Zirconium Alloys

                                             Zirconium alloy

Zirconium alloys : zirconium alloy is a zirconium matrix by adding other elements consisting of non-ferrous alloys.

Profile:  Zirconium alloys have good corrosion resistance, moderate mechanical  properties, low thermal neutron absorption cross section of atoms (Zr  0.18 barns) at high temperature and pressure water and steam 300 ~ 400  ℃, the nuclear fuel has a good compatibility,  and therefore can be used as a water-cooled nuclear reactor core  structural material (fuel cladding, pressure pipes, pipe brackets and  channel), which is the main use of zirconium alloys. Zirconium  on a variety of acid (such as hydrochloric acid, nitric acid, sulfuric  acid and acetic acid), alkali and salt have excellent corrosion  resistance, it is also used to make corrosion resistant zirconium alloy  components and pharmaceutical devices. Zirconium  and oxygen, nitrogen and other gases has a strong affinity, the  zirconium and zirconium alloys and also electric vacuum bulb industry is  widely used as a non-evaporable getter (see getter material). Zirconium has excellent light emission properties, and thus became the flash of fireworks materials.

Development:  pure zirconium on its strength and corrosion resistance, it can not  meet the nuclear fuel cladding tube and pressure requirements. The  late 1940s, the United States in order to explore the application of  zirconium in water-cooled reactors, to examine the zirconium-based  alloys. By  the mid-1950s, it has developed into an excellent overall performance  of Zr-2 alloy (Zirca-loy-2), and used as the world's first nuclear  submarine "Nautilus" nuclear fuel cladding material number, and later  made of Zr-4 ( Zircaloy-4), Zr-1Nb and Zr-2.5Nb alloy. Twenty  years, many other countries have also studied the zirconium alloy, but  the overall performance as good as the above-mentioned alloy, which is  not much use. Currently, from the sponge zirconium to zirconium alloy, it has  achieved national industrial production in the United States, Russia,  France, Germany, Canada and China.

Category: industrial-scale production of zirconium-based alloys have two series: zirconium and zirconium niobium-tin system. The former representative of the Zr-2 alloy, the latter on behalf of the Zr-2.5Nb alloy. Composition and properties shown in Table zirconium and zirconium alloys of three kinds. Zirconium  alloy elements selection principle is: not a significant increase in  zirconia thermal neutron absorption cross section; Second, we must not  be too damaging to improve process performance zirconium corrosion  resistance and strength at the same time. Tin-based  alloys zirconium, tin, iron, chromium, nickel added integrated (Zr-2  alloy), may improve the material strength and corrosion resistance,  thermal conductivity, corrosion film, reducing the surface state of the  corrosion susceptibility; zr-4  alloy does not contain nickel, and appropriate increase in iron  content, this hydrogen-absorbing alloy corrosion was only about half of  the zr-2 alloy. Zr-2 alloy is usually used for boiling water reactor, Zr-4 alloy used in pressurized water reactors. When  the zirconium niobium alloy, the temperature at the solubility limit of  α-Zr addition amount of Nb is reached, the best corrosion properties of  the alloy. Zr-1Nb and Zr-2.5Nb alloy niobium content is higher than the solubility limit of the use temperature. More  than niobium to a supersaturated state is present in α-Zr in the alloy  corrosion resistance negatively, while the second phase in the form of  β-Nb presence was much better. Most  of zirconium and zirconium alloy metal impurity elements required to  50ppm or less, thermal neutron absorption cross section of large  elements (such as boron and cadmium) shall not exceed 0.5ppm; nitrogen  serious damage to the corrosion resistance of not more than 80ppm; there  are some oxygen strengthen the role of its content by the strength requirement is generally 800 ~ 1600ppm.

Shift:  zirconium and zirconium alloys have allomorphs change, high temperature  phase is the β-Zr body-centered cubic structure, low-temperature phase  is a close-packed hexagonal structure of α-Zr. Pure zirconium transition temperature of 862 ℃. Alloys  and industrial zirconium transition temperature is influenced by  alloying elements and impurities elements: iron, nickel, chromium,  niobium added to narrow the α-Zr phase region, so that transformation  temperature is lowered; oxygen, nitrogen, tin expand α-Zr phase region,  so that transition temperature. Zr-2.5Nb quench hardening alloys have significant effects. This  alloy is heated to a zone or beta] (α + β) temperature region, after  quenching in water, its room temperature tensile strength of up to 90 kg  / mm 2; and then quenching the alloy aged at 500 ℃ appropriate, its  strength is not reduced , while the plasticity is significantly enhanced. This solution - aging alloys have excellent corrosion resistance.

Processing:  zirconium and zirconium alloy plastic, and can be made into pipe,  plate, rod and wire, wherein the pipe as the main products. Processing  zirconium and zirconium alloys of zirconium basic nature and depends on  the special requirements for nuclear reactors zirconium component. The  basic properties of zirconium is: easily contaminated oxygen, nitrogen,  hydrogen, etc., easy to stick mold, there allomorphs change. Zirconium nuclear reactor components requires a high dimensional accuracy, microstructure demanding performance and stability. The most widely used seamless zirconium tubes for the main processing  steps: preparation of a consumable electrode, casting, forging, hot  extrusion (tube), cold, finishing.
Vacuum consumable arc melting method is the most common method of zirconium and zirconium alloys of industrial production. With the correct method of alloying elements, the right mix of old and  new material and a reasonable proportion casting system in order to  obtain a high quality ingot.
Ingot  blooming ships in β phase region, which is conducive to deformation,  but also reduce the segregation of alloying elements. Secondary  forging temperature is lower than the breakdown, pure zirconium and  Zr-2 alloy in the α-phase region temperature zone forging, Zr-2.5Nb in  (α + β) phase region forging. Final forging temperature not lower than 700 ℃. Hot rolling temperature and the temperature is similar to the secondary forging, extrusion temperature lower. In order to prevent oxidation and stick mold, prior to extrusion billets to a copper-clad, or add a glass coating. Pure zirconium at liquid nitrogen temperature is still a good plasticity. At room temperature twice the amount of cold-rolled sheet annealing between up to 40% or more. Cold finished before the system has an important impact on the quality and performance of Zircaloy tubing. In order to obtain good overall performance pipe, a total reduction of  finished products before cold rolling should be more than 50%.
At room temperature was close-packed hexagonal structure of α-Zr in the cold deformation easy to form texture. Texture  zirconium tubes have an important effect on its strength, creep  properties, hydrides orientation, irradiation growth, etc. Zr-4  alloy cladding tube reactors in use, usually requires nearly radial  textured base (ie, a hexagonal lattice of C axis substantially parallel  to the tube diameter). Easy  to get this texture than the general orientation when the final cold  working step wall thickness to diameter shrinkage rate greater than 1. In  the Zr-2.5Nb pressure pipe production by controlling the extrusion  process parameters and extrusions organizations to control the texture. Zr-2.5Nb  hydride alloys in sheet form randomly oriented, plus the control  pressure pipe technology, will enable the sheet hydride was tangential  distribution to achieve the desired texture. Cold  plate through recrystallization annealing (about 650 ℃) after the  texture changes hydride orientation becomes confused (see preferred  orientation). Annealed cold material must be conducted in a vacuum oven, the vacuum should be higher than 10-4 Torr. Intermediate annealing temperature of about 700 ℃. Finish annealing is determined according to the performance requirements. The fuel cladding surface is very strict, which normally takes pickling. Pickling solution is an aqueous solution of hydrofluoric acid and nitric acid. Pickling  must completely remove the fluoride ion workpiece surface, otherwise it  will reduce the corrosion resistance of the material. It must be finished pipe straightening. If the straightening process is inappropriate, it will result in  non-uniform mechanical properties, low ductility blasting and hydride  orientation disadvantage.

Welding: zirconium and zirconium alloy has good welding properties. Commonly used methods of TIG welding and electron beam welding. Large-diameter thin-walled tube commonly used in welding fabrication. Fenxie flammable zirconium, zirconium during grinding and cutting products to pay attention to safety.

Inspection:  In the production of the cladding tube reactor with the pressure tube  and in the process, to be very strict inspection. Each  batch of products, in addition to routine chemical composition,  mechanical properties, dimensional accuracy and corrosion test other  items outside of each tube are required by NDT inspection.


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