A Study on the Effect of Thermal Ageing on the Specific-Heat Characteristics of 9Cr–1Mo–0.1C (mass%) Steel |
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Authors: | B Jeyaganesh S Raju S Murugesan E Mohandas and M Vijayalakshmi |
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Affiliation: | (1) Physical Metallurgy Division, Materials Development and Characterisation Group, Indira Gandhi Centre for Atomic Research, Kalpakkam, 603 102, Tamil Nadu, India |
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Abstract: | The effect of thermal ageing on the heat-capacity behavior of 9Cr–1Mo–0.1C (mass%) ferritic/martensitic steel has been studied
using differential scanning calorimetry (DSC) in the temperature range from 473 K to 1,273 K. The DSC results in the case
of slow cooled, normalized and tempered, and subsequently thermally aged samples (500 h to 5,000 h at 823 K (550 °C) and 923
K (650 °C), clearly marked the presence of both magnetic and α-ferrite + carbide → γ-austenite phase transformations that take place successively upon heating. Furthermore, for the case of fully martensitic
microstructure, an additional exothermic transformation at about 920 K(647 °C), arising from carbide precipitation is noticed.
This event is characterized by a sharp drop in C
P
. It is found that the α-ferrite + carbide → γ-austenite phase transformation temperature is only mildly sensitive to microstructural details, but the enthalpy change associated
with this phase transformation, and especially the change in specific heat around the transformation regime, are found to
be dependent on the starting microstructure generated by thermal ageing treatment. Prolonged ageing for about 500 h to 5,000
h in the temperature range from 823 K to 923 K (550 °C to 650 °C) contributed to a decrease in heat capacity, as compared
to the normalized and tempered sample. This is due to the increase in carbide volume fraction. The martensitic microstructure
is found to possess the lowest room-temperature C
P
among different microstructures. |
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Keywords: | Ferritic steel Heat treatment Martensite Specific heat Tempering |
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