Heat distribution in aluminothermic welding is vital for the solidification morphology of the weld. It is influenced by a variety of parameters such as the adjustment (height) of the preheating torch. The video shows the effects of varying the height of the preheating nozzle. The resultant heat distribution at the ends of the rails is visible after 40 seconds, the approximate time lapse between removing the preheating torch and pouring the molten steel into the mould.

A large number of aluminothermic welds, undertaken on a testing site of the Austrian Federal Railways (ÖBB), were measured periodically over a period of some years.

A comparison was made of two of these welds – the first boasted a high, initial quality (according to ÖBB regulations) while the second was significantly inferior.

The temperature of the aluminothermic welding process is generally specified as 2,500 degrees Celsius (°C). This temperature is, however, purely mathematical; the real temperature of the weld metal, when flowing out of the crucible, can be measured metrologically. The temperatures of various aluminothermic portions from varying manufacturers show temperatures (when leaving the crucible) of approximately 1,950 – 2,050 °C (measurement uncertainty: +/- 20 °C).

The aluminothermic steel enters the mould at a temperature of approximately 2,000 °C. The flow of the steel is divided, and runs downwards over the preheated mould walls. 

The temperature drop in the course of contact is negligible.

A sector typically damaged by head checks was inspected using computer tomography. A distinctive feature of the image gained is the crack running at an angle in a downwards direction – a crack that could potentially cause the rail to fracture.

In the course of comminuted rail fractures, the wheel has 'free flight' in the damaged section of the rail until it impacts once more into the remaining end, thereby triggering an impulse which moves as a shock wave through the rail. The support (the first concrete sleeper) collapses due to the overload. The critical zones of tension of the free end of the rail are easy to detect.

Slow bending fracture tests on Vignoles rails are intended to create a homogenous distribution of tensile stress in the area of the rail foot. In the case of grooved rails, such a distribution of stresses can only be achieved when the bending force is introduced in the shear centre of the profile. Transmitting the force beyond the shear centre causes the profile to twist, thus leading to an undesirable distribution of stress, particularly in the area of the rail web.

It is often helpful to show participants of welding courses this short experiment, which provides a simple illustration of how preheating has a positive influence on the stability of the weld.  

Note:

Those who are technically educated and who have a thorough knowledge of metallurgy, are requested to ignore certain simplifications – the end justifies the means; the didactic goal is to raise awareness in participants!