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Stainless steel surface composition

Stainless steel surface composition

(OP)
I am running incoming inspection of stainless steel bolts, to be specific, A286. I am using hand-held XRF, so it is surface sensitive. All elements meet the requirement except titanium. Instead of 2% Ti, I am getting 1.5% consistently. I wonder if passivation/pickling process is known to suppress Ti concentration. Or any other reason why Ti is lower on the surface? Thank you.

RE: Stainless steel surface composition

The XRF is only a qualitative process and you are well within the reach for %Ti for A 286 given existing surface conditions. I would accept the material.

RE: Stainless steel surface composition

first of all, you want to know if Ti at surface is really lower than inner given the qualitative nature of XRF, especially for slight Ti. I cannot think of a process that can lost Ti on surface unless it is severely oxidized to form scales. You may grind a bit to expose a fresh surface to do XRF again, or, use other accurate methods to confirm. If Ti is really as low as 1.5%, you will not be able to meet the strength req's. AMS 5525 specifies Ti min of 1.9%, our internal min is 2.0%.

RE: Stainless steel surface composition

1. Try verifying the machine's sensitivity by checking with a calibration block with known & certified chemical composition. Get a block with totanium value in the same range such as near 2%. This would help you determine your machine's sensitivity to detecting titanium in these ranges.

2. sometimes when detecting such micro-alloyed elements, PMI equipment also shows its own error of as high as 50% (in case your operating in the mode that indicates machine error), which indicates that the machine lacks the sensitivity to detect micro-alloyed elements.

3. If the sensitivity is proven by following point no. 1 above, then you can grind off the area as suggested by Magben to expose fresh material and check the material for acceptability.

Current generation XRF machines are quite sensitive and any mismatching of results should raise doubts. Next solution should be a lab based spectroscopy.

RE: Stainless steel surface composition

With Ti being a lighter element the energy is much easier to block.
But the first question is if you take a bar of A286 with a good freshly ground surface do you get 2%?
I am guessing that you don't.
Don't worry about it, this is good enough to verify that it is A286 and not some other alloy and that is all that a hand held unit is meant to do.

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P.E. Metallurgy, Plymouth Tube

RE: Stainless steel surface composition

(OP)
Thank you for your notes. Since the bolts are flight items, I could not arbitrarily sand-paper them. I needed a scientific reason to request light sanding. Finally, I got the customer's consent to lightly abrade the bolt head to expose the substrate. Ti got within range. I still don't fully comprehend how Ti becomes selectively lower on the surface. If MAGBEN would further explain heavy oxidation, I would appreciate.

In case anyone is not experienced with the up-to-date hand-held XRF, it is pretty accurate. You can modify zero point and gain for individual alloy. I have used hand-held XRF for years. It is not a scrap yard tool any more.

RE: Stainless steel surface composition

On all stainless there is thin surface oxide film, this is what makes them stainless.
The light elements (Al, Si, Ti, Mg) all have much lower energy of emission and are much more sensitive to interference and blocking.
Yes, the modern x-ray tube units are nice, but you really need to know what you are doing. You can set energy level (tube voltage), and analysis time by element, if you know how to handle XRF.

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P.E. Metallurgy, Plymouth Tube

RE: Stainless steel surface composition

(OP)
Thank you, EdStainless, for the reply. What I know is Ti was low as received and got within range after abrasion. Do you mean it can be explained with sentivitity/interference? I would like to hear. By the way, I have a metallurgy degree and am experienced with X-ray. You don't need to lay out the basics of sensitivity/interference. Thank you.

EDIT: I always use a certified reference block of the same grade for instrument check. Zero and Gain are adjusted, too.

RE: Stainless steel surface composition

Send one bolt that you have evaluated (and determined to be low in Ti content) out to a certified laboratory for ICP-AES analysis. Their results will tell you how far off your method is.

Maui

RE: Stainless steel surface composition

(OP)
Maui,
A certified A286 coupon was tested for accuracy. The instrument is not off. We are a full blown chemistry lab. We have ICP-OES. We just cannot destroy this flight fastener.

RE: Stainless steel surface composition

Then you are out of options.

Maui

RE: Stainless steel surface composition

I've always been taught that use of XRF really requires a little surface grinding on a non-critical area to remove surface oxides and contaminants. I would think it would be okay to make this measurement on the head.

RE: Stainless steel surface composition

Especially for light elements, having the same surface finish as the reference standard is critical.
A wet fine grind/coarse polish is preferred.
These devices also have a type standard function, so that you can shoot a known part and it will make corrections in the curve fits for that specific alloy.

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P.E. Metallurgy, Plymouth Tube

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