Cr-Mo鋼のクリープ脆化とその評価方法

抄録

This investigation was carried out to clarify the cause of cracks which are often detected in the 11/4 Cr- 0.5Mo steel chemical reactor vessel in use. Furthermore, the prevention of such cracks and rapid evaluation of creep embrittlement were also examined.
Firstly, appearance of cracks detected was investigated by the light microscope and SEM. The cracks were identified as the typical intergranular fracture initiated and propagated in the coarse grained HAZ adjacent to the fusion line of weldment. Grain boundary segregation of impurities such as Sn, Sb and S was revealed by Auger electron spectroscopy.
Secondary, several kinds of test including creep rupture test and high temperature tention test with slow extention rate technique (SERT) were conducted with the synthetic weld HAZ specimens directly sampled from the materials in which cracks were detected. Results of the creep rupture test indicated the creep embrittlernent behavior of specimens clearly by the low creep ductility and the small notch rupture strength ratio. Creep rupture strength of notched specimen coincided with the stress applied to the part of the reactor vessel where cracks were detected. As a result, it was verified that cracks detected were due to the creep embrittlement phenomenon.
Thirdly, it was confirmed that reheat treatment (900°C) was available to prevent the creep embrittlement of coarse grained weld HAZ.
Finally, SERT test using notched specimen was usefull for the accelerated evaluation of creep embrittlement by taking notice of fracture mode. In addition to high temperature atmosphere, such technique is also available for reappearance of damage caused by high temperature and high pressure hydrogen surroundings.