Investigation of the cause of failure and knifeline attack in the welds of tubes of the direct reduction unit's recuperator

In this study, failure causes of the welding side area of austenitic stainless steel pipes AISI۳۲۱ of the direct reduction unit’s recuperator were investigated. The main problemwas the impossibility of replacing the pipes after the fracture. For this purpose, the cracks created in the pipes were repaired by filler metal ER۳۴۷ and ERNiCr-۳ using the melting welding method (GTAW). The microstructures of base and weld metals were evaluated by light and scanning electron microscopes using a reverse detector. The cracks are knife attack grooves created next to the weld and about ۳ to ۵ mm in the weld area. The results showed that the way to prevent cracks is to modify the welding method by selecting the appropriate parameters and inlet temperature. It was found that the use of ERNiCr-۳ filler metal to repair cracks due to having sufficient amounts of alloying elements can have optimal metallurgical properties and Improves working life and does not cause Sensitivity in the base metal.