Amorphous alloy transformer cores have become the core component of energy-saving power transformers due to their excellent soft magnetic properties, such as ultra-low core loss, high magnetic permeability, and good temperature stability. Unlike traditional silicon steel cores, amorphous alloys are prepared by rapid solidification technology, which results in a disordered atomic structure and high internal stress. This internal stress seriously affects the magnetic properties of the core, making it impossible to directly meet the application requirements of transformers. The annealing process is the most critical post-processing technology for amorphous alloy transformer cores, which can effectively eliminate internal stress, optimize the atomic arrangement, and maximize the soft magnetic properties of the core. However, the annealing process is a complex thermal treatment process involving multiple steps, and each step’s parameter setting and operation standard directly determine the final performance of the amorphous alloy core. This article will deeply analyze the key steps of the annealing process that affect the performance of amorphous alloy transformer cores, elaborate on the influence mechanism of each step on the core’s magnetic properties, and provide a theoretical basis for optimizing the annealing process and improving the performance of amorphous alloy transformers.
The annealing process of amorphous alloy transformer cores mainly includes four core steps: pre-heating, temperature rising, heat preservation, and cooling. Each step has a unique role in eliminating internal stress and optimizing the structure, and any deviation in parameters such as temperature, time, and atmosphere will lead to a significant decline in the core’s performance. Among them, the temperature rising rate, heat preservation temperature, heat preservation time, and cooling rate are the most critical process parameters, and their reasonable matching is the key to ensuring the excellent performance of the amorphous alloy core.
The first key step affecting the performance of amorphous alloy transformer cores is the pre-heating step. The pre-heating step is mainly used to remove moisture, oil stains, and other impurities on the surface of the amorphous alloy core, and to reduce the temperature difference between the inside and outside of the core, avoiding thermal stress caused by rapid heating. Amorphous alloy materials are extremely sensitive to thermal stress; if the core is directly heated at a high rate without pre-heating, the surface temperature of the core will rise rapidly while the internal temperature remains low, resulting in a large temperature gradient and thermal stress. This thermal stress will overlap with the original internal stress of the amorphous alloy, leading to cracks or deformation of the core, and even irreversible damage to the magnetic properties. In addition, moisture and oil stains on the surface of the core will decompose at high temperatures, generating harmful gases that corrode the core surface and affect the insulation performance of the core.
The key to the pre-heating step lies in controlling the pre-heating temperature and heating rate. Generally, the pre-heating temperature is set between 100°C and 150°C, and the heating rate is controlled at 5°C to 10°C per hour. This slow heating rate ensures that the temperature inside and outside the core rises uniformly, and effectively removes moisture and oil stains. At the same time, the pre-heating process should be carried out in a dry and clean atmosphere, usually using nitrogen or argon as the protective gas to prevent the core from being oxidized at high temperatures. If the pre-heating temperature is too low or the time is too short, moisture and oil stains cannot be completely removed; if the pre-heating temperature is too high or the rate is too fast, it will still cause thermal stress, affecting the subsequent annealing effect. Therefore, the pre-heating step is the foundation of the entire annealing process, and its rational design is crucial to ensuring the stability of the core’s performance.
The second and most critical step is the temperature rising step. The temperature rising step is the core stage of the annealing process, which directly determines the degree of internal stress elimination and the optimization effect of the atomic structure. The internal stress of the amorphous alloy core is mainly generated during the rapid solidification process, including thermal stress and structural stress. The purpose of heating is to provide sufficient energy for the atomic movement of the amorphous alloy, so that the atoms can rearrange in a more stable state, thereby eliminating internal stress and improving the magnetic permeability of the core. However, the temperature rising rate and final heating temperature are the two core parameters that affect the effect of this step.
The temperature rising rate must be strictly controlled to avoid generating new thermal stress. Amorphous alloy materials have poor thermal conductivity; if the temperature rising rate is too fast (exceeding 15°C per hour), the surface and internal temperature of the core will form a large gradient, leading to new thermal stress, which will offset the effect of stress elimination. On the contrary, if the temperature rising rate is too slow (less than 3°C per hour), the production efficiency will be reduced, and the atomic rearrangement will be insufficient, resulting in incomplete stress elimination. Generally, the optimal temperature rising rate for amorphous alloy transformer cores is 5°C to 10°C per hour, which can balance the efficiency of stress elimination and production cost. In addition, the final heating temperature (also known as the annealing temperature) is the key factor determining the magnetic properties of the core. The annealing temperature of amorphous alloys is usually set between 350°C and 420°C, which is lower than the crystallization temperature of the amorphous alloy (about 550°C). If the annealing temperature is too low, the atomic movement energy is insufficient, and the internal stress cannot be completely eliminated, resulting in high core loss and low magnetic permeability. If the annealing temperature is too high, the amorphous alloy will undergo partial crystallization, forming a crystalline phase, which will significantly damage the soft magnetic properties of the core, leading to a sharp increase in core loss and a decrease in magnetic permeability. Therefore, the annealing temperature must be strictly controlled within the optimal range, and the error should not exceed ±5°C.
The third key step is the heat preservation step. After reaching the set annealing temperature, the core needs to be kept at a constant temperature for a certain period of time, which is called the heat preservation step. The purpose of heat preservation is to ensure that the internal and external temperatures of the core are completely uniform, and to provide sufficient time for the atomic rearrangement and internal stress elimination. During the heat preservation process, the atoms of the amorphous alloy gradually move to a more stable position, the internal stress is continuously released, and the magnetic domain structure is optimized, thereby improving the magnetic properties of the core. The heat preservation time is closely related to the thickness of the core and the annealing temperature: the thicker the core, the longer the heat preservation time required; the lower the annealing temperature, the longer the heat preservation time needed to achieve the same stress elimination effect.
For amorphous alloy transformer cores with a thickness of 0.02mm to 0.03mm, the optimal heat preservation time is 2 to 4 hours. If the heat preservation time is too short, the internal stress cannot be completely eliminated, and the atomic rearrangement is insufficient, resulting in unstable magnetic properties of the core. If the heat preservation time is too long, the production efficiency will be reduced, and the amorphous alloy may undergo slight oxidation or even partial crystallization, affecting the core’s performance. In addition, the atmosphere during the heat preservation process is also very important. It is necessary to maintain a stable protective atmosphere (such as nitrogen or argon) to prevent the core from being oxidized. The oxygen content in the protective gas should be controlled below 50ppm; otherwise, the surface of the core will be oxidized to form an oxide layer, which will increase the core loss and reduce the magnetic permeability.
The fourth key step is the cooling step. The cooling step is the final stage of the annealing process, and its cooling rate directly affects the final structure and magnetic properties of the amorphous alloy core. After the heat preservation is completed, the core needs to be cooled to room temperature. The cooling process must be carried out in a controlled manner to avoid generating new thermal stress and ensure that the optimized atomic structure and magnetic domain structure are stably retained. Amorphous alloys are sensitive to cooling rate; if the cooling rate is too fast, the atoms do not have enough time to rearrange stably, leading to the re-generation of thermal stress and the destruction of the optimized magnetic domain structure, resulting in an increase in core loss. If the cooling rate is too slow, the amorphous alloy may undergo partial crystallization during the cooling process, which will damage the soft magnetic properties.
The optimal cooling rate for amorphous alloy transformer cores is 3°C to 8°C per hour. In the initial stage of cooling (from annealing temperature to 200°C), the cooling rate should be strictly controlled to avoid thermal stress; in the later stage (from 200°C to room temperature), the cooling rate can be appropriately increased to improve production efficiency. In addition, the cooling process should also be carried out in a protective atmosphere to prevent the core from being oxidized during cooling. After cooling to room temperature, the core should be kept in the protective atmosphere for 1 to 2 hours to ensure that the temperature of the core is completely uniform and the internal stress is fully released.
In addition to the four key steps mentioned above, the annealing atmosphere is also an important factor affecting the performance of the amorphous alloy core. The annealing atmosphere is mainly used to prevent the core from being oxidized during the annealing process. Amorphous alloys are easily oxidized at high temperatures, and the oxide layer formed on the surface will increase the core loss and reduce the magnetic permeability. Therefore, the entire annealing process must be carried out in a protective atmosphere. Commonly used protective gases include nitrogen, argon, and hydrogen-nitrogen mixture. Among them, argon has a better protective effect, but its cost is higher; nitrogen is more economical and is widely used in industrial production. The purity of the protective gas must be guaranteed: the purity of nitrogen should be above 99.99%, and the oxygen content should be controlled below 50ppm. In addition, the flow rate of the protective gas should be reasonably controlled to ensure that the air in the annealing furnace is completely replaced, and the protective atmosphere is kept stable throughout the annealing process.
The interaction between each key step of the annealing process also has a significant impact on the performance of the amorphous alloy core. For example, if the pre-heating is insufficient, the temperature rising rate is too fast, which will lead to thermal stress during the temperature rising process, and even if the heat preservation and cooling steps are optimized, it is difficult to eliminate the generated stress, resulting in poor core performance. Similarly, if the annealing temperature is too high, even if the heat preservation time is appropriate, the core will still undergo partial crystallization, affecting the magnetic properties. Therefore, the annealing process must be comprehensively optimized, and the parameters of each step must be reasonably matched to achieve the best annealing effect.
In practical industrial production, the performance of amorphous alloy transformer cores is closely related to the annealing process parameters. Taking a 10kV amorphous alloy transformer core as an example, when the pre-heating temperature is 120°C, the temperature rising rate is 8°C/h, the annealing temperature is 380°C, the heat preservation time is 3 hours, and the cooling rate is 5°C/h, the core loss can be reduced by 30% to 40% compared with the unannealed core, and the magnetic permeability can be increased by 20% to 30%. If the temperature rising rate is increased to 20°C/h, the core loss will increase by 15% to 20%, and the magnetic permeability will decrease by 10% to 15%. If the annealing temperature is increased to 450°C, the core will undergo partial crystallization, and the core loss will increase by more than 50%.
In summary, the annealing process is the key to improving the performance of amorphous alloy transformer cores, and the pre-heating, temperature rising, heat preservation, and cooling steps, as well as the annealing atmosphere, are the core factors affecting the annealing effect. The pre-heating step lays the foundation for stress elimination and impurity removal; the temperature rising step determines the degree of stress elimination and atomic rearrangement; the heat preservation step ensures the uniformity of temperature and the completeness of stress elimination; the cooling step ensures the stability of the optimized structure; and the annealing atmosphere prevents the core from being oxidized. Only by strictly controlling the parameters of each key step and realizing the reasonable matching of each step can the internal stress of the amorphous alloy core be effectively eliminated, the soft magnetic properties be maximized, and the performance of the amorphous alloy transformer be improved. With the continuous development of amorphous alloy technology, the optimization of the annealing process will become more precise, which will further promote the wide application of amorphous alloy transformers in the field of energy-saving power grids.
