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Prediction of hardenability from isothermal transformation diagrams

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Journal of Heat Treating

Abstract

An analytical method for evaluation of quantitative hardenability of eutectoid steels from the corresponding TTT diagram was considered. The isothermal transformation kinetics of pearlite and cooling curves were combined by the additivity rule, and an equation expressing continuous cooling transformation of pearlite has been derived. Based on this equation, simple algorithms for the calculation of the widely used measure of hardenability, such as the critical cooling rates, the Jominy distance, and the ideal critical diameter from the corresponding TTT diagram were presented. It was shown that the derived equations give the above measures of hardenability with good accuracy. Finally, the relationships between J0 and D1 and among D1, D0, and H were reexamined taking cooling transformation into account. It was found that the formerly obtained relationships based on the half temperature time required correction. New relationships based on the cooling transformation are presented.

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Abbreviations

a:

extension of C curve in TTT diagram

b:

nose temperature (°C)

C:

coefficient of heat transfer at the metal-quenchant interface

c:

constant which indicates the relative position of C curve in time scale

d:

austenite grain size (μm)

D I :

ideal critical diameter (cm)

D 0 :

critical diameter (cm)

f 2 :

thermal diffusivity (cm2/s)

H:

severity of quench (cm-1)

h:

Biot number

J:

Bessel function

J0 :

Jominy distance (cm)

k:

thermal conductivity

m:

exponent expressing the effect of austenite grain size on isothermal transformation

n:

time exponent of isothermal transformation

t 0 :

half temperature time at nose temperature (s)

T 1 :

initial temperature of steel before quench (°C)

T 2 :

temperature of quenching medium (°C)

t h :

half temperature time (s)

U:

temperature fraction

X:

fraction of pearlite

αl :

lower critical cooling rate (°C/s)

αu :

upper critical cooling rate (°C/s)

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Author notes

  1. Minoru Umemoto & Imao Tamura

    Present address: Department of Metal Science and Technology, Kyoto University, Sakyo-ku, 606, Kyoto

  2. Nobuo Nishioka (associated with Graduate School)

    Present address: Mitsubishi Motors Co., Kyoto Works, Ukyo-ku, 616, Kyoto

Authors and Affiliations

    Authors
    1. Minoru Umemoto
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    2. Nobuo Nishioka
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    3. Imao Tamura
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    Additional information

    Nobuo Nishioka was formerly associated with Graduate School, Kyoto University

    Originally published inTrans. ISIJ. Permission to republish granted.

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    Umemoto, M., Nishioka, N. & Tamura, I. Prediction of hardenability from isothermal transformation diagrams. J. Heat Treating 2, 130–138 (1981). https://doi.org/10.1007/BF02833229

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