Whatever the processing route followed for the pro­duction of Zr metal, the sponge or the chips obtained by scrapping out the electrodes are the base products for alloy ingot preparation. The melting of the alloys is performed using the vacuum arc remelting (VAR) process. This process is specific to highly reactive metals such as Zr, Ti, or advanced superalloys.

For industrial alloy preparation, an electrode is prepared by compaction of pieces of base metal frag­ments (sponge or scraps) with inclusion of the alloy­ing elements. Typically, the elements to be added are the following: O (in the form of ZrO2 powder), Sn, Nb, Fe, Cr, and Ni to the desired composition. In addition, a strict control ofminor elements, such as C, N, S, and Si, is ensured by the producers, at concen­trations in the range of 30-300 ppm, according to their requirements to fulfill the engineering properties.

A few specific impurities are strictly controlled for neutron physics reasons: Cd and Hf due to their impact on neutron capture cross-section, U for the contamination of the coolant by recoil fission fragments escaping from the free surface of the clad­ding, and Co for in-core activation, dissolution trans­port, contamination, and g-irradiation.

The compact stack is melted in a consumable electrode electric vacuum furnace with water chilled Cu crucible. Electromagnetic fields are often used for efficient stirring of the liquid pool and reduced seg­regations. After three to four melts, the typical dimensions of the final ingots are 0.6-0.8 m diameter and 2-3 m length, that is, a mass of 4-8 tons.

2.07.3.2 Forging

Industrial use of Zr alloys requires either tube — or plate-shaped material. The first step in mechanical processing is forging or hot rolling in the p-phase, at a temperature near 1050 °C, or at lower temperatures in the a + p range or even in the upper a range. The high oxidation kinetics of Zr alloys in air at high temperatures restricts the high temperature forging process to thick components, that is, with minimum dimensions larger than 10 cm, at least. Final dimen­sions after forging correspond to 10-25 cm diameter for billets and 10 cm for slabs.

A p-quenching is usually performed at the end of the forging step. This heat treatment allows complete

dissolution of the alloying elements in the p-phase and their homogenization above 1000 °C, followed by a water quench. During the corresponding bainitic p to a transformation, the alloying elements are redistributed, leading to local segregations: O and Sn preferring the middle of the a-platelets, while the TMs (Fe, Cr, and Ni) and Nb are being rejected to the interface between the platelets.13 These segre­gations lead to plastic deformation strains highly localized at the interplatelet zones for materials having a p-quenched structure (heat-affected zones, welds, or p-quenched without further thermomechanical pro­cessing). As described later, this p-quench controls the initial size distribution of the precipitates in Zircaloy, and further recovery heat treatments should be per­formed below the p-a transus only.