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Al content in Type 431

Al content in Type 431

Al content in Type 431

Does anyone have experience in the effect of Al on machinability/polishability for Type 431. We currently have a heat with Al .004%, but customer wanted .001% stating "high" Al is detrimental. Is this anecdotal, or scientifically sound?

RE: Al content in Type 431

from this site

The effects of alloying elements​
The different alloying elements have specific effects on the properties of a stainless steel. It is the combined effect of all the alloying elements, heat treatment, and, to some extent, impurities that determine the property profile of a certain steel grade. It should be noted that the effect of the alloying elements differs to some extent between the different types of stainless steel.

Aluminum (Al)
If added in substantial amounts aluminum improves oxidation resistance and is used in certain heat-resistant grades for this purpose. In precipitation hardening steels, aluminum is used to form the intermetallic compounds that increase the strength in the aged condition.

RE: Al content in Type 431

Bogus, the Al is added for deoxidation at these levels.
It would take more than this to cause any surface issues.
There is more other trash in the structure than this small amount of Al2O3.

= = = = = = = = = = = = = = = = = = = =
P.E. Metallurgy, Plymouth Tube

RE: Al content in Type 431

The aluminum is probably a tramp element at this level. Anecdotal.

RE: Al content in Type 431

This level of Al is added as deoxidiser.

"Even,if you are a minority of one, truth is the truth."

Mahatma Gandhi.

RE: Al content in Type 431

Interestingly, after searching around my files I found one reference to aluminum deoxidation, which is in my copy of "Handbook of Stainless Steels" by Pecker and Bernstein, on page 3-6.

From Peckner and Bernstein, page 3-6, "On austenitic stainless steel grades, aluminum is seldom used; most American stainless steel producers consider the addition of aluminum as detrimental to the surface finish in polished strip. There is similar aversion to using aluminum in martensitic stainless steels destined for highly polished cutlery."

RE: Al content in Type 431

Ca is common in Austenitics, though a mixture also containing Si and Al is most common.
I have watched them add bails (3' cubes) of compacted soda cans into a melt for deox.

I would be a lot more concerned with the N content.
I can't see 0.005% Al having any impact.
They shouldn't have reported it since it isn't a specified element.
Though some mills are polite and report any element that they deliberately add.

= = = = = = = = = = = = = = = = = = = =
P.E. Metallurgy, Plymouth Tube

RE: Al content in Type 431

I have not seen Al deliberately used in austenitic stainless steel for deoxidation melting practice simply because silicon was primarily used on the miles of boiler tubing we purchased over the years and from audits of tube mills. Over the hundreds of MTR's, I found aluminum reported as either a tramp element or it would be unreported.

I have seen Ca and Si used together to control inclusion shape.

My only mention was that Pecker and Bernstein are highly regarded, and if mention was made about very small, tramp element concentrations of aluminum in stainless steel, this had to come from past experience, perhaps still anecdotal.

RE: Al content in Type 431

Thanks Guys! I was thinking it was anecdote too especially given that this is a micro-melt (e.g. from HIP powder), and O, S are high intrinsically. note there is another restriction in Ti that needs to be less than .002%, which makes sense to me, since the high N content.
By the way, Add Al for de-Ox is pretty common practice for both austentic and martensitic SS. However, for this alloy at such a low level, Al was not intentionally added for de-ox.

RE: Al content in Type 431

If this was melted from powder they will never get a good surface.
The O, N and S will form more inclusions that you can shake a stick at.
At least residual Ti would keep the O and N from forming with Cr, after all that's what 439 is.

= = = = = = = = = = = = = = = = = = = =
P.E. Metallurgy, Plymouth Tube

RE: Al content in Type 431

Ed, not melted from powder, but rather VIM+Gas atomization, HIP and Roll.

The applications are for very fine parts in which polish-ability is critical. Least non-metallic inclusions and very fine grain structure are targets. We successfully made .001%Al material previously on a best effort basis, and trying to convince customer .004%Al from current heat will not affect performance. People are very good at maths pointing out .004 is 4 times higher than .001! and so the perception is quality significantly sacrificed.

RE: Al content in Type 431

If you still have some of the lower Al content stock, work up a part or two (coupons) for fine polish, and also work up a part or two or three of the 0.004% Al current stocks, and polish these up also in precisely the same way, same duration, etc.

Put a hickey on each part, or a stamp mark, that allows you and only you to distinguish these parts.

Next, put all the parts in front of the customer, and have them pick out the ones with poor polished characteristics, if they can.
Do not be surprised at all if they get about 50% of the high Al stock parts picked out in the "good" pile, and put about 50% of the really low Al stock parts in the "bad" pile.

Then all you need to do is inform them of the results.

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