Straight from the baker to you…

Making steel is just like this sort of ...

1) Inclusions are on the inside, not on the outside surface…
2) Inclusions are non metallic materials entrapped within a solid metal matrix.
3) Inclusions that are typically expected include Sulfides (Type A), Aluminates (Type B), Silicates (Type C) and Globular Oxides (Type D)
4) Other types of inclusions are called exogenous  inclusions as they come from materials not expected to be entrained or entrapped within the steel-  typically slag or refractory that might have broken off during steelmaking.
5) Inclusions are measured and rated in North America according to ASTM method E45
6) Bearing Quality Steels use a number of different practices in order to minimize the inclusion content (because inclusions would wear differently than the host metal, thus nucleating premature wear and failure.)
7) Steel Cleanliness, Steel Microcleanliness, and Inclusion content are all  different ways of talking about the presence of these non metallic particles within the steel itself.
Three reasons inclusions are normally expected in  plain carbon and alloy steel bar products  in our shops:
‘8) Manganese sulfides are expected to be present as they aid machining.
9) Silicates are expected to be in non- free machining steels as silicon is added as a deoxidizer to assure the soundness (freedom from gas bubbles and voids) of the steel
10) Aluminates are also expected if the steel is ordered as Aluminum Fine Grain. the Aluminum scavenges Oxygen and  nucleates the formation of fine grains of austenite.
11) The Manganese Sulfides promote free machining as they provide a place for the chip to break and help control welding of material (built up edge) on the tool edge. In leaded steels, the lead is closely associated with these manganese sulfide inclusions.
12) The Silicates and Aluminates in our common steel grades are of high hardness, abrasive, and are a primary reason for tool wear and edge chipping in ordinary steels.
13) A quick look at the certification tells us whether or not we will find these kinds of inclusions- just look at levels of Manganese, Sulfur, Silicon, and Aluminum.
For machining, in keeping with the baking theme, I like to think of Manganese Sulfide inclusions as “kinda like the raisins in raisin bread.”
Bakers dozen photo credit.

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.”

Friday, May 16, 2008 (3).max
This is Coarse Grain Austenite. You like it for machining.

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?

  1. 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)
  2. Better Plastic Forming–  than Fine Grained Steels
  3. More Distortion in Heat Treat- than Fine Grained Steels
  4. Lower Ductility at the same hardness- than Fine Grained Steels
  5. 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.