Rotatable cutting tool and cutting tool body

A rotatable cutting tool carried in a bore of a holder wherein the holder has a forward surface surrounding a forward end of the bore. The rotatable cutting tool includes an elongate cutting tool body that has a central longitudinal axis, an axial forward end and an axial rearward end. The cutting tool body contains a socket in the axial forward end thereof whereby the socket receives a hard insert therein. The cutting tool body has an enlarged diameter collar mediate of the axial forward end and the axial rearward end. The mediate collar presents an axial forward facing surface and an axial rearward facing surface. The cutting tool body has an axial forward hardness region beginning at and extending a first pre-selected distance in an axial rearward direction from the axial forward end to encompass the axial forward facing surface of the collar. The axial forward hardness region has a hardness equal to or greater than a first hardness, as well as a first average hardness. The cutting tool body has an axial rearward hardness region beginning at and extending a second pre-selected distance in an axial forward direction from the axial rearward end to encompass an axial rearward section of the shank portion. The axial rearward hardness region has a third average hardness. The cutting tool body has a transition hardness region mediate of and contiguous with the axial forward hardness region and the axial rearward hardness region. The transition hardness region encompasses the axial rearward facing surface of the collar and an axial forward section of the shank portion. The transition hardness region has a second average hardness. The second average hardness is less than the first hardness, and the third average hardness is less than the second average hardness.