The Pirates made the moves before Friday night's game at Washington.
Pittsburgh claimed Axford off waivers Thursday from the Cleveland Indians. The 31-year-old was 2-3 with 10 saves and a 3.92 ERA in 49 games this season for the Indians.
For his career, Axford is 24-22 with 116 saves and a 3.38 ERA. He had 46 saves for the NL Central champion Milwaukee Brewers in 2011.
The 24-year-old Pimentel is 2-1 with a 5.40 ERA in 19 relief appearances this season.
Pittsburgh second baseman Neil Walker, who has missed 11 of the past 12 games with back tightness, was in Friday night's starting lineup.
"The muscle spasm is completely gone," Walker said before the game. "So we're in a pretty good place right now."