Neeves
Well-known member
With the increasing volume of stainless components coming out of China how do you know that something sold as 316 is 316?
Jonathan
Jonathan
How do you differentiate, easily, between 304 and 316?
- Thread starter Neeves
- Start date
<152587>
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Magnet exhibits nil attraction to 316, that is how our local scrappy applies his pricing structure.
savageseadog
Well-known member
If it really matters a scrapyard may agree to scan the item with an XRF spectrometer.
Neeves
Well-known member
I'm thinking of components on a yacht - shackles etc. Is a magnet reliable or are there other stainless steels with non magnetic characteristics.
macd
Active member
A magnet won't help much in distinguishing 304 from 316, since neither is magnetic, although some 304 shows very slight magnetism. (Both are austenitic steels, if you want to look it up. They're identical in composition but for the addition of molybdenum to 316. It's the nickel in both that makes them non-magnetic.) However, if your fitting is attracted to a magnet, it may suggest it's neither, as many cheaper stainless items are magnetic. On the other hand processes of working/forming and welding 304/316 can induce mild magnetism.
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vyv_cox
Well-known member
In theory both 304 and 316 are austenitic, which is non-magnetic. However, cheaply made versions of 304 may contain small amounts of ferrite, which is the stuff that carbon steel is made of, magnetic of course. Good quality 316 will have the chromium and nickel contents raised a little from their nominal 18/8 to maybe 20/10, plus the molybdenum of course, but 304 will often barely contain the nominal 18/8. If your fitting is attracted to a magnet it will corrode more readily, but the fact that it is not attracted to a magnet does not necessarily differentiate 304 from 316.
colhel
Well-known member
Well... if a shackle is magnetic it could be made from a tougher type of stainless called 17-4, more corrosive than 316 but much stronger.
Duplex and Super Duplex are stronger and more corrosion resistant than 316, and magnetic.
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vyv_cox
Well-known member
17-4PH shackles have been available from Wichard for 20 years or more. I have had two on my anchor and swivel for at least ten years, where they remain in excellent condition. One or two other manufacturers are making them now also. I have yet to come across shackles in duplex, of which 2205 is the most common for anchor shanks and shafts. S3i make them but they don't seem to advertise the price!
Neeves
Well-known member
I think you can source 'D' shackles, only, in Duplex - no bow shackles, or not that I have seen. Peteresen? They are not cheap.
But I would never use 316 shackles in the rode, to be strong enough and not deform easily they would need to be grossly oversized.
But it appears there is no easy way to differentiate 304 from 316, unless its rubbish 304?
Jonathan
But I would never use 316 shackles in the rode, to be strong enough and not deform easily they would need to be grossly oversized.
But it appears there is no easy way to differentiate 304 from 316, unless its rubbish 304?
Jonathan
colhel
Well-known member
The only way I can tell, but won't be much use here, is when machining 304 the swarf is ever so slightly magnetic.
I think the only safeguard is to buy from a trusted source.
I think the only safeguard is to buy from a trusted source.
PowerSlapper
New member
One test for molybdenum in 300 series stainless is to place equal drops of concentrated hydrochloric acid and concentrated nitric acid on the metal surface. Allow 3 to 4 minutes for reaction and transfer 2 drops to a spot plate.
b) Add 2 drops of 10% potassium thiocyanate.
c) Add 25% sodium thiosulfate drop-wise stirring until the red color begins to disappear. A violet color will appear if molybdenum is present.
or if you are "Old School", with an angle grinder, grind 304 and look at sparks; they will fork out at the end of their flight. 316 will spark but each spark will stay intact till the end of its flight. try it out.
b) Add 2 drops of 10% potassium thiocyanate.
c) Add 25% sodium thiosulfate drop-wise stirring until the red color begins to disappear. A violet color will appear if molybdenum is present.
or if you are "Old School", with an angle grinder, grind 304 and look at sparks; they will fork out at the end of their flight. 316 will spark but each spark will stay intact till the end of its flight. try it out.
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MM5AHO
Well-known member
This method of watching sparks is quite good but need experience. I remember in my early steelworks days that some old hands could stick a hand ladle into a furnace of molten iron and pour a little through air, watch the sparks and could tell the %C to within about 0.2% (in a range of 0-3%).
But the action of the sparks is a great indicator, even angle grinder sparks.
But the action of the sparks is a great indicator, even angle grinder sparks.
bikedaft
Well-known member
If you have time on your hands leave it on board for a year./monitor it for rust. Replace if rusty. From somewhere else...
GrahamHR
Well-known member
Surprising that titanium / titanium alloy items are not readily available nowadays at a reasonable price for marine applications. Pretty strong, tough and corrosion resistant from what I can gather.
Neeves
Well-known member
vyv_cox
Well-known member
One of my lecturers at college was very old school. He would put a sample of steel against a grinding wheel and make a pretty good assessment of carbon, chromium and nickel by looking at the colour of the sparks. Which is exactly what a spectrograph does, except it can analyse the colour spectrum rather more accurately than an eye can.
colhel
Well-known member
I think Ive got this the way round , 17-4 is tougher than duplex. Vyv?
villager
New member
Anopol Ltd of Birmingham sell a chemical kit called a Molytest that tells the difference between 304 & 316. I had one years ago but don't know the current price. It's on their website
vyv_cox
Well-known member
There are several grades of 17-4PH, dependent upon solution treatment. The strongest has UTS about 1500 MPa with elongation about 7%, weakest about 1100 MPa with elongation 10 - 11%. Corrosion resistance is normally reckoned to be about the same as 304.
2205 Duplex, which is almost the only choice that I have come across for marine purposes, is only about 600 MPa strength but the elongation is 25%. Super duplexes are complex, mainly intended for excellent corrosion resistance with strength between about 700 - 950 MPa, elongation also around 25%.