WHAT INFLUENCE THE SERVICE LIFE OF RAIL WELD

#thermit001 WHAT INFLUENCE THE SERVICE LIFE OF RAIL WELD.

To improve the quality of rail welds, the efforts were mainly focused on alloy hardening, post-weld heat treatment and welding procedures etc , and some investigation on porosity, fatigue performance.

Albeit, weld defects control the service life of rail welds.

How we can understand the formation mechanisms of rail weld defects.

Four main defects of rail welding:

Fins, shrinkage cavity, center line defect and micro porosity.

1). Fins formation: Inadequate fusion between weld metal and the surface of cold rail due to improper fixing of mould between weld metal and rail surface (Fig:1) can be avoided through the fixing of the mould and increasing pre heating time and temperature of molten metal.

??????????????????????????????????????????????????????????????????????????Fig:1?????????????

Four significant points that seriously influence inadequate fusion

i)???????????????????Pre-heating time.

ii)??????????????????Liquid temperature.

iii)????????????????Weld gap.

iv)????????????????Stick out dimensions.

2). Shrinkage cavity: It is an another serious weld defects and is often found in the rail web area of the rail weld ?(Fig:2) owing to thermal expansion and phase transformation, the volume of the weld metal changes constantly during cooling. The volume reduction of weld metal is greater in the high temperature region than in the low temperature region and, some volume contraction also occurs in the metal because microstructure changes. The influence of the welding conditions on shrinkage cavity formation, a high liquid temperature, longer pre heating time, and a wider weld gap can also promote the vertical solidification and it prevent the development of shrinkage cavities in rail welds.

Fig:2

3). Center line defect: It is like an accumulation of shrinkage type defects (Fig:3). Center line defect caused by the volumetric shrinkage and the liquid pressure drop in the weld zone during solidification. The most influencible welding parameter for the center line defect formation is weld gap.

Fig:3

4). Microporosity: Microporosity often observed in weld metals as small pores (Fig:4). Both spherical and irregular shape micropores can be found. Formation of microporosity involves both gas evolution during cooling and the liquid pressure drop inside the transition of solid and liquid temperature.

Fig:4