Test circuit for EU1640BCH12T / R Water proof ultrasonic sensors with Arduino?
I think that either the transducer Q factor is too high for the board (extra inertia due to the metal case), causing a pulse to become abnormally long, or the transducers fail to efficiently couple acoustically with the air (impedance mismatch), causing a weak ultrasonic pulse and even weaker reception. Internally a transducer looks like this:  

  

With the little horn being key to efficient coupling. At 40 kHz the wavelenght is just 330 m/s / 40 kHz = 8.25 mm.
Can you measure the terminals of the transmitter with an oscilloscope, and compare the results with the original transducer?

However I think that those transducers should work in open air with a proper controller as they are probably of the same design being used as parking sensors.  

Designing an ultrasonic rangefinder is not a trivial task and is beyond the scope of a reasonable answer.