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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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Chromium in Carbon, Alloy and Stainless Steels is NOT Hexavalent

Posted April 23, 2013 12:00 AM by Milo
Pathfinder Tags: alloys chromium stainless steel

The valency (oxidation state) of chromium metal as an alloying constituent of steels is Zero (0).

The Chromium atoms are present in 'substitutional' lattice positions, replacing iron atoms. These atoms are held in place in the lattice by shared electrons which make up a 'metallic bond'. Since there is no loss or gain of electrons, the valency state is Zero (0).

The Chromium in solid steels (Carbon, Alloy, and Stainless) should not be regarded as a health hazard. That is why stainless steels in particular can be used for food prep and bodily contact in medical and dental applications- the chromium is not available in an ionic state.

Hexavalent Chromium Cr(VI) is an ionic state and typically encountered as a 'chromate' or 'dichromate' salt. These hexavalent compounds are typically found in plating solutions. Hexavalent Chrome has been identified as a cause for health concerns and shown to be toxic.

Machining Steels-Carbon, Alloy, and Stainless- does not expose the operator to hexavalent Chromium. Hexavalent chromium is an ionic form of chromium in a chemical compound. Metallic Chromium is a form that is sharing electrons as part of a metallic bonding arrangement. The valence state of metallic chromium is Zero (0).

We've written about this before:

Chromium in Steel

Hexavalent Chromium Rule Finalized

British Stainless Steel Association Article on Chromium in Stainless

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

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#1

Re: Chromium in Carbon, Alloy and Stainless Steels is NOT Hexavalent

04/23/2013 2:07 PM

'....Machining Steels-Carbon, Alloy, and Stainless- does not expose the operator to hexavalent Chromium.....'

Do you consider grinding to be a machining operation?

Welding or brazing can also lead to hexavalent chromium exposure.

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#2
In reply to #1

Re: Chromium in Carbon, Alloy and Stainless Steels is NOT Hexavalent

04/23/2013 2:24 PM

Grinding of the material i am discussing are always done under coolant to prevent decarburization and the creation of an untempered martenite phase. These could create product failure.

The Hexavalent chromium from arc welding is a result of the chromium in the filler metal primarily being ionized.

"In responding to the second question about hexavalent chromium generated during welding processes we need only consider the high energy environment of the electric arc welding process. During SMAW or Gas Metal Arc Welding (GMAW) work, the end of the electrode or filler wire is melted off and deposited in the weld pool. As the weld metal being deposited during welding passes through the welding arc, electrons are stripped from some of the chromium atoms. If 6 electrons are stripped from a chromium atom, that atom becomes hexavalent chromium [xi] and this atom, being shy of a full deck of electrons, is appropriately referred to as an "ion."

"The CrVI ion is not stable. Rather it is highly reactive and will often rapidly combine with atmospheric oxygen to form Cr2O3. In the case of SMAW work and Flux Core Arc Welding (FCAW) work, the alkaline metals in the flux will tend to stabilize the CrVI so that it remains in the negative 6 valence state long enough to reach the breathing zone of the welder [xii]. This means that the highly reactive CrVI ion can be inhaled and can attack the lung tissue.

"Since most of the welding processes fume generated comes from the welding consumable (noted above), the filler metal employed during electric arc welding is the primary source of hexavalent chromium in welding fume. Consequently, when working to comply with the provisions of the OSHA Chromium VI standard (29 CFR 19101.1026) one would be well-advised to review the Material Safety Data Sheets (MSDSs) for the welding consumables to determine if chromium (that may be converted to hexavalent chromium during welding) is present in the consumables. One should also be aware that some carbon steels contain recycled metals that include chromium. Even though most of the welding fume comes from the electrodes/filler wire, some of the fume does come from the metal being welded. Consequently, there is a potential for hexavalent chromium in the welding fume from these steels. The MSDS for these steel may or may not include chromium because, when present, chromium may constitute only a fraction of a percent of the metal."

- http://www.air-quality-eng.com/weldingsafety.php

I stand by my original statement: '....Machining Steels-Carbon, Alloy, and Stainless- does not expose the operator to hexavalent Chromium.....'

Milo

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#3
In reply to #2

Re: Chromium in Carbon, Alloy and Stainless Steels is NOT Hexavalent

04/23/2013 6:19 PM

There are numerous grinding processes that are commonly used on stainless steel that are not done under coolant. Processes using cutting wheels and grinding wheels are very common in manufacturing food service equipment out of stainless. Small die grinders, angle grinders, and cut off wheels are commonly used for cutting and fitting.

It is not at all reasonable to suggest that all grinding operations on stainless are 'always done under coolant'. You original message suggests you are not limiting your statements to some small subset of manufacturers working with stainless.

.

The last paragraphs you site in your reply are in direct conflict with the claim about a lack of potential exposure to hexavalent chromium.

There also seems to be some confusion about the certainty with which CrVI will combine to form Cr2O3. While this is one common result, it is completely unrealistic to think there will be no hexavalent chromium formed. While not as prevalent as Cr2O3, there will almost certainly be some CrO3 formed, for example arc welding high chromium content alloys.

Welders are typically most at risk for hexavalent chromium exposure, but any operator in the vicinity of chromium being heated to a very high temperature in the same atmosphere, is likely to have increased exposure to hexavalent chromium.

A number of processes utilized for machining processes today involve bringing the base metal momentarily to a temperature high enough to yield some hexavalent chromium when used on certain material. Electron beam machining, high speed grinding, EDM, spot welding, laser cutting are all processes that might be found in a machine shop and used by non-welders.

.

The truth is there are numerous ways one can be exposed to hexavalent chromium. If you are claiming that no machine operators are exposed because you are excluding from machining any operation that happens to present such hazards borders on trading people's safety in order to write nice things.....

Down playing the importance or the risk has the potential to do real harm.

.

Luckily, if someone is aware of the danger, much can be done to minimize/eliminate the risk.

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#4
In reply to #3

Re: Chromium in Carbon, Alloy and Stainless Steels is NOT Hexavalent

04/23/2013 6:44 PM

Thanks. I should have considered that reposting on this general site (CR4) where people might confuse metal fabrication processes for precision machining. My post is aimed at precision machining. Production machining. NAICS 332721. I posted it originally on my industry specific site. In our industry the processes you describe are not typical in machining. Thanks for sharing your point of view. Milo

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#5
In reply to #4

Re: Chromium in Carbon, Alloy and Stainless Steels is NOT Hexavalent

04/23/2013 8:12 PM

I get it now. 'Speaking of Precision' at the top of the piece should have clued me in a little sooner.

The piece as I originally saw it seemed misguided and negligent to a dangerous degree: so out of place and atypical of everything else I have seen you write. Now that I understand it was aimed at precision machining, it makes much more sense.

Sorry, if what I wrote came off harshly.

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#6
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Re: Chromium in Carbon, Alloy and Stainless Steels is NOT Hexavalent

04/24/2013 10:40 AM

It seemed strident, but I thought that we could clear it up. I found it remarkable to be seen as on the failing to recognize the hazards side of this discussion when I am actually about the most prosafety person that I know.

No harm - no foul. Good discussions like this to arrive at the relevant issues help us all learn.

Appreciate your Feedback.

Milo

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#7
In reply to #6

Re: Chromium in Carbon, Alloy and Stainless Steels is NOT Hexavalent

04/24/2013 11:07 AM

Precision machining interests me immensely, and I knowledge base is pretty limited on the subject.

Out of curiosity, Have you done any articles on scraping?

Many years back I read an interesting piece on creating and truing precision surface plates starting from three flat(ish) cast iron surface, using a scraper (and of course some blue).

That process wasn't so complicated.

There was also instruction for making I don't remember the instructions for making very square and flat gauge blocks, and that was probably more complicated as I don't recall the process exactly, and it did require a little more in the way of tools (but still nothing that wasn't easily obtainable as I recall) than just a scraper and blue, a reference for accurate measurement must have been required as well.

.

Have you done much scraping? That is considered precision machining, right?

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#8
In reply to #7

Re: Chromium in Carbon, Alloy and Stainless Steels is NOT Hexavalent

04/24/2013 11:16 AM

When rebuilding a machine, they often will hand scrape the ways.

I have seen it done.

I am not "that" guy, however.

My claims to the "black arts" in my field are the ability to sort grades using sparktest, and a few drop type wet chemical tests, Sort mixed status (hardness usually) parts with a hammer or files, and grind a tool without burning (decarburizing) it.

I have set up Browne and Sharps single spindle screw machines, and operated lathes mills and blanchard grinders, but make no claims to be a machinist- just a guy who can whittle a part out if given sufficent time.

I'll put together a post on the lost art of hand scraping. thanks for the tip.

Milo

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#11
In reply to #8

Re: Chromium in Carbon, Alloy and Stainless Steels is NOT Hexavalent

04/24/2013 9:30 PM

Very interesting.

.

I'd really like to hear about some of the tests you use for sorting (not asking you to divulge your best secrets or anything) in an upcoming blog. That skill has broad value well outside precision machining.

.

In my case specifically, occasionally when I travel to Orlando, Florida, I stop by a shop called SkyCraft (anyone who appreciates CR4 topics, should definitely make it point to visit.. look it up, or you can see what looks like a flying saucer out of the side of a building where Fairbanks meets I-4) which is a vast assortment of inexpensive surplus items I see no where else, from Litz wire to rolls of ptfe, power supplies to acrylic tubes, yagi antennas to stepper motors, gyros to carbon brushes...on and on...all for pocket change.. (I really have no connection with Skycraft, it is just such an amazing find, I can't help but spread the word to the type of people I know will enjoy it so much)

Anyway SkyCraft has a wild assortment or various metal pieces, typically parts that were machined and not used or damage in machining or just someones overstock or whatever. This isn't the typical pile of galvanized scrap.

You can find 6/4 and 3/2.5 Ti pieces and occasionally some billet. There is often 316 and 316L stainless, 6061 T6 billet, etc. All of it is just silly cheap and a good portion has labels (otherwise I'd have difficulty saying if it was 304 or 316 or 316 L for example.)

Some of it without labels, I can still be fairly certain of...at least knowing broadly (not specifically) what I'm handling.

.

But there are numerous items that are real puzzles, and Skycraft staff doesn't know either. For example there are these panels that appear to probably be stamped. the material is exceedingly light, lighter than aluminum, about like magnesium, and damn stiff. These are pieces are really thin, but you can't noticeably deform the panels by hand. They are typically powder coated yellow and sometimes grey (and are still freakishly light even with the powder coating). These might be from Lockheed, but no confirmation.

So my guesses were that it could be magnesium (but it still seemed really stiff even for magnesium), an MMC (which would account for the stiffness), or maybe beryllium or an aluminum beryllium alloy.

I think the chance of it being beryllium is very slim (approaching none), but the prospect of berylliosis has me concerned enough I didn't make any additional scrapes in the coating....

I'd appreciate any tips for distinguishing pieces like those or others. I have a decent feel for what things like cast magnesium, Monel, Tungsten, Brass, Bronze, Aluminum are generally, but I'd love to know quick tricks to be more certain, or know more specifically.

Hey, and just to be certain we are on the same page. These are merely suggestions for future blogs, I'm not requesting that you go out of your way to work up a tutorial for me... its just something I think a lot of people would find interesting. But I've also been known to completely misjudge what 'a lot of people would find interesting' so I certainly wouldn't be insulted if it isn't something you devote time to writing about in the future.

.

Anyway, thank for the discussion so far. Forgive me if I seem to be trying to pick your brain too aggressively.. it just looks like that is some fascinating stuff up there.....

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#9

Re: Chromium in Carbon, Alloy and Stainless Steels is NOT Hexavalent

04/24/2013 2:41 PM

to milo and truthisnotacompromise: I really appreciate CR4 because of give-and-take such as presented here.

I have spent most of my career in Semiconductor Manufacturing but now am in a field which has me looking at manufacturing efforts ranging from low-tech 3 man shops to fairly sophisticated large facilities. Reading these blogs exposes me to a broad cross-section of disciplines which help me to appreciate and understand what I am seeing in real life.

The best compliment I get is when an engineer, after a meeting, says to me, "You ask good questions." It's this kind of stuff that let's me ask good questions.

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#10
In reply to #9

Re: Chromium in Carbon, Alloy and Stainless Steels is NOT Hexavalent

04/24/2013 3:01 PM

Well thanks for the feedback.

When we can understand that there might be a reason for a misunderstanding besides that the other person just doesn't get it, and demonizing them, we can improve how we communicate.

Improvement is what I think it is all about.

So rather than take comments as offensive or critical, I always try to see what might the issue be that causes the difference. That patience helped me to see that Truthisnotacompromise was considering metal fabricating, not just machining in his thinking.

So now I have relevant user feedback about the need to be more explicit when I post about processes- especially involving human safety. And I got a great Idea for a new post from him.

WIN. Win Win.

Thanks.

(If you are in the precision machining space, with your work, we probably ought to connect via Linked in. Send me a CR private message and we can connect.)

Milo

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