METHOD OF IMPROVING FATIGUE RESISTANCE OF THERMITE WELDING OF RAILWAY RAILS

摘要

"Preventing or reducing defects" improves "endurance limit" and "fatigue resistance" of alumino thermit welding of railway rails. "Thermal insulation of two rails on both sides of mould up to one meter length on each side starting from mould wall" retards dissipation of "heat energy that conducts to two rails from weld cavity" into ambient air via surfaces of "heat dissipating zones" of two rails and retains heat energy in two rails. Retention of heat energy in the two rails causes: a) Prevents thermal contraction of "heated and thermally expanded regions of two rails" during "solidification phase of welding" and thereby prevents occurrence of "tearing apart tensile stresses" and thereby prevents "hot tearing" and formation of"centerline defect"; b) Reduces thermal gradient and thereby reduces conductive heat flux from weld cavity to two rails and thereby: i) Causes "increase in depth of melt-back of stick-out rail ends", which prevents "cold-lap defect formation", ii) Demotes "progression of parallel dendritic solidification fronts of rails 51 and 52 towards each other along longitudinal axis of rails", which prevents formation of liquid weld metal pool locked amidst nascent solidus and thereby prevents "shrinkage cavity defect formation", iii) Reduces criticality of "initial temperature of superhot thermite liquid metal tapped into weld cavity" and thereby reduces quantity of dissolved gases in thermite liquid metal, which reduces severity of "microporosity defect". c) during solidification phase of welding increases conductive heat flux to the bottom of mould due to dissipation of heat energy via surfaces of heat dissipating fins present at the bottom of mould-assembly and thereby promotes vertically upward directional solidification of fused liquid metal present in weld-cavity and thereby prevents shrinkage cavity defect formation.