+1 for an angle grinder with suitable cut off disc.
But that CAN be a bit in-accurate, so if a hack saw is a must (accuracy) then the basic rules is at least 3 teeth in contact all the time (plus, as said above, use the saw for the FULL length, NOT "jabbing"quickly backwards and forwards in just the middle of the blade).
There's a long "tutorial" at the top of the Metal Working section (by yours truly, sorry) which goes into detail, but in a nutshell:
1. With a 12mm bar, 18 TPI may well keep minimum 3 teeth always in contact (try it); if not, then a 24 TPI blade;
2. If you're confident that you can keep sawing straight without flexing (and therefore, most likely shattering the blade), then a HSS "all hard" blade every time. If not, then a "bi-metal" blade;
3. "Best" brands IMO are Eclipse, Sandvik, Starret (in no particular order). All 3 do both "all hard" and "bi-metal" blades;
4. Do NOT bother with non-branded "£ shop" type blades.
Edit for a P.S. No cutting fluid required for any of the above. Just adds mess and does nothing to reduce temps (IF you're cutting at sensible speed, full blade stroke).
HTH