The invention relates to a tool for applying torque to a hammer union having a hammer union wing with wing nuts. The tool comprises a wrench having an arcuate or circular shaped wrench head that can be placed around the hammer union wing, the wrench head having wing nut cavities in the inner surface for receiving the hammer union wing nuts, and sockets around the outer surface for receiving one or more lever members such as a torque bar and/or hammer lug for applying torque to rotate the wrench head. The lever member(s) may be positioned in any of the sockets in the wrench head, thereby allowing the lever member(s) to be repositioned during use with respect to the wrench head without having to remove the wrench head from the hammer union. The wrench head may be held in place on the hammer union wing using magnets.