rotational axis for successively changing the position of laser beam 9 application the same periphery may be heat treated by rotating the mirror 10 while the rotor 2 is set in a fixed position as shown in fig3 if desired the mirror 10 in fig3 may be made tiltable so as to direct the laser beam 9 across the periphery or both the rotor 2 and the mirror 10 may be tiltable and or movable furthermore instead of changing the location of application of the laser beam 9 in a continuous manner along the rotor periphery the laser beam 9 may be applied in a successive spot to spot manner along the periphery as a further alternative the laser beam 9 may be directed first to an arbitrarily selected spot or area 34 a 34 see fig4 and subsequently to the spot or area 34 b 34 which is farthest away from the spot 34 a 34 along the groove 3 and then to the spots 34 c 34 and 34 d 34 and so on in intermittent sequence so that each shot of the laser beam 9 is applied to the area which is farthest from that to which the immediately preceding shot was directed though the above mentioned experiment on rotor surface hardening was made using a co 2 laser other types of lasers such as a yttrium aluminum garnet yag laser or ruby laser may be employed other appropriate forms of radiant energy such as an electron beam might also be used in place of the laser beam 9 in the same way and.