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.

Aluminum is a critical ingredient of  steel in our shops, not just as a stand alone material for machining.

And it makes a darn nice container for pressurized carbonated beverages like Pepsi...

Aluminum metal is used to make many parts produced by precision machining, and is finding increasing application in automotive because of its light weight and high strength to weight ratio.
But aluminum plays a key role in some steel applications that you should know about.

  • Aluminum is used as a deoxidizer.  Aluminum scavenges Oxygen from the melt reducing porosity in the solidiied steel.
  • Aluminum is used to produce a fine austenitic grain size. (Aluminum is the most effective element to control grain growth in steel.)
  • Aluminum is also used as an alloying addition in the amounts of 0.95- 1.30 weight 5 to make Nitriding steel. Nitriding increases the hardness of the steel by the formation of a hard, stable aluminum nitiride compound.

This is what nitrided steel looks like under the microscope.

Aluminum’s ability to scavenge Oxygen results in tiny aluminum oxide particles dispersed throughout the steel. As aluminum oxide is hard and abrasive, Aluminum is not deliberately added to free machining steels where it would destroy tool life.
Aluminum is more effective at grain growth control than elements like vanadium, titainium and zirconium. These three elements adversley affect hardenability because they form carbides that are both  quite stable and difficult to dissolve in austenite prior to quenching.
In the nitriding steel, this recipe is relatively distortion free at the temperatures up to the nitriding temperature. 

    Nitralloy “N” Nitralloy 135
C   0.22-0.27 0.38-0.43
Mn   0.50-0.70 0.50-0.80
P   0.035 0.025
S   0.040 0.025
Si   0.15-0.35 0.20-0.40
Ni   3.25-3.75 0.25
Cr   1.00-1.35 1.40-1.80
Mo   0.20-0.30 0.30-0.40
Cu   0.35
Other   Al, 0.95-1.30 Al, 0.95-1.30
Source   ASTM A355-89 AMS 6470J

So yes Virginia, you may have more Aluminum in your shop than the number of aluminum bars, soda cans and foil wrappers might lead you to believe. Hiding in your steel!
 Nitride structure
Nitralloy Table

 Silicon plays many roles in steel but its most important is deoxidation; it is detrimental to tool life, machinability and surface quality in low carbon and free machining steels.   

Silicon is an important ingredient for quality steel.

Silicon makes up about a quarter of the earths crust. It is mined as sand, quartz, mica, talc, feldspars, vermiculite, and others; silicon is a key ingredient in glass, computer chips, and certain gemstones- rock crystal, agate, rhinestone, amethyst. Opal.
Opals are primarily silicon, but too precious to use for steel deoxidizing.

 The human body contains approximately one gram of silicon, ranging from 4 ppm in blood, 17 ppm in bone, and up to 200 ppm in various tissues. Cereal grains are our primary source of dietary silicon.
Silicon is seldom found as a pure element, because it has a high affinity for oxygen. It is this ability to scavenge oxygen that makes silicon important in steelmaking.
Silicon’s primary role in steel making is as a deoxidizer. It makes steel sound, by removing oxygen bubbles from the molten steel. The percentage of silicon in the analysis was related to the type of steel, rimmed and capped steels (made by the ingot method) had no silicon intentionally added. Semi-killed steels typically contained up to 0.10% max silicon, and fully killed steels could have up to 0.60% maximum. Commercial practice in the US and Canada throughout my career was 0.15-.35 % silicon in SAE carbon and alloy steels.
In addition to deoxidiation silicon also influences the steel five different ways:

  1. Silicon helps increase the steel’s strength and hardness, but  is less effective than manganese in these functions.
  2. In electrical and magnetic steels, silicon helps to promote desired crystal orientations and electrical resistivity.
  3. In some high temperature service steels, silicon contributes to their oxidation resistance.
  4. In  alloy grades, silicon also increases strength (but not plasticity!) when quenched and tempered.
  5. Silicon also has a moderate effect on hardenability of steel.

But there are always less desireable aspects of any element in an alloy

  • Silicon is detrimental to surface quality in low carbon steels, a condition that is especially magnified in low carbon resulfurized steels.
  • Silicon is detrimental to tool life in machining as it forms hard abrasive particles which increase tool wear and thus lower the steel’s machinability.
  • Bottom line, on plain carbon and alloy bar steels, silicon contents of 0.10, 0.15-.35 weight percent are typical; On resulfurized , and resufurized and rephosphorized  free machining steels, silicon analysis above 0.02 wt % is cause for concern, due to potential surface quality and certain tool life issues.
    Silicon metal photo

    The role of Manganese in steel in our precision machining shops.

    Manganese ore like this comes from Turkey.

    Carbon is a chemical element that is the primary hardening constituent in steel. Manganese is a chemical element that is present in all commercial steels, and contributes substantially to a steel’s strength and hardness, but to a lesser extent than does carbon.

    1. The effectiveness of Manganese in increasing mechanical properties depends on and is proportional to the carbon content of the steel.
    2. Manganese also plays an important role in decreasing the critical cooling rate during hardening. This means that manganese helps to increase the steel’s hardenability. It’s effect on hardenability is greater than that of any of the other commonly used alloying elements.
    3. Manganese is also an active deoxidizer, and is less likely to segregate than other elements.
    4. Manganese improves machinability, by combining with sulfur to form an soft inclusion in the steel that promotes a steady built up edge and a place for the chip to break.
    5. Manganese improves yield  at the steel mill by combining with the sulfur in the steel, minimizing the formation of iron pyrite (iron sulfide) which can cause the steel to crack and tear during high temperature rolling.

    Manganese is an important constituent of today’s steels.
    Now you know a few reasons why Mn (the abbreviation for Manganese) is the second element shown on the chemical analysis report (right after carbon).
    It’s That Important!
    Mn Ore Photocredit.