Austenite Formation in Low Carbon Microalloyed Pipeline Steels

• Author / Creator

  Hintze Cesaro, Alejandro

• A study of the kinetic and microstructural aspects of austenite formation in
  low carbon pipeline steels during continuous heating and a model capable to
  predict the interface velocity are presented in this study. The overall goal of
  this work is to increase the level of understanding concerning the austenite
  reversion in low carbon pipeline steels. This work builds on top of the theory
  of solute drag and a Gibbs energy balance approach, and expands this theory
  to account for the effect of Mn redistribution at slow heating rates.
  Due to the unique characteristics of commercial pipeline steels, given by
  a refined chemical composition and tailored steel processing conditions, the
  initial microstructure previous to the austenite transformation was carefully
  studied to ponder all the relevant transformation conditions during continuous heating. Dilatometry experiments were employed to construct a continuous heating transformation (CHT) diagrams and study the kinetics and
  microstructural aspects of two commercial X80 pipeline steels.
  Experimental results suggested that orthoequilibrium calculations are
  poor predictor of the transformation temperatures and that the mechanisms
  controlling the transformation kinetics are susceptible to the heating rate. To
  obtain a better understanding of the transformation kinetics, a Gibbs free
  energy balance across the interface was employed to calculate the boundary
  migration rate as a function of the available driving force. This energetic approach was supported by an original methodology to calculate the chemical
  driving force under paraequilibrium conditions coupled with a modification
  of the solute drag model. The calculation was applied for continuous heating
  transformation at 1 ◦C/s and 200 ◦C/s.
  The results of this work serve to bring a deeper understanding on how
  austenite is formed during continuous heating. In particular to highlight the
  differences between slow (conventional heat treatments) and fast (welding)
  heating rates.

• Subjects / Keywords

  • austenite
  • pipeline
  • phase transformation
  • kinetics

• Graduation date

  Fall 2022

• Type of Item

  Thesis

• Degree

  Doctor of Philosophy

• DOI

  https://doi.org/10.7939/r3-5yn7-8c45

• License

  This thesis is made available by the University of Alberta Library with permission of the copyright owner solely for non-commercial purposes. This thesis, or any portion thereof, may not otherwise be copied or reproduced without the written consent of the copyright owner, except to the extent permitted by Canadian copyright law.