抄録
Creep-rupture tests have been performed on the austenitic heat-resisting steel LCN 155. The rupture elongation of the solution-treated or insufficiently aged specimens of this kind of steel considerably decreased under a certain range of stress (or over a certain range of creep-rupture time). The rupture took place along the grain-boundaries and marked intergranular corrosion occurred by etching in such a specimen ruptured with small elongation. These features are common to many other austenitic steels or super alloys. Therefore, the present paper is intended to throw light on the mechanism which induced a marked decrease in rupture elongation and on the reason why this decrease was inevitably attended with marked intergranular corrosion. The investigations were performed in details on LCN 155 steel and the electro-chemical examination of intergranular corrosion was also made on chromium steels, from which the following points have been made clear.
(1) In the rupture test chromium atoms, which are usually added in rather great quantities to austenitic heat-resisting steels to give them oxidation resistance, diffuse to grain-boundaries and precipitateas carbides. Therefore, the chromium is depleted in the matrix near grain boundaries and thereby marked intergranular corrosion by etching takes place.
(2) The chromium distribution in the depleted zone was determined from the distribution of anodic equilibrium potential in the microscopic region of matrix in the vicinity of grain-boundary carbides. It was made clear that the concentration in the matrix at the interface with the carbide precipitate growing in a boundary was less than 1 wt% approximately.
(3) According to the calculation based on the measured chromium concentration gradient, the Kirkendall effect was supposed to occur with the diffusion of atoms during the precipitation of carbides. The relative excess vacancy concentration due to the Kirkendall effect was calculated and the excess ratio of more than 102 was obtained for LCN 155 at the aging period of 5 hr at 700°C. The value of about 10 was found to continue for a fairly long period of time.
(4) The above excess vacancies may promote the formation and growth of voids in the vicinity of grain boundaries in the rupture test under a stress which exceeds a eartain value. The voids develop to cracks and eventhally result in rupture. Therefore, one may expect that the reduction in the creep-rupture elongation of solution-treated or insufficiently aged austenitic steels is due to this mechanism.
