According to the Guiness Book of World Records, Eddie Feigner, a famous softball player and the founder of the "King and His Court" 4-man softball team, is supposed to have been clocked at 104 miles per hour. In a game against Major Leaguers Feigner K'd Willie Mays, Roberto Clemente, Brooks Robinson, Willie McCovey, Maurey Wills, and Harmon Killebrew, all in a row.
and yet...arm injuries for pitchers are quite uncommon for softball players. ( knees and throwing injuries for infielders and outfielders are much more common.) there is some schools of thought that young baseballplayers should throw underhanded until 14 years old.
I have been a fastball pitcher for about 10 years. Other than a minor rotator cuff problem I have not had an injury at all. One thing that both my massage therapist and chiropractor noticed is that the shoulder of my pitching arm is rotated slightly inward (they tell me it is nbecause of my pitching).
As long as you have proper technique, I would think that the risk of injury would be fairly low.