Investigations on Bending Condition for Welded Carbon Steel Pipe by High Frequency Induction Heating

Abstract

The induction heating bent pipes of carbon steel welded pipes are used for the piping in nuclear power plants, in place of elbows. This application is useful to suppress the radiation exposure at in-service inspection. The quality of the bent pipes are controlled on the technical standards of welding for electrical equipments. However, the influence of the bending condition has not been yet sufficiently understood on the mechanical nronerties of the bent nines.
The purpose of this investigation is to establish the appropriate bending dondition for the carbon steel weld pipe which corresponds to the carbon steel pipe STPT 42 in JIS G 3456, in relation to the transformation of the structures of the base metal and the weld metal durnig bending. The results are summarized as follows:
(1) The maximum heating temperature should be set in the range from 900°C to 1000°C, in order to assure the high Charpy impact properties.
(2) The maximum heating temperature which is lower than 900°C causes the imperfect transformation of the base metal and the weld metal, then is likely to spoil the Charpy impact properties.
(3) Higher heating rate causes the increase of A_c1 point, remarkably for the base metal which has higher carbon content than weld metal.
(4) Higher cooling rate causes hardening of the base metal and weld metal, however, the transformation temperature does not change remarkably, except for the A_r1 point of base metal.