by Terry Primos
The naming convention for steels can get quite confusing at times. Some are named with a
series of letters and numbers, others are named with just numbers. There actually is
some degree of order among the chaos.
Listed in this installment are two of the methods used in the classification of steels.
One is a system devised by the American Iron and Steel Institute (AISI) in cooperation
with the Society of Automotive Engineers (SAE). This system groups tool steels by their
purpose or unique properties. The second method, called the Unified Numbering System (UNS)
uses a series of 4 to 5 digits to classify steels according to their primary alloying
element, the approximate content of the primary alloying element, and the approximate
carbon content in hundredths of one percent.
I will apologize in advance to those looking for some rhyme or reason in the naming
conventions of various stainless steels such as 440C, 154CM, ATS-34, etc. You won't find
it here. This was neither an oversight, nor an intended omission. The fact is that after
several hours of research on the Internet, perusal through the books in my library and
a couple of phone calls including one to a steel mill, I was unable to get any information
providing a correlation between the names of these steels and the group they fall into.
Classification of Tool Steels
||Description or Notable Properties
||Die steel, air hardening, high chromium
||Hot work, chromium, tungsten, and/or molybdenum
||Tungsten alloy, high speed steel
||Molybdenum alloy, high speed steel
||Low alloy, special purpose
||Carbon-tungsten, special purpose
||Mild steels, low carbon and other types
Using this chart we can make some sense out of names like O1, A2, D2, W2, L6, etc. But
what about steels like 1084, 1095, 5160, and 52100? Where in the world did they come up
with that? These steels are classified under the Unified Numbering System, which in my
opinion provides us with more pertinent information as you'll see below.
Under this system, steels are assigned a series of 4-5 numbers. The first number tells
us the primary alloying element or elements, with 1 being plain carbon steel containing
no significant alloying element. The second number represents the approximate percentage
of the primary alloying elements. The final numbers indicate the approximate carbon content of the
steel in hundredths of one percent. Let's take a look.
1 - Plain Carbon (not an alloy steel)
2 - Nickel
3 - Chromium and Nickel
4 - Molybdenum
5 - Chromium
6 - Chromium and Vanadium
7 - Tungsten
8 - Nickel, Chromium and Molybdenum
9 - Silicon and Manganese
Let's start with an easy one. With 1084 the first digit tells us that this is a plain
carbon steel. The second digit shows that there are no alloying elements. The final two
digits show that the steel contains approximately .84 percent carbon. Pretty simple. How
about 52100? The first digit shows that the primary alloying element is chromium. The
second digit means that there is approximately 2 percent chromium (this is rounded off).
The last group of numbers show that the carbon content is roughly 1 percent.
One thing that puzzled me for awhile was the second digit. If a steel is classified as
50xx, then is it a chromium steel with no chromium? No. It is a low chromium steel.
For example, 50100 contains about .45 percent chromium. The .45 is not enough to
round up to 1 percent, so it gets the value of 0. 52100 usually contains about 1.5
percent chromium, so it gets rounded up to a value of 2. A good way to look at the 5xxx
types of steel is:
- 50xx = low chromium
- 51xx = medium chromium
- 52xx = high chromium
I hope that this information will be useful to you. Please understand that I'm no expert
in this area. If any of my observations are technically or grossly inaccurate, please let