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|Title:||Kinetics of Carbonitride Precipitation in Microalloyed Steels|
|Keywords:||Materials Science and Engineering;Materials Science and Engineering|
|Abstract:||<p>Transmission electron microscopy (TEM) and scanning transmission electron microscopy (STEM) have been used to investigate the morphologies, distributions, compositions, particle size distributions and growth kinetics of niobium carbide and titanium-niobium carbonitride precipitates in steels containing low levels of Ti, Nb, C and N. The microalloyed steels were solution treated at 1390°C for 2 hr and quenched, and then aged at 1000°C and 1100ºC for various times.</p> <p>The precipitates, mostly with spherical shapes for short aging times and polyhedral and cubic shapes for long aging times, were found to be not quite uniform due to preferential sites for heterogeneous nucleation. After rapid growth within the first few minutes, insignificant ripening takes place for precipitation at 1000ºC while fast coarsening occurs at 1100ºC. During the aging, only complex carbonitride precipitates of the form (Tiᵪ Nb₁₋ᵪ)(CyN₁₋y) were found among the newly nucleated and growing particles. There is a strong size dependence of particle compositions for short aging times. The youngest of these particles. which approach the size of critical nuclei. tend to be Ti-rich. Titanium nitride tends to form at very high temperatures and titanium-niobium carbides go to completion at low temperatures.</p> <p>A thermodynamic model was applied to the alloy systems to predict the compositions and mole fractions of the complex carbonitrides. The calculated precipitation time-temperature (PTT) diagrams for carbonitrides exhibit a C shape for all of the steels. A model predicting the growth kinetics of carbonitrides and composition variation within the precipitates for the initial growth stage (before coarsening) has been developed based on equilibrium thermodynamics with the inclusion of capillarity and multicomponent diffusional kinetics. In this theory the precipitates in Fe-Mi-Xj austenite containing two substitutional transition metals M (Ti, Nb or V) and two interstitial elements X (C and N) have been considered. Satisfactory agreement with the experimental results has been demonstrated.</p>|
|Appears in Collections:||Open Access Dissertations and Theses|
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