3d Key not strong enough
That is correct! Take your raw ABS plastic and try to bend and break it. Do it again with a few lengths. Pretty easy to break it right?

You issue is the material is not up to par with your goal. Your heavy duty lock takes too much force to turn.

Your solutions are to 


Find a stronger material. These are considered stronger than PLA / ABS


PVA
PET
Polycarbonate
PETT (SHATTERS!)
POM, Acetal

Use the PLA to make a lossless cast of the key and use metal
Find a easier lock and or invest in oil for the lock.