The Effect of Molybdenum on Niobium Diffusivity in Austenite and Microalloy Precipitate Evolution in Vacuum Carburizing Steels
EMSL Project ID
42190
Abstract
To capitalize on the increased processing temperature capability and reduction in processing times associated with high-temperature vacuum carburizing of steels, existing commercial carburizing alloys must be modified for high temperature grain stability due to the deleterious effects of abnormal grain growth on fatigue performance. Grain refinement may be achieved by pinning mobile grain boundaries with a distribution of microalloy precipitates, e.g. (Nb, Ti)(C,N), present at elevated temperature. Current literature is lacking a unified explanation of the specific contribution of molybdenum (Mo) to microalloy precipitate stability at austenitic temperatures, and several studies allude to both an effect of Mo on niobium (Nb) diffusivity in austenite and preferential segregation of Mo to precipitate/matrix interfaces. Mo could possibly slow the diffusion of Nb at elevated temperature, thereby slowing the rate of Nb-containing microalloy precipitate dissolution and subsequent abnormal grain coarsening. The proposed study aims to investigate the effects of Mo on bulk and grain boundary Nb diffusion in iron (Fe) upon reheat to austenitic temperatures. A total of 16 thin film diffusion couples will be prepared to investigate the effect of Mo on bulk and grain boundary diffusion of Nb in binary Fe - Mo alloys. Diffusion profiles will be analyzed by time-of-flight secondary-ion mass spectroscopy (ToF-SIMS), and a diffusion coefficient of Nb in each Fe-based layer will be determined for each diffusion temperature.
Use of EMSL facilities is necessary to achieve the desired physical depth and accuracy with SIMS analysis of the prepared thin film diffusion couples. Specifically, the dual beam depth profiling capability of EMSL's ToF-SIMS provides the necessary mass resolution (>10, 000) and depth resolution (~1 nm) to clearly resolve metallic species of Mo and Nb within a submicron diffusion profile without convoluting edge effects common to single beam analysis. The results will be used to determine the effect of Mo on microalloy precipitate coarsening by its interaction with Nb bulk and grain boundary diffusivity. By obtaining a fundamental understanding of the behavior of Mo in austenite with respect to microalloy precipitate evolution, it becomes increasingly feasible to tailor precipitate distribution and composition to retard the onset of abnormal grain growth at vacuum carburizing temperatures.
Project Details
Project type
Exploratory Research
Start Date
2011-01-31
End Date
2012-02-05
Status
Closed
Released Data Link
Team
Principal Investigator