Abstract: The oil cracking furnace of a refinery plant has been in operation safely for 24 years.In the synthetical nondestructive inspection of weld metal,62 defects exceeding the limits set for this type of equipment were found.For the purpose of getting a correct maintenance programme,the authors made destructive examinations of some samples with typical welding defects.The testing results showed that these defects were all caused by welding.They are cold cracks,solidification cracks,pores and lack of fusion.Fractographic analysis proved that there was no propagation of these defects during the period of operation.Safety assurance analysis by fracture mechanics indicated that these defects do not have to be removed or repaired.At present the said oil cracking furnace is in full operation.