which stainless steel for clevis pins and similar parts

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Rum Run

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Here's one for the metallurgists!
I'm refitting an boat built in 1968. The chainplates, bow and stern fittings are original, made from some type of stainless steel and look fine. the bottle screws are bronze and look fine. The clevis pins may or may not be original but two have been lost and replaced with bolts anyway. This raises a question: What is the best stainless steel to use for clevis pins?
I know people will say A4 / 316, but most fasteners sold in the swindleries are A2 and who knows what the ones sold online are! And is A4 the best anyway - what about Duplex?
With the amount of material in each pin being pretty small, it seems worth optimising material quality without compromise on cost so I'm looking for advise before potentially buying some rod and machining my own (I have the necessary kit)
 
The clevis pins nowadays are usually either Nitronic or less usually Duplex, but I'd stay the hell away from 316 or 304, purely because of the likelihood of them getting crevice-corrosion. Making your own would be false economy because they're available in the proper grade from most swindleries. Bolts are a big negative, because the metallurgical changes weaken them against corrosion more than raw bar stock IMHO.
 
Adamastor said:
The clevis pins nowadays are usually either Nitronic or less usually Duplex, but I'd stay the hell away from 316 or 304, purely because of the likelihood of them getting crevice-corrosion. Making your own would be false economy because they're available in the proper grade from most swindleries. Bolts are a big negative, because the metallurgical changes weaken them against corrosion more than raw bar stock IMHO.
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Not sure why crevice corrosion should be an issue with rigging clevis pins. More likely to suffer from wear than crevice corrosion, although if correctly sized that should be minimal.
 
All my clevis pins are 316 made by myself and I have no problem.

I would not use 304 as it will have get crevice corrosion due to sea water getting in between the bearing surfaces and loosing the oxygen content to replace the chromium oxide at the joint that is removed due to small movement of the pin/joint.

I have had several A2 bolts fail due to crevice corrosion between the head and shank when se water has splashed onto then even inside.

I'm in the process of replacing the A2 bolts supplied with my 316 3 part ball valves.
 
Adamastor said:
The clevis pins nowadays are usually either Nitronic or less usually Duplex, but I'd stay the hell away from 316 or 304, purely because of the likelihood of them getting crevice-corrosion. Making your own would be false economy because they're available in the proper grade from most swindleries. Bolts are a big negative, because the metallurgical changes weaken them against corrosion more than raw bar stock IMHO.
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The S3i website shows their clevis pins as 316. They are a respected supplier AFAIK but I was a bit surprised. Who makes them Nitronic?
I'm considering making my own because the sizes may well be difficult to obtain, though I need to measure the diameters with a vernier rather than a Homebase tape measure to be certain that they are imperial, and because I can be sure of the material, and because I have a workshop full of kit for the job so if nothing else is paying my labour is "free".
 
Rogershaw said:
All my clevis pins are 316 made by myself and I have no problem.

I would not use 304 as it will have get crevice corrosion due to sea water getting in between the bearing surfaces and loosing the oxygen content to replace the chromium oxide at the joint that is removed due to small movement of the pin/joint.

I have had several A2 bolts fail due to crevice corrosion between the head and shank when se water has splashed onto then even inside.

I'm in the process of replacing the A2 bolts supplied with my 316 3 part ball valves.
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Could your experience of SCC be because of a warmer climate accelerating corrosion?
 
304, 316 and duplex stainless steels will all suffer crevice corrosion. Not sure about Nitronic but as it depends upon the formation of a passive chromium oxide film for corrosion resistance, as do the other three, I suspect that it does also. The only stainless material I know of that does not is the high nickel 904 but its cost excludes it for most purposes (other than Rolex watch cases!)

My rigging clevis pins could well be 30 years old, almost certainly 316 with no sign of crevice corrosion because they are rarely wet for significantly long times. They are a little distorted due to rigging loads but not worn.
 
I replaced some bolts which a previous owner of my old dayboat had used as clever pins. The thread on the bolts had worn the holes in the chain plates to an oval shape. Fortunately the new rigging screws had larger diameter pins so I simply had to bore out the oval holes to round.
 
My new Sta-Loks are 316. I'm sure my original Hasselfors were too, and had minimal wear and no corrosion after 15 years. Would've happily reused the old if the new toggles hadn't already come with new clevis pins. Gave them a polish and put them in the rigging spares box.
 
Answer to the question "What is the best stainless steel to use for clevis pins?" depends whether you need very high strength. I mean, 6mm dia clevis pins in a mirror dinghy will be fine in almost any metal so you might as well choose best corrosion resistance ie 316. But if you want high strength and in order to achieve that you can tolerate 304 levels of corrosion resistance rather than 316, you need 17-4ph stainless steel. It has UTS of around 1100-1200 MPa, compared with about 600MPa for 316 and can be machined, welded and mirror polished the same as 316. I always buy 17-4ph products for reasonably mission critical items like for example tie down eyes for a heavy tender where, say, you want a smallish diameter swivel pin but very high strength.
 
Don't forget that clevis pins are in shear and not tension so it is the ultimate shear strength that is more important than ultimate tensile strength.
 
Rogershaw said:
Don't forget that clevis pins are in shear and not tension so it is the ultimate shear strength that is more important than ultimate tensile strength.
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Two things:
1. They correlate. UTS is just easier to google online. An alloy that has 2x the UTS of another material is going to have broadly aroundabout 2x the shear strength too. The point of my post was the ~2x multiple, not the absolute value
2. Except where the 2 parts joined by the shear pin fit together with virtually no gap, a clevis pin is in bending, which is somewhat different from shear, and UTS is highly relevant
I'm sticking with my story!
 
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jfm said:
Two things:
1. They correlate. UTS is just easier to google online. An alloy that has 2x the UTS of another material is going to have broadly aroundabout 2x the shear strength too. The point of my post was the ~2x multiple, not the absolute value
2. Except where the 2 parts joined by the shear pin fit together with virtually no gap, a clevis pin is in bending, which is somewhat different from shear, and UTS is highly relevant
I'm sticking with my story!
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I take your point but as a mechanical design engineer ex CEng if a clevis pin has any significant bending moment the arrangement is a very bad and dangerous design.

One of the worst points about current clevis pin design on rigging is that the design allows the pin to rotate in the part with the smallest bearing area thus allowing extra wear the pin.
 
Rogershaw said:
I take your point but as a mechanical design engineer ex CEng if a clevis pin has any significant bending moment the arrangement is a very bad and dangerous design.

One of the worst points about current clevis pin design on rigging is that the design allows the pin to rotate in the part with the smallest bearing area thus allowing extra wear the pin.
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I'm not going to get into a qualifications competition :D. Yep I agree re design, but it happens, esp on smaller boats. Indeed on small boats where the shroud has a thimble loop in the steel wire, the clevis is always in bending due to the curve in the thimble, no matter how apparently snug is the fit of the shroud into the chain plate. I don't love it, but it happens.

My point remains though: I was merely wanting to convey that 17-4ph is ball park twice as "strong" as 316, and on par with 304 for corrosion resistance. Just as an example, the ties downs from Monitor Marine, of which I have a few and they are lovely things, are made in 17-4ph
 
 
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