stainless steel testing

[Piero]

Hi
I dont claim to know enough about my metals, hence this very school boy question,

But I was given a shed load of odds and ends of alloy, steel and stainless steel.

is the magnet test a sure and final way to tell.

only some very shinny bits are only just slightly magnetic

 

[Parm]

An expert will be along shortly but I'm sure ive read somewhere that some flavors of SS are magnetic

 

[taxi ray]

You can usually tell the difference from looking at the different pieces, aluminium will look slightly dull, stainless is mostly shiny and more of a silver colour. I think most stainless is non magnetic, if the magnet sticks to it, it could possibly be chrome iron. I would only expect stainless to have a slight pull on a magnet.

 

[brianblock]

Austenetic stainless isn't magnetic

 

[Piero]

might the old file test help

wont stainless (even if slightly magnetic ) still be harder than steel

 

[KemppiFrog]

Park the box outside for a day or so. Light bits are ally, rusty bits steel and any still shiney heavy bits are stainless 304 is slightly magnetic, 316 not.
Yes to the file test, but only if it is mild steel. SS is not that hard to file, comparisons with different types of harder steel might confuse.
KF

PS Parm, not an expert, just do a bit of stainless fabbing for boats.

 

[nick952]

Use the "Spark Test" method (I was taught this as a school boy)
https://en.wikipedia.org/wiki/Spark_testing

 

[Richard.]

The alloy is an easy one as weight alone will be the biggest clue there along with its visual appearance.
Slightly magnetic would suggest it's a stainless steel.
Brianblock is dead on austenitic grades are none magnetic like your 304's 316's etc
Your ferritic and martensitic grades (usually 4**) these can be slightly and very magnetic depending on Fe content. As we know steel is very magnetic so all slightly and non megnetic would be stainless.
After that all the very magnetic material can be told apart with a spark test.

 

[KemppiFrog]

Um, Richard. 304 is slightly magnetic. Before replying to the previous post, I actually tried a bit. Held it up by two fingers and put a magnet to it. Gentle attraction to it. Not much, but there.
KF

 

[optima21]

the magnetic attraction of non magnetic stainless steels also depends on if it is spring stainless steel, that has been hardened by temper rolling (or cold working). as the stainless steels gets harder, it becomes more magnetic.

 

[gaz1]

what about the scratch test ?

stainless wont mark easily whilst steel will but alloy scratches easily and so does other metals like lead aluminum

on top of the magnet test it helps to sort into piles

 

[hotrodder]

FWIW i've just had a load of 2B (cold rolled, softened & descaled and then lightly rolled again) 304L laser cut and cannot detect the tinyest hint of attraction with a magnet. Look up some data sheets for 304 and 316 and the magnetic permeability numbers will be the same for both alloys. 316 has slightly more Ni (along with the addition of some moly) and is maybe(?) "more non-magnetic" on paper if that makes sense but the differences ain't something that can be used to tell them apart hence the moly spot test kits used to confirm that 316 is 316 and not 304 or cheapnese tat

Cold work either of them (or any of the austenitics) enough and they become more magnetic (still only slightly) as some of the austenite gets transformed to martensite... go round a stainless kitchen sink with a magnet and you'll probably find a few areas that show more attraction while flater areas that haven't been drawn much/as deeply will show less or even none.
Weld any of the 3xx and the welds will also show a slight attraction to a magnet. More so/more reliably if filler wire is used because the ferrite content of wire is typically slightly higher IIRC to reduce crack sensitivity (or they're alloyed to slightly increase the ferrite count)

Leaving stuff out in the rain doesn't really work reliably if everything is a bunch of mixed up random offcuts... grab any 3xx stainless off cuts from a mates workshop and chances are they'll already have rust stains on them from contamination. If they don't they will do if left out, granted not as much as a piece of mild steel but a spot test with a drop of water will often leave a 'rust' stain on a sheet of 304 or 316 that 1) has mill ID codes on it and 2) was a mint offcut nicked from the local laser cutters until they got their mits on it

Even with lots of experience spark testing really needs some known samples to compare against i.e pretty much anyone can tell some titanium from steels and even CI from low carbon steel but telling the difference between 3xx and 4xx series stainless from the sparks is in a different league. Kinda pointless though given a magnet will do that and stick like a limpet to the common suspects like 430

Oh, and "alloy" isn't short for aluminium. In fact the aluminium bits might be the only ones that aren't alloys unless there's some commercially pure copper or Ti in there... [/pedantic]

ETA pic is some 1050-h14 Al, 304 2B stainless and grade 5 Ti (going clockwise from right)...

The oxide layer on the Ti has thickened a bit (hence me tickling a bit of it with some scotchbrite) but the differences in colour, weight and stiffness are more than enough to ID their basic flavour. Narrowing things down to a specific alloy is a very different proposition though

 

[Richard.]

Nice post hotrodder.
Ref the spark test
I wasn't suggesting it as a test to tell various stainless grades apart as you say that's a league of its own but more to tell the magnetic stainless from the steel.
All stainless grades spark very differently to steel FME and this would be easy to quickly seperate them if the magnet is picking them all up.

 

[hotrodder]

Cheers. Most of it's already been done but FWIW there's a bit more depth, including why hardness/scratch testing alone isn't that trustworthy, as well stuff on the different chemical tests which are typically either spendy and/or involve playing with nasty acids for the most part... http://www.bssa.org.uk/topics.php?article=175