ON THE REAPPEARANCE OF SHARP YIELD POINT OF MILD STEEL BY AGING.

Abstract

(I) The grain-boundaries of mild steel are harder than the interior of grains. This structure may be called "skeleton structure." When a piece of mild steel is cold-worked, the skeleton breaks, and thereupon appears the jog of yield point. Mild steel, thus cold-worked, bears slip bands both in the skeleton and in the interior of grains. When such steel is subjected to aging, the crack of skeleton are remedied by thermal motions of atoms, and precipitations are formed along the slip bands both in the skeleton and in the interior of grains. And thus the steel becomes harder. This is the phenomenqn of strain-aging.
(II) As the skeleton is remedied and becomes harder, the jog of yield point reappears.
(III) Isothermal curves for the increases of yield point and tensile strength during aging have hyperbolic forms, and they are expressed by the equations (2) and (3) in this paper. The velocity constants derived from these equations vary with absolute temperature, as shown by equation (5). If we calculate the activation energies from equation (5), we get 20, 000cal./mol. for the increasing velocities of both yield. point and tensile strength. This fact suggests that the atomic prgcesses are the same tor the precipitations both in the skeleton and in the interior of grains.
(IV) The change of the length of jog during aging goes parallel to equation (7). The reappearance velocities of yield point are expressed by equation (9) or (10). It is demonstrated theoretically that the activation energy derived from the reappearance velocities of yield point is approximately equal to that calculated from equation (5).