mentioned previously and the operations are repeated continuously in the above described embodiment the clamp rolls 33 are formed into an inverted frustoconical shape and each pipe 5 is clamped in a wedge gap formed by each of the clamp roll pairs which are arranged at a plurality of points in the lengthwise direction of the pipe thereby preventing the pipe from being ejected oscillated or bent also a unidirectional rotating mechanism is provided for the clamp rolls 33 allowing each clamp roll 33 to rotate only in the direction of an arrow 35 and consequently each clamp roll 33 is prevented from rotating in the opposite direction due to an internal pressure produced when the cooling water is supplied into the pipe 5 thus the pipe 5 is held in position and there is no danger of the nozzle 41 disengaging with the pipe and causing leakage of the high pressure cooling water and cracks in the pipe itself or the adjacent pipe or pipes when the pipe 5 shrinks upon cooling each of the clamp rolls 33 rotates in the direction of the arrow 35 in response to the shrinkage and thus there is no danger of damaging the outer surface of the pipe 5 then when the pipe 5 is quenched so that the pipe 5 is elongated while slipping over the surface of the clamp rolls 33 there is no danger of damaging the outer surface of the pipe 5 since the pipe outer surface has already been hardened by the quenching note that where the pipes can be clamped satisfactorily by the pipe body clamps alone depending on the quenching conditions the pipe end clamps need not be provided while in the embodiment described above water is contained in the quenching.