From LoL wiki:
PENETRATING BULLETS
+30% armor penetration
Unique Passive - Dodge Piercing: Turret attacks can not be dodged
Unique Passive - Warming Up: Turrets gain 37.5% damage each time they strike a champion (Max 75% bonus damage).
Unique Passive - Heated Up: After the turret is fully heated, consecutive attacks against the same champion deal 25% additional damage (Max 50% bonus damage).
Against champions, Warming Up and Heated Up combine to deal 100% damage on the first hit, then 137.5%, 175%, 200%, and finally 225% on the fifth hit and beyond. If the turret attacks another champion, only Heated Up stacks will be reset, and the damage will stay at 175%. As soon as the turret is not targeting any champion, both passives reset and damage returns to 100%.