STRUCTURE DEGRADATION OF 25CR35NI HEAT-RESISTANT TUBE ASSOCIATED WITHSURFACE COKING AND INTERNAL CARBURIZATION

Microstructures of 25Cr35Ni heat-resistant cracking tube after service were investigated and degradation mechanism of tube material was disc ussed. Results reveal that three distinguished zones, that is, interna l oxide, carbide free, and internally carburized zones, will gradually develop in the inner wall of a cracking tube during service. Carbide free and internally carburized zones are formed primarily in relation to the periodic spalling and regeneration of surface oxide scale, and diffusion velocity of carbon and carbide forming elements in matrix an d the solubility limit of carbon in alloy. The formation and growth of filament coke can aggravate structure degradation of the inner wall o f the cracking tube, while deposition of lamellar and spheroidal coke may slow structure degradation to some extent, Surface coking and deco king cycles strongly aggravate the structure degradation of tube mater ial and damage the service life of the cracking tube.