Only one thought. I remember from somewhere or other that you should not apply the setter to the base of the tooth but rather part way up. Supposedly applying the tooth setter at the base increases the chances of breaking the tooth. I have generally followed this advice and (knock wood) have never broken a tooth.
You now have me wondering whether steel can get brittle. I am pretty ignorant about this, but doubt that it can become brittle unless heated in a way that changes the structure of the steel.