Stainless steel is an extremely common metal, and it’s highly useful since it’s so good at resisting corrosion.
There are a lot of different grades of stainless steel. About 150-200 in all, but only around 15 of them are common. We’ll just be focusing on the SAE grades of stainless, since this is what you’ll most commonly come across in North America.
Probably the most useful thing will be how to tell what kind of stainless you’re working with based on its application and other recognizable characteristics. I’ll also add in some other useful info into the mix, like how to handle welding it and tips on machining and heat treating, too.
I’ve also made the ultimate reference chart for grades of stainless steels (printable). It includes descriptions and applications of every grade of stainless that I could find info on.
I made this for myself a while ago and refer to it pretty regularly. You can print it out and stuff it in your toolbox like I did. I’ve also posted a version of it at the bottom of this article.
Five Categories of Stainless
It’s really helpful to have a basic understanding of the 5 categories (or families) of stainless. 3 of them are main categories, 2 of them are combinations of the categories.
The categories are based on the crystalline structure of the metal. By understanding the characteristics of these 5 categories, you’ll have a much easier time identifying grades.
Austenitic Stainless Steel
Austenite is the primary structure for these types of stainless, and it’s stabilized by nickel, manganese and nitrogen.
They are not hardenable by heat treating and they are generally non-magnetic. I’ll get more into magnetism later on.
These make up the 200 and 300 series of stainless, and this is by far the most common type you’ll run into.
Ferritic Stainless Steel
Ferrite is the primary structure, and this type of stainless is primarily made of just iron and chromium.
It’s cheap and it handles high temperatures well (it’s resistant to Stress Corrosion Cracking aka SCC) and can be hardened by heat treating depending on the grade. A lot of the 400 series of stainless are in this category.
While not the most common, you’ll definitely run into this kind of stainless on a regular basis.
Martensitic Stainless Steel
Martensite is the primary structure. It’s heat treatable and strong. The corrosion resistance is moderate.
This stuff machines well but is terrible for welding and forming. This is what the rest of the 400 series of steel are made of. Knifemakers generally like this type of stainless.
Precipitation Hardening (PH) Stainless Steel
This is a mix of martensitic and austenitic grades. You can have some that fit both categories.
Precipitation hardening is actually a really cool and complicated process, and I’m working on putting together an article that explains it simply. I’ll post a link here when I’m done. This is, in my nerdy opinion, the most interesting kind of stainless.
This stuff is an absolute workhorse of the stainless world. They can typically take 3-4 times the strain of 304 stainless. This is because they’re made with nickel plus some other elements to fine-tune the characteristics.
PH steels generally machine well and they’re weldable under the right conditions (annealed, solution treated, and overaged).
They don’t really get all that hard, though, so they’re not good for anything that requires good wear resistance or a knife edge.
This is a more exotic type of stainless so it’s not too likely that you’ll run into it on a regular basis. 17-4 PH is the most common, and it also goes by the grade designation 630 (although less commonly).
Duplex Stainless Steel
These ones are specialized. They’re a combination of austenitic and ferritic stainless types.
They’re used when higher strength is needed, but you also have to have a higher corrosion resistance. It’s another kind of high-performance, exotic metal.
You can have lean-duplex, standard-duplex, and super-duplex. The lean-duplexes are cheaper versions of the standards, and super-duplexes are really complicated, specialized and tricky to make.
This is arguably the least common type of stainless that you’ll run into, but it does have some really interesting applications.
Grades of Stainless and Common Applications
This is generally my preferred way of identifying any kind of metal. There are typically a small handful of metals that are common in a particular application. That means that if you know what metal is used for what, you can generally narrow down the possibilities quite a bit.
The Most Common Grades of Stainless
Hands-down, the most common grade of stainless is 304. You will literally find it everywhere. It’s economical and performs really well in general applications.
It goes by a few different names, depending on application and location.
For example, it’s often called 18-8 in cookware or occasionally by the old trade name “Staybrite”. Outside the US, you might hear it being called A2 stainless (especially with fasteners). It could also be UNS S30400 in North America, DIN 1.4301 in Germany or SUS304 in Japan. It’s all the same basic stuff.
Either way, it is by far the most common, and you’ll find it everywhere that doesn’t have demanding engineering requirements for strength or corrosion resistance.
The next most common stainless is 316. It’s really corrosion resistant. It handles salt water no problem and is extremely easy to clean without staining.
It sometimes is called A4 stainless steel or marine grade stainless. 316L is the common grade for saltwater.
It’s more expensive than 304, which is why it isn’t used quite as much. Usually, you can guess which stainless is being used by the application.
Common applications of 316 stainless are:
- Anything in contact with salt water or other chlorides
- Food processing and kitchen equipment
- Pharmaceutical equipment
- Medical equipment
- Chemical equipment
Basically, it’s a bit more expensive but it’ll last longer in corrosive environments.
Less Common Grades of Stainless
There are a handful of other kinds of stainless that are worth knowing about, and here’s a quick overview.
201 is a kind of a poor man’s stainless. It’s frequently used in lower-end consumer goods and cheap banding. It’s much more prone to rust than 304, although it is pretty tough stuff. The reason it’s cheap is that it has a low nickel content.
303 is a common one. It’s almost the same as 304, but they add sulfur to make it more machinable. Actually, a lot of technical drawings will list 303 as an acceptable substitute for 304.
309 is used in high-heat applications, and it’s common for high end exhaust systems, burners, furnaces, etc. It resists corrosion (note that this is different from being corrosion-free) up to 1900 F. It’s not meant for wet applications.
416 is another kind of stainless that’s used for machinability. It’s a martensitic stainless with sulfur and phosphorous added. It’s inexpensive, but it’s got lower corrosion resistance compared to 304. It’s magnetic and heat treatable.
The most common application of 416 is in pump shafts and sometimes in after-market exhaust systems.
17-4 is found in high-strength shafting, as well as in miscellaneous surgical and aerospace applications. It’s really strong but it doesn’t get too hard (and brittle).
440 is a common one for things like kitchen utensils and knives. It is great for heat treating, and it maintains a good cutting edge. There are a few variants of 440 that are designated by a suffix of A, B, C and F.
The main difference between the types of 440 stainless is the carbon content. Carbon is what makes the steel hard when heat treated. A has the least amount of carbon, B is medium, and C and F have the highest amount of carbon.
440C is commonly used in knives, as well as other cutting tools that are meant for wet or corrosive environments.
What the Suffixes Mean
There are a few common suffixes that are important details when it comes to the properties of the stainless.
For some of the grades, the suffix means something specific to that grade. For example, 440 (which we just went over) has four common suffixes: 440A, 440B, 440C, and 440F. The F is a free-machining variation of 440C
There are a few suffixes that are more generic. Here’s an overview:
L or S are added to indicate low carbon. Generally, it’s a bit weaker but more weldable. You’ll see that the L designation is common with 304 and 316, and an S is used for 310.
LN is a low carbon but with nitrogen added. This counteracts the strength reduction from lower carbon but maintains weldability
Ti is a suffix you occasionally find with 316. It stands for titanium, which is added for heat resistance. You’ll find this used in some chimney liners.
F means free-machining, meaning that it puts less wear and tear on cutting tools. It will also take a better polish more quickly. This is often used for things when cosmetics are important, like in watches and other consumer goods.
The Magnet Test
This is really worth considering here because there’s a lot of confusion about this test.
Many people think that to test if a metal is stainless or carbon steel, you just need to see if it’s magnetic.
If it’s magnetic, it’s carbon steel. It’s nonmagnetic, it’s stainless.
So, half of that statement is true. If it’s not magnetic (and not another obvious kind of metal, like titanium, zinc, aluminum, or brass) then it is very likely stainless.
HOWEVER
Stainless can be magnetic, too. Actually, all stainless types are magnetic except the austenitic grades.
But austenitic grades, like 304 and 316, can become magnetic. This is done through cold work, like bending, pressing, blasting and cutting.
Basically, if you buy a rod of 304 stainless, it will likely be non-magnetic. But if you start doing things with it, it will start to become magnetic.
That said, it will never be as magnetic as the other types of stainless or as regular carbon steel. So you should feel a difference by how magnetic it is.
The point is, you’ll need to develop a bit of a feel for this test, and even then it can be inconclusive. Other factors, like the strength of the magnet, the size/thickness of the steel, and the actual grade of stainless, can all mess with your judgement.
Giant List of Stainless Grades
I consider this chart to be my greatest feat in scraping information from the Internet. It took days worth of work and several dozen different websites to compile this complete of a list.
Types-and-Applications-of-Stainless-SteelIf you’re having any trouble viewing the document, you can open it directly by clicking on this link.