Zirconium tube

Material: R60702, r60704, r60705, ZR1, Zr2

Executive standard: ASTM b550 ASTM b551 GB / t21183-2007

Zirconium tube is an alloy tube material made of zirconium and other metals. Due to the excellent nuclear and corrosion resistance of zirconium, zirconium alloy tubes and rods are often used in the nuclear fuel assemblies and in core structures of water-cooled reactors. Pure zirconium tubes and rods are mainly used to manufacture chemical equipment, because the price is expensive and the consumption is not much. The chemical composition and application of commonly used zirconium tube and bar are shown in the table, and the amount of zirconium tin alloy tube and bar is more.

Process characteristics

(1) Zirconium has a strong affinity for gas. It reacts with oxygen, hydrogen and nitrogen at about 200, 300 and 400 ℃ respectively, and reacts rapidly at higher temperature. Therefore, melting, casting and heat treatment should be carried out in vacuum furnace. When hot working in the atmosphere, proper protective measures should be taken and the operation time at high temperature should be shortened.

(2) The friction coefficient between zirconium and die is high, and zirconium is easy to adhere to die surface; the thermal effect of plastic deformation is significant, and good lubrication conditions are required during processing.

(3) Processing specification has a sensitive effect on the performance of products. In particular, attention should be paid to the control of the cold working and heat treatment process before the finished product to meet the use requirements.

(4) The requirements of dimensional accuracy and surface quality of tube and bar are very high. Therefore, the process equipment should have good precision, and the design and manufacture of the die should be advanced and reasonable.

casting

Sponge zirconium is used as the raw material in the ingot for processing tube and rod. The spindle of nuclear power engineering materials should be made of nuclear grade zirconium sponge, and the content of more than 20 impurity elements such as HF, B, CD and u should be strictly limited. Alloying elements can be added in the form of high purity metal or master alloy.

Sponge zirconium and alloy elements are pressed together to form rod-shaped electrode, which can be melted twice in vacuum consumable electrode arc furnace to obtain ingot with dense structure and uniform composition. Before the rod electrode is pressed, the sponge zirconium and alloy elements should be mixed uniformly. The pressed electrode should have proper density. The chemical composition of the spindle obtained by secondary smelting should be within the specified range.

In order to avoid that the low melting point alloying element (SN) may flow ahead of time in the first melting, the addition of alloying element in the form of master alloy is more conducive to the uniformity of ingot composition than that of pure metal. Zr Sn CR Fe or Zr Sn CR Fe Ni master alloys are used for casting Zircaloy. It is easier to avoid segregation of refractory element (NB) when Zr NB master alloy is added than pure NB.

The master alloy is usually refined by vacuum non consumable electrode arc furnace. The weight of zirconium alloy ingot prepared by vacuum consumable melting method is about several hundred kg, its composition uniformity is easy to control, and it is also suitable for the needs of plastic processing. With the development of modern casting technology, more than 1 t homogeneous ingot can be produced.

The technology of induction melting and casting of zirconium and its alloys has entered the practical stage. The technical difficulty of this method is to overcome the contamination of zirconium by crucible materials. The solutions are as follows: using composite coating crucible or cold crucible induction melting and using shell casting. For example, zirconium uranium alloy rod for pulse reactor can be produced by two or three times of vacuum consumable melting and ingot re extrusion, or directly cast by vacuum medium frequency induction furnace.

The blank of plastic processed zirconium alloy tube and bar is prepared by hot forging and hot extrusion, which has good plasticity and low deformation resistance.

The ingot is heated by box type electric furnace before forging. In order to reduce high temperature oxidation, the spindle can be protected by glass coating. When oil fired furnace and gas furnace are used for heating, the heating should be carried out in neutral or slightly oxidizing atmosphere to avoid direct flame on the ingot surface. Large deformation forging is carried out in β phase (BCC) and precision forging is carried out in high temperature region of α phase (dense hexagonal). The forging equipment can be air hammer, steam hammer or hydraulic press.

The tube blank and bar blank are produced by the method of jacketed extrusion. Copper is often used as a coating to improve lubrication and avoid high temperature oxidation of zirconium. Both ingots and forgings can be used as extruded billets. The box furnace or power frequency furnace is used for heating, and the heating temperature is controlled in the middle high temperature region of α phase.

Generally, it is extruded with conical die to ensure the continuity and integrity of copper sheath. The upper limit of heating temperature before extrusion is the lowest temperature at which eutectic material may be formed between the cladding material and zirconium (according to the Cu Zr binary phase diagram), so as to avoid deterioration of the surface of the extrusion. For example, the heating temperature should be lower than 885 ℃. When high temperature extrusion is carried out, the smooth ingot can be heated by glass bath furnace or salt bath furnace.

Tube and bar products are produced by cold rolling, rotary forging, spinning, drawing and other cold working 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 die. The surface of the drawing blank should be phosphated or oxidized. In order to obtain the ideal surface quality, tubes are usually produced by cold rolling instead of drawing.

For the cladding tube of nuclear fuel element, the quantity is more and the requirement is higher, so it can be rolled by two roll or multi roll cold rolling mill. Zirconium alloy pipes are produced by two roll cold rolling mill in Europe and America. The size range of zirconium alloy cladding tube for PWR is 6-15mm × 0.4-0.7mm. The rod used as fuel element end plug is not usually produced by cold drawing method, and the extruded rod blank is produced by cold rotary forging. The diameter of the end plug is the same as the outer diameter of the cladding tube, and the machining allowance of the end plug should be reserved for the finished bar specification.

The final rolling of semi-finished pipe after the last intermediate annealing should be mainly wall reduction, and the ratio of wall reduction rate and diameter reduction rate (q) must be greater than 1. For the tube with Q > 1, the c-axis of hexagonal lattice is almost parallel to the tube diameter.

After the finished tube works in the reactor and absorbs hydrogen, it can be seen from the metallographic analysis of the cross section that the distribution of hydride is generally tangential. The tangential analysis of hydrides is beneficial to crack resistance of pipes. For the tube with Q < 1, the distribution of hydride is almost radial. Therefore, the Q value of the rolled pipe from the last intermediate annealing to the finished product specification should be ≥ 2 ~ 3.

heat treatment

It was carried out in a vacuum furnace with a vacuum degree of 1 × 10-2 ~ 1 × 10-3 PA. Only thick workpieces with complete protective coating or wrapping are allowed to be rapidly heat treated in the atmosphere. The intermediate annealing temperature of Zr Sn alloy tube bar is about 700 ℃, and the recrystallization begins at about 500 ℃. The grain size of Zr Sn alloy tube bar is obviously coarsened at 600 ℃. The stress relief annealing of the finished pipe is 450-500 ℃, and the recrystallization annealing of the finished pipe is 530-600 ℃.

The final intermediate annealing of Zr Sn alloy is changed to heating to high temperature region of β phase or α phase for rapid cooling, and then the final cold working and heat treatment of the finished product result in better corrosion resistance. The reason may be that the second phase in the metallographic structure of the finished pipe is fine and dispersed, and the solid solution content of alloy elements in α - Zr matrix is increased. The operation of the reactor shows that the above heat treated Zircaloy tube, as the cladding tube of the nuclear fuel element, can alleviate the nodular corrosion of the fuel cladding during the operation of the reactor. However, because of the difficulties in the manufacture of the tube quenching furnace and the cleaning treatment of the tube surface after quenching, it is not easy to implement the semi-finished tube quenching in industry. Therefore, the producers often only heat and quench the massive billet in the high temperature zone of β phase or α phase after forging or before extrusion. Although the effect of this treatment is slightly poor, the corrosion resistance can still be optimized.

Zirconium niobium alloy is strengthened by heat treatment, and its corrosion resistance can be improved by quenching cold working and aging treatment.

Product performance inspection: before leaving the factory, the indexes of chemical composition, corrosion performance, mechanical properties, surface quality, hydride orientation, metallographic structure, dimensional deviation and non-destructive testing should be examined to ensure that the indexes of chemical composition and corrosion performance are the same and the mechanical performance indexes are similar. The thermal neutron capture cross section of zirconium material is very small, which is one of the excellent nuclear physical properties and is guaranteed by the chemical composition of the material. The thermal neutron capture cross section of Zircaloy tube and rod is not more than 0.24b and that of pure zirconium tube and rod is not more than 0.18b. In addition, the ability of anti radiation growth is better, which is another nuclear physical property of zirconium material. It is related to not only the reactor condition, but also the cold working capacity and crystal texture of zirconium material.

Before leaving the factory, samples of Zircaloy tubes and rods should be taken and tested in 400 ℃ and 10.3MPA steam for 3 days or 14 days. It is found that the weight gain in 3 days should be less than 22mg / DM2 and that in 14 days should be less than 38mg / DM2. A black and bright protective film is formed on the surface. The mechanical properties of the recrystallized zirconium tin alloy pipes are as follows: room temperature strength limit ≥ 413mpa, yield strength ≥ 241mpa, elongation ≥ 20%. The mechanical properties of the pipes in high temperature strength and stress relief state are determined by the supplier through negotiation. Although the corrosion weight of Zr Nb alloy tube and rod is slightly larger than that of Zr Sn alloy tube and rod, but the strength is higher. It is also an ideal material for PWR.