Hello Shaju
308 and 347 would be appropriate fillers if the material is 321 or equivalent. Without a few facts to go with it I would say work with a 347 filler. Although this "sounds" like a boiler repair.......To get solid advice from the experts you on here should provide pictures of the failures and give a solid description of the service environment. I.E. Pressure in service, operating temps vs. not in service temps, any mechanical loads applied, chemistry of what is in the tubes?