Device (10) for hardening tubes (11), in particular for hardening square tubes using laser beams, said device comprising a tube (11) with clamping devices (14, 15) at both ends (12, 13), and a processing head (16) which moves relatively to the tube (11). To improve the device (10) in order to perform a hardening process as free of delay as possible, the device (10) is designed so that the clamping devices (14, 15) are provided with elastic sealing devices (18) sealing the tube inner cavity (17) and said clamping devices (14, 15) have at least one coolant line (19) connected to the tube inner cavity (17).