Abstract
A manifold pipe made of the wrought Incoloy 800H and handling gaseous hydrocarbons at 820 °C has developed longitudinal cracks leading to unscheduled plant shutdown short of about 17% of the expected service life. The respective hydrocarbons result from steam reforming of methane to produce hydrogen in a series of furnace tubes attached to the pipe and externally heated by outside burners. It is shown that the cracks are of intergranular nature and have resulted from massive precipitation of grain boundary carbides particularly the extremely hard and brittle M7C3-type. This is correlated with substantial absorption of elemental carbon released by chemical reactions among carbonaceous gases and subjecting the pipe to carburization attack. However, the role of possible higher than normal external firing temperature in accelerating the carburization and creep rates could not be ruled out. This can occur to compensate for the insulating effect of coke deposited at the inner surface.
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