Abstract Scope |
Zirconium (Zr) is essential in alloys used in nuclear applications such as supercritical water cooled reactor (SCWR) which require joining of stainless steel (SS) and Zirconium (Zr). In the Canadian SCWR concept, the in-core portion of the pressure tubes transition from a Zr-alloy to a SS to allow welding these to a tube sheet. However, there are a considerable number of challenges associated with joining Zr-alloy to SS due to the differences in parent material properties and the formation of brittle intermetallic compounds which can lead to poor joint integrity, embrittlement, and cracking. The rotary friction process allows a short welding cycle time while keeping the components in the solid state, and this may help to minimize the thickness of the interfacial reaction layer which make it an excellent candidate for joining Zr and SS. In current study, Zr solid bar was successfully welded with solid SS bar using rotary friction welding. Preliminary testing involved trials with a ± 20% range bounding the targeted nominal rotation speed and burn off pressure combinations, while conditioning pressure and forging pressure were kept constant at 70 MPa and 150 MPa respectively. A burn-off displacement of 0.5 mm was used. Weld sampled were produced without preheating the SS blank and SS blanks preheated to 600°C for 30 min in order to promote increased deformation and flow of the SS faying surface. In addition, the weld samples produced from preheated SS blanks were post-weld heat treated to 600°C for 1 hr to study the influence of post-weld heat treatment in microstructure and properties. Optical, SEM and TEM analysis were performed to characterize the joint and chemical composition of the joint was investigated using EDS mapping. Tensile tests were performed on the sample produce from joint while strain behavior was monitored using Digital Image Correlation. The completed weld exhibited a continuous 1 μm thick interface without any delamination. The interface can be divided in four distinct zones. Heat affected zone (HAZ) in the SS side, reaction layer on the SS side, a second interfacial reaction layer on the Zr side, and subsequently the HAZ of the zirconium. The second interfacial reaction layer in Zr side can be further divided into two zones based on boundary barrier and grain size. Each of the above zones contain varying compositions of zirconium, iron, manganese, nickel and chromium. It is observed that nickel from the stainless steel diffused towards the zirconium to form Ni2Zr7, which was confirmed by electron diffraction. It is also worth noting that nano-twinning could also be observed in the interlayer, in regions that correspond to Fe2Zr. In tensile tests, a maximum joint strength of 250 MPa was achieved following post-weld heat treatment, though ductility was low due to the brittle nature of the joint. A post-weld heat treated joint between Zr-702 alloy and 304-SS joint has been characterized using microscopy and mechanical testing. The result shows three distinctive zones at the interface of the weld joint. Post-weld heat treatment showed potential for improve tensile strength, although the joint exhibits negligible ductility. |