Abstract
	Contribution analyse structure, mechanical properties and failure of lowalloyed steel pipes which serves in creep conditions. To creep failure especially perlite sphereoidization and coagulation of carbide particles take place often 90 000 h. of service at 500°C. After 180 000h of service structure of pipes materials and competed mechanical properties guarantee safety service. The broken of pipes was occurred often certain time always in weld joints. The high deformation fatigue enhanced by corrosion was reason of failure.
	The damage of the tube at welded joint tube - tube plate (fig.4) and variable stresses provoked thermal stresses which rises during start and shut down of the plant. It can not be eliminated, that there is different temperature of upper and lower tubes, this leads of the thermal stresses and development of bending moment and considerable growth of peak stresses at welded joint. 
	In the case of waste pipe (fig.7) besides assuming of thermal stresses due to temperature changes of pipe mantle (shut down, start) the decisive role has oscillation of stress which comes from bending moment (vibrations) originated during mud discharge of the plant. The damage in the direction of longitudinal axis of tube number 3 (fig.11) besides of thermal stresses originated from shut down and start of plant, also tensile stress increase due to longitudinal thermal expansion (as result of the bending  of tube at the intake to header chamber).  During temperature changes at this place the cyclic tensile - compressive stresses occures.