Zirconium has a α→β alloisomer transformation at 862°C.
The α phase has a close-packed hexagonal structure, and the β phase has a body-centered cubic structure.
The solid solubility of stannum,
niobium, ferrum, chromium,
nickel and other elements in the β phase is mostly greater than their solid solubility in the α phase, and even in the α phase, the solid solubility decreases as the temperature decreases.
The solubility of the alloying elements Fe, Cr and
Ni in the second phase particles in α-
Zr is generally lower than or equivalent to the level of impurities in unalloyed zirconium.
Therefore, at operating temperature, these elements always exist in the form of precipitation.
In the β zone above 950°C, the transition elements Fe, Cr, and Ni dissolve in the solid solution. In order to prevent the formation of sub-microscopic precipitation nuclei, it can be kept in a supersaturated solid solution by quenching.
Modern processing includes a quenching stage in the manufacturing process to establish the initial size distribution of the precipitated phase. All subsequent thermal processing will increase the average size of the precipitated phase in the material. Therefore, the control and adjustment of the processing parameters that determine the size and distribution of the precipitated phase is Very important.
The purpose of Zr-Sn alloy quenching is to re-dissolve the second phase formed by forging cooling into the β phase, and the alloying elements are supersaturated and dissolved in the α phase after rapid cooling.
In this way, on the one hand, the distribution of alloying elements can be made uniform.
On the other hand, it can also improve the resistance to boil-like corrosion of the finished pipe.
If the quenched castings are subjected to the 400℃, 10.3MPa steam corrosion test, it can be found that the uniform corrosion performance at 400℃, 10.3MPa/72h is very poor, but the resistance to boil-like corrosion at 500℃ is very good.
β quenching can improve the resistance to boil-like corrosion, because the heat preservation in the β phase area during the β quenching treatment makes the alloying elements solid solution and homogenization. The cubic β phase will be transformed into needle-like α phase after quenching. The grain size is less than 0.5μm, and the intermetallic compound particles with a diameter of 20-25nm are uniformly distributed on the grain boundary, in a network shape, and the network intermetallic compound maintains the conductivity of the oxide film, thereby preventing furuncle corrosion from occurring.
The effect of β quenching on furuncle corrosion depends on the cooling rate. The critical cooling rate of Zr-2 and Zr-4 alloys is about 50°C/s. The quenching rate of Zr-4 alloy has a great influence on its corrosion performance.
Generally, zirconium alloys will be hardened to a certain extent when quenched in the β zone, but hardening is not the main purpose of quenching. β solution heat treatment quenching makes
Nb, Fe, Cr and other β stabilizing elements supersaturate in the β phase, which improves the corrosion resistance of the alloy.
When the quenching rate is faster, a martensite structure with twins and dislocations is obtained, and when the quenching rate is slow, a widmanite structure with deformed α-Zr and the second phase is obtained.
The quenching rate has little effect on the grain change, indicating that the β-phase grain size is mainly determined by the annealing temperature. As the β-quenching temperature increases, the grain size increases after quenching, and the corrosion rate of zirconium alloys is not affected by the β-quenching temperature.
However, when the quenching rate is slow, a second phase is formed, and the supersaturated content of Nb, Fe, and Cr in the matrix is reduced, thereby improving the uniform corrosion resistance of the alloy.
It can be seen that increasing the amount of supersaturated solid solution Nb, Fe, and Cr in the matrix will make the uniform corrosion rate faster.
However, it is not desirable to form a second phase during β-quenching, because β-quenching is to dissolve the elements in the matrix. In order to obtain good uniformity, annealing heat treatment is usually carried out after quenching, so that the Fe and Cr alloying elements will be precipitated and grown in the supersaturated solid solution a phase and distributed uniformly.
Homogenization in the β phase region completely dissolves all the second phase particles, but causes significant grain growth. The grain size reached a few millimeters after being kept at 1050°C for 30 minutes.
Due to the slow cooling rate of large alloy castings, the β grains undergo a bainite transformation during water quenching to transform into α needle-like structures. The β eutectoid element will be repelled to the front of the phase change due to the phase change, and precipitate on the α needle boundary.
The cold processing and intermediate recrystallization annealing can further control the size distribution of the precipitated phase.
Zr-2 alloy is forged above 1010°C (phase zone) and quenched immediately. After α treatment and processing and recrystallization treatment, the second phase particles can be distributed in dots, thereby reducing corrosion and hydrogen absorption.
The influence of Zr-4 alloy on β quenching is more sensitive than that of Zr-2.
These effects of β treatment are mainly due to the content of Fe.
