Will Bolts Be Significantly More Brittle in Moderately Low Temperatures?

Many years ago I learned 2 fundamental rules regarding structural materials; Don't use any more carbon than necessary and don't make it any harder than necessary. 30 or more years ago, embrittled grade 8 bolts caused the largest recall in the history of GM up to that time. Those bolts were used in the undercarriage and failed in a brittle manner in cold climates with snow, ice, salt, etc.

The best and toughest structure for your tow hitch bolts is tempered martensite. My guess is that the fractures in your bolts are flat plane and brittle with little evidence of ductility. You might examine the fracture surface and see if you can find the origin at perhaps a crack or other stress riser.

The key to determining the effects of cold temperatures on the bolts is determine their material and temper (hardness) and check the corresponding ductile to brittle transition temperature (Charpy V-notch, or equivalent). Going over speed bumps might have caused an impact load on the bolts.

Harder and stronger is not necessarily better.

Did the heads fall off or were the fractures random?

It was the heads on both occasions.

I had a similar problem with thousands of bolts where the head fell off. It was the result of improper heat treat during manufacture. Cost me a bundle with the recalls and so forth. Solved the problem for good by getting a Mechanical Certificate on all subsequent orders. The bolts tightened just fine but being under stress due to the fault within one year the heads parted company with the shank. I had a very good designer draftsman at the time and he recognized the problem. I had a similar occurance in heavy industry where the heads were falling off Allen socket bolts used to hold hydraulic flanges together but the superiors at the time would not listen and continued using same bolts for a while. We performed a lot of unnecessary cleanup for this reason.

Sorry, I was not logged in. It was the heads that came off both times.

Thanks for the advice Roy Hammy.