Stainless Steel Grades

[Guest]

There seems to be some confusion about this. I understood that 316 is the top grade, when applied to the material. When applied to nuts and bolts, the equivalent to 316 is A4, and A2 is a lesser grade not to be used below the waterline. Can anyone confirm or correct my understanding

 

[jfkal]

316L is supposed to be even a little better. For nuts and bolts we go at 316 as well. There is a (German?) alternative numbering system V4A for 316 and V2A for 314. Different numbers same stuff.

 

[Gordonmc]

Stainless steel needs oxygen to create a barrier to corrosion, so may not be suitable for under waterline use, whatever the grade.
One of the best reference pieces I have come across (in eedjit-proof English) is on the Classic Marine website. www.classicmarine.co.uk/articles/cutmetals.pdf
No- I am not a shareholder!

 

[vyv_cox]

There is no such thing as a "top" grade, there are plenty with more chromium and nickel than 316 but each needs to be specified for the duty under consideration.

18/8 stainless steel (18 CR, 8 Ni, balance Fe) is the minimum that qualifies to be an austenitic, non heat-treatable grade. This is 302 for general, less demanding use, and 304 grade. For many purposes it is perfectly adequate, but it suffers one particular problem: pitting in aqueous chlorides. This can be improved by the addition of 2% molybdenum and the resulting material is grade 316. Most marine fittings are made of this material. Welding these stainless steels can lead to problems due to the formation of chromium carbides at grain boundaries, denuding the surrounding material of chromium and leaving it susceptible to corrosion, a condition known as knife-line attack. The answer to this is to reduce the carbon content as far as practicable. Stainless steels for welding applications are known as 304L and 316L (L for low carbon).

Grade 304 is also known as A2, and 316 as A4.

 

[Guest]

I can recommend an excellent book that answer your question and many others

'Metal Corrosion in Boats' by Nigel Warren ISBN 0-7136-4869-4.

And no I do not have any connection with the author or publisher.

 

[Guest]

vyv,,spoken like a true metalurgist. Whats the new number for the old EN58J? I'd be interested as I've been out of that side of engineering for sometime now.

 

[mica]

Do these grades also apply to rigging wire? We had new life lines fitted a couple of seasons ago and already there are signs of rust, where they go into the swage terminals.

What is the best method of coping with these rust outbreaks. Mike cleaned the standing rigging with hydrochloric acid (silly man ended up with quite bad burns on his hand) then coated them with Waxoyl. Haven't inspected them lately, another job for the winter layup, but would be interested to know what you do as you seem to know quite a lot about stainless steel..

MICA

 

[Guest]

If using stainless steel bolts, beware of nuts jamming if nut is same grade as the bolt. Using two different grades can ease the problem but need to be aware of your corrsion resistant needs. Option can be to coat the thread with teflon or similar if you need to disassemble in the future.

 

[mulligan]

I would think that if you electropolished most high grades of st/st you would get the corrosion resistance required for most boat/yacht requirements, if you then wanted to go further you could of course passivate in a 20-25% nitric acid solution for a couple of hours.

 

[vyv_cox]

The rust you are seeing is one of the most common complaints about stainless steels. This is "crevice corrosion" caused by differential aeration. Where there is a narrow gap, for instance in threads, cable ends, clevis pins, etc, you are likely to see brown staining. Sometimes staining occurs on apparently flat surfaces and this is due to the presence of minute crevices that are not visible to the eye. Rigging wire is particularly prone to this because it is hard drawn to increase strength and the die causes microscopic tearing of the metal surface. Polishing is the best way to reduce the effect but it should be remembered that it is normally only staining, not particularly damaging.

Anything that reduces the crevice effect may be effective. Waxoyl is very good in this respect, as is warm grease. I would definitely not clean with hydrochloric acid as stainless steels are particularly susceptible to chloride pitting. Either polish open surfaces or fill crevices without cleaning off the stains. Once the air is excluded the corrosion will stop. On bolted stainless fittings I always use Sikaflex to exclude air, including the bolt threads.

 

[cats]

Hi vyv_cox
Have read with interest your post on stainless steel.
I have a worn prop. shaft where it passes through the cutless bearing and was considering having the area metal sprayed and turned down to the original diameter, however having read your article mentioning the problem of welding
stainless I am wondering if my idea is feasible.
I would appreciate your comments, also is there a grade of steel commonly used for prop.shafts?
Derek.

 

[vyv_cox]

Derek,

Stainless prop shafts are typically 316 in my experience, but it would not surprise me to find some in 304.

Metal spraying could be an option for building up worn shafts but you need to interrogate the sprayer very carefully. Accept no other preparation than grit blasting with aluminium oxide. No other medium will do. Ensure that the person has good experience of the coating/shaft metal combinations in seawater, as after flaking your next potential problem is under-coating corrosion. The shaft carbon content will not have a significant effect as spraying temperatures are too low to modify the stainless steeel.

Another possibility is weld overlaying, but again, a number of precautions needs to be taken and a high-carbon shaft material may lead to further problems.

Chromium plating is yet another option, but not one I have come across on boats.

My experience in this area is on much larger equipment, like seawater pumps for example. I have some doubts whether you will find any economic advantage in repairing a small boat shaft compared with having a new one made, particularly when the increased risk of subsequent problems is taken into account.

 

[vyv_cox]

Peter,

You must be nearly as old as me if you remember En58J!!

This is a strange one, En58J spec seems to fall right in the middle of 316 and 320. 320 is 17 Cr, 12 Ni, 2.5 Mo plus a trace of Ti.

Vyv

 

[cats]

vyv_cox,
Many thanks for your trouble and information,

Derek.