Debasis Creep Fissure / Stress Rupture / Creep related damages in high temp super alloys (non-magnetic) becoming magnetic ( getting ferritic phase to pick up magnetic signals - measured by magentic permeability variations) is a complicated subject to discuss in this forum. To my mind, no straight answer or guideline value should even be attempted to be given in a discussion forum without a study of the particular plant operational / maintenance / inspection schedule results or data. For Ex: in Pyrolysis furnace - Ethylene cracker service - using HP alloys - creep related damages start from 20,000 hr service. Further deterioration depends on the operational temperature variations (temperature excesses or abuses each plant works - mainly from achieving certain production targets) , burner configurations or burner profile variations, catalyst performance in service and also from production limitaions or production achieving targets. Some companies knowingly use higher temperature of operation than design (in some cases) even taking into consideration certain known allownace percentage of replacement tubing as a routine shut down procedure. Some companies admit to local higher firing and higher temperature excesses in operation and plan to retire particular tubes or repair tubes with excess magnetic permeability. Magnetic permeability detection guideline, based on bore carburization seen, either for retiring or for repair welding depends on the type of expertise the inspection companies possess to diagnoze the degradation and magnetic permeability signals to recommend either for replacement or repair welding. To answer your question to give a bench mark guideline value limits from magnetic permeability measurements or permeability values diagnozed for retiring or replacement is difficult. This depends on the type of equipment used for magnetic permeability measurement, type of expertise the inspection company whom you engage possesses, type of operational fluctuations in temperaturre your particualr heater tube had seen from last inspection to current inspection schedules, the types of upset shut downs the heater furnace had seen etc. As a general guideline, you can attempt and do successful repair welding in pyrolysis furnace tubes if the bore carburzation had not penetrated greater than 0.6 to 0.9 mm deep. If you had seen visible cracks, i guess the tube had come to discard limit and may not be serviceable at all for use in such high temperature service. Each Plant / User should evaluate the risks involved and the residual life expected from the tube - if bore carburization exceeds 1.0 mm deep. Again it depends on the internal expertise the paltn people possess, the external expertise the plant expects from inspection companies and also on the general company management philosophy for retiring or repairing tubes for a given shut down time frame to have a peaceful operation for the next two years time. Trust this is of help to you C.V.Srinivasan Nishi Engineers Pvt Ltd India Dec 31, 2005 E-mail: nishi@vsnl.com >Cast H39WM and H46WM Ethylene Pyrolysis Heater Tubes are >high temperature superalloys and are principally non >magnetic ( mu <= 1) in nature. But w.r.t. continued use they >suffers from bore carburisation and gradually become >magnetic which can be measured by Magnetic Permeability >Meters from OD. Presence of mu values justifies existance of >bore carburisation. Higher the mu values means higher is the >depth of carburisation. Higher the mu values - higher is the >risk of weld cracking i.e. lower is the weldability. > >Now request forum expertise to share their experience and >ideas that what should the benchmarked mu values over and >above which weldability could be significantly affeted by >forming visible cracks ? > >regards > >D.Mitra >Sr. Engineer ( Inspection) >Haldia Petrochemicals Ltd. >West Bengal ; India >Mob - 94342 36119