Originally Posted by Dave Brand
Accountants over-rule engineers!
Unfortunately this is often true (though i enjoy being able to tell them to f@*k off whenever i have a saftey related issue) and is why most things don't work as well as they could; from your kettle to your car - or even the trains & 'planes you use...
As for why Stainless Steel fastners aren't used everywhere, there are several reasons.
Firstly is obviously cost. This is, however, not entirely true since low stressed stainless bolts could be re-used whereas corroded steel bolts need to be binned.
Secondly, and most importantly, is the Ultimate Tensile Strength (UTS) of SS is lower than a steel alloy. The A4-80 is equivalent(ish) to an 8.8 grade bolt, but you then have 10.9 & 12.9 grade alloy steel bolts. 10.9 & 12.9 are classed as high strength steel alloy and require more care in use, but you can apply MUCH more load. For example you can torque an M8 8.8 grade bolt to 30Nm whereas a 12.9 grade will take 50Nm. In general you're looking at 70% more applied torque.
Thirdly is material compatability. If you clamp steel components with stainless fastners you get a galvanic reaction and resultant corrosion.
Fourth is that SS fastners have lower fatigue life than steel
Fifth is the galling effect of SS fastners can resist tightening and give false clamping loads for a given indicated torque applied. (Fit with anti-seize copper grease and use 10% less torque)
Otherwise they're great...
(Off to find an accountant to kick