Question

The welding process used for joining railway tracks is

• A. TIG welding
• B. Spot welding
• C. Submerged welding
• D. Thermit welding

Hint:

Whenever a question references railway track alignment, joining rails on-site, or an exothermic chemical powder reaction, the answer is uniquely Thermit Welding.

Answer 1

The Correct Option is D

Concept: Joining thick, heavy structural steel sections on-site, such as railway tracks, requires a process that provides high heat input without requiring heavy external power generation equipment.

• TIG / Submerged Arc Welding: Requires heavy electrical power sources and multi-pass runs, which makes them difficult to implement efficiently across remote railway networks.

• Spot Welding: A resistance welding technique designed for thin sheet metal components.

• Thermit Welding: A exothermic chemical casting and fusion process that uses a chemical reaction to produce superheated liquid steel.

Step 1: Examine the underlying chemical mechanism of Thermit welding. The process relies on a highly exothermic reduction-oxidation (redox) reaction between a metal oxide (typically iron oxide) and a reducing agent (aluminum powder). The standard balanced chemical formula for this reaction is given by: \[ Fe_2O_3 + 2Al \rightarrow Al_2O_3 + 2Fe + \Delta H \quad (\text{Heat}) \] This chemical reaction releases enough thermal energy to raise the temperature to approximately $3000^\circ\text{C}$.

Step 2: Apply the process to railway track joining. The iron oxide and aluminum powder mixture is placed into a crucible positioned directly above the aligned gap between two rail segments enclosed in a sand mold. Once ignited, the reaction rapidly creates liquid iron, which sinks to the bottom due to its high density, while the aluminum oxide slag ($Al_2O_3$) floats to the top. The superheated liquid iron is then tapped into the mold gap, melting the rail ends and fusing them into a continuous joint upon cooling.