Why does Permalloy core turn black after annealing?

The reasons why the Permalloy core turns black after annealing may involve many aspects. The following is a detailed analysis of this issue:

1. Oxidation reaction
During the high temperature annealing process, if Permalloy comes into contact with oxygen, it is easy to undergo oxidation reaction. This oxidation reaction will form an oxide film on the surface of the alloy, causing the surface of the iron core to turn black. The composition and thickness of the oxide film depend on factors such as the chemical composition of the alloy, annealing temperature and time.

2. Improper annealing process
Improper annealing process is also an important reason for the blackening of Permalloy core. For example, if the annealing temperature is too high or the holding time is too long, the elements in the alloy will diffuse and change phases, resulting in new compounds or structures. These changes may cause the color of the alloy surface to change. In addition, if the cooling speed after annealing is too fast, it may also cause stress inside the alloy, which will affect the surface color.

3. Alloy composition and microstructure
The chemical composition and microstructure of Permalloy also have an important influence on its color after annealing. Some elements in the alloy (such as iron, nickel, etc.) easily react with oxygen at high temperatures to form oxides. At the same time, the microstructure of the alloy (such as grain size, phase distribution, etc.) will also affect the oxidation reaction and the formation of oxides.

4. Measures to prevent blackening
In order to prevent the Permalloy Core from turning black after annealing, the following measures can be taken:

Strictly control the annealing atmosphere: During the annealing process, the atmosphere in the furnace should be ensured to be an inert atmosphere or a reducing atmosphere to reduce the chance of contact between oxygen and the alloy.

Optimize annealing process parameters: According to the chemical composition and performance requirements of the alloy, formulate reasonable parameters such as annealing temperature, holding time and cooling rate to reduce the occurrence of oxidation reaction and the generation of internal stress of the alloy.

Surface treatment: Before annealing, the alloy should be properly surface treated, such as cleaning and rust removal, to improve its oxidation resistance. After annealing, polishing or sandblasting can also be performed to remove the oxide film on the surface and restore the original luster of the alloy.

Choose appropriate annealing equipment: Ensure that the temperature of the annealing equipment is accurately and evenly controlled to reduce uneven annealing and increased oxidation reactions caused by equipment problems.

In summary, the reasons why the Permalloy core turns black after annealing may involve oxidation reaction, improper annealing process, alloy composition and microstructure, etc. In order to prevent the core from turning black, it is necessary to strictly control the annealing atmosphere, optimize the annealing process parameters, perform appropriate surface treatment and select suitable annealing equipment.