Table of Contents
Types of Stainless Steel: Magnetic vs. Non-Magnetic
Stainless steel can be categorized into different types based on its structure at room temperature:
-
Austenitic Stainless Steel
- Common Grades: 304, 316, 321, 310
- Properties: Generally non-magnetic or weakly magnetic.
-
Martensitic or Ferritic Stainless Steel
- Common Grades: 430, 420, 410
- Properties: Strongly magnetic.
Among these, 304 stainless steel (austenitic) is the most commonly used material for screws and decorative products. While 304 is generally non-magnetic, there are specific circumstances where it can exhibit weak magnetism. Let’s break down the reasons.
Why Can Austenitic Stainless Steel (e.g., 304) Become Magnetic?
-
Chemical Composition Variations
During the smelting process, slight component segregation or improper heat treatment can occur. This may introduce a small amount of martensite1 or ferrite phases into the material’s structure, which are magnetic. -
Cold Working and Deformation
Cold working2—such as bending, forming, or rolling—alters the microstructure of 304 stainless steel. Under stress and deformation, the material’s austenitic structure can partially transform into martensite, which is magnetic.- The greater the deformation, the more martensite forms, increasing the material’s magnetism.
- For instance:
- A Φ76 tube (larger diameter) may have minimal magnetism.
- A smaller Φ9.5 tube shows more noticeable magnetic properties because of the higher deformation during processing.
- Square and rectangular tubes, especially at their corners, experience significant deformation, making their magnetic properties even more pronounced.
How to Eliminate Magnetic Properties in Stainless Steel?
If stainless steel becomes magnetic due to cold working or improper processing, its original non-magnetic austenitic structure can be restored through a high-temperature solution treatment3. This heat treatment relaxes the material’s internal structure and eliminates magnetism.
The Difference Between 304’s Weak Magnetism and Other Materials
It’s essential to distinguish between the weak magnetism of 304 stainless steel4 and the strong magnetism of ferritic stainless steel (e.g., 430) or carbon steel.
- 304 Stainless Steel: Always exhibits weak magnetism at most.
- Ferritic Stainless Steel or Carbon Steel: Displays strong magnetism similar to traditional magnetic metals.
If a stainless steel screw shows strong magnetism like carbon steel, it is likely not made of 304 or 316 stainless steel.
Understanding Common Stainless Steel Screw Materials and Magnetism
Here’s a breakdown of commonly used stainless steel materials and their magnetic properties after cold working:
| Material | Magnetism After Cold Working | Characteristics |
|---|---|---|
| 316 | Very weak (≤1.01u) | Superior corrosion resistance5; best for critical environments. |
| 304HC | Weak (1.01u-1.6u) | Good balance of strength and formability. |
| 304M | Slightly stronger (1.6u-2.0u) | Excellent tensile strength and easy cold working. |
Magnetic Properties Comparison (Weak to Strong):
316 < 304HC < 304M
Is the Magnet Test Reliable for Stainless Steel Screws?
Many buyers bring a magnet to test stainless steel screws, assuming non-magnetism equals high quality. However, this method is unscientific because:
- Stainless steel is non-magnetic in its natural state,
- Slight cold working can introduce weak magnetism,
- True non-magnetism can only be achieved in vacuum states or after specialized treatment.
Therefore, magnetic properties alone are not a reliable indicator of quality.
Final Takeaway: Choose the Right Stainless Steel Screws for Your Needs
When selecting stainless steel screws, focus on the material properties that meet your specific requirements:
- 316 Stainless Steel: For environments requiring high corrosion resistance and minimal magnetism.
- 304HC or 304M: Ideal for general use, offering a balance of strength, durability, and workability.
Understanding the causes of magnetism in stainless steel helps you make informed decisions and avoid misconceptions. At FHT, we ensure that every stainless steel screw meets the highest standards for performance, reliability, and quality—no matter the application.
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Footnotes
- Martensite: The magnetic phase formed during chemical or physical alterations in stainless steel.
- Cold working: A process where deformation causes changes in stainless steel’s microstructure.
- High-temperature solution treatment: Restores stainless steel’s original non-magnetic structure.
- 304 Stainless Steel: Explains weak magnetism compared to other stainless steels.
- Superior corrosion resistance in 316 stainless steel makes it ideal for harsh environments.
