ANNEALING AND AGING OF INTERSTITIAL-C IN ALPHA-FE, AS MEASURED BY INTERNAL-FRICTION

The diffusion of interstitial C in alpha-Fe has been studied by the dy namic mechanical relaxation method, and the isothermal aging kinetics of the removal of interstitial C by precipitation from its saturated s tate has been measured at several temperatures. The height of the Snoe k peak is found to decrease according to the relation Q(max)(-1)(T,t) = Q(max)(-1)(T,t --> infinity) + Delta Q(max)(-1)(T) exp {-t/[k(0) exp (E/RT)]}, where t is the aging time, k(0) a time constant, and E the activation energy for the precipitation of C atoms. The anelastic rela xation time is independent of the amount of C in alpha-Fe and varies a ccording to tau = 3.45 X 10(-15) exp [80.0 (kJ/mol)/RT. The diffusion coefficient varies according to D = 6.61 x 10(-3) exp [-(80.0/RT) cm(2 ) s(-1), over the temperature range 303 to 357 K. The effect of chemic al composition on k(0) has been determined and a procedure for determi ning the constant of proportionality between Q(max)(-1) and the amount of dissolved carbon in steel is described. A concept of accumulated e quivalent aging time is introduced and theoretically justified on the bases of time-temperature superposition of aging effects. Calculations and experiments show that a substantial decrease occurs in the C cont ent of steel when it is heated to high temperatures and that this effe ct alters the shape of the Snoek peak.