The effect of normalization on the structure and mechanical properties of rails aluminotermitic welded joints

The article is devoted to improving the quality of rail aluminotermitic welded joints. The analysis of the causes of rail aluminotermitic welded joints failure is given. The results of an experimental research of the effect of normalization on the hardness and structure of metal rail welded joints are studied. It is shown that without heat treatment the hardness of rail welded joints increases from 24 HRC to 38 HRC in the fusion zone of the weld metal and rail metal. The hardness is confirmed by microstructural analysis. Microstructural analysis showed the differences in the grains sizes of metal welded zone and heat affected zone. The structure of welded metal is acicular dendritic. Owing to a difference between structures of the welded joint zones the probability of occurrence of cracks on the boundary of fusion weld and metal is increased. It is found that the normalization of rail top aluminotermitic welded joints at temperature 850…900 °C provides the formation of ferrite-pearlite structure in the welded zone. The normalization formed fine-grained metal structure with a hardness, which approximate to the hardness of rail in the heat affected zone (along the fusion line). The normalization eliminates the harmful effects of overheating of the metal during welding. It is noted that heat treatment of rail aluminotermitic welded joints slightly changes mechanical properties, such as hardness, in the welded zone. But, hardness decreased by 8..10 HRC in the heat affected zone. This investigation has shown that the normalization of rail aluminotermitic welded joints reduces the probability of occurrence of brittle destruction in the heat affected zone during the operation of continuous welded rail.

rails, aluminotermitic welding, heat treatment, microstructure, hardness, rails joints