Thermit (Aluminothermic) Welding — Identify the Key Advantages In fabrication and track maintenance, thermit welding produces molten steel at the joint using a chemical reaction. Which set of benefits correctly describes why this process is preferred for heavy sections and field repairs?

Introduction / Context:
Thermit (aluminothermic) welding is widely used for joining heavy sections such as railway rails and large steel components. The process forms molten steel in situ by an exothermic reaction, then feeds it into a properly prepared mold around the joint. Understanding its advantages clarifies where and why it is selected over arc-based methods.

Given Data / Assumptions:

• Chemical heat source: aluminum powder reacting with iron oxide to produce molten steel.
• Molded joint geometry with preheating of the ends to be joined.
• Focus on metallurgical and thermal characteristics of the resulting weld zone.

Concept / Approach:
Because the weld metal is poured as a liquid into a mold, the entire joint cross-section is filled with molten metal almost simultaneously. The mass cools more uniformly than a narrow arc-fused bead, which helps reduce steep thermal gradients and the associated residual stresses. The large heat reservoir and controlled feeding further minimize porosity and shrinkage problems when the mold, gating, and preheat are correctly designed.

Step-by-Step Solution:
Recognize that the process casts a joint rather than fusing a thin bead.Uniform molten pool formation → relatively uniform cooling.Lower thermal gradient → reduced internal residual stresses.Therefore the most comprehensive choice is “all of the above”.

Verification / Alternative check:
Railway practice routinely uses thermit welding to achieve full-section joints with good stress distribution compared to localized arc passes.

Why Other Options Are Wrong:
Choosing any single advantage ignores the combined benefits that drive adoption.“None of these” contradicts well-established thermit welding characteristics.

Common Pitfalls:
Assuming thermit welds automatically have zero residual stress; they are minimized, not eliminated, and proper preheat and mold design still matter.

Final Answer:
all of the above