How to get the magnetic properties of 304 stainless steel castings
September 26 11:22:18, 2025
In general, cast products made from non-magnetic 304 stainless steel scrap can become slightly magnetic. But why does this happen? Here are the main reasons:
1. **Improper control of chemical composition**
To reduce costs, many manufacturers set the Ni content at the lower end of the range, typically between 8.0% and 8.2%. When the Cr/Ni ratio reaches a certain point, some ferrite forms in the microstructure, which is magnetic. This ferrite can be completely dissolved into austenite through solution treatment at temperatures between 1050°C and 1080°C, making the material non-magnetic.
2. **Cold working or work hardening**
During cold working, austenitic stainless steel can transform into deformed martensite. This increases the strength of the material, but the deformed martensite is magnetic. Solution treatment or annealing can eliminate the martensite, but it reduces the strength of the steel.
If you want to maintain good strength while minimizing magnetism, you can use the following demagnetization methods:
1. **Adjusting the alloy composition**
Based on phase diagram principles, lowering the Cr/Ni ratio by increasing Ni and Mn levels to their upper limits can help. Before cold working, perform a high-temperature solution treatment and control grain size to around level 4 (while ensuring surface quality). This helps reduce magnetic properties after cold working.
2. **Heat treatment after cold working**
Standard 304 stainless steel often shows slight magnetism after cold working. If the material is struck or subjected to impact, the austenite may transform into martensite, resulting in magnetic properties. Heating the material to 1050°C and then quenching it in water can eliminate the magnetism effectively.
**Notes:**
1. The concept of “Cr/Ni reaching a certain value†refers to the ratio of Cr equivalent to Ni equivalent.
- Cr Equivalent = Cr% + 1.5(Si%) + Mo% + Nb% - 4.99
- Ni Equivalent = Ni% + 30(C%) + 0.5(Mn%) + 26(N% - 0.02) + 2.77
When the Cr Equivalent / Ni Equivalent is less than 0.9, the structure becomes single-phase austenite, and there is no magnetic property.
2. Adding nickel, manganese, or nitrogen, and reducing chromium or silicon can all help reduce magnetism.
3. There are "alloy degaussing agents" available on the market that can convert residual ferrite into austenite, achieving a demagnetization effect. These agents also improve corrosion resistance and enhance salt spray test performance.
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