A method for induction hardening of an annular surface of a circular mechanical component part, comprising the steps of: - arranging at least four inductors, in pairs, in tandem with respect to said annular surface to be hardened, associated to a first cooling shower arranged immediately adjacent to a second cooling shower; - feeding and simultaneously rotating the inductors, relatively to each other and with respect to the annular surface to be hardened, moving therewith the showers (16,18); characterized in that, in combination: i)- the inductors are arranged to form a first and a second pair of inductors, each pair formed by a first inductor (5;7) and by a second inductor (6;8) arranged immediately adjacent the first one, wherein the first inductor of the first pair is operatively associated to the first shower and the first inductor of the second pair is operatively associated to the second shower; ii)- the first and second pair of inductors are arranged in tandem, immediately adjacent to each other, with the first shower arranged immediately adjacent to the second shower; iii)- the step of feeding and simultaneously relatively rotating the inductors is carried out so that the second pair of inductors leaves the first pair from the side of the first shower to be repositioned in tandem adjacent to the first pair on the opposite side, while cooling the surface to be hardened, previously heated by the inductors, by means of the showers (16,18) associated with the first inductors (5,7); the method further comprising the steps of: - ceasing to feed and moving away from the surface to be hardened the second inductor (6,8) of each pair (15,17) when the second inductors (6,8) of the first and second pairs are immediately adjacent to each other, while the relative rotation between the first inductors (5,7), the corresponding showers (16,18) and the surface to be hardened continues until the first inductors (5,7) of the first and second pairs (15,17) are immediately adjacent to each other; - ceasing to feed and moving the first inductors (5,7), either subsequently or simultaneously with respect on each other, away from a portion of the surface to be hardened on which they insist at that moment, along with the first and second showers (16,18); - cooling said surface portion to be hardened with a third shower which is positioned thereon in replacement of at least the last inductor (5,7) moved away - arranging at least four inductors, in pairs, in tandem with respect to said annular surface to be hardened, associated to a first cooling shower arranged immediately adjacent to a second cooling shower; - feeding and simultaneously rotating the inductors, relatively to each other and with respect to the annular surface to be hardened, moving therewith the showers (16,18); i)- the inductors are arranged to form a first and a second pair of inductors, each pair formed by a first inductor (5;7) and by a second inductor (6;8) arranged immediately adjacent the first one, wherein the first inductor of the first pair is operatively associated to the first shower and the first inductor of the second pair is operatively associated to the second shower; ii)- the first and second pair of inductors are arranged in tandem, immediately adjacent to each other, with the first shower arranged immediately adjacent to the second shower; iii)- the step of feeding and simultaneously relatively rotating the inductors is carried out so that the second pair of inductors leaves the first pair from the side of the first shower to be repositioned in tandem adjacent to the first pair on the opposite side, while cooling the surface to be hardened, previously heated by the inductors, by means of the showers (16,18) associated with the first inductors (5,7); - ceasing to feed and moving away from the surface to be hardened the second inductor (6,8) of each pair (15,17) when the second inductors (6,8) of the first and second pairs are immediately adjacent to each other, while the relative rotation between the first inductors (5,7), the corresponding showers (16,18) and the surface to be hardened continues until the first inductors (5,7) of the first and second pairs (15,17) are immediately adjacent to each other; - ceasing to feed and moving the first inductors (5,7), either subsequently or simultaneously with respect on each other, away from a portion of the surface to be hardened on which they insist at that moment, along with the first and second showers (16,18); - cooling said surface portion to be hardened with a third shower which is positioned thereon in replacement of at least the last inductor (5,7) moved away