Type of Document Dissertation Author Al-Hajeri, Khaled Author's Email Address hajerikf@hotmail.com URN etd-04082005-082222 Title THE GRAIN COARSENING AND SUBSEQUENT TRANSFORMATION OF AUSTENITE IN THE HSLA STEEL DURING HIGH TEMPERATURE THERMOMECHANICAL PROCESSING Degree Doctor of Philosophy Program Materials Science and Engineering School School of Engineering Advisory Committee
Advisor Name Title Anthony J. DeArdo Committee Chair C. Issac Garcia Committee Co-Chair Ian Nettelship Committee Member Luis E. Vallejo Committee Member Mary Grace Burke Committee Member Keywords
- PSN
- inclusions
- heavy steel sections
- particle stimulated nucleation
- austenite
- conditioning of austenite
- IGF
- ferrite refinment
- intragranular ferrite
Date of Defense 2005-04-13 Availability unrestricted Abstract In heavy steel sections, fundamental properties such as strength and toughness are improved remarkably by refining and homogenizing the final ferrite microstructure. Conventionally, the microstructural refinement of the final ferrite depends mainly on the austenite grain size, morphology and composition prior to the phase transformation as well as the cooling rate during the transformation. The production of heavy sections by universal-type rolling mills takes place at elevated temperature (above 1200 ºC) followed by low cooling rates (<0.1 ºC/sec). These processing conditions produce coarse austenite grains and subsequently coarse ferrite grains in addition to other undesirable low temperature transformation products. In heavy steel sections, therefore, the refinement of the ferrite microstructure through modern TMP techniques is not feasible.The main objective of this study is to investigate the feasibility of refining the ferrite microstructure and eliminate the high-carbon low transformation products in heavy steel sections using the particle-stimulated nucleation, PSN, mechanism to enhance the nucleation of the intragranular ferrite, IGF.
In this investigation, the MnS and Ti-oxide inclusions were used to promote the intragranular ferrite (IGF) nucleation mechanism in a typical ASTM A572 grade 50 Steel. This work included the study of the decomposition behavior of coarse grained austenite and nucleation of the IGF as a function of very slow cooling rate. In addition, the effects of the inclusions type, size and volume fraction on the kinetics (nucleation and growth) of IGF were investigated.
The main findings of this research were that the refining of ferrite microstructure is achieved, successfully, through enhancing the IGF nucleation with implementing of the PSN mechanism. Additionally, the efficiency of the inclusion as an IGF nucleation site is mainly related to the inclusion size and type.
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