Josehua Gau Jalong Sensitized S.S 304 subjected to long exposure at 600 deg Cent would have undergone metallurgical transformation. Hardness increase and ferromagnetic effect seen could be due to precipitated phases of Chromium (some transformed phases of chromium carbides, chromium nitrides). To repair weld successfully and most satisfactorily, ideal approach would be to mill anneal at 980-1000 deg cent or best to solution anneal at 1100 - 1120 deg cent before repair welding in a conventional way. This approach would have made all unwanted precipitates go into complete solid solution i.e., all unwanted precipitates due to sensitization and also due to small SCC (due to Polythionic acid formation during shut down of the vessel without normal soda ash or caustic injection prior to air entry in the vessel) would have been dissolved and made into super saturated solid solution at that temp of holding. Your successful attempt to hold at 350 deg cent and welding with pre heat maintained at 200 deg cent might have helped all surface fissuring trend to soften (due to SCC from Polythioinic acid) or reduce surface high hardness areas due to precipitated phases of chromium rich phases from long term holding at 600 deg cent. For SCC to appear due to Polthionic acid - your vessel might have been opened without normal caustic soda wash during depressurization and before entry of air in the vessel. The reason why your first attempt to weld with pre heat at 175 deg cent could be due to surface SCC from polythionic acid formation on an already sensitized vessel part (Plate or nozzles or pipe attached to the vessel). Hope this helps. C.V.Srinivasan Nishi Engineers Pvt Ltd India April 21, 2008 E-mail: nishi@vsnl.com >(i) I would like to know from other facilities/bodies >experiences how repair is carried out on Austenitic >Stainless Steel (e.g. AISI 304 Controlled Carbon) component >of a pressure vessel or piping that is suspected to have >sensitived i.e. due to long term exposure at temperature >approx. 600 deg. C. The component hardness have increased >significantly and material ferromagnetism also increased >(i.e. attract magnet). Repair attempted using normal >austenitic stainless steel repair/welding technique but >several cracks (in all direction) developed adjacent to the >HAZ as well as the surroundng base metal. Besides >sentization, Polythionic Acid SCC is also one of the >potential damage mechnism. How is repair carried out on >Austenitic Stainless Steel (e.g. AISI 304 Controlled Carbon) >component of a pressure vessel that is suspected to have >sensitived i.e. due to long term exposure at temperature >approx. 600 deg. C. >(ii) Instead of the usual austenitic stainless steel >repair/welding technique, where moisture/contaminant are >removed prior to welding and limiting the interpass >temperature at 175 deg. C., the repair was successfully >carried out using the following technique/steps. The area to >be repaired was heated up to 350 deg.C and held for 3 hours, >and the temperature was reduced to 200 deg. C. The welding >was carried out at maximum interpass temperature maintained >at 200 deg. C. Upon completion of the welding the repaired >area was covered with insulation blanket and cooled down >naturally. No crack detect after using this technique. What >is the effect/significance of this repair technique to the >material properties? >(iii) Are there any references where similar technique have >been applied on sensitized/aging austenitic stainless steel? >(iv) Any other techniques to repair sensitized SS?

Josehua / gwg The problem posed by Josehua is not only related to repair of a sensitized S.S material. This as it looks, from description given, is combined with ferromagnetic phase + SCC due to polythionic acid formation - most probably the vessel was opened without the normal precaution of alkaline wash before the shut down and opening of all nozzles and man holes during a shut down. For a purely sensitized S.S. 304, one could think of circular ( 360 degree) or staggered welding over lay to introduce as much compressive stresses with minimum tensile stress loading. In a situation of SCC already developed on the surface, due to polythionic acid, + sensitized S.S 304 with ferro magnetic phase also already noted due to high hardness, the best possible solution is to consider solution or mill anneal to bring the precipitated phases in super saturated solid solution conditions. This approach of solution anneal or mill anneal before welding would ensure good weldabiilty and the success would be both long term and also avoid / removal of all precipitated phases. What is done for a nuclear reactor piping with only sensitized zones and weld overlay (circular - 360 degree over lay) may not be strictly applicable for this vessel with already sensitized (magnetic hard layers) + SCC developed on the surface. The fact that some success had been achieved with improved welding conditions indicate that surface cracks seen may not be quite deep and is controllable. Hope this helps to approach the problem from a combined perspective. C.V.Srinivasan Nishi Engineers Pvt Ltd India May 27, 2008 E-mail: nishi@vsnl.com >As far as I know, the only effective method to repair >sensitized 304 stainless steel pipe material in place is to >deposit a 360 degree weld overlay (build-up) and in effect >build a new pressure boundary, similar to what the Nuclear >Generators have done on IGSCC damaged 304 stainless steel >reactor recirc piping. The weld overlay in effect >encapsulates the sensitized material and provides a new >pressure boundary. If done with water in the pipe, the weld >overlay can place the original pipe ID in compression and >arrest or prevent crack growth.