Austenitic Grain Size is a material characteristic that is usually reported on test reports and certification documents for the steel materials that we machine in our shops.
Coarse Austenitic Grain Size is a result of NOT ADDING grain refining elements to a heat of steel. Because these Grain refining elements have not been added, the steel has a “Coarse Austenitic Grain Size.”
Typically this practice is applied to free machining grades such as 11XX and 12XX steels. These steels are sold primarily for their ability to be machined at high production rates.
What does Coarse Austenitic Grain Size imply for the parts that you make?
Better Machinability– Coarse Grained Steels are more machinable and provide longer tool life than Fine Grained Steels. (The elements added to make the Austenitic Grain size fine create small, finely dispersed hard abrasive particles in the steel)
Better Plastic Forming– than Fine Grained Steels
More Distortion in Heat Treat- than Fine Grained Steels
Lower Ductility at the same hardness- than Fine Grained Steels
Deeper Hardenability– than Fine Grained Steels
Coarse Austenitic Grain Size will show up on the test report as an ASTM value of 1-5. Values of 5 and higher are called Fine Grained Steels, and are the result of additions of Aluminum, Vanadium, or Niobium in North American commercial practice for most Carbon and Alloy steels.
The methods for determining Austenitic Grain Size are detailed in ASTM Standard E112, Standard Test Methods For Determining Average Grain Size.
A nice discussion can also be found HERE.
While we think that chemistry may be the controlling factor for machining performance of the steel in our machines, the contribution of austenitic grain size is also important. As long as you are ordering your free machining steels (11XX and 12XX series) to Coarse Grain Practice, Austenitic Grain Size should not be an issue in your shop.
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.
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?