The home side dominated from the off in Tuesday night's League One clash at Bramall Lane and were never seriously tested by Darren Ferguson's Posh side.
Stephen McGinn had a shot on the run saved by Bobby Olejnik and Callum McFadzean's low drive from the edge of the area was deflected wide.
McGinn then went close to scoring with a low drive which beat Olejnik but came back off the keeper's right-hand post. Billy Paynter, following up, shot straight at Olejnik.
As the home side continued to press, Shaun Miller made a good run into the area before firing over.
Davies (47) struck the opener with a superbly flighted free-kick over the defensive wall - the ball going into the top right-hand corner of Olejnik's net.
After McGinn fired off-target from a good position and Conor Coady shot just over, substitute Porter (88) added a second goal with a good finish after being sent clear through the middle.
The Blades are now 10th in the table, eight points behind sixth-placed Peterborough with 13 games remaining.