CAUSES OF BREAKDOWN OF THE STEAM-PIPE WEL DED JOINT AT THE TEMPERATURE OF 545-DEGREES-C

In the paper there are described structural, thermodynamic and diffusi on parameters and hardness of break-down and on the other side an inta cted circumferential welded joint of steam-pipe with 258 mm in diamete r and 32 mm wall thickness. The lifetimes influence of welded joints w ith long-term (19700 h) expose to temperature 545-degrees-C and pressu re 18,2 MPa was studied on the 12 020/17 134 (12Cr-1Mo-1/4V) pipe join t. The carbon steel 12 020 was confounded with original proposed creep -resisting low-alloy steel 15 128 (1/2Cr-1/2Mo-1/4V). From the view of the service factors it is demonstrated, that the crack initiation was under pipe axial stress and interaction of low cycles fatigue mechani sm (parameter R < 0, 5) in the decarburized zone of the low carbon ste el 12 020 controled. It is assumed that the crack in the decarburized zone low carbon steel 12 020, at the no considerably difference of car bon effective diffusion coefficient in welded joints E-B 329 (2 1/4Cr- 1Mo)/12 020 - D(Ceff) = 3,35 . 10(-10) cm2 . s-1 and 17 134 (12Cr-1Mo- 1-4V) /E-B 329 - D(Ceff) = 5,34 . 10(-10) cm2 . s-1, is connected with full absence of precipitation, substitution, dispersion and interstit ion strengthening of the decarburized zone 12 020. In comparison with the break-down welded joint, the crack has been not observed in the hi gh strength weld metal E-B 329 with substitution elements Cr, Mo. The results form the base for estimation of the residual service life of w elded joints.