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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. Details of the microstructural findings and a proposed mechanism of stress relief cracking will be discussed.
In the refining industry, Incoloy 800H (UNS N08810) has been employed for components operating in the intermediate temperature regime (500~700℃). 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.
Key words: 800H (UNS N08810), Relaxation Crack, Oxide
Various alloys subjected various heat treatments were examined in-service and tested to determine their susceptibility to stress relaxation cracking.
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The research was focused to assess the key factors concerning relaxation cracking and how to control it. It included the effect of chemical composition of the base materials (both Fe and Ni base alloys), heat to heat variation, grain size, cold deformation, welding, operating temperature and heat treatments.
This paper discusses reactors in hydrocarbon service that experienced numerous cracking problems over a 8-year period, where cracks were confined to the welded zones. The material is TP347 stainless steel, welded with E347-16 consumables.