What temp puts too much brittle stress on metal tools?

I got an interesting question from Ken today where he asked me the following question:

"Shop temperature this time of year does tend to run low. I've been wondering what is the safe cold temperature to work with metal tools, such as router bits, table saw, planer, etc.?

What temp puts too much brittle stress on metal tools?"

I told Ken that I could find a good answer to his question, so I thought I'd ask for some help from everybody.  If you know the answer, simply add your commit below.  

I don't think that the temperatures in most shops would even come close to causing a concern for metal tools.  But, what do I know? 

Let's here your take on the subject.

-Craig