The effect of molybdenum on niobium, titanium carbonitride precipitate evolution and grain refinement in high-temperature vacuum carburizing alloys

Existing commercial carburizing alloys can be processed at higher temperatures and shorter processing times utilizing vacuum carburizing due to the suppression of intergranular oxidation. To provide resistance to undesired grain coarsening at these elevated temperatures and associated reduction in fatigue performance, microalloyed steel variants have been developed which employ fine Ti- and Nb-carbonitrides to suppress grain growth. Grain coarsening resistance is believed to be limited by the coarsening resistance of the precipitates themselves at high temperature, so further alloy/processing developments to enhance microalloy precipitate coarsening resistance based on a greater mechanistic understanding of solute interaction with microalloy precipitates would be beneficial. Molybdenum is known to affect microalloy precipitate evolution during processing in ferrite and austenite, but a unified explanation of the role of Mo in precipitate evolution is still lacking.;Accordingly, the effect of molybdenum on microalloy precipitate size and composition evolutions and the associated onset of abnormal grain growth in austenite was investigated in Mo-bearing and Mo-free, Nb,Ti-microalloyed SAE 4120 steels. Molybdenum additions of 0.30 wt pct to alloys containing Nb additions of 0.05 and 0.10 wt pct Nb delayed the onset of abnormal grain growth in hot-rolled alloys reheated and soaked at 1050 °C and 1100 °C. The coarsening rate of microalloy precipitates was also reduced in Mo-bearing alloys relative to Mo-free alloys during isothermal soaking at 1050 °C, 1100 °C, and 1150 °C. The observed microalloy precipitate coarsening rates exceeded those predicted by the Lifshitz-Slyozov-Wagner relation for volume-diffusion-controlled coarsening, which is attributed to an initial bimodal precipitate size distribution prior to reheating to elevated temperature. Heat-treatments of hot-rolled alloys (tempering and solutionizing) prior to reheating to elevated temperature in austenite beneficially affected precipitate size distribution prior to reheating, lowered precipitate coarsening rates, and delayed the associated onset of abnormal grain growth during soaking at elevated temperature. Thermo-kinetic simulations support experimentally observed effects of prior precipitate distributions on precipitate coarsening. Investigations of microalloy precipitate composition evolution indicated that Mo is incorporated into fine microalloy precipitates (<40 nm) following hot-rolling and cooling to room temperature. The molybdenum concentration gradients observed in fine precipitates in hot-rolled alloys are attributed to the precipitation sequence of microalloy carbonitrides prior to reheating. The molybdenum concentration in microalloy precipitates also varies as a function of precipitate size and total Nb addition in hot-rolled alloys reheated to 900 °C. Further reheating to 1100 °C and soaking results in a reduction of Mo concentration in microalloy precipitates due to Mo partitioning to austenite. Thermodynamic calculations support observations of reduced Mo incorporation in microalloy precipitates in austenite relative to ferrite. Possible mechanisms for the effect of Mo on Nb-rich precipitate coarsening and associated grain growth were investigated. No measurable segregation of Mo to the carbonitride-matrix interface was observed, and solute Mo is shown to have a negligible effect on Nb diffusion activation energy. It is hypothesized that Mo reduces the coarsening of microalloy carbonitrides either through a reduction in the diffusion frequency factor, particle matrix surface energy, or a combination of these mechanisms enhanced by Mo partitioning during soaking in austenite.