304 vs 316 on deck

[thinwater]

Originally Posted by Chotu

... When it comes to aluminum, very preliminary check says that I have to have three times the thickness for the aluminum sheet that I would for the stainless. So it doesn’t save any weight.....

Of course, common sense should tell you this is not true or we would have stainless masts and stainless airplanes, and no aluminum anchors. It depends on the alloy and the details. Pure aluminum is quite soft.


But the reason in my mind is corrosion. I did not like the way the stainless was corroding the fitting you showed. A bolt does not do the same, because the area is much less.

[Chotu]

Originally Posted by thinwater

Of course, common sense should tell you this is not true or we would have stainless masts and stainless airplanes, and no aluminum anchors. It depends on the alloy and the details. Pure aluminum is quite soft.


But the reason in my mind is corrosion. I did not like the way the stainless was corroding the fitting you showed. A bolt does not do the same, because the area is much less.

Even though you’re always accurate with everything, I’m going to have to go with celestialsailor on this one. And some reports that I was reading.

Celestialsailoris a bit of an expert in this area, even if he’s not mentioning it.

It definitely depends on alloys, but at the same time, it is true. It takes a lot more aluminum to create the same strength as stainless. Talking about the typical alloys. 6061 – T6. 304. 316.

We don’t have stainless airplanes and stainless masts because no one could afford one.

If you check, the only person rich enough to create stainless aeronautical stuff is Elon musk. One of those rockets he has been working on is made of stainless. Otherwise, it’s too cost prohibitive (and probably not malleable enough) if you’re not the most rich person in the world.


I am a little bit confused by the last part of your post. Mentioning the corrosion. Talking about a bolt being different. I see a bolt holding it in place. So I’m not sure what you mean. Can you expand on that?

In any case, that piece of aluminum that is corroding in the picture is 3/4 of an inch thick on my boat. It will also be electrically isolated.

[coopec43]

Chotu
I don't blame you for looking at alternatives.

8 July 2021 — Since March 2020, steel prices are up a staggering 215%. The benchmark price for hot-rolled steel hit another all-time high last week, climbing .

https://fortune.com/2021/07/08/steel...ing-up-bubble/

"4 Mar 2022 — Spot prices for Chinese stainless steel have risen in recent days and shot up by up to 4% on Friday. Again, a significant increase in demand and

https://steelnews.biz/chinese-stainl...significantly/

[phorvati]

Originally Posted by Chotu

No kidding!



How often do you have to watch the rust?



Would polishing it once a year do the trick? Maybe every six months?



I use this stuff on stainless steel usually. It works great. And it leaves a little protective barrier.



https://www.amazon.com/Collinite-850...26039990&psc=1

It all depends how you did your finish work. I did not mean the stainless polish at the end. I mean how good is your metal finishing work. For me it started with cold worked stock 304. You can also pay more and get a better finish 304. I started with 60grit belt sander and worked my way up. Around 400grit, you can start wet sanding, and work up to 1200 or even 2000 before switching to rouge. There are various rouge colors for various levels of finish. Its been 4 years and a lot of seaspray, but I only need to rinse it with fresh water and a seasonal stainless polish like 3M stuff you buy at defender or west marine. But mirror finish is still there, I can basically still shave using it. Many folks cut corners, some go to electro polish and do plating. Salt will punish you if you leave crevices and surfaces that are not mirror finish. No free lunch here. But 304 can be just as good as 316. In fact 316 will have problems if you leave it partially finished. Again you need mirror finish. Any scratch will trap salt and corrode. So finish is more important than 304 vs 316.

[s/v Jedi]

Originally Posted by Celestialsailor

No...Yield strength of 6061 Alum is appr. 6,600psi and 304 stainless is appr. 30,000psi. A factor of appr. 4.5.

I’m confused… I can find info that says a T6 temper is even stronger than 304:

[coopec43]

Originally Posted by s/v Jedi

I’m confused… I can find info that says a T6 temper is even stronger than 304:

Isn't there a new line of Stainless Steels that has been developed that is far superior to the 300+ SS?

Maybe this has something to do with it?
https://www.metal-am.com/oerlikon-in...inless-steels/

OR THIS?

https://greensteelworld.com/outokump...tainless-steel

304 vs 316
https://www.reliance-foundry.com/blo...tainless-steel

[coopec43]

Maybe this is the article?

AN N’GENIUS NEW INVENTION IN STAINLESS STEEL TECHNOLOGY
A new series of high strength austenitic stainless steels has been developed that could be the most significant innovation in stainless steel technology for more than a century.
The N‘GENIUS Series™ of High Strength Austenitic Stainless Steels, patent protected in 30 countries around the world, represents the total reinvention of conventional austenitic stainless steels – specifically developed to out-perform, supersede and replace the majority of existing grades in the 300 Series.

https://bssa.org.uk/an-ngenius-new-i...el-technology/

[pcmm]

Originally Posted by Chotu

My machine shop is FINALLY doing the attachment points for my crossbeam.

I have attached a picture of this type of thing.

The plans specify them in stainless. I don’t know how to change the material to aluminum. I’m not sure what thickness aluminum would have the same structural properties.

The 316 stainless is $700 just for the sheet of stainless to make these.

Would 304 last?

Lastly, what about changing them to aluminum? How can I change the specs so they would have the same physical properties?

They are made from 3/8” 316 stainless in the plans.

This can easily be "meatball" engineered... If the tangs in the plans are 3/8" stainless that pretty beefy already as the bracket is 2 plates of stainless to sandwich the tang on the end of the cross beam. to Convert to Aluminum just make the tangs on the bracket out of the same thickness aluminum as the tang on the end of the cross piece. Its imprecise but you end up with twice the thickness on the bow bracket as the cross beam tang. More than strong enough.

[Chotu]

Originally Posted by pcmm

This can easily be "meatball" engineered... If the tangs in the plans are 3/8" stainless that pretty beefy already as the bracket is 2 plates of stainless to sandwich the tang on the end of the cross beam. to Convert to Aluminum just make the tangs on the bracket out of the same thickness aluminum as the tang on the end of the cross piece. Its imprecise but you end up with twice the thickness on the bow bracket as the cross beam tang. More than strong enough.

Ha! That's exactly what I reasoned out bit was afraid to post for fear of ridicule.

If the big 3/4" tang carried the load, the part attaching to the hull could be made from the same thing. Stands to reason.

[pcmm]

Originally Posted by Chotu

Ha! That's exactly what I reasoned out bit was afraid to post for fear of ridicule.

If the big 3/4" tang carried the load, the part attaching to the hull could be made from the same thing. Stands to reason.

Actually if you did that (by reason) the hull part would be twice the strength of the tang on the cross beam just by having collectively twice the thickness.

[Celestialsailor]

Originally Posted by s/v Jedi

I’m confused… I can find info that says a T6 temper is even stronger than 304:

Hi Jedi…T6 refers only to its temper, AFAIK. The yield remains the same. Untempered Al. Would bend to its psi yea d number. A tempered Al. would remain somewhat rigid then snap. I hope I’m not wrong on this or I have been fooling myself for a time

[Celestialsailor]

In case anyone is interested in boring information, 300 series cannot be hardened but 400 series can. This is because the f the amount of iron in it

[wyb2]

Originally Posted by Celestialsailor

Hi Jedi…T6 refers only to its temper, AFAIK. The yield remains the same. Untempered Al. Would bend to its psi yea d number. A tempered Al. would remain somewhat rigid then snap. I hope I’m not wrong on this or I have been fooling myself for a time

Yield is the stress that the permanent bend starts at, ultimate is the stress that full break occurs. Usually you design to yield unless it’s something that only needs to work once.

I think you are half right, tempering tends to increase the yield strength faster than the ultimate. So you get a part that is much harder, but not quite as much stronger.

Looking at 6062 in the chart linked below (they don’t have numbers for untempered 6061), yield goes from 5,000 psi to 35,000 psi (7x). But ultimate goes from 14,000 psi to 38,000 psi (2.7x).

http://https://www.engineeringtoolbox.com/properties-aluminum-pipe-d_1340.html

Though I believe there is another reason to use the lower number, which is the weld heat will ruin the temper. I don’t have any experience with aluminum welds, so not sure if it brings it all the way back to baseline, or if there are tricks around it, etc.

[NPCampbell]

You can use the general PREN number of stainless to gauge corrosion resistance. A PREN of 32 means that the metal is generally resistant to seawater corrosion. There are several formulas for PREN:

PREN = %Cr + (3.3 x %Mo) + (16 x %N)
PREN = %Cr + 3.3 x (%Mo + 0.5%W) + 16 x %N
... and these aren't all of them.

Neither 304 or 316 has a PREN above 32. However, from the formula, you can see that each % of moly is much more significant than each % of chromium.

304 has no moly, 316 has 2-3% so it is generally much more corrosion resistance than 304 by a lot. 316 will tea stain considerably less than 304.

Another factor to consider is weldability. The welds on 316 lose their corrosion resistance due to the buildup of chromium carbides. Annealing (heating and quenching) is required to dissolve them. You might consider the slightly weaker, low carbon, 316L grade which requires no annealing but maintains the same corrosion resistance.

[s/v Jedi]

I think that when they list yield strength as equal to 304, that this means that the yield strength is actually equal to 304, not only 25% of it.

It would not make any sense to have equal yield strength numbers for two materials, where one has a yield strength actually 4 times higher.

Still confused, but suspect 6061 T6 may be equal to 304. Read the attached image: