At first glance all hack saw blades may look alike, but there actually is a difference. The number of teeth per inch differentiates one blade from the next. Which blade you choose should depend on which metal you will be cutting.
For heavy-duty cutting jobs like steel reinforcing rod or pipe, an 18-teeth per inch blade would be the best choice.
For a job that requires medium-duty cutting, like a thin wall electrical conduit, a 24-teeth per inch blade would do a better job.
When cutting light metals like aluminum, a 32-teeth per inch blade should easily do the trick.