owen
Member
is it possible to tell the difference between different grades of stainless or does one just rely on the supplier/manufacturer??
A2 A4 or 316
- Thread starter owen
- Start date
fisherman
Well-known member
When we examined the spec we found that it is possible for good A2 to be better than poor A4. I seem to remember it was down to the % of chrome and nickel. No way of telling which we had got.
VicS
Well-known member
Both A2 (aka 304) and A4 (aka 316) are austenitic high chromium, high nickel stainless steels both are non-magnetic so the only way of distinguishing them is by chemical analysis or by testing with a Metascope.
There is some info on stainless steels on the A2A4 website HERE
When Cliff or one of the other metallurgists come along they will add some detail.
In the meantime there is some good reading In Wikipedia and more if you follow the links at the end of the article
There is some info on stainless steels on the A2A4 website HERE
When Cliff or one of the other metallurgists come along they will add some detail.
In the meantime there is some good reading In Wikipedia and more if you follow the links at the end of the article
cliff
Active member
304 and 316 SS come in many subgrades depending on Carbon and Nitrogen content but as you say A2 is roughly equivelent to 304 and A4 is roughly equivelant to 316.
The main difference between 304 and 316 grades is the addition of 2.5% Mo in 316 grade material. This has the effect of increasing high temperature creep resistance and increasing resiatance to brines (aka salt water).
There is no such thing as "bad" A2 or "bad" A4 - the problem arises with cheap imports of material branded as A2 or A4 or whatever but not complying with the material specification. When specifying material it is important to get the correct grade and condition to ensure long service life. To buy screws and fastenings manufactured by "one hung low's" nut, bolt and screw factory in some distant part of the world is not a really a good idea. Unfortunately there are many suppliers in the UK offering "cheap" stainless fasteners.
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"Artificial intelligence is no match for natural stupidity"
sailroom <span style="color:red">The place to auction your previously loved boatie bits</span>
The main difference between 304 and 316 grades is the addition of 2.5% Mo in 316 grade material. This has the effect of increasing high temperature creep resistance and increasing resiatance to brines (aka salt water).
There is no such thing as "bad" A2 or "bad" A4 - the problem arises with cheap imports of material branded as A2 or A4 or whatever but not complying with the material specification. When specifying material it is important to get the correct grade and condition to ensure long service life. To buy screws and fastenings manufactured by "one hung low's" nut, bolt and screw factory in some distant part of the world is not a really a good idea. Unfortunately there are many suppliers in the UK offering "cheap" stainless fasteners.
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sailroom <span style="color:red">The place to auction your previously loved boatie bits</span>
owen
Member
thanks for the replies.. i bought some a4 fixings from a reputable online dealer recently. i was surprised how soft the steel was compared with previous stainless I had bought from a chandler.
boatmike
Well-known member
Re: One Hung Low......
When I was in Thailand I condemned every fastener in the stores and had new ABS certified stock sent in from the USA. Most were marked A2 but you could pick em all up with a magnet!
Moral - know the source of your stainless steel!
When I was in Thailand I condemned every fastener in the stores and had new ABS certified stock sent in from the USA. Most were marked A2 but you could pick em all up with a magnet!
Moral - know the source of your stainless steel!
owen
Member
Re: One Hung Low......
boatmike i would be interested how you condemned the fittings. was it on how they looked ,their machining or their hardness??
boatmike i would be interested how you condemned the fittings. was it on how they looked ,their machining or their hardness??
vyv_cox
Well-known member
Magnetic A2
AISI 304 can acquire magnetism when highly worked. If yours were cold rolled their material spec could be OK despite their being picked up by a magnet. They would also have had superior strength due to their manufacturing method.
AISI 304 can acquire magnetism when highly worked. If yours were cold rolled their material spec could be OK despite their being picked up by a magnet. They would also have had superior strength due to their manufacturing method.
boatmike
Well-known member
Re: Magnetic A2
Look chaps, I don't condemn loads of boatyard stock without good reason. These were all magnetic, had a high carbon content, (we had some analysed in the USA) and some were rusty. Buy in the far east at your peril! They were NOT stainless steel at all but were marked and marketed as such. .... As Cliff implied, there are counterfeit screws, hinges, fittings and other bits out there. There is also a lot of Chinese and Eastern bloc steel that has a very dubious pedigree and is outside of the spec of the material it is supposed to be. If building to Lloyds, DNV, ABS or any other certification authority, they insist on seeing at least a certificate of conformity from a reputable supplier and in critical cases an analysis showing full tracability of the material used. Where do your screws come from??????
Look chaps, I don't condemn loads of boatyard stock without good reason. These were all magnetic, had a high carbon content, (we had some analysed in the USA) and some were rusty. Buy in the far east at your peril! They were NOT stainless steel at all but were marked and marketed as such. .... As Cliff implied, there are counterfeit screws, hinges, fittings and other bits out there. There is also a lot of Chinese and Eastern bloc steel that has a very dubious pedigree and is outside of the spec of the material it is supposed to be. If building to Lloyds, DNV, ABS or any other certification authority, they insist on seeing at least a certificate of conformity from a reputable supplier and in critical cases an analysis showing full tracability of the material used. Where do your screws come from??????
wagenaar
New member
Re: Magnetic A2
The usual fabrication method for bolts is via cold-forming, not machining as many people seem to think. In this process the head is severely deformed as well as the zone of the thread and it could well be that this causes the magnetism. It should also pointed out that 304/A2 is rather close to contain small amounts of ferrite, which would cause it to be magnetic. In particular when the C-content as near the upper limit of the prescribed range exists this danger. There exists also a type 304L, where the C-range is lower than for the "normal" 304. Just for general information: A2 and A4 come from the german normalisation(DIN)and 304 and 316 come from the AISI (American Iron and Steel Institute) and the SAE (Society of Automotive Engineers).
It should also be pointed out that in good practice one should not only quote the type of steel, but also the appropiate standard.
The usual fabrication method for bolts is via cold-forming, not machining as many people seem to think. In this process the head is severely deformed as well as the zone of the thread and it could well be that this causes the magnetism. It should also pointed out that 304/A2 is rather close to contain small amounts of ferrite, which would cause it to be magnetic. In particular when the C-content as near the upper limit of the prescribed range exists this danger. There exists also a type 304L, where the C-range is lower than for the "normal" 304. Just for general information: A2 and A4 come from the german normalisation(DIN)and 304 and 316 come from the AISI (American Iron and Steel Institute) and the SAE (Society of Automotive Engineers).
It should also be pointed out that in good practice one should not only quote the type of steel, but also the appropiate standard.
charles_reed
Active member
A4 = 316
A2 = 303 or 304.
A4 is the only one to use below the waterline, most nuts and bolts are marked, if not assume A2.
A2 = 303 or 304.
A4 is the only one to use below the waterline, most nuts and bolts are marked, if not assume A2.
owen
Member
oh dear i hope you are not right. i have just checked my purchases and the nuts are marked A4 but the self tappers and bolts have no markings
vyv_cox
Well-known member
Re: Magnetic A2
Never suggested that you would, but to be fair, your post only said they were magnetic. I was simply pointing out that 304 can be magnetic in case readers felt obliged to chuck out every fitting that their magnet was attracted to.
Never suggested that you would, but to be fair, your post only said they were magnetic. I was simply pointing out that 304 can be magnetic in case readers felt obliged to chuck out every fitting that their magnet was attracted to.
nick1612
New member
This problem of identifying whether the grade of stainless is correct is a daily routine in manufacturing equipment for the offshore oil business (where I work). It is resolved by what is called "Positive Material Identification" (PMI for short). Using a pistol-like device you get a reading of the chrome (and other elements) content which is accurate enough to distinguish 316 from 304, etc. PMI is rroutinely specified to be performed on receipt of materials and also later on in the manufacturing (building) process, to ensure correct materials at the latest possible stage. Unfortunately, the PMI analyser is quite expensive and also emits X-rays, requiring operator certification in most industrial environments. Not much help for the individual boater, I'm afraid, but at least you now know that you are not alone with this problem!
nick1612
New member
PS - here is a link to a PMI device supplier:
http://www.bruker.com/products/x-ra.../handheld-xrf/applications/pmi.html?utm_id=10
http://www.bruker.com/products/x-ra.../handheld-xrf/applications/pmi.html?utm_id=10
vyv_cox
Well-known member
8 years on, the problem most probably doesn't exist any more.
Birdseye
Well-known member
Yes. In the old days we did a spark test and nowadays you would use much more sophisticated equipment. Neither are available to Joe p[ublic so you have to go on what the supplier says. However, sup[pliers rarely if ever lie. If you are insistant ,on knowing the spec of what you are buying insist on a factory test certificate coming with the goods. This was a routine matter when I worked in the stainless steel industry.
A1, A2, A4 etc are specifications for grades of austenitic stainless steel that correspond to types 303, 304, 316 but the only thing that needs to concern you as a yottie is A2 vs A4. The latter doesnt discolour in salt water whilst the former does so. But the discoloration is cosmetic for the most part
Stainless bolts and screws are almost all cold forged to form the heads and this is made easier if the steel is soft. So we used to add in 2% copper to the steel as well as copper plating the wire itself to help the fastener maker. The threads might be formed by rolling which work hardens the material so you end up with a much softer head than body on a bolt. And the roll forming distorts the internal structure which in turn makes it slightly magnetic. The lower the alloy content within the spec, the more magnetic it is so dont be surprised if you can pick up a stainless spring with a magnet. The same applies to stainless self tappers which are likely to be highly roll formed.
But whichever grade you buy, dont put it below water in an environment where there isnt a good flow of fresh water. For example, dont use stainless steel bolts into the rudder skeg. In such a situation if stagnant water gets in it will corrode as badly as mild steel.
vyv_cox
Well-known member
The difference is pitting resistance rather than discoloration. 316 contains 2% molybdenum, giving it far better pitting, and more crucially crevice corrosion, resistance. Even better is 317, which has 3-4% of molybdenum, although I have yet to see it generally available.
Birdseye
Well-known member
You are right of course Vyv , but the reality for most yotties using A2/ 302/304 above deck is that the only problem they have is surface discoloration / rust. Which usually surprises them because they assume that stainless cannot rust. As for 317 I cannot remember ever supplying that to any fastener maker. If 316 wasnt adequate the next step usually was nickel alloys. OK it some years ago now but I seem to remember 317 mainly going as MIG welding wire but I could well be wrong.
vyv_cox
Well-known member
I met the owner of C-sail a while ago and discussed materials with him. He supplies stainless steel skin fittings, a lot going to the Middle East. As you say, nickel alloys offer the best resistance to crevice corrosion, his ultimate is AISI 902, the stuff that Rolex use for their cases. Googling it tells me that it is also used for more, erm, personal matters.
