Structural Interpretations on the Torsional Creep Charactors of the Cold-Worked Heat Resisting Steel in Tension (Report 4)

Behaviors of over-strained metals

Abstract

In this report the structural interpretations are given for the torsional creep charactors, especially the transition phenomenon of the increasing creep rate with the augment of prestrain, of prestrained heat resisting steels to various extent in tension at room temperature. They are as follow :
1. Cells were observed more easily on the prestrained steels above the transition range. The structure was not prominent and not numerous of the transition range, and it was numerous but not prominent at heavy increasing of prestrain above the transition. range.
2. Considering the creep caused by the formation of cell, the transition phenomenon can be interpreted by the results mentioned above. And the approximately constant creep rate ofter the transition seems to be the sequence of the cancel of the effects of the movement of more numerous dislocations, which is deduced by the increasing of the number of cell, and of the insufficient movement of dislocations to cellboundary, which is deduced by the obscure of the structure.
3. It can be comprehended by the difference of easiness on the formation of cell at two temperature groups that the equation,
γ=k(p)ⁿ
p=τ, e-Q/RT
where K and n are the constants, y is the creep rate, y is the shear stress, T is the absolute temperatute, Q is the activation energy and R is the gas constant, is satisfied separately within the two groups of [300°C] and [450°and 600°C]. The difficult formation of cell at latter temperature group is related with the comparative increasing of visible slip band and grain-boundary displacements for total creep strain.
4. The tendency to the reproduction of extinguished striation or, at least, the arrest of the extinguishment was observed from the X-ray diffraction photographs of prestrained steels to the transition range. It confirms the comparative easiness of the formation of cells. The striations, however, were not observed at heavy increasing of prestrain obove the transition range because the size of cell was small and the recovery was not sufficient.
5. The formation of cells is associated with the undulation observed on the surface of prestrained specimen and the easiness of the formation of cells in the prestrained steels above the ultimate-point-strain is deduced from the prominent undulation in thos steels.
6. Recrystallization did not seem to play an important role in the charactors of this experiment.