Speaking of Precision Blog

Speaking of Precision

Speaking of Precision is a knowledge preservation and thought leadership blog covering the precision machining industry, its materials and services. With over 36 years of hands on experience in steelmaking, manufacturing, quality, and management, Miles Free (Milo) Director of Industry Research and Technology at PMPA helps answer "How?" "With what?" and occasionally "Really?"

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A Baker’s Dozen Facts To Know About Inclusions In Steel

Posted March 22, 2011 8:54 AM by Milo

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.

Editor's Note: CR4 would like to thank Milo for sharing this blog entry, which originally appeared here.

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#1

Re: A Baker’s Dozen Facts To Know About Inclusions In Steel

03/23/2011 8:30 AM

As just a point of clarification for posterity, the term "inclusion" normally refers to a substance within the heat that is undesirable. When we add the elements referred to in your post to enhance certain capabilities, and thus are desired, we refer to those not as "inclusions", rather they are "alloys". That said, I found your post informative and creative. Good work!

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#2
In reply to #1

Re: A Baker’s Dozen Facts To Know About Inclusions In Steel

03/23/2011 9:47 AM

Good morning Weldeng449. Thanks for your comment.

As a professiional metallurgist, I accept your sincerity in trying to further define the terms, however I take exception to your use of the term "alloy" as a valid descriptor for chemical substances that find there way into a heat iof steel. Lead for instance is deliberately added to enhance machinability. However it is NOT an alloying element. Silicon added as a deoxidizer does not make the heat now a "silicon alloy," same with Aluminum. Copper is a residual or tramp element that is higher in scrap fed electric furnace steels than with new melt BOF steels. THe copper is limited to 0.35 wt % typically. This does not make the steels that show this level of copper "copper alloys."

Because Lead does not alter Mechanical properties such as Tensile strength , Yield strength, hardness etc, it is not considered an alloying element. But it is in fact added to both plain carbon and alloy steels. There are no LEAD alloys of steel, as compared to chrome moly, or nickel chrome moly alloys.

Your statement that the the "normal" use of the term inclusion to mean a substance within the heat that is undesireable may in fact be the case for you as a professional welder. However, inclusion materials such as manganese sulfides are NOT undesireable in resulfurized steels, and in fact are a result from the specified chemical composition requirements to assure their presence. In the case of a high quality bearing steel, they would be undesireable, and the chemistry spec would be adjusted to try to minimize their presence.

As I pointed out in my post, silicates and aluminates are also part and parcel with the chemistry needed to assure freedom from porosity and to assure fine grain size. The addition of silicon or aluminum to say grade 1045 does not make it a silicon alloy nor an aluminum alloy. From a machininsts point of view these may be undesireable as they reduce tool life, but as they are chemically specified by the Composition per AISI, SAE, ASTM, etc, it is wrong to claim them to be undesireable when they are in fact the known legitimate resultant of meeting the specified chemical composition for grade.

I am certain that your professional experience gives you the grounds to insist that inclusions are undeireable, and I would agree in welding. But in the steels to be welded, they are normal expectancy, which I wanted to explain to people as one of the goals of my post.

Perhaps another way of looking at this is like lumber: When you order knotty pine, it is with the understanding that you may not reject the wood because it has knots in it. The steels typically encountered by my audience of precision machining shops will always have a normal expectancy of inclusions such as Sulfides, silicates, and aluminates. Globular oxides are also a function of the steelmaking process.

Thanks again for sharing your thoughtful point of view.

Milo

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#3
In reply to #2

Re: A Baker’s Dozen Facts To Know About Inclusions In Steel

03/23/2011 11:57 AM

Milo,

Thanks to you for your reply! Also thanks for the distinction of perception. I can now see where my perception as a "professional welder" would differ from that of a "professional metallurgist" as to the term NORMALLY used for inclusions and alloys.

I suppose I should have clarified that for those of us who are not "professional metallurgist", inclusions are NORMALLY, (as opposed to ALWAYS) seen as detrimental; whereas if an alloy is added for purpose, it is usually for a desirable effect.

Please forgive my ignorance and thanks for setting me straight!

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#4
In reply to #3

Re: A Baker’s Dozen Facts To Know About Inclusions In Steel

03/23/2011 12:16 PM

Hey there. Its not about ignorance. Just different words and understanding. Different "lenses" by which we get our view on things.Your lenses have to reduce harmful light rays from the arc or flame. Our lenses needed more light (first metallograph I worked on actually had carbon arc light source. Bellows was as long as an alligator...) to get an image through a microscope.

Of course we'll have different perspectives- THATS what is cool about this site- everyone can bring their facts confidently, only to see that things "ain't necessarily exactly the way we thought."

As far as I know, Inclusions are a real big bad boy in weld integrity, which was your perspective, so I totally understand. Thanks for contributing your opinion. it raised the level of discussion.

Milo

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#5
In reply to #4

Re: A Baker’s Dozen Facts To Know About Inclusions In Steel

03/23/2011 1:14 PM

I understand and couldn't agree more. Dealing with other's perceptions can be both enlightening and frustrating. The more you can make it the former rather than the latter, the better off you are! That's what I try to do anyway.

Again, thanks for your perspective. It was enlightening, interesting, and informative. Also much appreciated!

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