Effects Of Heat Treatment On Microstructure And Properties Of The Welding Zirconium Alloy Joint

Zircaloy-4 is widely used as fuel tubes in pressurized water reactors due to its excellent physical and mechanical properties. The welding quality of the zircaloy-4 is becoming more important with the constant advancement of reactor technology and the focus on nuclear safety.In this paper, zircaloy-4 was welded by micro-plasma arc welding in a small cabinet filled with argon to avoid the negative impact of the air. A copper cooling fixture was made to accelerate heat lost and reduce the width of the joint and the grain size. The joint presented a golden color, in line with the quality requirements. The welded joint was contained within 4mm, the gain size increased as the location moved from base material to welded zone.Once the reactor goes wrong, air may flow into the power plant and the cladding material would be oxidized by it, just as what had happened to the Fukushima nuclear power plant. Based on that consideration, an investigation was carried out on how the heat treatment of the welded joints impact its oxidation behavior. The results indicate that oxidation kinetic curves of non-heat treating joint, quenched joint and tempered joint were similar. An oxidation transition happened to all curves. At the early stage of oxidation, the curves obey the parabolic law, while the later stage of oxidation, the curves follow the linear law. The oxidation transition time of the quenched joint was the shortest, while the oxidation transition time of the tempered joint was the longest, the oxidation time of the non-heat treating joint was in the middle. Which indicated the protective oxidation film formed on the tempered joint lasted a relative long time. The tempered joint showed an excellent oxidation resistance ability because of the large amount of dispersedly distributed second phase Zr(Fe,Cr)2. While the quenched joint had the worst oxidation resistance performance because of the existence of β-Zr formed due to the fast cooling rate. Of all the joints, the fusion zone has the thickest oxide film, which indicated that large grain size had an negative impact on the oxidation resistance performance. The oxidation of the joints were connected to the microstructure of the base material. The results also showed that the oxidation of zircaloy-4 was caused by the zirconium nitride and its re-oxidation, which also developed a large number of cracks that accelerated the oxidation behavior of zircaloy-4 by letting more air have access to the base material.The mechanical properties of zircaloy-4 joints after heat treatment was studied, results show that micro-hardness at different locations of the joint varied. The hardness of the fusion zone was the biggest, the hardness of the HAZ was smaller than fusion zone but bigger than the base material. The joint after quenching had improved tensile strength and yield strength but reduced plastic property. The tempered joint had good plasticity but poor tensile strength and yield strength, the non- heat treating joint had good plasticity and tensile strength but poor yield strength. The fracture type of the quenched joint was brittle rupture when axial stretched at room temperature and the fracture mechanism was cleavage fracture. The fracture type of non- heat treating joint and tempered joint were ductile fracture and the fracture mechanism were micro-void coalescence fracture.