There’s a tonne of different types of stainless steel and all of them are coded through a series of confusing numbers. It’s enough to trigger old math-related nightmares. Don’t worry, we’ll guide you through the basics. No math or chemistry expertise required!
Even when we cut out the science, it’s a very complicated topic. In this blog we’ll focus on the broad families of stainless steel. Later, we’ll hone into the differences between specific metals.
Counting by 3’s
Generally speaking, stainless steel types are named through a three digit number. The first number is the series – family – that they belong to. Type 410 stainless steel belongs to the 400 series and 201 stainless steel belong to the 200 series.
Each series of stainless share common overarching physical traits which are derived from their chemical and molecular structure.
Counting by 2’s
Down a digit, the second digit doesn’t differentiate the types of steel further. Instead the last 2 numbers function together to label a steel’s type within the family. Think of the first digit as your family name and the remaining 2 digits as your first name.
Harry Brearley (inventor of stainless steel) is Harry from the Brearley family. Type 210 is 10 of the 200 series.
Around the same time Brearly came up with his alloy, other metallurgists came up with metals that could be considered stainless steel. Harry might not be the true inventor of stainless!
Counting by the numbers
Some series of steels don’t follow this coding altogether (a conspiracy to confuse us all!). You’ve got exceptions like J405 and 2205. You’ve got subtypes like 316L and 316Ti. For the sake of simplicity (sanity) let’s focus on the 3 stainless steel series that are most commonly used: 200s 300s and 400s – with special attention to the last 2 series, which are especially common.
Molecular structure plays a big role in differentiating the series of stainless steel. That’s because, when a metal has a different chemical composition, molecules interact with each other in different ways – leading to a different molecular structure. Which means that to have a different molecular structure also means to have a different chemical composition.
There are four main structures of stainless steel: Austenitic, Ferritic, Martensitic and Duplex. Even on a molecular level, they have an almost identical crystalline structure. The major difference between the structures is the metallic compound in the centre of the crystal.
If you want a detailed explanation of the chemistry behind each of these metals, you’re barking up the wrong steel post! We’re going to focus on the different uses of these steels and leave the science for Wikipedia
Chromium is one of the most important elements for rust protection
Ferritic Steel, Martensitic Steel, and the 400s
400 series stainless is a bit of an oddball compared to the other two series. Not only are the 400 series not austenitic, it also doesn’t belong exclusively to one molecular structure. Within the 400 series are steels with Ferritic structure and Martensitic structure. The majority of 400 series stainless types are ferritic steels. Chemically speaking, 400 series steel carries high amounts of Chromium and low amounts of nickel.
400 series stainless steel is typically seen as ok-quality steel for cheap. Nickel is the most expensive metal in stainless steels. The low amounts of nickel mean that 400 series steels can be made at a lower cost. The high amounts of chromium give 400 series a higher tolerance against rust and chemical corrosion – like the 300 series but not as effective.
While both the 400 and 300 series stainless steels both carry nickel, 300 series steels also have small amounts of another rust-resisting element: molybdenum.
The ferritic nature of most 400 series stainless steels means that they can go through a process called quenching. 400 series steels can be made stronger by heating them up and rapidly cooling them down repeatedly.
To sum up: 400 series have these characteristics:
- Magnetic: due to the lack of nickel
- Somewhat rust resistant: due to Chromium. Not as good as 300 series.
400 Series steels are known as engineering steels because they are tough, cheap and easy to form. Technically speaking, 400 series steel can do the work of 300 series steel in specific environments but we advise against it because it’s a bad long-term investment and, in some industries, it’s illegal.
The Austenitic structure is one of the four main stainless-steel structures. It’s normally achieved by adding nickel, manganese and nitrogen into the alloy. This molecular structure gives the metal two distinctive physical properties:
- Non-magnetic: magnets won’t stick to it
- Not hardenable: Rapidly heating and cooling these steels won’t make the metal stronger.
Of the 2 major series, 200 and 300 series stainless are both Austenitic.
The difference between 200 and 300 is not 100
The difference between these two groups of metals how they achieve their crystalline (Austenitic) structure. 200 series stainless steels do this by adding manganese and nitrogen to their alloys and 300 series stainless steels do this by adding nickel.
A different chemical process creatives different side effects. The high nitrogen content in 200 series makes these steels stronger – especially in cold temperatures – and more resistant to impact. The higher nickel in 300 series steel means more chromium (a common element in stainless steel) in the alloy. Chromium makes 300 series steel more resistant to rust and chemical erosion. For this reason, 300 series steel is the preferred family of stainless for sanitary environments like kitchens and medical practices.
In another blog, we’ll spiral down the rabbit hole even further to explore the Types of steel within the series.
Phew! That’s the basics done! There are more differences between the families of stainless steel but, hopefully, we’ve given you enough that you can easily spot a 400 series steel from a 300 series steel (hint: throw a magnet at it) and that you understand what each steel series is good for. This is just part 1 of a multi-blog series. Stay tuned as we dive deeper and explore the different types of stainless steel.