Kinetics of Precipitation from Quenched Low Carbon Steel
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I.
INTRODUCTION
Q U E N C H aging of a low-carbon steel is often characterized by a two-stage curve of internal friction or electrical resistivity during annealing from room temperature to about 300 ~ ~Each of the two stages is correlated to each characteristic precipitate. The most reliable correlation is that the first stage corresponds to so-called cluster precipitation and the second stage to e carbide precipitation in the temperature range of 35 ~ to 75 ~ while above 100 ~ the first stage indicates e carbide precipitation and the second stage cementite (Fe3C) precipitation. 2 There have been many studies on precipitation kinetics. However, most of them describe in detail the mechanism of the initial stage terminating the aging curves at an arbitrary point based on the assumption that only one kind of precipitate appears during the first stage. 3-7 Actually, precipitates of different composition and structure simultaneously appear, and the measured physical quantities are to be a sum of contributions from at least two precipitates. Therefore, the separation of the experimental curves into elementary curves of precipitation might lead to a better understanding of the various precipitation processes in low-carbon steels. The present paper describes the quench aging curves of low-carbon steel by using equations which are derived by considering the contributions from each kind of precipitation to the physical quantities throughout the process. The comparison between calculated and experimental curves of electrical resistivity, thermoelectric power, and hardness which were reported by Abe and Suzuki 2 is carried out, and the physical meaning of the new equations is discussed.
II.
EXPERIMENTAL PROCEDURE
The experimental data reported by Abe and Suzuki 2 are used. Their experimental procedure and results are summarized as follows: The chemical composition of the material is given in Table I. The specimen of low-carbon steel was austenitized for two hours at 920 ~ in an argon-flow furnace, cooled at a rate of 50 ~ per hour down to 650 ~ and rapidly cooled. K. HANAWA, formerly Graduate Student, Institute of Industrial Science, Tokyo University, Minato-ku, Tokyo 106, is now with Showa Denko, Ltd., Shiojiri, Nagano 399-64, Japan; T. MIMURA, formerly Graduate Student, Tokyo University, is now with Kobe Steel, Ltd., Kakagawa, Hyogo 675-01, Japan. Manuscript submitted March 7, 1983. METALLURGICALTRANSACTIONSA
Table I.
Chemical Composition of Material (Wt Pct)
Si 0.046 0.01 C
Mn 0.35
P
S
Insol. AI
Sol. N
0.020
0.018
0.010
0.0060
After cold rolling, material was machined into the specimens, which were immersed for 20 minutes in a salt bath kept at 700 ~ and quenched into iced water. Abe and Suzuki estimated the carbon content in supersaturated solid solution to be about 0.011 wt pct. The specimens were isothermally aged at 35, 50, 75, 100, 150, 200, 250, and 300 ~ The electrical resistivity at liquid nitrogen temperature, the absolute thermoelectric power at 0 ~ and Vickers hardness at room temperature were m
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