Debasis Carburization effect : differs from pyrolysis furnace tube from that of a methane or ammonia producing fertilizer plant even for the same HP modified or micro alloyed HP modified tubes. For ammonia or methanol production plants- pressure (35 atm) is higher but with lower temperature (temp: 900-1000 deg cent) compated to pyrolysis furnace tubes. For pyrolysis tubes- ethylene production plants - temperature is more ( 1100 deg cent) with pressure of operation is cosniderbly lower (5-10 atm). Carburization :Threshold values will vary depending on the temperature excursions seen in each heater tube. If it is a new tube, temperature excursions will not affect drastically. If it is an used tube with > 30,000 - 50,000 hr service history, temperature excess of even 10-25 deg centigrade would drastically alter the carburization index. HP mod tubes can usually take more abuse variations in temperature excesses than simple HP/HT/HK varieties in the same furnace design. Also, international studies still debate whether carburization precedes or oxidation with carburization is the predominant mode for increasing carbon content or in other words carburization index in such high temperature materials exposed to temperatures 900-1150 deg cent. For repair of pyrolysis heater tubes, it is considered good for repair welding if carburization index is 1.0. i.e., with carbon content of 1.0 % success in repair welding would give good life after repairs. For repair in ammonia or methanol plant furnace tubes, although 1% carbon pick up will give ease in repair welding (successful to get a near 40,000 - 60,000 hr service life after repairs) it depends on the temperature excesses and cyclic effects the furnace had seen. Pompey -France, Europe's one of leading heat resistant casting manufacturers- had done some pioneering work on carburization of HR cast heater tubes. You may get some research information on actual theoretical carburization index factor (laboratory studeis) or carburization index curves at different temperatures for high temperature heat resistant casings used in these furnaces. I do not remember (off hand) all publication details. Trust this will help you C.V.Srinivasan Nishi Engineers Pvt Ltd India June 10, 2007 >Carburization of Heater Tube occurs under redox potential / >reducing environment above a case specific CO/CO2 ratio. >Nickel improves resistance to carb. and so assisted by Cr, >Si, Al. Now some literature measures Resistance to >Carburisation by an Index formula ( %Ni + 9 x %Si). > >I would like to know what could be the threshold minimum >value of the Index to mitigate carburization susceptibility >of HP-Mod cast Tubes in principle. > >regards