<VV> Bolt Grades

[Chris & Bill Strickland]

As part of the continuing saga of the broken bolts, I ran across this 
article, with comments by bolt manufacturers (do you really trust the 
guy that makes the product?) about fatigue of Grade 8 bolts and some 
high strength stainless fasteners. 
http://www.enginebuildermag.com/Article/23859/fastener_facts.aspx

Seems like one of the issues with fatigue failures has to do with using 
a bolt in instances where it is not torqued solid to full design 
specification and then it is loaded cyclically. Perhaps the outer spring 
links fall into this category?

Bill Strickland

“One of the issues with Grade 8 bolts is that there are some areas where 
you really don’t want to use them,” says Doc Hammett, [of] Totally 
Stainless [http://www.totallystainless.com/]. “If there’s a cycling load 
on them you could start to get into trouble. A classic example was on 
the old belt drives where street rodders were using Grade 8 for 
accessories and they were breaking bolts all of a sudden. Many were left 
scratching their heads until someone figured out the bolts were 
fatigued. The higher the carbon steel the more they are prone to 
fatigue. Fastener manufacturers add other alloys to carbon steels and 
change the properties to suit their specific needs. This is one of the 
most interesting things about steel: you can add a little bit of 
something and make the properties change drastically.”

Hammett says one of the biggest things that Totally Stainless has done 
recently is to introduce large high-strength stainless bolts. Stainless 
steel by definition is anything with at least 12 percent chromium in it. 
“There are over 1,000 different alloys of stainless,” says Hammett. 
“What people generally think of is 300 series stainless is generally a 
low strength material and is not heat treatable. The most common 300 
series is an 18-8. It’s 18 percent chrome and 8 percent nickel. The 
tensile strength for 1/2˝ and larger 18-8 stainless bolts is no more 
than 80,000 psi and the yield strength is only 45,000 psi. We use 17-4 
PH for our high-strength bolts, this material is heat treatable and has 
a tensile strength of 200,000 psi and a yield strength of 175,000. We 
electro polish them, which makes them more corrosion resistant.”