Development of Manufacturing Process of PNC-FMS Wrapper Tube with SUS316 Short Joint

When a ferritic-martensitic stainless steel (PNC-FMS) wrapper tube having far greater swelling resistance against neutron irradiation is applied in the JOYO or MONJU reactor, it becomes necessary to weld it with SUS316 austenitic stainless steel (entrance nozzle and handling head). Such welding between PNC-FMS and SUS316 causes the delta (δ) ferrite formation at heat-affected zone, which leads to significant toughness degradation. In addition, bending of wrapper tube caused by their differential thermal expansion should be straightened. For preventing those problems, manufacturing process of the complex wrapper tube was developed. This process involves TIG-welding with SUS316 short pipe joints in 50mm length to both ends of a PNC-FMS round tube, and then performing the drawing and normalizing and tempering. Normalizing induces complete disappearance of the δ ferrite in the course of wrapper tube manufacturing. The mechanical properties of PNC-FMS/SUS316 welded zone were confirmed to be equivalent to those of the base metal even after thermal aging.