Straight from the baker to you…
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.”