Have you always just assumed that stainless steel isn’t magnetic because your magnets don’t stick to your refrigerator?
You may be surprised to hear that stainless steel is available in both magnetic and non-magnetic forms.
Two conditions can make stainless steel magnetic:
- Iron in the chemical structure
- Quartz structure in a ferritic or martensitic structure.
Stainless steel that has an austenite structure is non-magnetic.
Austenitic grades such as 304 and 316 typically exhibit low levels of magnetism because they contain high amounts of austenite (face-centered cubic crystal structure).
On the other hand, martensitic and ferritic grades like 430 and 409 tend to be magnetic because they have a higher concentration of ferrite in their microstructure.
Simply put, the answer to “Is Stainless Steel Magnetic, comes down to the composition of the alloy.
Are All Stainless Steels Magnetic?
Stainless steel, which is an alloy, is a combination of different metals (or alloys), with Chromium being the main alloying element.
The ability of stainless steel to resist rust and tarnishing is due to the various elements – manganese, nitrogen, carbon, silicon, and chromium.
To be identified as stainless steel, the composition needs to be max 1.2% carbon and not less than 10.5% chromium.
Chromium is what makes stainless steel resistant to corrosion. Additionally, it is what makes some types of stainless steel magnetic.
If there is more than 11% chromium in the alloy, the stainless steel will be non-magnetic.
However, if there is only between 8-11% chromium in the alloy, the stainless steel may or may not be magnetic.
So what does this mean for you?
Well, if you’re looking for stainless steel that is guaranteed to be non-magnetic, you should look for an austenitic grade of stainless steel (such as 304, 316, etc.), as these have higher chromium levels (>18%) and therefore will be non-magnetic.
If you’re looking for stainless steel that ‘may’ be magnetic, you should look for a ferritic or martensitic grade of stainless steel (such as 430, 420, etc.), as these have lower chromium levels (<14%) and therefore may or may not be magnetic.
The bottom line is this: if you need non-magnetic stainless steel, austenitic grades are your best bet.
If you’re okay with the possibility of a little magnetism, go with a ferritic or martensitic grade.
What Types Of Stainless Steel Are Magnetic?
Stainless steel is available in more than 57 types allows. Countless custom alloys are also produced by manufacturers.
Each type’s composition varies. Depending on microstructures and metallurgical properties, stainless steel is categorized into;
- Duplex stainless steel – This one combines both ferritic and austenitic crystals to provide the best of both worlds. It is magnetic. Furthermore, it boasts greater corrosion resistance compared to the austenitic 316 and 304 stainless steel.
- Austenitic stainless steels – These have non-magnetic properties. They are what most people refer to when they say “stainless steel”. The 300 series alloys contain chromium and nickel as the primary alloying elements. They are non-hardenable by heat treatment.
- Precipitation-hardened steel – Precipitation-hardened steel is magnetic. The 17-4PH alloy is the most common type of precipitation-hardened steel. It contains chromium, nickel, and copper as primary alloying elements.
- Ferritic stainless steel – They have a body-centered cubic crystal structure like that of carbon steel. As such, they are magnetic. The 400 series alloys have chromium as their primary alloying element and can be hardened by heat treatment.
- Martensitic stainless steels – If iron is present, this type can have a ferromagnetic crystal structure. Martensitic steel’s magnetism is mainly because iron is the primary component.
Magnetic Properties of 316 And 304 Stainless Steel
Different stainless steel families boast different physical properties. The elements added to the alloy inform the different magnetic properties.
Basic stainless steel features a ‘ferritic’ structure that makes it magnetic. This steel, which is formed from adding chromium, can be hardened via the addition of carbon, thus making it ‘martensitic.’
‘Austenitic’ is the most common type of stainless steel. This type of stainless steel features a higher chromium content. It also contains nickel, which modifies the physical structure of the alloy, making it theoretically non-magnetic.
304 and 316 stainless steel are common austenitic grades. 304 contains 18% chromium and 8% nickel whereas 316 contains 16% chromium, 10% nickel and 2% molybdenum.
They are non-magnetic when annealed.
However, they can become magnetic when cold worked. This is because the changes to their structure result in the alignment of their atoms which creates magnetic domains.
The magnetism can be removed via heat treatment. The process involves heating the metal above what’s known as the ‘Curie point’ and then cooling it rapidly.
So what are the magnetic properties of 316 and 304 stainless steel?
304 Stainless Steel
Under annealed or cold-worked conditions, 304 has a slightly higher strength than 302. It also has better corrosion resistance in most environments making it ideal for food and chemical processing equipment, as well as medical devices.
When cold-worked, 304 becomes slightly magnetic due to the martensitic transformation that takes place. However, annealing this steel will return it to its non-magnetic state.
316 Stainless Steel
This austenitic grade contains molybdenum which increases the steel’s overall resistance to pitting and crevice corrosion, especially in chloride-rich environments. As a result of these properties, 316 stainless steel is commonly used in food processing equipment, marine applications, and chemical processing.
When cold-worked, 316 also becomes slightly magnetic. However, annealing this stainless steel grade will return it to its non-magnetic state.
In summary, the austenitic structure of both 304 and 316 stainless steel makes them non-magnetic when annealed and slightly magnetic when cold worked.
Does Stainless Steel Stick to A Magnet?
Stainless steels are not generally attracted to magnets because they contain little or no iron. However, some grades – such as 304 and 430 – may become slightly attracted to a magnet when cold worked.
Other grades, such as 316 and 2205, are non-magnetic in all conditions.
Some stainless steel can be mildly attracted to a strong magnet, but this is usually because of a small amount of ferrite (a type of iron oxide) in the microstructure of the metal.
If you need a stainless steel that is guaranteed to be non-magnetic, look for the ‘316’ or ‘2205’ grades. These are both austenitic grades of stainless steel and they contain very little or no ferrite in their microstructure.
These are both austenitic grades of stainless steel and they contain very little or no ferrite in their microstructure.
If you’re not sure whether a particular grade of stainless steel is magnetic, you can test it with a magnet. If the metal is attracted to the magnet, it’s likely to be ferritic. If it’s not attracted, it’s probably austenitic.
Why Don’t Magnets Work On Certain Stainless Steels?
It simply has to do with the magnetic properties of austenite and ferrite, which are what give stainless steel its strength.
Austenite is non-magnetic, but it can become magnetic when cold-worked. Ferrite, on the other hand, is magnetic.
So what happens when you try to magnetize stainless steel? If the steel has more austenite than ferrite, it won’t be attracted to a magnet.
If it has more ferrite, it will be attracted. But there’s another wrinkle.
Even if a piece of stainless steel is mostly austenite, it can still be attracted to a magnet if it has been cold worked—that is, if it’s been drawn, bent, or otherwise shape-changed by processes that introduce stresses into the metal.
Cold working aligns the austenite molecules in such a way that makes the metal magnetic.
While all types of stainless steel are non-magnetic in the annealed condition, different types of stainless steel can become magnetic when cold worked.
Ferritic stainless steels and martensitic stainless steels are the only types of stainless steel that are magnetic. If you’re looking for non-magnetic stainless steel, austenitic stainless steel is your best bet.
When austenitic stainless steel is cold worked, the molecules become aligned in such a way that makes the metal magnetic.
So, if you’re looking for non-magnetic stainless steel, austenitic stainless steel is your best bet.
Here are the different features of various series of stainless steel:
- 200 series – These aren’t magnetic because they contain no nickel. They will become slightly magnetic when cold worked
- 300 series – The 300 series contain chromium and nickel, but they’re non-magnetic because the microstructure is austenitic. They will become slightly magnetic when cold-worked.
- 400 series – The 400 series contain chromium, manganese, and silicon, but they’re non-magnetic because the microstructure is austenitic. They will become slightly magnetic when cold-worked.
Types of Stainless Steel That Are Not Magnetic
Several types of stainless steel are not magnetic.
Some examples include austenitic stainless steel such as grades 304, 316, 321, 310, and 330. Ferritic stainless steels such as grades 430, 409, and 410 are also non-magnetic.
You’ll want to avoid long term storage of citrus juice, vinegar, or tomato sauce in your stainless steel containers. These can cause your stainless steel container to develop small pits on its surface and ultimately shorten the life of your containers.
It is the chromium and nickel content of these alloys that make them non-magnetic.
Inconel, titanium, and duplex alloys are also examples of non-magnetic stainless steel.
These metals contain little to no iron which is what makes a metal magnetic.
Stainless steels that have a high carbon content can also be non-magnetic because the increased carbon inhibits the formation of ferrite in the microstructure of the metal.