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Characterization of TiN-rich Particles in Thick Walled X70 Microalloyed Steels

  • Author / Creator
    Arjomandpur, Mehdi
  • Microalloyed steels derive their superior properties from the introduction of microalloying elements (titanium (Ti), niobium (Nb), and vanadium (V)). These elements form various carbonitride particles at different stages of production. Titanium nitride (TiN) has the lowest solubility limit in the liquid and solid, resulting in the formation of coarse TiN-rich particles (micron size) in the liquid during solidification and fine TiN-rich particles (some tens of nanometers in size) in austenite. The coarse TiN-rich particles decrease the toughness of microalloyed steels by acting as initiation sites for cleavage cracks. Conversely, the fine TiN-rich particles increase the toughness of microalloyed steels by hindering austenite grain growth during homogenization and welding. This study considers the effect of Ti and N concentrations on the sizes, compositions, and volume fractions of the coarse and fine TiN-rich particles in several as-rolled steels. Also, the type of precipitates in an as-cast steel was studied. Scanning electron microscopy (SEM) equipped with energy dispersive X-ray (EDX) analysis was used to measure the sizes and compositions of the coarse TiN-rich particles. Scanning transmission electron microscopy (STEM) equipped with EDX was employed to measure the sizes and compositions of the fine TiN-rich particles. Matrix dissolution was performed to extract the coarse and fine TiN-rich particles and measure the concentration of Ti in the matrix through inductively coupled plasma (ICP) analysis of the supernatant solutions. Quantitative X-ray diffraction (QXRD) analysis was conducted on the XRD patterns of the particles extracted via matrix dissolution to measure the volume fractions of the particles. The Scheil-Gulliver solidification model was employed to gain insight into the coarse TiN-rich particle characteristics. The results showed that an increase in the product of the nominal Ti and N concentrations led to an increase in the size and volume fraction of the coarse TiN-rich particles. The compositional analysis indicated that the coarse TiN-rich particles were not pure TiN but contained Nb. The amounts of Ti and N bound in the coarse TiN-rich particles were subtracted from the nominal concentrations to obtain the adjusted Ti and N concentrations. An increase in the product of the adjusted Ti and N concentrations led to a decrease in the size and an increase in the volume fraction of the fine TiN-rich particles. The compositional analysis indicated that the fine TiN-rich particles contained a larger concentration of Nb in comparison with the coarse TiN-rich particles. The characteristics of the coarse or fine TiN-rich particles at the quarterline and centerline locations were compared, and no significant differences were observed. The study of the precipitates in the as-cast steel showed that the fine TiN-rich particles were absent in the as-cast steel, indicating that they form during the reheating (homogenization) stage. The measured volume fractions of the coarse TiN-rich particles were in good agreement with the predicted values obtained using the Scheil-Gulliver solidification model, except for the steel with the lowest nominal Ti and N concentration product.

  • Subjects / Keywords
  • Graduation date
    Fall 2021
  • Type of Item
    Thesis
  • Degree
    Master of Science
  • DOI
    https://doi.org/10.7939/r3-qpnf-tt44
  • License
    This thesis is made available by the University of Alberta Libraries 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.