metals in boating
 

I broke a bolt holding my outboards so I am looking into what the BEST metals for new bolts would be as I have obviously brought the ones I have to failure.

Heres an interesting little article about monel

Monel's corrosion resistance makes it ideal for marine applications such as piping systems, pump shafts, seawater valves, trolling wire, and strainer baskets. Some alloys are completely non-magnetic and are used for anchor cable aboard minesweepers,[4] housings for magnetic-field measurement equipment. In recreational boating, Monel wire is used to seize shackles for anchor rodes, Monel is used for water and fuel tanks, and for under water applications. It is also used for propeller shafts and for keel bolts.

However, because of the problem of electrolytic action in salt water (also known as Galvanic corrosion), in shipbuilding monel must be carefully insulated from other metals such as steel. The New York Times of August 12, 1915 published an article about a 215 foot yacht, "the first ship that has ever been built with an entirely monel hull," that "went to pieces" in just six weeks and had to be scrapped, "on account of the disintegration of her bottom by electrical action." The yacht's steel skeleton deteriorated due to electrolytic interaction with the monel.[5]

Alloy A-286

Alloy A286 is a precipitation hardenable alloy, which is used extensively in the petro-chemical, aerospace and oil field industries, especially for fasteners, stud-bolts

The mechanical properties detailed are typical of most oil-field equipment companies requirements. The main difference between these figures and the properties of ASTM A638 Grade 660 is that the 0.2% offset yield strength is increased from 85,000 psi up to 105,000 psi.

To obtain this a secondary precipitation treatment is often necessary. The maximum hardness figure of 35 HRC is from NACE MR-01-75 and differs from ASTM A638 grade 660 which is 248 HBN minimum. Alloy A-286 is also used for extrusion liners and dies.



Tensile Strength, min

145,000

1000

-



Yield Strength
(0.2% offset), min

105,000

724

-



Elongation in 2"
(or 50mm) or 4D, min

-

-

15



Reduction of Area

-

-

25



Hardness(Rockwell)

24-37

-

-

A286 is a very good choice for high strength and good corrosion protection. Ftu of A286 is usually around 130-160ksi depending on heat treat.

Even better would be something in a 300-series stainless (especially 316L) for high strength and excellent corrosion resistance. Ftu of 316 is typically around 75ksi annealed but can reach upwards of 150ksi fully hardened.

Maybe you can find some of the information that you're looking for here...http://arp-bolts.com/

Speaking of outboard bolts, this reminds me of a similar incident from a boat I was on last summer. We had the same situation occur on the steering system.

Check out the cyclical fatigue failure of these bolt threads. This is precisely why you should never load up threads in shear.
You can tell from the corroded lines that this bolt was failing little by little over time. The final failure occurred when the last remaining bit tore off (where there's no rust).

BTW, if you want the ultimate high-strength, corrosion resistant bolt, find yourself some NAS67xx series bolts.

How did the bolt break and what do you attribute the failure to? I'd say depending on what the bolt is going through (casting or billet or fabrication, etc) that you're better off staying with commonly available 316L SS and going to a larger size, especially IF you have ruled out overtorquing or other installation issues and you think it is simple overstress.