In the refining industry, Incoloy 800H (UNS N08810) has been employed for components operating in the intermediate temperature regime (500~700 °C). However, it is known to suffer from relaxation cracking in the HAZ after short duration in service. In the present study, detailed microstructure of the crack-tip region of a failed tube was examined using SEM, TEM and EBSD to clarify the relaxation crack mechanism. The current findings suggest that dislocation enhanced M23C6 precipitation near grain boundary promoted oxidation near grain boundary, and then a crack propagated along the oxide layer or interface between the oxide layer and base metal. Details of the microstructural findings and a proposed mechanism of stress relieve cracking will be discussed.

Subject
Scanning electron microscopy, Dislocation, Tubes, Oxide layers, Transmission electron microscopy, Density, Stress relaxation cracking, Slip, Grain boundary, Cracks, Grain, Austenite, Oxidation
Keywords:
800H (Uns N08810), Relaxation Crack, Oxide
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2018
Association for Materials Protection and Performance (AMPP)
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