Zirconium alloy processing annealing process

After the material is cold worked, it must be annealed in order to recover its plasticity. The annealing temperature is generally 530-700°C to obtain a recrystallized structure. The microstructure is equiaxed α grains and precipitated phases located on the α grain boundaries and dispersed in the crystals. This is recrystallization annealing (RX). In order to make the product have better mechanical properties, the final annealing temperature can be lowered to avoid recrystallization, which is stress relief annealing (SR). It is characterized by elongated grains and high density of dislocations because of its high mechanical strength.

Annealing is divided into intermediate vacuum annealing and finished vacuum annealing.

Intermediate annealing

Intermediate vacuum annealing: It is the vacuum annealing for the softened metal to obtain continuous processing ability during the cold working of zirconium alloy. The intermediate annealing is recrystallization annealing.

The recrystallization temperature is related to the annealing time for the amount of cold working deformation. The larger the cold working amount and the longer the annealing time, the lower the recrystallization temperature.

However, the high recrystallization temperature is likely to cause damage to the boil-like corrosion of zirconium alloys. There are many factors that affect the boil-like corrosion of zirconium alloys, but as an internal factor of metallurgy and processing, the influence of heat treatment is the key.

An empirical method of controlling the size and distribution of intermetallic compounds. Based on controlling the time and temperature of the last β-phase quenching and all α-phase annealing processes-the cumulative annealing parameter A Σ Ai; Ai=tiexp(-Q/RTi), Q is the activation energy, R is the gas constant titanium and titanium are respectively The temperature and effective time of the i-th annealing process. ——It has been used in actual production activities.

Cumulative annealing parameter A. An increase in A means an increase in time or temperature in the α annealing phase area, so the cumulative annealing parameter is related to the size of the precipitate phase and is a parameter that affects the performance of furuncle corrosion.

Since the intermediate annealing temperature has a significant effect on the boil-like corrosion of Zr-Sn alloy, the low-temperature processing technology of Zr-4 alloy is developed. Reduce the traditional annealing temperature from 700-750°C to 600-650°C.

For zirconium niobium alloys, it is different. Zr-Nb alloys undergo an enveloping reaction at 610°C. Annealing above this temperature will cause the Nb-rich β-Zr to remain and deteriorate the corrosion resistance of the alloy, so it should not be higher than 600°C.

The annealing temperature of Zr-1Nb alloy and M5 is 580℃, the annealed structure is almost completely recrystallized α-Zr, and the fine and dispersed β-N precipitates in the α-Zr grain boundary and matrix, which does not contain β-Zr The organization has high corrosion resistance.

E635 is a multi-element zirconium alloy (Zr-1Nb-1.3Sr-0.4Fe), which has better corrosion resistance than Zr-Sn alloy in a specific environment. When used as a pressure tube, it is recommended to be used in a partially recrystallized state , When used as a cladding tube, it is used for complete recrystallization. The second phase particles in the alloy are Zr(Nb, Fe)2 or (Zirconium, Niobium)3Fe.

The intermediate annealing temperature should not exceed a certain temperature, otherwise β-Zr will be formed that is not resistant to corrosion, but when the annealing temperature is low, complete recrystallization cannot be achieved.

As the intermediate annealing temperature increases, the size of the precipitate becomes larger. With the increase of annealing temperature, the Nb-containing zirconium alloy decreases the amount of N in α-Zr. Prolonging the intermediate annealing time below the segregation temperature (about 610°C) can stabilize the β-Nb produced and improve the corrosion resistance. It can be seen that controlling the intermediate holding time and choosing the intermediate holding temperature have an important effect on improving the corrosion resistance of the alloy.

Final annealing
Finished vacuum annealing: zirconium alloy finished processed materials must be annealed before use to obtain a good combination of strength and plasticity. According to different requirements, it is divided into recrystallization annealing and stress relief annealing.

The stress relief annealing temperature is generally performed at 500°C. Compared with cold working, the strength changes little after stress relief annealing, but the plasticity is obviously improved.

The annealing temperature for recrystallization is generally 530-580°C. Under recrystallization conditions, as the annealing temperature increases, the strength decreases and the elongation increases.

The finishing annealing temperature has an important influence on the creep properties of the zirconium cladding tube.

When the annealing temperature of the Zr-4 tube is increased from 490°C to 575°C, its creep rate under 350°C/100MPa stress drops three times. When the annealing temperature of the Zr-1Nb tube is increased from 500°C to 650°C, its creep rate under the stress of 400°C/100MPa decreases by 10 times. For this reason, although there was a requirement for the stress annealing (cutting) state in the early days, it is basically changed to the recrystallization annealing (taking) state or the partial recrystallization annealing state.

The final annealing temperature of the Zr-2 or Zr-4 cladding tube has an obvious effect on the corrosion performance of the 500℃ superheated steam outside the reactor. The corrosion weight of the recrystallization annealing tube is always higher than that of the stress-relieving cladding tube. ——This is because the temperature of the stress relief annealing is lower than that of the recrystallization annealing, so that the Fe and Cr in the solid solution α-Zr matrix will not be further precipitated, and it also restricts the further dissolution and precipitation of the second phase And grow up.

The final annealing of the zirconium alloy cladding determines the final microstructure. For the Zr-4 alloy, the recrystallization annealing treatment of the cladding tube has a smaller corrosion weight than the stress relief annealing cladding tube, while the Zr-2 alloy cladding is removed Stress annealing is used to improve the resistance to boil-like corrosion. For Nb-containing zirconium alloys, the second phase is affected by the annealing temperature after β solid solution. Annealing below the segregation temperature can produce Zr3Fe, Zr-Fe-Nb, For second phases such as B-Nb, annealing above the segregation temperature will produce a second phase. As the Nb content increases, Zr-Fe-Nb decreases.

This is because the change of N content changes the solid solubility of Fe, and the type of N-containing precipitation is different, and the corrosion law is also different. The corrosion rate of Zr-Mb alloy increases with the increase of β-Zr phase and decreases with the decrease of β-Nb phase. And reduce.

Annealing below the eutectoid temperature, if the Nb content is lower than the equilibrium solid solubility in the matrix, with the extension of the final annealing holding time, the corrosion resistance will not change much;

If the Nb content is higher than the equilibrium concentration of the N matrix, the long-term heat preservation will promote the solid solution Nb in the matrix to reach the equilibrium concentration, thereby affecting the corrosion resistance of the zirconium alloy.

It can be seen that annealing heat treatment can avoid the existence of metastable phase and make the precipitation reach a thermodynamic equilibrium state. During aging, after β quenching, supersaturated solid solution Nb is precipitated, and the size of the second phase increases with the extension of the aging time;

Aging at a lower temperature (480°C), the formation of the second phase of Nb-containing Zr-Nb-Fe is the reason for the increase in corrosion resistance;

When the aging temperature is higher (580℃), the Nb content in the second phase increases, while the N content in the matrix decreases. Since the second phase grows when the Nb content of the matrix decreases, the effect of aging on improving corrosion resistance is offset.