Application of a new model to the interphase precipitation reaction in vanadium steels
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INTRODUCTION
H I G H strength low alloy and micro-alloyed vanadium steels can be significantly strengthened by precipitation of vanadium carbide particles at the austenite/ferrite interphase boundaries during the transformation of austenite to ferrite. The interphase precipitation reaction can occur during both isothermal and continuous cooling transformations after austenitization, and results in a characteristic microstructure of sheets of vanadium carbide (VC) precipitates containing predominantly one variant of VC. The strength and toughness of the resulting microstructures have recently been shown by Todd and Li I11 to depend on the austenitization temperature, VC solubility in austenite, volume fraction of VC available for precipitation in ferrite, the size and spacing of the resulting precipitates, and the ferrite grain size. Optimum combinations of strength and toughness were achieved in a Fe-0.2C-1V alloy by isothermal transformation at temperatures above the nose of the ferrite C-curve. In contrast, transformations at temperatures below the ferrite C-curve nose resulted in high strength but catastrophically brittle microstructures. The mechanical properties could be correlated with the spacings between the sheets of interphase precipitates and the size of the VC particles. Yield strengths correlated well with Melander's model for critical resolved shear stress when all the available vanadium and carbon precipitated as interphase vanadium carbide. [2] In order to optimize the strengths and toughnesses of vanadium containing steels, it is essential to understand the mechanisms governing the formation of the interphase precipitates and to predict their sizes and spacings as a function of transformation temperature. The existing theories of interphase precipitation in vanadium steels have not yet adequately addressed this problem. I3-111 This paper re-examines the current mechanisms and models for the interphase precipitation reaction and shows how a new solute balance model, developed recently by Todd, Li, and Copley, I121can be applied to explain many aspects of the interphase precipitation of vanadium carbide in ferritic steels. P. LI, Research Engineer, and J.A. TODD, Assistant Professor, are with the Departments of Materials Science and Mechanical Engineering, University of Southern California, Los Angeles, CA 90089-0241. Manuscript submitted October 5, 1987. METALLURGICALTRANSACTIONS A
EXPERIMENTAL PROCEDURE
The chemical compositions (wt pct) of the steels used in this study were Fe-0.19C-1.14V-0.45Mn-0.001N and Fe0.2C-1.0V-3.0Ni-0.5Mn. The alloys were prepared from high purity elements (99.9+ wt pct) and induction melted under argon in a magnesium oxide crucible. A 10 kg, 7 cm diameter ingot was cast in a copper mold in an argon atmosphere. The ingots were upset forged at 1200 ~ in air and cross-rolled to rectangular sections 7 cm wide by 1.5 cm thick. Specimens were isothermally transformed at temperatures in the range 600 ~ to 750 ~ The isothermal time temperature transformation (TTT) and
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