Permeability measurement is an old practise to assess the value of inside Magnetism by measurement from Tueb outside for Ethylene Cracker and others. The carburisation front i.e. the depth of carburisation dictates the permeability value and from this value we can only assess the degree of carburisation and not the depth. An 1976 NACE Corrosion Paper dictates that a 25Cr35Ni heater tube will be weldable upto a Permeability value of 1.4. I would like to know different industrial practice in this regard i.e. what Perm value Plant users are taking as maximum for declaring a weldability. regards D.Mitra Debasis.Mitra@hpl.co.in

Debasis Bore carburization is influenced by factors of heat input to the ethylene cracker tube. Magnetic permeability is perhaps one of the oldest techniques to detect change in permeability in non magnetic austenitic S.S materials. It is not a fool proof technique but still is used by some plant users. This technique can give qualitative indication of bore carburization depth - depending on the experience of inspection company assessing the damage and the quality of instrumentation used for the assessment. Indicative grading can be done / is still done , by selective plant users, using the magnetic permeabiity change and the depth of carburization effect on the 25/35 Ni ethylene cracker tube . With this repair welding attempt is done by users - based on their inherent capabiilty to repair at site - and with known limitation by the user to utilize repaired tubes for a short period of 1.5 - 2 years after successful repairs or with minor cracks left out in the tubes. Minor crack assessment is done by conventional N.D.T and if required by surface metallography on site repair welded ethylene cracker tubes. In general, as a rule of thumb, carburized bore case depth of 0.5 mm to 0.8 mm may be successfully repair welded with a known limitation that the tubes repair welded may serve the user till the major turn-around of the ethylene cracker - say a life of 1.5 to 2 years. I remember a pioneering research study was done by a centrigugal cast high temperature alloy manufacturer in France on bore carburization and oxidation effect and the effect of exposure to temperature variations in high temperature cast alloys used in this service. Off hand, i do not remember to give details of this publication. If NACE paper had given substantiative information on magnetic permeability as the only guiding criteria for successful repair weld, you can assess based on this guideline. Magnetic permeability - sensitivity to assess bore carburization and locations will depend largely on the quality of instrumentation and the capability of the inspection company assessing bore carburization depth and - utilizing magnetic permeability change - for assessing repair welding of used ethylene cracker tubes of 25/35 alloys. Trust this is of help to you C.V.Srinivasan Nishi Engineers Pvt Ltd India May 31, 2006 E-mail: nishi@vsnl.com >Permeability measurement is an old practise to assess the >value of inside Magnetism by measurement from Tueb outside >for Ethylene Cracker and others. The carburisation front >i.e. the depth of carburisation dictates the permeability >value and from this value we can only assess the degree of >carburisation and not the depth. > >An 1976 NACE Corrosion Paper dictates that a 25Cr35Ni heater >tube will be weldable upto a Permeability value of 1.4. I >would like to know different industrial practice in this >regard i.e. what Perm value Plant users are taking as >maximum for declaring a weldability. > >regards >D.Mitra >Debasis.Mitra@hpl.co.in