Thermodynamics of the Fe-Mo-C system at 985 K

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I.

INTRODUCTION

SINCE chromium-molybdenum ferritic steels are widely used as a material for pressure vessels, their thermodynamic properties have become an important subject in connection to the hydrogen attack resistance of steel at elevated temperatures. Thermodynamic evaluation of carbon activities with relation to the molybdenum partitioning in the ferrite and the carbide phases will provide a basic knowledge for quantitative analysis of hydrogen attack in these steels. Thermodynamic properties of the Fe-Mo-C system in the ferritic temperature range have not been established. Recently, Lobo and Geiger j measured carbon solubilities in ferritic iron and Fe-Mo alloys with a gas equilibration method at 0.48 and 1.16 wt pet Mo. Their results showed that the addition of molybdenum decreased the graphite solubility (ae = 1) below the eutectoid temperature and increased it above the eutectoid temperature. The phase equilibria in the Fe-Mo-C system were studied by Takei 2 and Kuo. 3 Sato et al. 4 constructed a carbide phase diagram of Fe-Mo-C alloys at 973 K. They emphasized, however, because of slow kinetics of carbide formation the diagram was not a quantitative one and proposed separate stable and metastable diagrams for the Fe-Mo-C system. Chatfield and Hillert s computed isothermal sections of the Fe-Mo-C system including those in the ferritic temperature range. In this study, the equilibrium carbon contents and molybdenum partitioning between ferrite and carbides were measured under known carbon activities at 985 K. Thermodynamic parameters were determined from the experimental results.

lI.

EXPERIMENTAL PROCEDURE

A. Materials

Five Fe-Mo alloys were prepared in the composition range from 0.2 to 3 wt pet Mo. The alloys were melted in an induction furnace in fused alumina crucibles under argonHARUE WADA is Associate Research Scientist, Department of Matedais and Metallurgical Engineering, University of Michigan, Ann Arbor, MI 48109. Manuscript submitted January 23, 1985. METALLURGICALTRANSACTIONS A

3 pet hydrogen flow. Specimens of 0.5 x 10 x 100 mm were prepared by cold rolling in the same way as described for the Fe-Cr-C alloys. 6 Table I shows the purities of the starting materials Fe and Mo. B. Equilibration with CH4/H2 Gas Mixture

Cold rolled 0.5 mm thick specimens were equilibrated with fixed ratio CH4/H2. A gas flowing method was employed for higher carbon activity levels and a sealing method was used for lower levels. The carbon activity was in the range of 0.045 to 1.0. The carbon activity was controlled by adjusting the gas ratio in the gas flowing method, and by adjusting the amount of Fe-C alloys which were included as a carbon source in the sealing method. Equilibration by the gas flowing method has been described in the previous paper. 6 In the sealing method, fourteen specimens including pure Fe and the Fe-C specimens were sealed into a 28 mm I.D. quartz tube. The specimens were separated from each other .with thin alumina tubes and tied together with 0.1 mm iron,wire. The one end closed quart