A Positron Study of the Quench and Tempering Process in AISI 4340 Steel

Dopper broadening measurements of the gamma-ray spectra produced by positron annihilation have been made on an AISI 4340 (Unified Numbering System [UNS] G43400) steel as a function of both isochronal tempering over the temperature range of 150 to 700°C and isothermal tempering at 250°C for times ranging from 1 to 1100 min. Changes in the positron annihilation line shape parameter Sare correlated with the corresponding structural transformations that accompany tempering of the as-quenched steel. Of particular significance is the increase in the Sparameter in the temper embrittlement regime. This increase in Sparameter is postulated to be related to the precipitation of a transition iron carbide that alters the defect structure of the surrounding ferrite phase.