Failure Investigation of Boiler Water Wall Tubes of a Thermal Power Station

Failure investigation was carried out on boiler
water wall tubes of a thermal power plant through visual
inspection, chemical analysis, and metallurgical analysis.
Failure was in the form of thin/micro cracks along the
length of the tubes which were located at the girth welding
joint of tubes. Experimental results revealed that the
cracking was from inward to outward of the tube thickness.
Discontinuities/cavities were observed in the welded
region which might have occurred due to lack of fusion of
base metal and the weld metal. Cracks were initiated from
the sharp corner/crack tip of the cavities/discontinuities
present at the welded region under the action of hoop/
thermal stress existed during the operation. Nature of the
crack propagation indicates the case of typical hydrogeninduced
cracking. Moreover, the presence of the cavities/
discontinuities reduced the cross-sectional area of tubes
resulting increased stress intensity. Increased stress beyond
the flow stress of the material assisted by hydrogen-induced
effect resulted the cracking of the tubes. In order to mitigate
the problem, proper welding of tubes joints should be
carried out followed by proper inspection after weld.
Secondly, hydrogen dissolution during welding should be
prevented and treatment for its removal after welding
should be carried out.