In this post we’ll take a look at the significance of the last 2 digits of the steel grade designation. We will be discussing carbon, not alloy grades.
The first 2 digits give us an idea about whether the grade is a plain carbon or alloy steel. See our post here. So 1018 is a plain carbon grade;  1137 is a resulfurized carbon steel; 4140 is an alloy steel.
So lets look at those last two digits in the 4 digit  AISI/ SAE grade designation, and what they mean in the carbon grades we see in our shops.
The secret to understanding real estate  is location, location, location.
In steel, the secret to understanding is carbon, carbon, carbon.

Carbon is so important in understanding a steel’s characteristics, that in the North American nomenclature system, the last two digits of the grade are the average carbon content expressed as weight  %. Carbon is the most important indicator or predictor of a steel’s properties and response to processing.
So in that 1018 steel, 0.18 weight % carbon (on average) is implied; in 1137, 0.37 wt % carbon (on average) is implied; in 1144, the average carbon content we expect is, you guessed it, 0.44 wt.%.
So what does that mean to us as machinists?
Very low carbon. Grade 1008-1010. The low carbon content makes these steels low strength and very ductile. Typically used for cold heading. Cold forming. The machinist would characterize these as gummy. Chips are stringy, continuous, and soft.
Low carbon. Grade 1018; 1022. Low carbon means low strength. The non alloys in this range are weldable, and all of these grades are cold formable with out  the need for an anneal.  Grades in this carbon range are often carburized to achieve a high surface hardness. Not a good choice for machining, difficult to get chip to break.  Chips are somewhat continuous, and soft to semi-soft. Parts made from these grades tend to have low stock removal, and look like the bar that they were made from- bolts, light duty shafts, tie rods,  pins.
Medium carbon.  1045, 1137, 1144. Medium carbon means medium strength. Usually cold drawn. Can be heat  treated. Not recommended for cold heading. Welding requires special practices and residual control.  (Do not weld 11XX grades due to high sulfur content!)
Chips are continuous and semi hard (1030), continuous and tough (1035), and continuous and start to become springy or hard (1045-1050).  Small shafts, forgings, and kingpins are typical of these grades. Not usually annealed. Heavy draft (cold work) followed by a stress relief operation can get yield strengths into the 100,000 psi minimum. ASTM A 311 class B is one such designation, Stressproof (TM)  is Niagara LaSalle’s trademarked name for a similar product.
High carbon. Above 0.50 carbon, most of us start to describe steels as “high carbon.” Depending on the application, and carbon content, an anneal may be required for processing. My rule of thumb for carbon grades  is at 0.60 and above, an anneal is required prior to cold drawing. ( For alloys, generally annealing is required at 0.40% carbon.)   So a 1060 bar would be annealed prior to cold drawing.   The type of anneal for these steels would be called a lamellar pearlitic anneal. It would help to develop a  predominately coarse lamellar  pearlitic structure in the steel. Chips are continuous and range from hard (1060) to tough (1070) to springy (1080 and above).
Very high carbon. At 0.90  carbon and above, (drill rod and bearing steels) a different kind of anneal is called for. It is called spheroidize annealing, and results in a greater mean free path of ferrite between the hard carbide particles in these steels. Very high carbon steels are most machinable in the spheroidize annealed condition.
Adding sulfur to carbon steels is called resulfurizing. This addition provides a way to break up the chip, thus escaping the continuous chip that we get from 10XX steels. This is why 11XX and 12XX steels are so machinable.
Click here to learn how  steel chemistry might have contributed to the sinking of the Titanic.
Photomicrograph from JOM, 50 (1) (1998), pp. 12-18.