what r the causes of residual stress development in the welded joint?how this generated stress can be relieved

Answer 1

Most welded joints are not reheated, at least in structural steel welding. The residual stress is caused by the heat of the welding process causing the steel to expand while the weld metal is laid down. Later, the weld and the steel both cool, and the tension that develops is a result of the shrinkage being restrained by the rest of the steel surrounding the weld.

Steel in general, due to rolling, has residual stresses as well, typically with flanges in compression because they cool faster by being exposed to air more, and webs in tension as they cool last and are restrained by the flanges. The amount of residual stress is typically 0.3Fy (in American designs), but varies based on possible reheating of the steel for other purposes, as well as if the steel is oil-quenched, quenched and tempered, etc.

Answer 2

The major cause of residual stress in welded joints occurs from cooling of the joint after welding.
These stresses are usually relieved after welding by heating the welded assemblies to about 1100 - 1200 Deg. F (for carbon steel) for about 1 hour and then air cooling the assemly.