If you ask someone “is stainless steel magnetic?,” you are likely get a wide range of responses. Some people believe that stainless steel is a completely non-magnetic material. Others believe that stainless steel must be magnetic because it contains iron.
So, do magnets work on stainless steel? Yes and no. Like most things in life, the answer lies in the grey zone, in that murky in-between. The fact is, there are many types of stainless steel alloys, each with magnetic and non-magnetic variants. There is no correlation between magnetism and corrosion resistance.
We can determine whether a piece of stainless steel is magnetic most often by observing its chemical composition.
When alloyed, ferrite and iron crystal structure give stainless steel magnetic properties. High austenite levels offset the ferrites, rendering these stainless steel alloys non-magnetic. And even in these cases, non-magnetic grades can become magnetic once worked on (i.e., bent, deep drawn or formed into a tube).
Still other stainless steel alloys gain partial magnetism after forming or undergoing heat treatment.
And then there are the category of stainless steels which are highly resistant to corrosion and carry a magnetic field (such as 420 and 430 grades). The inverse also holds true – some are corrosion-resistant but non-magnetic (such as 316).
Depending on your requirements, some types of stainless steel are better suited than others for specific applications.
What is stainless steel?
Stainless steel is a metal alloy comprised of iron, chromium, silicon, carbon, nitrogen and manganese. It is primarily known for its resistance to tarnishing and excellent corrosion resistance, a key consideration for certain applications. To be recognized as ‘stainless,’ said alloy must contain at least 10.5% chromium and no more than 1.2% carbon.
Based on their internal composition, we can divide stainless steels into five different categories:
- Austenitic stainless steels
- Ferritic stainless steels
- Martensitic stainless steels
- Duplex stainless steels
- Precipitation-hardened steels
Criteria for stainless steel magnetism
Stainless steel grades play a contributing factor, but in general, for stainless steel alloys to be magnetic, they must meet the following requirements:
- The alloy must contain iron (Fe).
- The crystal structure of the stainless steel alloy must be arranged in a martensitic or ferritic structure.
First and foremost, since stainless steels are a type of steel, they must contain iron in their chemical composition. This makes stainless steel magnetic.
The second requirement is that the stainless steel must have its crystal structure be arranged in a ferrite or martensite structure. If a stainless steel is mostly comprised of an austenite structure, then it will not be magnetic.
As we’ve already mentioned, corrosion resistance has no bearing on magnetism. In stainless steels, corrosion resistance is dependent on the concentration of chromium (and sometimes molybdenum). The more of each, the better the corrosion resistance.
Which types of stainless steel are magnetic?
Not all stainless steels are the same. Below are the main family characteristics that differentiate stainless steel types from one other:
Austenitic stainless steels
This is the most widely used family of stainless steel. Most austenitic stainless steel are non-magnetic due to the greater presence of austenite. Even though stainless steel grades such as 304 and 316 stainless steel have high amounts of iron in their chemical composition, austenite renders them non-ferromagnetic.
With special thermal treatment or work hardening, you can render some austenitic stainless steel alloys in this branch partially magnetic, as this process changes the underlying crystal structure, forming ferrite in some locations.
Ferritic stainless steels
The most cost-effective family of stainless steel. Ferritic stainless steels have magnetic properties due to the alloy’s large ferrite composition.
Ferrite is a compound of iron and other elements. The combination of a ferritic crystal structure with iron makes ferritic stainless steels (such as the 400 series) magnetic.
However, some variants of this ferritic stainless steel exhibit a weak magnetic pull.
Duplex stainless steels
One of the most recent alloys of stainless steel that offers a nice balance of greater corrosion resistance, easy fabrication, and price savings because of the absence of nickel (an element used to make grades non-magnetic).
The mixture of austenitic and ferritic crystals within Duplex stainless steel contributes to their magnetic pull; the substantial amount of ferrite (which is magnetic) is also important. The best known Duplex grade on the market is 2205.
Precipitation hardening & martensitic stainless steels
Used for hardened edge (cutting) applications. The material becomes magnetic after hardening.
Conversely, many martensitic stainless steels are innately magnetic. This includes common grades such as 410, 420 and 440. The unique crystal structure of martensitic stainless steels can be ferromagnetic if iron is present. Since stainless steel is a type of steel, there is an abundant amount of iron in its composition – this is the primary reason why martensitic stainless steel are magnetic.
How does steel magnetism work?
Magnetism occurs primarily due to the uneven distribution of electrons in atoms of certain metal elements, including iron. A shift occurs within said atom, creating magnetic dipoles – a by-product of the irregular rotation of electrons.
Which types of stainless steel are magnetic?
The ferritic, duplex, martensitic, and precipitation hardening families are all magnetic.
What types of stainless steel are not magnetic?
The following variants of stainless steel lack magnetic properties:
- Austenitic steel grades 301 and 304 are not magnetic but can become partially magnetic with cold forming.
- Austenitic steel grades 302HQ, 303, 310, 316, 321 253MA, 904L.
Why is magnetism important in stainless steel?
Magnetic materials can have a huge effect on the intended performance of said material, depending on its application. If a material needs to be quickly sorted from other materials, than having one material be magnetic simplifies the sorting process.
But when we’re talking about manufacturing processes like welding and fabrication operations, magnetic material can have a detrimental effect.
Why? For one, electric currents can exhibit odd behavior in magnetic materials. Secondly, magnetic fields interfere with the welding spark and can create poor welds.
Sure-fire test for magnetism
Shopping for stainless steel and not sure if it’s magnetic? Bring along a magnet. The higher the quality of stainless steel, the more likely it’s non-magnetic (which also means it costs more). That’s not to say magnetic stainless steels aren’t useful, but more a case-by-case situation.
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