structural stainless--unobtanium?

[HgTX]

I have a designer who wants stainless steel for corrosion protection & aesthetics, but assumed he could use typical carbon steel properties in the design. So he assumed yield strength of 50 ksi and ultimate tensile strength of 125 ksi.

I fear I may not be able to make this work.

These are rod & hanger assemblies for a bridge. Fatigue loading, some components welded, some machined. Outdoor exposure, but not to salt water and probably very little road salt, if any.

The bridge design specs say that the SS has to come from ASTM A 176, A 240, A 276, or A 666, or something that meets the chemical & mechanical requirements of something listed in one of those specs. Or "other published specifications that establish its properties and suitability and that it is subjected to analyses, tests, and other controls to the extent and in the manner prescribed by one of the listed specifications". Which I choose to interpret as "if I need to go elsewhere to make it work, so be it".

I am assuming we need some kind of austenitic SS, preferably 300 or 400 alloy group. 304 or 316 type would be best.

For starters, we need 1.75" threaded rod/bar.

Nothing in A 176 meets the strength requirements.

Nothing in A 240 meets both yield and ultimate strength requirements.

Nothing in A 666 meets the strength requirements unless I get into the cold-worked materials, but I think those are all just sheet, not plate or bar.

In A 276, I see:
XM-21 (S30452?), S30454, S31654, Condition B ("relatively severe cold work")
Are any of those available as 1.75" rod/bar? And threadable? (And available as melted & manufactured in the U.S.?)

But wait, there's more. That's for the threaded bar. There's also 1.5" and 1.75" plate that needs to be weldable, a pin with a 2.5" diameter head (needs to be machined down to 2" diameter where the head isn't), a clevis that needs to be machined from material originally at least 6.125" in diameter (not to be welded), and...rectangular tubing, 5"x4"x3/16", weldable. So far I can't find that rectangular SS structural (as opposed to ornamental) tubing even exists.

So...does any of this stuff exist? And would they be better off with a casting for the clevis?

Hg

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[MikeHalloran]

A reasonable design yield for stainless that you can actually get and can actually machine is around 27ksi.

Your designer needs to start over, with a metals catalog and technical reference at hand.


I have had some fairly fancy clevises flame-cut from 7" mild steel plate, 40 years ago. Any steel service center should be able to do as well today, stainless or not.



Mike Halloran
Pembroke Pines, FL, USA

 

[HgTX]

Turns out where the Fy=50,Fu=125 came from was that he started out with some proprietary British system, then picked out the one alloy he could find (S20161) that had somewhat similar strengths. (And given that, I'm really not sure what he thought he was asking me when he asked me what steel he should specify.)

I've suggested he start over with Fy=30, Fu=75. That seems to match a lot of 300 steels out there, including machinable 303. How's the corrosion resistance on 303, by the way?

S20161 I don't think has the same corrosion resistance as 304/316 and I'm told it may not be weldable. If he insists on sticking with it, I'm still not sure what shapes it's available in. I have my doubts about the tubing (which may end up being a non-issue as it is just being used as conduit so any weldable 304 should do) and the 6.5" round (or any other section from which a 3"x5" block can be machined).

My best bet for forcing a redesign is the availability issue. I must confess I have now gotten testy with the designer, who is a nice man.

Hg

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[MikeHalloran]

If selecting material is part of his job, I would be more than a little testy with a designer who used imaginary material.

I would, however, do any appropriate yelling in private.






Mike Halloran
Pembroke Pines, FL, USA

 

[desertfox]

Hi HgTX

I would look at a stainless steel 316L its machineable, weldable if you keep to the low carbon content and better corrosion resistance than 304

http://www.roymech.co.uk/Useful_Tables/Matter/Strength_st.htm

theres a few good links to other sites too if you look around at this reference above.

desertfox

 

[SnTMan]

Well the threaded bar and pin could be (S)A-193-B8x in various flavors, depending on the strength required. B8Cl2 is one of the higher strengths in austenitic SS.

A-240 plate will be weldable in 304 & 316, but strengths will be lower than 50 / 125.

Clevis maybe machined from A-182 forging (may be dual certified as bar, don't remember the spec) or plate, depending, again strengths are what they are.

Sounds like a redesign around material you can buy to me.

Regards,

Mike

 

[HgTX]

A193 Gr. B8M was recommended, but doesn't meet those strength assumptions. I really hope he goes for the redesign.

Hg

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[HgTX]

@desertfox: how low is low carbon?

Hg

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[desertfox]

Hi HgTX

0.03% max for 316L see this article also:-

http://www.bssa.org.uk/topics.php?article=110

 

[SnTMan]

I suppose if you are not designing to any codes, standards or any assumption of liability you could use fictitious allowables, otherwise I don't see how.

Regards,

Mike

 

[HgTX]

What's a guideline for not ideal but tolerable machinability? I'm not sure I can talk him into a 25-ksi steel.

Hg

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[MikeHalloran]

Just barely tolerable machinability is what you get with any stainless in the annealed condition. In any other condition, it's really not machinable.



Mike Halloran
Pembroke Pines, FL, USA

 

[moltenmetal]

Forget about using 304L or 316L- that's old advice. These days, in most product forms it's as easy or easier to find 304/304L or 316/316L dual grade as to find the straight L grade, and for no additional cost. Dual grade gives you the superior strength of the straight 316 grade with the low carbon of the L grade to reduce HAZ sensitization after welding.

316 doesn't give you the yield or UTS you require anyway. A duplex stainless steel might, but at some serious cost, IF you can find someone to make the product forms you require. A re-design with more reasonable yield and UTS assumptions IS required if you wish to use material that will be of reasonable cost in the product forms you require.

303 is "free machining" but neither weldable nor particularly corrosion resistant. It is similar to 304 in resistance but in some services is inferior to 304. Sounds like corrosion resistance isn't your biggest concern in this application.

 

[EdStainless]

There has been a lot of bridge work done with duplex stainless, usually 2205. Here you get 65ksi min yield.

There are papers published on this. Try contacting TMR in Pittsburgh.


= = = = = = = = = = = = = = = = = = = =
Plymouth Tube

 

[HgTX]

Sounds like 2205 will save us, if the designer can live with Fu=90 rather than 125 ksi. It looks to be available in the shapes we need. Good Fy, says it's weldable, machinable, corrosion-resistant. Magic steel?

Hg

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[unclesyd]

MikeHalloran,
I wouldn't worry about machining any of the Austenitic SS. We do this everyday as the majority of our equipment is SS. All it takes is a ridged setup and the proper tooling.

HgTx

Nitronic 60 might be a possibility with 105 TS and 55 YS and is avaiable along with 2205.

Mike Jones at Ameribolt said you should contact try these suppliers as to delivery times.

http://atlrod.com/tie_rods.htm

http://www.wecallinc.com/index.html

http://www.dysoncorp.com/specialtyfasteners.html

 

[SnTMan]

HgTX, one caution about 2205 is potential for brittle fracture. For PV work, very difficult to exempt from impact testing, at least under Sec VIII, Div 1.

Regards,

Mike

 

[moltenmetal]

2205 is expensive. Unless you're buying mill-run quantities, it is expensive beyond what you'd guess from the alloying constituent composition in our experience.

Do any of the lean duplexes meet the required yield and tensile? These can be much cheaper, and you don't need the corrosion resistance of 2205 for your application by the sound of it.

Duplexes have higher minimum temperature limits than the austenitics- best to check if that's a problem in your application.

Most alloys are "weldable, machinable and corrosion resistant" in the seller's literature- it's a matter of degree! Duplex is all three, to a degree, but so are the austenitic grades. Welding duplex IS trickier than welding austenitics if you wish to obtain the same corrosion resistance in the welds and HAZ as the parent metal. Machining speeds/productivity rates will be nowhere near those for carbon steel. You can get duplexes in plate and bar easily enough, but finished product forms are very likely going to be custom-made for you in any duplex grade, whereas in plain-Jane 18-8 (304/L) you may find most of what you need in stock somewhere. Whether that's a big cost driver for the job or not depends on how much you're using of course.

In my business, cost is never an object- it's usually the SUBJECT. The notion of finding a material to meet someone's arbitrary selection of some physical properties is totally foreign to me- unless it's a few minor aesthetic pieces on a large job, it'd be "back to the drawing board, buddy!".

 

[HgTX]

I suspect that most of what we need will not be in stock somewhere. It's a custom design. They didn't like any of the off-the-shelf designs they saw. We were planning all along to buy rounds & plate and cut/machine as needed.

I think 2205 may very well be the generic version of the proprietary system he fell in love with (Macalloy 460 for the rods). We're still working on him to go to some other more normal material for the plates & clevis but that may not happen. I'm thinkin' it won't.

Cost...this is a "signature" project and the steel is not the largest part of it. It's not exactly that money is no object, but they're already not making the cheapest bridge they can build. How much of a price premium (rough %) are we talking about here?

Also, what does 2205 look like?

Hg

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[HgTX]

Here's the latest, based on some other bridge that someone is making for someone else:

"Pin should be through bored with a threaded rod for the caps on both ends.

The clevis would be cast from ASTM A 743, CF8M.

All welds would be TIG welds.

The bar, clevis and link plate would be 'hot finished and solution annealed after welding.' "

I question the TIG requirement and would prefer to leave that up to the fabricator as long as they meet the requirements of D1.6. Also I'm not sure what they mean by "hot finish". Or whether they want that and the annealing whether or not they weld.

Hg

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