View Full Version : Keel Bolts - What grade steel?

07-02-2017, 03:34 PM
Evening all, I'm replacing a couple of keel bolts in an iron keel. What grade mild steel should I be looking at (4.6, 8.8, 10.9 etc) I see strengths vary. As ever thanks for the advice.

wizbang 13
07-02-2017, 03:55 PM
I dunno anything about grades of steel.
Does one end of the scale take galvanizing better than the other?
A 'weaker" steel just may be better.

MN Dave
07-02-2017, 09:38 PM
I would go with a lower grade of steel. Keel bolts need more toughness than strength and the lower strength grades hsve less problems with things like hydrogen embrittlement and stress corrosion cracking. http://www.boltscience.com/pages/the-stronger-the-better-is-not-necessarily-the-case-for-fasteners.pdf

I wouldn't call 8.8 mild steel. High strength low alloy steel is more like it. FWIW, the strength of a metric bolt is in the grade number. The number before the decimal is the tensile strength in hundreds of megapascals, and the number after the decimal is the yield strength as a percentage of the tensile strength. So a 9.8 has a tensile strength of about 900MPa and a yield strength of 80% of 900, or 720. SAE grades you have to google for the strength. So I wouldn't call 9.8 mild and you might call me pedantic. So, with apologies to Wiz, I will forge ahead and say that the stainless grades like A4-70 also has the strength in the grade, 70 means 700 MPa ultimate, the A4 is the alloy and that's another google lookup.

EDIT, Non-bumpworthy: I am not convinced that wrought iron's relative corrosion resistance is all it is cracked up to be. If you want to find it, this may be the only consistently available source: https://www.realwroughtiron.com/about-wrought-iron/what-is-wrought-iron/

len hornick
07-02-2017, 11:12 PM
Hi Real wrought iron,It is available with some looking ,Gets along with cast iron ,not as strong as steel ,but tough ,Resists seawater corrosion.

07-03-2017, 01:11 AM
I have only replaced a few keel bolts, and used "off the shelf mild steel". I think wrought iron is far more suceptable to shock load cracking/shear in accidental groundings, where basic mild steel has a certain amount of stretch within its own ultimate strength. I coated replacements with epoxy coal tar, though galvanize is better if you can get them back in without damaging the coating. 25mm/1in studded bar/rod has been used in oversize holes an an epoxy pour with success.