Aluminium, SS or Galvanised Screws.

[Hunson]

Following an incident with my (new to me, first) boat. I am in need of obtaining some screws to refasten the rail to the toe rail that my rubbing strake ( D section) clips onto.

Now in my limited metallurgic knowledge and from what I have read on the forums over the past few weeks I understand that I have to becareful about what metals I put together.

My problem starts with the following pics.

The first screw on the left came from the rail along with the 3 others. Now I was checking to see what sort I needed believing that I would need aluminium as this is what I believe my rail to be. On testing with a small magent the first screw is picked up (indicating a steel content) but the other three are not attracted at all again leading me to believe that they are aluminium.

So I tested on some stainless steel screws and found they were not attracted either. Is this right. Or could what I think are SS just be shiny aluminium.

Also is it possible from the pics to get an idea of what size they are.

Many thanks for your help and knowledge.

 

[tillergirl]

Some stainless steel grades are magnetic, others not. The 'marine' grades of 312 (A2) and 316 (A4) are not magnetic. Ain't never seen aluminium screws (not that that means they don't exist) but don't use galvanised. I suppose the perfect answer would be to use marine grade stainless electrically insulated from the rail - by some rubber grommet.

 

[Hunson]

Thanks, I take it then also that my rail is also probably one of these types as well rather than my percieved aluminium. .

 

[Guest]

Post deleted by danfoley

 

[ccscott49]

A2, (304) are magnetic. A4 (316) arent.

 

[Hunson]

From what you say about the colouring, its ali, as its sort of a whitish/greyish matt, whereas I would expect the ss to be more polished. And its reasonably rigid/flexible compared to what I would expect of ss.

Many thanks all.

 

[VicS]

[ QUOTE ]
A2, (304) are magnetic

[/ QUOTE ] Are you sure about that. Certainly not magnetic like mild steel. in fact the only way you can tell A2 and A4 apart is by chemical or other instrumental analysis.

A2 is 304, not 312 as stated else where.


316 (A4) is the material to use for this job and it should be used in conjunction with zinc chromate paste (AKA yellow duck poo). Readily available from good chandlers is "Duralac" paste, which is actually barium chromate.

Some useful info on stainless steel (at least A2 and A40) can be found on
A2A4 's website

Also the British Stainless Steel Association website and the International Stainless Steel Forum website.

 

[cliff]

[ QUOTE ]
A2, (304) are magnetic. A4 (316) arent.

[/ QUOTE ]Not quite correct. Let me explain......

Magnetic permeability (the ability of a material to carry magnetism) is indicated by the degree to which it is attracted to a magnet. All stainless steels, with the exception of the austenitic group, are strongly attracted to a magnet.

All austenitic grades have very low magnetic permeabilities and hence show almost no response to a magnet when in the annealed condition; the situation is, however, far less clear when these steels have been cold worked by wire drawing, rolling or even centreless grinding, shot blasting or heavy polishing. After substantial cold working Grade 304 may exhibit quite strong response to a magnet, whereas Grades 310 and 316 will in most instances still be almost totally non-responsive.

The change in magnetic response is due to atomic lattice straining and formation of martensite. In general, the higher the nickel to chromium ratio the more stable is the austenitic structure and the less magnetic response that will be induced by cold work.

Any austenitic (300 series) stainless steel which has developed magnetic response due to cold work can be returned to a non-magnetic condition by stress relieving. In general this can be readily achieved by briefly heating to approximately 700 - 800°C (this can be conveniently carried out by careful use of an oxy-acetylene torch). Note, however, unless the steel is a stabilised grade it could become sensitised to carbide precipitation. Full solution treatment at 1000 - 1150°C will remove all magnetic response without danger of reduced corrosion resistance due to carbides.

Many cold drawn and/or polished bars have a noticeable amount of magnetism as a result of the previous cold work. This is particularly the case with grades 304 and 303, and much less so for the higher nickel grades such as 310 and 316. Even within the chemical limitations of a single standard analysis range there can be a pronounced variation in the rate of inducement of magnetic response from cold work.

Clear? or is everyone even more confused? /forums/images/graemlins/grin.gif
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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>

 

[ccscott49]

Arent screws etc cold worked? The bolts I was using for my passerelle, were A4, I couldnt get two in A4 so used A2, those showed attraction to a magnet.

 

[cliff]

[ QUOTE ]
Arent screws etc cold worked? The bolts I was using for my passerelle, were A4, I couldnt get two in A4 so used A2, those showed attraction to a magnet.

[/ QUOTE ]Not necessarily, and even if they were cold worked during manufacture, they may also have been solution annealled (or stress relieved) to remove any magnetic properties induced. To say 304 screws are magnetic is incorrect - they MAY be magnetic but just because they are 304 it does not follow that they are magnetic.
--------------------

"Artificial intelligence is no match for natural stupidity"
sailroom <span style="color:red">The place to auction your previously loved boatie bits</span>

 

[Hunson]

With the ability of these to be able to become magnetised. If I was planning on relocating a couple of the holes in the rail, what type of drill bit should I use.

Or is it a case that you can rub some small objects against each other to magnetise and the fact that the rail is long that the localised drilling wouldn't create a manetised effect. ?

 

[misterg]

For what you're doing, you can safely ignore most of the above /forums/images/graemlins/smile.gif (with due respect to Cliff, VicS, et al)

They look like 1/2" x No.8 Pan head self tapping screws. (Maybe 12mm x 4mm in metric). The best thing is to verify the size by comparing them against some new ones in the blister packs in a hardware shop, or the chandler (don't have to be SS just for comparison), then order a job lot of new ones. I couldn't get A2A4's site to work for self tappers, but even Screwfix do them in A2 (which I understand is OK for use on deck). here

Your toe rail is probably aluminium, so you should use duralac or a similar product to stop corrosion due to dissimilar metals.

Any decent quality drill bit should be OK to drill the holes, but you may need to experiment with the exact size to get the screw to go in without shearing off, yet still have a decent hold. I would start with ~1/8", and try on some scrap first. You may or may not need to drill clearance holes in the part that the screws pass through. If you're drilling stainless steel, the cobalt drill bits are better. (For SS, the bit needs to turn relatively slowly, but you need to keep steady pressure on it so that it produces one long continuous bit of swarf, and use some oil as a lubricant - works for me, but others will recommend differently - just don't drill SS dry. Not an issue for aluminium)

Andy

p.s. The screws are almost certainly not aluminium, though I've got some aluminium nails (and plastic ones, too).

 

[VicS]

[ QUOTE ]
with due respect to Cliff, VicS, et al

[/ QUOTE ] What do we do with this fellow Cliff? Al's not too pleased either! [ QUOTE ]
I couldn't get A2A4's site to work for self tappers

[/ QUOTE ] Do a search on there for self tapping you'll find them then alright! Loads of sizes to choose from and slotted or Pozidriv heads.

Another source is Sea Screw but check their minimum order value.

Also Anglia Stainless ltd



I agree that they are 1/2" x 8. the nearest on a2a4 is 13mm x8. Othersites may list them as 4mm rather than no.8

 

[Hunson]

Thanks guys n gals.

Today I have learnt alot thanks to your replies.

misterg and VicS you are both quiet right that they are No8 1/2". A phone call to a nice gentleman at Sea Screw to ask the preverbial question of how do I mesure a screw /forums/images/graemlins/smile.gif and all orderd.

Again many thanks.

Gary.

 

[wotayottie]

I cant remember ever supplying any 310 for any fastener application. Leaving aside the copper containing 302 type alloy (and the martensitic and ferritic grades), all were either 304 or 316. None of the self tapping screws you show in your piccie will have been heat treated after roll forming the thread - if they had been, the thread would be soft and the screw not self tapping.

As far as I can see from your piccie, those screws are stainless steel. I'd be surprised to see any aluminium or galvanised in that application. Should be easy to tell anyway - aluminium are light weight and both ally and galv are silvery whilst austenitic tends to look slighly yellowish when put at the side of a galv screw.