The addition of Nb significantly retards recovery and recrystallization through solute drag and precipitation pinning effects. It is important to describe the precipitation behaviour precisely because finely distributed precipitates retard the restoration process abruptly. The isothermal precipitation behaviour has been studied frequently. However, during industrial hot deformation processing, the temperature decreases continuously, so that isothermal data cannot be applied directly to predict the precipitation under these conditions. The focus of this study is therefore on the CCP (Continuous-Cooling-Precipitation) behaviour of Nb carbonitride in austenite. / To calculate the CCP behaviour, isothermal PTT (Precipitation-Time-Temperature) data in terms of the P$ sb{ rm s}$ (precipitation start) and P$ sb{ rm f}$ (precipitation finish) times are first required. In 1987, Dutta and Sellars developed a thermodynamic model to predict P$ sb{ rm s}$ times for the precipitation of niobium carbonitride, and in 1989, Liu and Jonas developed a model for titanium carbonitride. In the present work, the L-J (Liu-Jonas) model is used to calculate the P$ sb{ rm s}$ time at a given temperature from experimental data. Since the prediction models are only limited to the P$ sb{ rm s}$ times, a new calculation method for the P$ sb{ rm f}$ time, based on reaction kinetics and classical nucleation and growth theory, is proposed in this study. Two models are developed to follow the precipitation process, and the time exponent and rate constant for the kinetics are formulated for each model. The additivity rule, which was developed by Scheil in 1935, is then used to calculate the extent of precipitation during continuous cooling. / Isothermal precipitation rates for 0.040% Nb steels are measured experimentally by the stress relaxation method. The CCP behaviour is then calculated and is then examined by continuous cooling testing, using a hot deformation dilatometer. Precipitates are observed by transmission electron microscopy of specimens quenched after a period of cooling at various cooling rates. The P$ sb{ rm s}$ and P$ sb{ rm f}$ times estimated from the particle size data show good agreement with the calculated CCP behaviour.
| Identifer | oai:union.ndltd.org:LACETR/oai:collectionscanada.gc.ca:QMM.70199 |
| Date | January 1991 |
| Creators | Park, Sung-Ho, 1957- |
| Publisher | McGill University |
| Source Sets | Library and Archives Canada ETDs Repository / Centre d'archives des thèses électroniques de Bibliothèque et Archives Canada |
| Language | English |
| Detected Language | English |
| Type | Electronic Thesis or Dissertation |
| Format | application/pdf |
| Coverage | Doctor of Philosophy (Department of Mining and Metallurgical Engineering.) |
| Rights | All items in eScholarship@McGill are protected by copyright with all rights reserved unless otherwise indicated. |
| Relation | alephsysno: 001259622, proquestno: AAINN72023, Theses scanned by UMI/ProQuest. |
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