2001 Volume 19 Issue 1 Pages 60-69
Two types of intergranular fractures (type I and II) were observed in the specimens after the creep-rupture test on the synthetic HAZ of Cr-Mo steel. The types I and II fractures, respectively, may be induced by the weld-thermal-cycle, and by some mechanism of secondary segregation. The authors assumed that the type I fracture will be brought about by the segregation induced by the α/γ transformation ("the transformation-induced segregation") during welding. The possibility of arising the transformation-induced segregation was examined by the experiments on the synthetic HAZ and also by the calculations. The grain boundaries of prior-austenite was analyzed by using two methods ; "the EDX analysis in a micro-region" and "the grain boundary etching method". The experimental results informed that phosphorus was concentrated in the grain boundary at the time when the α/γ transformation occurred. The greater the heating rate, the larger the quantity of phosphorus in grain boundary. The segregation of phosphorus will be induced by a great heating rate such as 1000 K/s. The mechanism of this type of segregation could be explained from the view point of the distribution of phosphorus in transformed phase (austenite) and original one (ferrite). Phosphorus is accumulated at the α/γ interface as the distribution coefficient ko (=[P]γ/[P]α) is smaller than the unit.
