The Basics of the North American Steel Grade System

In North America, the AISI/SAE steel grade nomenclature system is widely used.
In this system, 4  numeric digits (XXXX)  describe the base grade. The first two digits tell you whether the steel is a carbon or alloy grade.
If the first digit is any number other than a “1”, that steel is an alloy steel. We’ll discuss alloy steels in a later post.
If the first digit is a ” 1 “, the steel is a carbon grade. 10XX is the template for the plain carbon steels. We’ll explain those last two digits at the end of our post. (Exception: if the second digit is a “3”- then its one of the  alloy manganese 13XX grades- grades we don’t encounter very often these days.)
If the second digit is a “1”, the steel is a resulfurized  carbon steel.  11XX. Guess how many “extra” elements were added to the grade? If you guessed 1- thats right. Sulfur is  the one element added  to promote machinability in the 11XX grades of steel.
If the second digit is a “2”, the steel is called a rephosphorized and resulfurized steel. Both sulfur and phosphorus,-2 elements- are added to make these free machining steels.  1215 and 12 L14 are the grades we mostly see today. (As many of you know, that “L” as an infix tells us that there is a lead addition in the 12L14 steel.)
If the second digit is a “5” the grade is a high manganese carbon steel.  Grades 1524, and 1541 come to mind as the principal 15XX grades seen by our industry.
A “B” infix tells us that the steel has been treated with boron. This makes it especially adept at being heat treated. 15B21 is used  to make fasteners  that are heat treated.
So, now that you know what the first 2 digits mean in a US grade designation for steel, what about the last two?

The last 2 digits in the grade are the mean or average carbon content of the steel. In weight percent.  
So grade 1018, is a plain carbon steel, 0.18% average  carbon content.
1144 is a resulfurized 0.44% average carbon content steel for higher strength and machining.
And 1215, well- 1215 is a resulfurized, rephosphorized 0.09 max weight % carbon steel for machining. 0.09% max!
 Don’t you just love exceptions?