Arvind Garg You have not specified the service duty, the temperature and pressure of operation / design. Incoloy 800 H material can stand upto 800- 816 degree centigrade for close to 100,000 hr. It is normally used in Reformer furnace transfer headers as spool pieces or at the outlet of reforming furnaces as hair pin tubes and also as ferrules at the outlet of secondary reformers . Incoloy 800 H material if the temperature run away had exceeded 850-900 deg centigrade can see catastrophic oxidation and also see bore carburization in reforming furnace outlets. In such a case, stress rupture failures will normally result. Micro structure analysis will reveal the type of cracks, bore oxidation or carburization in used Incoloy 800 H material. If the Incoloy 800 H had been supplied by chance in mill-annealed condition then stress rupture failure will occur even at a lower temperature of operation around 800-815 deg centigrade. Grain size estiamtion will help you to know if it had been supplied in mill annealed condition. Normally Incoloy 800 H designation material calls for supply in solution-annealed condition with grain size ASTM 5 or larger. If a solution annealed Incoloy 800 H had failed then the cause is due to over heating around 850-900 deg centigrade. Micro structure analysis will help you to determine the grain size, bore carburixation / oxidation, the morphology of cracking. Trust this is of help to you C.V.Srinivasan Nishi Engineers Pvt Ltd Chennai 20 India Nov 3 E-mail: nish@vsnl.com/nishi@hathway.com >An Incolloy 800H pipe ruptured and appeared to be result of >overheat. What features, microscopically, could confirm this >?