Explain what is meant by temper brittleness?

Answer 1

temper brittleness is most often used in reference to the resilient strength of the metal.

the more brittle the metal is, the easier it is to break. the less brittle, the harder to break.

And at the other end from brittle, it becomes too easy to deform.

A bolt with improper tempering for the task its put to is likely to either snap, or have the threads damaged by the nut