We have an autoclave which seemed to have sprung a leak at a weld. Non-destructive testing was performed, including X-rays, but only potential defects were noticed. We decided to have our welder repair the suspected defects by grinding the them with a non-ferrous tool and re-weld. Unfortunately, the welder HAS used a carbon steel grinding tool to remove the suspect defective weld. He subsequently welded over with stainless steel rods. I believe that pitting or galvanic corrosion will occur. But my question is how to correct it. I do not believe that simple passivation of the weld will solve the problem. Do we need to remove the weld with a nonferrus grinder (more than was originally removed to ensure all contaminents are removed) and re-weld again with stainless steel rods? Or is there a simpler method? Thank you in advance

Auto claves - SS to C.S will develop pin hole leaks at the HAZ or at the weld after some years . The possible cause is SCC or high hard layers in the HAZ or improper selection of weld electrode for the first run between Carbon steel and Stainless Steel. Repair welding should not give you a major problem by using steel grinding wheels to remove the complete weld between the S.S and C.S . To be on the safer side, try Aluminia based wheels for removing the old weld areas between Carbon steel and S.S. Proper joint preparation is essential - as per code needs . Try build up with first layer using S.S 310 or S.S 309 stabilized grade filler wire / elecrodes preferably smaller dia of 2.5 mm dia or 2 mm dia. build up first layer can be done using TIG process. Use clean Stainless steel wire brush. Dye Penetrant inspection to be done after this first run - i.e, the weld between carbon steel to stainless steel. Cover run can be same S.S 309 / S.S. 310 or S.S 316 L or S.S. 317 L with Mo / Cb or Ti stabilized grades of 2.5 mm or same TIG process done. Do Dye Penetrant inspection of the cover runs also with providision for air vent ( a small 6 mm hole to take away all occluded gases due to welding) Make a hole for proper venting of accumulated gases to escape so that any occluded gas of H2 is not trapped at the weld. This can be later seal welded once a pneumatic soap bubble air test is done on the integrity of the weld. If you wish you can take Gamma ray for record purpose of the weld. In auto-clave you can expect SCC trend after some years. Trust this information is useful Chakravarthi V Srinivasan Tech Dierctor Nishi Engineers Pvt Ltd Chennai 20 India March 26, 2004