Thermodynamic properties of carbides in 2.25Cr-1Mo steel at 985 K
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I.
INTRODUCTION
THERMODYNAMIC properties of Fe-Cr-C l and Fe-MoC z alloys have recently been measured by the author at 985 K, a temperature at which the matrix of the alloys is ferrite. Thermodynamic analyses have been performed on alloy carbides for these two systems. The results may be applied to analyze the Cr-Mo steels if the interactions between Cr and Mo, particularly those in the carbide phases, are experimentally determined. Thermodynamic properties of alloy carbides in the ferritic steel containing Cr and Mo are largely unknown. A recent study of 2.25Cr-1Mo steel3 showed that the thermodynamic evaluation of the system had to be based on the thermodynamic properties of the alloy carbides and ferrite phases. Alloy carbides are influenced by the coexisting ferrite phase as well as the carbon activity. The amounts of carbon and alloying elements in ferrite phase have also been related to the mechanical properties of steel. 4,s Carbide species and their precipitation sequence in 2.25Cr-lMo steel were reported by Baker and Nutting6 and slightly modified recently. 7 Their results 7,s indicate that M23C6, M6C, M7C3, and MEC carbides are possibly present after 1000 hours tempering at 985 K. The 2.25Cr-lMo steel exhibits resistance to hydrogen attack at elevated temperatures because of the stable carbides formed by Cr and Mo. Hydrogen attack is known to be caused by a reaction of hydrogen and carbon to form nondiffusible methane bubbles in steel. Since carbon atoms are consumed to form methane, the stability of the carbides and the carbon activity have important influences on the susceptibility of steel to hydrogen attack. Several investigations3'8'9 have been conducted on hydrogen attack of this steel. Parthasarathy and Shewmon3 recently examined the effects of tempering on
HARUE WADA is Associate Research Scientist with the Department of Materials Science and Engineering, The University of Michigan, Ann Arbor, MI 48109. Manuscript submitted August 23, 1985. METALLURGICALTRANSACTIONSA
the carbon activity and hydrogen attack kinetics of 2.25Cr-1Mo steel. Hakkarainen et al.a concluded from their observations by TEM that methane bubbles were formed at the carbide-matrix interfaces of 2.25Cr-lMo steel exposed to hydrogen. Sakai and Kaji9 found that 2.25Cr-lMo steel showed lower methane bubble density than other steels and estimated the internal pressure in the bubbles. This study aims to analyze 2.25Cr-lMo steel as a combination of Fe-Cr-C and Fe-Mo-C alloys and to develop a thermodynamic model that enables analysis of alloy carbides containing Fe, Cr, and Mo. Experimental work was conducted to determine the relationship among the carbon activity, carbide species, and their alloy compositions. Based on the experimental results, the thermodynamic parameters were computed for the ternary alloy carbides, M6C and M23C 6.
II.
EXPERIMENTAL WORK
A. Equilibration of Specimens Commercial 2.25Cr-lMo steel was cold rolled into a 0.5 mm sheet and cut to 10 • 100 mm specimens. Table I shows the chemical composition of the st
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