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THE GRAIN COARSENING AND SUBSEQUENT TRANSFORMATION OF AUSTENITE IN THE HSLA STEEL DURING HIGH TEMPERATURE THERMOMECHANICAL PROCESSING

Al-Hajeri, Khaled (2005) THE GRAIN COARSENING AND SUBSEQUENT TRANSFORMATION OF AUSTENITE IN THE HSLA STEEL DURING HIGH TEMPERATURE THERMOMECHANICAL PROCESSING. Doctoral Dissertation, University of Pittsburgh.

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    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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    Item Type: University of Pittsburgh ETD
    ETD Committee:
    ETD Committee TypeCommittee MemberEmail
    Committee ChairDeArdo, Anthony Jdeardo@engr.pitt.edu
    Committee CoChairGarcia, C Issacgarcia@engr.pitt.edu
    Committee MemberNettelship, Iannettles@pitt.edu
    Committee MemberVallejo, Luis Evallejo@engr.pitt.edu
    Committee MemberBurke, Mary Grace
    Title: THE GRAIN COARSENING AND SUBSEQUENT TRANSFORMATION OF AUSTENITE IN THE HSLA STEEL DURING HIGH TEMPERATURE THERMOMECHANICAL PROCESSING
    Status: Unpublished
    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.
    Date: 21 June 2005
    Date Type: Completion
    Defense Date: 13 April 2005
    Approval Date: 21 June 2005
    Submission Date: 08 April 2005
    Access Restriction: No restriction; Release the ETD for access worldwide immediately.
    Patent pending: No
    Institution: University of Pittsburgh
    Thesis Type: Doctoral Dissertation
    Refereed: Yes
    Degree: PhD - Doctor of Philosophy
    URN: etd-04082005-082222
    Uncontrolled Keywords: austenite; conditioning of austenite; ferrite refinment; heavy steel sections; IGF; inclusions; intragranular ferrite; particle stimulated nucleation; PSN
    Schools and Programs: Swanson School of Engineering > Materials Science and Engineering
    Date Deposited: 10 Nov 2011 14:35
    Last Modified: 25 Apr 2012 11:32
    Other ID: http://etd.library.pitt.edu/ETD/available/etd-04082005-082222/, etd-04082005-082222

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