Abstract
The aging process of patented, rapidly cooled, and subsequently slowly cold drawn eutectoid carbon steels has been investigated by the methods of internal friction, electrical resistivity, and room temperature tensile test.
The results obtained are summarized as follows.
The strain aging occurs in three stages.
The first stage aging occurs below about 150°C. Supersaturated carbon and nitrogen in α iron lock dislocations. In fine lamellar pearlitic steel, dissolved carbon content in a iron is small after patenting and drawing, and there is no significant contribution to the first stage by carbon. Large amounts of nitrogen can, on the contrary, dissolve in lamellar pearlitic ferrite and cause the increase of tensile strength by aging at this stage.
The second stage appears only in heavily prestrained specimens at temperatures from about 150°C to 230°C The dissolution of some portion of lamellar cementite and the dislocation locking or pinning by carbon supplied therefrom take place. An appreciable increase of tensile strength results. The activation energy of 28 kcal/mol was obtained experimentally. This value is believed to be the sum of the activation energy of carbon diffusion in ferrite and the binding energy of cementite in ferrite.
The third stage corresponds to overaging from the view point of tensile strength.
