Identifying stainless bolts

[zeusfaber]

Apologies if this is an obvious question - but it's an end of engineering I don't normally work in:

Been landed with a piece of kit held together with a number of stainless bolts that are far too tatty to put it back together with (some would benefit from still having a head, for starters).

I'm not having too much trouble identifying them as M6 x 1.0 x 20mm with a 10 mm AF hex head, but I'm not doing very well working out which grade of bolt I'm after. The only marking on the head is a very large figure "8". It's a marine application.

A couple of hours of Googling hasn't turned up anything very convincing. Anyone recognise the grade marking?

Thanks.

A.

 

[desertfox]

Hi zeusfaber

Are you sure they are stainless bolts have a look at these markings for stainless bolts:-

http://industrialhardware.com/excel/gradeMarkings.htm

http://www.shopdawg.com/Boltgrademarkings.htm

An 8 on its own sounds like a carbon steel bolt to be sure
you could try a magnet on one of the bolts most stainless bolts aren't magnetic

regards
desertfox

 

[unclesyd]

One has to be careful with using a magnet as some SS fasteners have varing degrees of attraction due to work hardening during manufacture.

If you have access to a lab if a fastener is magnetic a drop of 50/50 V of HNO3 will tell the tale.

Or take a torch and heat to cherry red, air cool and if stainless the fastener will lose it's attraction when a magnet is applied.

 

[drawoh]

unclesyd,

Austenitic stainless steel (300 or 18-8) is non-magnetic and not heat treatable. Most stainless steel screws I have encountered are this grade. Given its superior corrosion resistance, it is a good fastener for water craft. I usually point out to people that the heat treatable 400 series stainless steel is duller in colour and usually magnetic.

zeusfaber,

Carbon steel metric bolts would be identified as 5.6, 8.8, 9.8, 10.9 or 12.9, indicating the strength. Metric bolts would be A2 or A4. Could you be looking at bolts custom made for the manufacturer?

Try Maryland Metrics at

http://mdmetric.com.

JHG

 

[btrueblood]

drawoh,

Syd knows what he is saying. 18-8 (type 300) stainless steels become magnetic as they are cold-worked (work hardened); most fasteners nowadays are made by the thread-rolling process since it adds beneficial strength by the cold work done on the metal, thus most stainless fasteners are perceptibly magnetic (i.e. a magnet will stick to 'em).

 

[drawoh]

btrueblood,

Thanks.

Would they be magnetic after annealing? Most of the 300 series stainless steel fasteners I encounter are annealed. I usually work with machine and cap screws up to 1/4" and M6.

JHG

 

[unclesyd]

No, any Austenitic SS fastener that is magnetic at room temperature will be non-magnetic after an anneal.
The cherry red that I mentioned only gets the metal above the curie point. This is all that is required for SS to loose it's magnetic properties.

It is desirable to have some SS screws to magnetic. I have a friend who install gutters and flashing and would like the have Austenitic SS magnetic for all attaching all Al products, they will work better in a magnetic driver bit.

 

[GBor]

zeusfaber,

Are you sure it isn't an "infinity" symbol? Like a manufacturer's marking?

And how certain are you that it is stainless?

Can you describe your "marine application" a little more? Salt water or fresh water?

It wouldn't be a grade 8 carbon fastener in a salt water environment due to the possibility of hydrogen embrittlement. Besides, grade 8 marking on a hex head is 6 radial lines. And metric carbon steel would, as drawoh suggested, have more of an 8.8.

Any chance you can take a picture and upload it?

 

[rconner]

If this is any kind of significant forensic investigation, chemical analyses (e.g. spectrograph "shot" etc. of a piece of bolt metal) by a competent laboratory might go a long way to narrowing this down. If the bolts are e.g. an austenitic grade of stainless steel (e.g. Type 18-“8”/302 or 304… etc. see

http://www.estainlesssteel.com/corrosion.shtml

), I suspect this might show Cr in the range of say 18-20% and Ni somewhere maybe 8-12%. If they are something else, the levels of these elements will likely be significantly less.

 

[zeusfaber]

Pretty sure the marking is an eight - one lobe on the marking is bigger than the other.

Application is a saltwater cooling jacket on a cylinder head. I'm assuming stainless partly based on the colour, and partly based on the corrosion pattern (bit of staining where the threads have been fretting, but general absence of significant rust.

Come to think of it, I've got a contractor with one of those handheld X Ray fluorescence spectrometers - might ship one up to him and see if he'll do me a favour.

Thankfully, this isn't forensic - just an overdue maintenance task on an underloved, undermaintained and under documented piece of kit that has started to feel a bit jealous.

A.

 

[GBor]

It is a marine application and you are listed as a military engineer. Are you sure it isn't monel?

 

[zeusfaber]

Many thanks to everyone who's contributed here.

Found time this afternoon to get a closer look and have a play.

What I actually appear to have is carbon steel bolts with a bright zinc plate. I'm surprised how well they've done given the environment they're in and how much neglect they've seen.

I think the stamping on the head must be for the convenience of the OEM - looking further, I've found some similar bolts on the same machine with "6" (or perhaps that's "9") on the head - there's also some evidence of the same numbers on different diameters of bolt.

Conveniently, the OEM has stamped torque settings into most places, and I think I'll be able to use these to give me enough peace of mind to replace what I've got with A4 stainless. They're not heavily loaded fasteners - the main cause of failure seems to be seizure followed by overenthusiastic spannerwork rather than overstress.

GBor: These bolts are definitely steel, but Monel was a smart bit of lateral thinking - I've got lots of the stuff floating around.

Thanks everyone.

A.

 

[markarnoldTN]

A simple slightly destructive test is to take the bolt and carefully touch a grinding wheel in motion. Carbon steel will have a large pattern of fan shaped sparks. Stainless will be much more linear and closer together. The larger the carbon content, the more the sparks will fan out. I have done this many times as we could not always rely on the magnet test.