1. Inadequate control of chemical composition, particularly the Ni content. To cut costs, many manufacturers set the lower limit of nickel between 8.0% and 8.2%. When the Cr/Ni ratio reaches a certain level, some ferrite forms in the microstructure, which is magnetic. However, by performing a solution treatment at 1050–1080°C, the ferrite can be fully dissolved into austenite, making the material non-magnetic.

2. Cold work hardening. During cold working, austenitic stainless steel may transform into deformed martensite, which increases strength but also introduces magnetism. This type of martensite can be eliminated through solution treatment or annealing, though this may reduce the material’s strength. If you want to maintain good mechanical properties while reducing magnetism, consider the following methods:

1. Adjust the Cr/Ni ratio by increasing Ni and Mn levels to their upper limits. Perform a high-temperature solution treatment before cold working, and control grain size to around 4 levels. This helps minimize magnetic properties after deformation.

2. It's common for 304 stainless steel to show slight magnetism after cold working. Impact or striking can cause austenite to transform into martensite, resulting in some magnetic characteristics. Heating the material to 1050°C and then quenching it in water can effectively remove the magnetic properties.

Notes:

1. The concept that "Cr/Ni reaches a certain value" refers to the balance between chromium and nickel equivalents. The formula for calculating these values is as follows:

Cr Equivalent = Cr% + 1.5(Si%) + Mo% + Nb% - 4.99

Ni Equivalent = Ni% + 30(C%) + 0.5(Mn%) + 26(N% - 0.02) + 2.77

If the Cr Equivalent/Ni Equivalent ratio is below 0.9, the structure becomes fully austenitic and non-magnetic.

2. By adding more nickel, manganese, or nitrogen, or by reducing chromium and silicon, you can achieve better demagnetization results.

3. There are commercial "alloy degaussing agents" available on the market that can convert residual ferrite into austenite, thus reducing magnetism. These agents also improve the corrosion resistance of castings, with excellent performance in salt spray tests.

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Tag: stainless steel casting

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