Should I replace my mild steel keel bolts with stainless steel?
When considering replacing keel bolts, or any other fasteners, with a different material, a lot of things need to be considered. The designer and manufacturer will have specified a certain fastener for a reason, and that is rarely just a question of cost or ease of manufacture. There are two popular misconceptions about stainless steel. First that it is stronger than mild steel and second that it doesn’t corrode. Both are very significantly wrong.
The subject of corrosion is covered in another of my FAQ’s on this site. Basically, stainless steel corrodes (or more accurately erodes) where you can’t see it, whereas mild steel corrodes where you can see it. Furthermore, stainless steel corrosion creates crevices which can lead to fractures in the metal whereas mild steel corrosion is usually little more than a surface defect.
The issue of strength can be summarised by observing and understanding the head markings of the bolts. Mild steel metric bolts are marked with two numbers separated by a dot. The most common high tensile bolts being 8.8, 10.9, 12.9 and the strongest 14.9. Imperial sizes were marked with radial lines but we will concentrate on the more popular metric bolt grading system.
The first digit is the tensile strength, which is the load at which the bolt will fail. The second digit is the yield strength or elastic limit. This is the load at which the bolt will stretch by a certain percentage to absorb the load without suffering any permanent deformation. It can be seen from this that the tensile strength will always be more than or equal to the yield strength.
Stainless steel bolts are graded in a similar way but are marked differently. There is only one digit for the tensile strength because there is no reliable yield strength in stainless steel. It is often thought to be harder than mild steel and therefore stronger. The opposite is true. Hardness makes the metal more brittle and therefore unable to stretch to absorb loads without breaking.
The most common markings for stainless steel are A2 or A4, which relate to the alloy, and then 60, 70 or 80. These equate to the first digit in the mild steel series with an additional zero to denote the lack of a recordable yield strength. It must be noted that the stainless steel bolt with the highest tensile strength is equal to one of the lowest grades of high tensile mild steel bolts.
So when considering replacing mild steel keel bolts with stainless steel, remember that the designer specified the size of bolts, the number of bolts AND the grade of steel bolts in a complex calculation of loads and stresses that they will be subjected to, he wasn’t just trying to save some money.
Also, think about this. Why is it that the motor industry never uses stainless steel bolts? It is very frustrating trying to disassemble a car suspension or drive train with mild steel bolts that have been covered by years of salt or mud. Stainless steel would be so much more sensible … or would it? Think about that next time you hit a pot hole at 60mph!