オーステナイト系ステンレス鋼のラプチヤー強度と電顕直接観察

抄録

To obtain the high temperature strength data of Austenitic stainless steels, creep rupture tests at 650°C for as long as 10000 hours are in progress with five steels-two 18Cr-12Ni-2.5Mo type steels, one 18Cr-12Ni-0.25Ti type steel, and two 18Cr-12Ni-1Nb type steels, heat treated at 1100°C for 1 hour and water-quenched.
Three specimens were tested under each stress level to investigate the scattering of data. The results obtained from the experiments which were conducted for 3000 hours are reported in this paper.
The change of the tensile and the impact properties of these steels during the aging and creep at 650°C were also investigated. The impact values of these steels fell to one half of its initial values during several thousand hours of aging at 650°C and the 18Cr-12Ni-2.5Mo steel showed a larger drop of impact value and more increase in tensile strength during the aging than the two other types of steel.
To study the strengthening mechanism in austenitic stainless steels at elevated temperature, the precipitation and dislocation behaviour during the aging and creep of a 18Cr-12Ni-2.5Mo and a 18Cr-12Ni-1Nb steel were observed by transmission electron microscopy.
In the specimens of both steels, solution-treated at 1 100°C, the dislocation density was rather high because of the thermal strain produced in the quenching in water.
In the 18Cr-12Ni-2.5Mo steel, M₂₃C₆ formed on dislocations during the aging, and after aging for 1000 hours, the dislocation loops surrounding the coarsened precipitates of M₂₃C₆ were observed, which suggested that the precipitates threw-out dislocations during its growth (“precipitation growth dislocations”).
In the specimens aged under the creep stress, much more dislocatinos tangled around the precipitates than in those aged without stress, and this showed that moving dislocations were arrested by the precipitates during the creep deformation.
In the 18Cr-12Ni-1Nb steel, many dislocations were produced around the large undissolved NbC particles in the solution treatment, because of the difference in the thermal expansion coefficient between NbC and the matrix. The NbC formed on dislocation during the aging, and the NbC precipitates also threw-out precipitation growth dislocations”, on which very fine NbC precipitates were formed.
In the specimens deformed under creep stress, numerous dislocations tangled around the precipitates of NbC which formed on dislocations produced during the solution treatment and in the earlier stage of creep deformation.
From these observations, it was comfirmed that in these steels, precipitation of M₂₃C₆ or NbC contributed to the strengthening against creep at 650°C in typical precipitation hardening mechanism.
In both steels, very thin platelets of M₂₃C₆ or NbC formed on stacking faults were observed, which appeared more frequently in the specimens aged under creep stress than in those aged without stress.
Segregation or precipitation of the alloying elements on stacking faults also seemed to have some role in the strengthening of austenitic stainless steels at elevated temperature.
Fewer dislocations were observed in the specimens ruptured after the creep deformation, and this would suggest the possibility of escaping of dislocations from precipitates during the ternary creep, but further studies are required about the sudden increase of creep strain and the initiation of cracks in the beginning of tertiary creep.