Effects of Metallurgical Variables on Creep Embritllement of Steels

Abstract

The effects of metallurgical variables on creep embrittlement of steels for use of boiler and chemical reactor vessels were investigated.
The correlation between laboratory studies and failure cases of actual vessels was examined. The following results were obtained.
(1) There existed the lowest creep ductility range for a steel at a certain Larson-Miller's parameter.
(2) Most severe embrittlement ocurred in coarse grain region of HAZ (heat affected zone) of weldment.
In order to improve the embrittlement of the HAZ, treatments as stress relief annealing of higher temper parameter or rapid heating similar to heat cycles in fine grain region of HAZ were most effective.
(3) Detrimental effects due to impurities on the creep ductility ocurred only in synthetic HAZ specimens for 11/4Cr-1/2Mo steels. The effect of P on the creep embrittlement was smaller than that of Cu and Sb. The degree of the embrittlement corresponded to notch weakening as for the creep rupture strength.
(4) Examination of a defect found by NDI at the welded joint in a reactor vessel showed that the cracks initiated at the root of the weldment and that the segregation of impurities on the grain boundaries in HAZ was extremely high.