What issue can arise in the weld due to movement in the rail during the solidification process?

The presence of movement in the rail during the solidification process of a thermite weld can lead to the formation of a hot tear. As the molten metal begins to solidify, it transitions from a liquid to a solid state. If there is movement or shifting in the rail at this critical time, it can place undue stress on the solidifying metal. This stress may result in fractures or tears within the weld, which are termed "hot tears."

Hot tears generally occur when solidification is partially complete but the solidifying material is still not strong enough to withstand stress or strain. This can compromise the integrity of the weld, making it weaker and more susceptible to failure under service conditions. The phenomenon is primarily associated with the characteristics of the material and the thermal contraction that occurs during cooling. Addressing movement during the solidification phase is crucial in ensuring a sound and reliable weld.