Metals are magnetic, right? Well, some metals aren’t, including certain types of stainless steel. Let’s find out which ones.
Some stainless steels are magnetic, and others are not. The defining factor of magnetism comes down to the steel‘s microstructure. There are magnetic and non-magnetic stainless steels, depending on the composition.
To be magnetic, it must meet certain requirements. In order to better understand stainless steel and its magnetic properties, let’s examine what stainless steel is.
What is stainless steel?
As an alloy, stainless steel is made up of a combination of metals with the balance being iron but the main alloying element being Chromium.
Stainless steel resists tarnishing and rust because of the elements contained within: iron, chromium, silicon, carbon, nitrogen, and manganese. It must be composed of at least 10.5% chromium and at most 1.2% carbon to be recognised as stainless steel.
Chromium (combined with nickel) is the component that gives stainless steel its corrosion resistance.
While stainless steel is frequently called stainless, it is not really stainless, because the chromium builds up on its surface making it able to withstand abuse much longer than regular steel without chromium.
Passivation is the process of applying chromium to stainless steel to keep the shiny, silver finish.
Is Stainless Steel Magnetic?
There are various types of stainless steel commonly available. Some of them are magnetic, and some are not. This property depends on several aspects, including alloying components and crystalline microstructure.
At its base, stainless steel is magnetic and has a ferritic microstructure. This is due to the inclusion of chromium, which is present in all forms of stainless steel. To harden this basic stainless steel, we add carbon, changing the microstructure to martensitic. Both of these variations are magnetic.
Increasing the chromium content to around 18% and adding nickel at about 10% to the alloy changes the microstructure to austenitic and makes the metal non-magnetic. This is the most commonly used stainless steel.
The chromium increases the alloy’s corrosion resistance, while the nickel improves the grade of stainless steel.
The two variants of stainless steel most commonly used are 304 and 316, both paramagnetic. Paramagnetism is a phenomenon where metals generally display non-magnetic behavior, except when in the presence of a powerful magnet.
Here, the magnet’s movement relative to the metal induces an electrical current inside the metal, causing it to react to the metal. This induced magnetism disappears when the magnet is removed.
304 stainless steel has an austenitic microstructure and contains chromium and nickel at minimum 18% and 8%, respectively. This metal could contain other alloying elements in trace amounts, including carbon, manganese, phosphorous, silicon, and sulfur.
316 stainless steel is alloyed mainly with molybdenum at 2% and a few other alloying elements in trace amounts. These include carbon, manganese, phosphorous, sulfur, and silicon.
Is stainless steel 304 magnetic?
No, 304 stainless steel is not magnetic. Its austenitic microstructure is responsible for this. Austenite forms at high temperatures and usually reverts to the ferritic microstructure during the cooling process.
The ferritic microstructure is magnetic. However, 304 stainless steel is alloyed with nickel, stabilizing the austenitic microstructure and enabling it to remain intact during the cooling process. That is how we can have a non-magnetic austenitic microstructure at room temperature.
304 stainless steel has a slight susceptibility to magnets, but this is not strong enough to be considered magnetic. Note that any process that this metal undergoes that changes its microstructure to ferritic will render it more magnetic.
Here, we include cold working and welding. In some cases, austenite could also spontaneously revert to martensite at low temperatures. Martensite is also magnetic, so your beautiful 304 or 316 stainless steel will now interact with magnets.
What Makes Stainless Steel Magnetic?
It is necessary that steel contains iron and has either a martensitic or ferritic crystal structure in order to be magnetic.
Two conditions will cause stainless steel to be magnetic:
- Stainless steel contains iron.
- Stainless steel has a crystal structure arranged in a ferritic or martensitic structure.
All stainless steel contains iron, which should theoretically make them magnetic. However, not all stainless steel has a ferritic or martensitic crystal structure. Some have an austenite structure, which makes them non-magnetic.
Is Stainless Steel A Permanent Magnet?
Stainless steel is generally not a permanent magnet. Some stainless steels can be magnetic, but they lose their magnetism once the external magnetic field is removed. Permanent magnets, like those made from iron or cobalt, retain their magnetism over time.
What Affects the Magnetism of Stainless Steel?
We have known what determines if the stainless steel is magnetic. Here are factors that affect the magnetism of stainless steel:
- Alloy Composition: The types and amounts of alloying elements, such as chromium and nickel, can affect the magnetic properties of stainless steel.
- Heat Treatment: Different heat treatment processes can change the crystal structure of stainless steel, which in turn influences its magnetic properties.
- Cold Working: Processes like cold rolling can introduce magnetism in stainless steel by altering its crystal structure.
- Temperature: Stainless steel’s magnetic properties can change with temperature; it might become more or less magnetic when heated or cooled.
- Presence of Impurities: Impurities can disrupt the uniformity of the crystal structure, potentially reducing or altering magnetic properties.
Which Stainless Steel Types Are Magnetic?
If we follow the logic stated above, then the following types of stainless steel would qualify as magnetic:
- Ferritic stainless steel, including grades like 409 and 439
- Martensitic stainless steel, including grades like 410 and 440
- Duplex stainless steel (a combination of ferritic and austenitic), including grades like 2205
Below are a few examples of magnetic and non-magnetic, given as a quick summary.
- Ferritic stainless steel grade 430 is magnetic
- The austenitic stainless steel grade 304 is not magnetic but it may become a little magnetic in areas that have been cold working (bending, deforming, etc.).
- Austenitic stainless steel grade 316 is not magnetic
Ferritic Stainless Steel
Ferritic is composed of iron and other elements. Since ferritic stainless steel typically contains large amounts of ferrite, they are usually magnetic. However, the strength of the magnetic pull varies among the different ferritic stainless steel types.
Ferritic stainless steel is typically less expensive with better engineering properties. However, with their lower chromium and nickel content, they are less resistant to corrosion.
Grades of Ferritic Stainless Steel
- 409 is an automotive exhaust grade.
- 430 and 430F are resistant to mildly corrosive environments.
- F20S is a ferritic alternative to grade 304 or 304L.
- F18MS / 444 is a ferritic alternative to grade 316 or 316L.
Martensitic Stainless Steel
Martensitic stainless steel contains chromium but no nickel. If iron is present, the crystal structure can be ferromagnetic. As a result, many of the different martensitic stainless steel types are magnetic.
Martensitic stainless steel is resistant to corrosion.
Grades of Martensitic Stainless Steel
- 410 is a standard grade suitable for low-duty hardened applications.
- 416 is a free-machining bar grade.
- 420 has a higher hardness level making it suitable for cutlery and cutting tools.
- 431 is primarily used for shafting due to its hardness and toughness.
- 440A, 440B, and 440C are very high hardness grades primarily used in the construction of cutting tools.
Duplex Stainless Steel
Duplex stainless steel is a blend of austenite and ferrite. Ferrite is the larger component of the two, so duplex stainless steel tends to be magnetic.
Due to the presence of austenite, the magnetic strength of this type is typically lower than that of the previously discussed types of stainless steel.
Grades of Duplex Stainless Steel
- 2101 is an economical alternative to 304 and 316.
- 2304 is an alternative to 316.
- 2205 is highly resistant to pitting and stress corrosion.
- 2507 and 2507CU are also highly resistant to pitting and stress corrosion.
Austenitic Stainless Steel
Austenitic stainless steel contains a large quantity of chromium and nickel. The increased nickel content makes most austenitic stainless steel non-magnetic.
Austenitic stainless steel is the most common type of stainless steel and often used in appliances, kitchen equipment, automotive trim, chemical equipment, and pharmaceutical equipment.
Grades of Austenitic Stainless Steel
- 301, 301L, and 301LN are suitable for roll-formed structural components due to their high strength.
- 302HA is a low work hardening rate grade.
- 303 and 303Se are free-machining bar grades.
- 304, 304L and 304H are high corrosion and oxidation resistant
- 310, 310S and 310H are high temperature resistant.
- 316, 316L and 316H have increased resistance to pitting corrosion in chloride environments.
- 321, 321H and 347 are suitable for heavy section welding and higher temperature applications.
- 253MA is resistant to high temperature.
- 904L is highly resistant to general corrosion and pitting.
Why Is Magnetism An Important Factor to Consider for Stainless Steel?
Magnetism is an important factor to consider for stainless steel because it affects the compatibility and performance of stainless steel in various applications, particularly in environments sensitive to magnetic interference.
Additionally, magnetism influences the complexity and difficulty of fabrication processes such as machining, forming, and welding. For example, magnetism complicates machining due to metal chips sticking to magnetic surfaces.
Also, magnetic materials can cause arc blow during welding, which leads to poorer weld quality. Therefore, magnetic properties must be considered to ensure precision and quality in fabrication processes.
Other Magnetic Steel Types Beside Stainless Steel
Apart from stainless steel, there are several magnetic steel types. Check the table below for a quick comparison between them:
| Steel Type | Electrical Steel (Silicon Steel) | Maraging Steel | Alloy Steel | Carbon Steel | Tool Steel | Stainless Steel |
| Magnetism | High | High | Moderate to High | Moderate | Moderate | Generally Low |
| Magnetic Permeability | Very High | High | Moderate to High | Moderate | Moderate | Low |
| Coercivity | Low | Low | Moderate | High | High | High |
| Application | Transformers, electric motors, generators | Aerospace, tooling applications | Various industrial applications | General engineering, construction | Tools requiring high hardness | Corrosion-resistant applications, cookware, medical instruments |