acceleration, which isunquestionably very severe for this kind of electric conductor equipment.

3.

Copper Thermite Welding Bond

The outline of copper thermite welding bond of Furukawa
Power Components will be presented here.
3.1 Product1) Copper Thermite Welding Powder
Copper oxide and aluminum powder are encapsulated

BOND FOR TRACK CIRCUITSThe type and use environment of track circuit bond will bebriefly described as follows.
2.1 Type of Bond
Rail for railroads serves not only as a track on which trains
travel, but also as a conductor like electric wire to guidedifferent kinds of currents. Electric currents on the rail
track include the return current from electric train motors,
as well as the signal current used for train operation control such as location detection and speed control.
The bond for the former, i.e., for return current use iscalled a rail bond and for the latter, i.e., for signal current
use is called a signal bond. More specifically, as shown in
Figure 1, a rail bond is used to electrically connect
between rails to effect smooth conduction of the returncurrent. On the other hand, as shown in Figure 2, a signal
bond is used to branch the signal current on the rail tolead the current to the lead wire of control equipment.

Figure 1

Figure 2

Rail bond installed on a rail.

Signal bond installed on a rail.

Furukawa Review, No. 33

200818Introduction to Copper Thermite Welding Bondin a ceramic container. Figure 3 shows the appearance.
2) Bond
The bond comes in different conductor sizes andforms according to application and current capacity.
Figure 4 shows the appearance.
3) Mold and Clamp
It consists of a crucible for combusting copper thermite welding powder and a clamp for fixing the crucible to the rail. Figure 5 shows the appearance.
3.2 Principle
The copper thermite welding is a welding method thattakes advantage of a thermochemical reaction called athermite reaction. In this reaction, the copper thermite
powder comprising copper oxide and aluminum powder
is ignited in the crucible and the generated heat is used tojoint the bond to the rail. The welding process is shown inFigure 6.
3.3 Welding Procedure
Figure 7 shows the welding procedure.
3.4 Features
1) As illustrated in the welding procedure above
described, the jointing work on the rail is simple andskill-less.
2) The molten copper generated by the thermite reaction
reaches a temperature as high as approximately
2,000℃. Due to this high temperature, the bond terminal and the rail matrix partially melt to make a reliable
joint, so that the bond joint has a high mechanicalstrength leading to superior durability against shockand vibration at the time of train passage. On the other
hand, unless treated by suitable means, the rail matrix
at the bond joint is subjected to rapid heating andcooling, so that a residual tensile stress is generated
on this spot resulting in disadvantageously aCopper thermite
welding powder

SetupCombustion

Crucible
Runner channel

Figure 3

Thermite welding powder.

BondIgnitionMelting

Moltencopper

Fusion weld

Figure 6
Figure 4Thermite welding process.

Typical rail bonds.
① Grinding of rail

② Preheating of railand crucible⑤ Welding completedFigure 5

Mold and clamp.

Figure 7③ Pouring of welding
powder

④ Ignition

Welding procedure.

Furukawa Review, No. 33

2008

19Introduction to Copper Thermite Welding Bonddecreased fatigue resistance.
Remedial Measures Proprietary to Furukawa
Power Components for Improvement of Rail
Fatigue Strength
To summarize and repeat the description above, the hightemperature copper thermite welding has, in spite of its
strong welding strength, thermal effects on the rail tolower the fatigue strength. It is said that this is the reason
why copper thermite welding failed to be widely used inrail track applications. Furukawa Power Components hasdeveloped a new method to suppress the decrease in thefatigue strength of welded rails, thereby succeeding in
expanding the application area of copper thermite welding for rail tracks. In this method, depressed areas areformed around the bond that is welded on a rail, thus
removing the residual tensile stress on the welded portionof the rail so as to improve the fatigue strength. The
depressed areas are formed by impact blowing and compression. The method is called “peening”, and is standardized as shown in Figure 8. The peening method has
expanded the application of copper thermite welding
bonds, resulting in the widespread use of today.

Depression

3.5

RailBondFigure 84.

Peening tool

Peening method.

CONCLUSION

Although, as mentioned at the beginning, the copper
thermite welding bond has received a certain extent ofrecognition as a countermeasure for reinforcing track circuits, it is still a developing product. Installation works on
the rail are usually done at night, so that the working tools
are required to be low in noise, non-power driven andlightweight. However, it is true that the welding tools ofFurukawa Power Components have to be improved still tomeet these requirements. We intend to focus on theimprovement of the welding tools, thus responding to thecustomers’ requirements.
For more information, please contact:
No. 5 Sales Department, Furukawa Power Components		 TEL: +81-46-869-3491
FAX: +81-46-869-3494

Furukawa Review, No. 33200820