Early Stages of Aging and Tempering of Ferrous Martensites
- PDF / 1,730,073 Bytes
- 9 Pages / 603.28 x 788 pts Page_size
- 25 Downloads / 232 Views
I.
INTRODUCTION
IN April of
1980, an informal tempering seminar dealing especially with the early stages of aging and tempering was held at Massachusetts Institute of Technology. Unpublished MIT thesis research was reexamined in the light of recent X-ray diffraction data of Winchell and co-workers as well as atom-probe field-ion microscopy (FIM) studies at Oxford University. A wide range of alloy compositions (covering 0.1 to 2.0 wt pct C) has been investigated by several experimental techniques, including electrical resistivity, transmission electron microscopy, and Mi3ssbauer spectroscopy. Detectable stages of aging are found to occur near room temperature. However, kinetic differences over the span of carbon contents investigated have made it difficult to correlate the diverse experimental findings. We here present a tentative framework for such a correlation of these observations, based on the informal tempering seminar discussions in which Professor P. G. Winchell was an active participant. While important information concerning basic aging/ tempering phenomena may be obtained from comparison of the behaviors of carbon and nitrogen in ferrous martensites, we now focus on the behavior of carbon because of the wider range of available experimental observations for this case. Meaningful correlation of different experiments is facilitated by the selection of observations performed under well-defined, reproducible experimental conditions. We will therefore give primary consideration to experiments performed by heating of "virgin" martensites in which carbon diffusion during the martensitic transformation is avoided. This is generally accomplished through the use of Fe-Ni-C alloys with sufficiently low Ms temperatures. Nickel is believed to exert relatively little influence on the kinetics of aging and tempering, although it apparently increases the rate of cementite (0-carbide) precipitation somewhat at higher temperatures. ~ As many of the experimental observations reflect the "average" behavior of G.B. OLSON, Principal Research Associate, and MORRIS COHEN, Institute Professor Emeritus, are both with Massachusetts Institute of Technology, Cambridge, MA 02139. This paper is based on a presentation made at the "Peter G. Winchell Symposium on Tempering of Steel" held at the Louisville Meeting of The Metallurgical Society of AIME, October 12-13, 1981, under the sponsorship of the TMS-AIME Ferrous Metallurgy and Heat Treatment Committees.
METALLURGICALTRANSACTIONS A
relatively macroscopic specimens, we will deal mainly with phenomena occurring in the major volume fraction of the material rather than at special locations such as twinned midribs or retained austenite. After a review of electrical resistivity and X-ray diffraction evidence for separable stages of structural change during aging and tempering, dimensionless diffusion kinetic parameters will be proposed to allow correlation of measurements in this structural sequence. Available Mrssbauer spectroscopy, transmission electron microscopy, and atom-probe mic
Data Loading...
