Metal treatment – Process of modifying or maintaining internal physical... – Heating or cooling of solid metal
Reexamination Certificate
2001-07-25
2003-07-01
King, Roy (Department: 1742)
Metal treatment
Process of modifying or maintaining internal physical...
Heating or cooling of solid metal
Reexamination Certificate
active
06585835
ABSTRACT:
THE BACKGROUND OF THE INVENTION AND PRIOR ART
The present invention refers to a method for producing a component suited to be subjected to an increased radiation in a corrosive environment, wherein the component includes an alloy, which at a first, high temperature has a BCC-structure and which at a second, lower temperature has an HCP-structure, wherein the alloy includes at least one alloying element which has a low solubility in the HCP-structure and wherein the alloy is rapidly cooled from the first temperature to the second temperature while secondary phase particles, which include said alloying element and which contribute to improved corrosion properties of the alloy, are separated in the HCP-structure.
Methods of this kind are previously known for the production of components, which include zirconium-based alloys and which are to be provided in nuclear power plants, or more precisely in the area of the core of such plants. The rapid cooling involves cooling of the zirconium-based alloy from a first, high temperature, where it is present as a &bgr;-phase with BCC-structure, to a second, lower temperature at which the alloy is present as a &agr;-phase with HCP-structure. The method is well known and is called &bgr;-quenching. One purpose of the &bgr;-quenching is the possibility to remove a directed texture, which the alloy normally will have in the production thereof and which results in a differing tendency of the alloy to grow in different directions when it is subjected to the radioactive radiation in a nuclear power plant. Thanks to the &bgr;-quenching it is thus possible, for instance, to avoid growth of such components as cladding tubes in their longitudinal direction and the risk that such components are seriously damaged due to bending as a result thereof. Another example is spacer sheets which thereby are prevented from growing in such directions that unnecessary large plays are present between these and the cladding tubes which they are arranged to hold, which plays result in a disadvantageous wear of the cladding tubes. By the &bgr;-quenching of sheets for boxes in nuclear power plants, it is also possible to avoid any preferential growth in axial direction when being radiated. Such a growth would otherwise lead to bending of the box, which in its turn results in known resulting problems in the reactor core.
The &bgr;-quenching also means that the result of previous heat treatments from the structure of the alloy are removed, which makes it possible to remove rough, structures which are not desired. In the &bgr;-quenching &agr;-lamellae are formed, or more precisely packages of &agr;-lamellae in the &bgr;-particles which the alloy has in the &bgr;-phase area, i.e. the first higher, temperature. In order to obtain an advantageous structure of relatively short and thin &agr;-lamellae and a plurality of &agr;-lamellae packages in each original &bgr;-phase particle, the &bgr;-quenching ought to be relatively quick. Ordinary &bgr;-quenching therefore involves immersing of the component in a water pool, which results in a very rapid cooling of the alloy and consequently thin &agr;-lamellae.
Outer surfaces of the component, at least some of which will be in immediate contact with a corrosive medium at the same time as they are subjected to radiation when the component is used, thereby will come into immediate contact with the cooling medium and be cooled significantly faster than core areas in the sheets, tubes, bars, etc., which are cooled in this manner. The secondary phase particles which are formed at the particle border areas in the &agr;-phase will thereby not have time to grow to an optimal size with regard to their contribution to the corrosion resistance of the alloy. Outer layers having insufficient corrosion resistance are thus produced. According to previous technique this problem has been overcome by mechanically machining or if possible etching away such layers of said components.
Such a machining for removing such layers involves however a further operation stage in the production of such components, wherein this operation stage contributes to make the production of the component more difficult and in addition more expensive.
SUMMARY OF THE INVENTION
An object of the present invention is to provide a method which reduces or completely eliminates the need of machining away the outer layers of a &bgr;-quenched alloy of the kind initially defined due to the fact that these layers have achieved an insufficient corrosion resistance due to the cooling conditions to which the component is subjected during the &bgr;-quenching. The method is to make it possible to obtain a desired secondary phase particle size and distribution at said layer already at or immediately after the &bgr;-quenching.
These objects are obtained by a method of the initially defined type, which is characterised in that the rapid cooling is performed in two stages, wherein the first stage includes a rapid cooling at a relatively lower intensity and the second, subsequent stage involves a rapid cooling at a relatively higher intensity.
By such an initial less intensive cooling it is possible to give the secondary phase particles at the surface time to grow to larger particles during the cooling than during the more intensive cooling, whereby the corrosion resistance is improved in the area of said surface. By surface is meant, preferably, also the layer or layers of the alloy which are located most closely beneath said surface. However, it is logical to assume that these are cooled down more slowly than the surface and that these layers may be assumed to have a sufficient secondary phase particle size when the alloy at the surface has a sufficient secondary phase particle size with regard to the contribution of the secondary phase to the corrosion resistance. The secondary phase particles are preferably separated in the particle border areas of the second structure. Preferably, the first stage is controlled in such a way that the cooling velocity at least at an alloy surface of the component which is intended to be in direct contact with the corrosive environment is lower than what it should have been if the cooling was performed by ordinary quenching of the component in water. Thanks to the more slow cooling than what is normal of said alloy surface or the surface of the component, the secondary phase particles are given time to grow during the cooling to larger particles than during ordinary cooling by water.
The first structure is a BCC-structure and the second structure an HCP-structure. A typical such material is zirconium-based alloys. Such alloys have such properties that they are used in applications where they are subjected to an increased radiation in a corrosive environment, in particular in nuclear power plants. In addition, such alloys normally require a &bgr;-quenching in order to remove a directed texture which they have achieved in a previous producing stage.
According to a further preferred embodiment, the alloy is a zirconium-based alloy. Such alloys are frequently employed in components in nuclear power plants and have thereby surfaces which are in immediate contact with the surrounding corrosive environment, i.e. the surrounding corrosive medium, and which at the same time are subjected to an increased radiation.
According to a further preferred embodiment, said cooling velocity is less than 100° C./seconds, preferably less than 50° C./seconds, and most preferably less than 25° C./seconds. These cooling velocities are advantageous to zirconium-based alloys in general and zirconium-based alloys according to the specifications for Zircaloy-alloys, such as Zircaloy-2 and Zircaloy-4 in particular. Thanks to a sufficiently low cooling velocity, the particles are given sufficient time to grow to a size which promotes the corrosion resistance of the alloy.
According to a further preferred embodiment, a cooling medium with a lower cooling performance than water is employed at least during an initial phase of the cooling in order to obtain said control cooling velocity. Alterna
Dahlbäck Mats
Wikmark Gunnar
King Roy
Swidler Berlin Shereff & Friedman, LLP
Wessman Andrew
Westinghouse Atom AB
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