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Process characteristics of zirconium tube
(1) Zirconium has strong affinity with gas. It reacts with oxygen, hydrogen and nitrogen at about 200, 300 and 400 ℃ respectively. It reacts rapidly at higher temperature. Therefore, melting and casting and heat treatment should be carried out in vacuum furnace. When thermal processing is carried out in the atmosphere, proper protective measures shall be taken and the operation time at high temperature shall be shortened.
(2) Zirconium has a high friction coefficient with the die and is easy to adhere to the die surface; The thermal effect of plastic deformation is significant, and good lubrication conditions are required during processing.
(3) Processing specifications have a sensitive impact on the performance of products. Special attention shall be paid to the control of cold processing and heat treatment process before finished products to meet the use requirements.
(4) The dimensional accuracy and surface quality of tube and bar are very high. Therefore, the process equipment should have good accuracy, and the design and manufacture of tools and dies should be advanced and reasonable.
The ingot used for processing tube and rod is made of sponge zirconium. Spindles of materials for nuclear power engineering should be made of nuclear grade sponge zirconium, and the content of more than 20 impurity elements such as HF, B, CD and u should be strictly limited. Alloy elements can be added in the form of high-purity metal or parent alloy.
Sponge zirconium and alloy elements are pressed into rod electrodes and melted twice in vacuum consumable electrode arc furnace to obtain ingots with dense structure and uniform composition. Before pressing the rod electrode, the sponge zirconium and alloy elements should be roughly evenly mixed. The pressed electrode shall have appropriate density. The chemical composition of the spindle obtained from secondary smelting shall be within the specified range.
During the melting and casting of Zr Sn alloy, in order to avoid the early flow of low melting point alloy elements (SN) during the first melting, the alloy elements are added in the form of parent alloy, which is more conducive to the uniformity of ingot composition than pure metal. Zr Sn CR Fe or Zr Sn CR Fe Ni master alloy is used for melting and casting zirconium tin alloy. When smelting zirconium niobium alloy ingot, the alloying elements are added in the form of Zr NB master alloy, which is easier to avoid the segregation of refractory elements (NB) than pure niobium.
The master alloy is usually refined by vacuum non white consumable electrode arc furnace. Zirconium alloy ingots prepared by vacuum consumable melting generally weigh about hundreds of kilograms. Its composition uniformity is easy to control and is also suitable for the needs of plastic processing. The progress of modern melting and casting technology has been able to produce homogeneous ingots weighing more than 1t.
The induction melting and casting technology of zirconium and its alloys has begun to enter the practical stage. The technical difficulty of this method is to overcome the pollution of crucible material on zirconium. The solutions include: using composite coating crucible or cold crucible induction melting and shell casting. For example, the zirconium uranium alloy rod used in pulse reactor can be prepared by 2 ~ 3 times of vacuum consumable melting and ingot re extrusion, or directly cast in vacuum medium frequency induction furnace.
The blank of plastic processing zirconium alloy tube and rod is prepared by hot forging and hot extrusion. The hot processing plasticity is good and the deformation resistance is not very high.
The ingot shall be heated in a box electric furnace before forging. In order to reduce high temperature oxidation, the spindle can be protected with glass coating. When heating with oil furnace and gas furnace, the heating shall be carried out in neutral or slightly oxidizing atmosphere to avoid direct flame on the ingot surface. Large amount of deformation in forging β Phase (body centered cubic), precision forging in α The high temperature region of phase (dense hexagonal) is carried out. The forging equipment can be air hammer, steam hammer and hydraulic press.
The tube blank and rod blank are made by cladding extrusion. Red copper is often used as a sheath to improve lubrication and avoid high-temperature oxidation of zirconium. Ingots and forgings can be extruded. Use box furnace or power frequency furnace for heating, and the heating temperature shall be controlled within α Medium and high temperature region of phase.
Generally, conical die is used for extrusion to ensure the continuity and integrity of the copper coating of the extruded part. The upper temperature limit of heating before extrusion is the lowest temperature at which the cladding material and zirconium may produce eutectic materials (according to the Cu Zr binary phase diagram), so as not to deteriorate the surface of the extrusion. For example, when extruding zirconium and its alloy with copper-clad sleeve, the heating temperature should be lower than 885 ℃. When high-temperature non jacketed extrusion is carried out, the smooth ingot can be heated by glass bath furnace or salt bath furnace.
The finished tube and bar products are produced by cold rolling, rotary forging, spinning, drawing and other cold processing methods. Zirconium and its alloys have good plasticity and medium deformation resistance during cold working. The cold working performance is similar to that of low carbon steel, but it is difficult to lubricate and easy to stick to the mold. The surface of the drawing blank should be phosphated or oxidized. In order to obtain the ideal surface quality, pipes are generally produced by cold rolling without drawing.
For the cladding tube of nuclear fuel element, it can be rolled by two roll or multi roll cold rolling tube mill because of its large amount and high requirements. In Europe and the United States, two roll cold rolling mills are widely used to produce zirconium alloy tubes. The size range of zirconium alloy cladding tube for PWR is acoustic φ 6～15mm × 0.4～0.7mm。 The rod used as fuel element end plug is usually not produced by cold drawing, and the extruded rod blank is made by cold rotary forging. The diameter of the end plug is the same as the outer diameter of the cladding tube, and the finished bar specification shall have allowance for the machining of the end plug.
The final rolling of semi-finished pipe after the last intermediate annealing shall be dominated by wall reduction, and the ratio of wall reduction rate to diameter reduction rate (q) must be greater than 1. For the tube with Q > 1, the c-axis of the hexagonal lattice of metal crystal is almost parallel to the diameter of the tube.
After the finished tube works in the reactor and absorbs hydrogen, when sampling for cross-section metallographic analysis, it can be seen that the distribution direction of hydride is generally tangential. This tangential analysis of hydride is beneficial to the crack resistance of the pipe. For the pipe with Q < 1, the hydride is almost radially distributed. Therefore, the Q value of rolled pipe after the last intermediate annealing and before the specification of finished product shall be ≥ 2 ~ 3.