PREMATURE BREAKDOWN OF THE STEAM-PIPE THR OUGH THE ROOT DEFECTS OF WELDED JOINT AND THROUGH THE AXIAL ASYMMETRY STRAIN

In the paper the premature break-down of steam pipe welded joint with 18336 h exposure temperature of 480 degrees C is described. The steam pipe of creep-resisting low-alloy steel (0.5 wt.% Mo) was welded with the electrodes of the same base (0.3 wt.% Mo). It was assumed that the crack developed immediately from the root defect on the inner surface of the circumferential welded joint through the effects of weld-notch and water corrosion. The crack arised through the axial asymmetry add ition load. Calculated amplitude of axial strain in pipe was 70,4 MPa and is three times greater as amplitude of axial strain of steam press ure changes. The weld-heat affected zone of base material was strained from the bend moment to the break-down without influence of diffusion processes between base material and weld metal as a consequnce of ver y low value of effective diffussion coefficient in welded joint (D-Cef f = 5.33.10(-14) cm(2).(-1) by width of decarburized zone 38 mu m). Fr om the view of the service factors the technological procedure (weldin g the root layer with argon shielding in pipe) was proposed to suppres s and minimize the root defects and the freedom of pipe movement in su pporting places was used to suppress the axial asymmetry strain from t he bending moment on very low level. The results form the base for est imation of influence of welded notch on the service life of welded joi nts with long term exposure to temperature 480 degrees C.