Quantitative microstructural characterization of microalloyed steels

• Author / Creator

  Lu, Junfang

• Microalloyed steels are widely used in oil and gas pipelines. They are a class of high strength, low carbon steels containing small additions (in amounts less than 0.1 wt%) of Nb, Ti and/or V. The steels may contain other alloying elements, such as Mo, in amounts exceeding 0.1wt%. Microalloyed steels have good strength, good toughness and excellent weldability, which are attributed in part to the presence of precipitates, especially nano-precipitates with sizes less than 10nm.
  Nano-precipitates have an important strengthening contribution, i.e. precipitation strengthening. In order to fully understand steel strengthening mechanisms, it is necessary to determine the precipitation strengthening contribution. Because of the fine sizes and low volume fraction, conventional microscopic methods are not satisfactory for quantifying the nano-precipitates. Matrix dissolution is a promising alternative to extract the precipitates for quantification. Relatively large volumes of material can be analyzed, so that statistically significant quantities of precipitates of different sizes are collected. In this thesis, the microstructure features of a series of microalloyed steels are characterized using optical microscopy (OM) and scanning electron microscopy (SEM). Matrix dissolution techniques have been developed to extract the precipitates from the above microalloyed steels. Transmission electron microscopy (TEM) and x-ray diffraction (XRD) are combined to analyze the chemical speciation of these precipitates. Rietveld refinement of the XRD pattern is used to fully quantify the relative amounts of the precipitates. The size distribution of the nano-precipitates (mostly ≤10 nm) is quantified using dark field imaging (DF) in the TEM. The effects of steel chemistry and processing parameters on grain microstructure and the amount of nano-precipitates are discussed. Individual strengthening contributions due to grain size effect, solid solution strengthening and precipitation strengthening are quantified to fully understand the strengthening mechanisms of the steels.

• Subjects / Keywords

  • Microalloyed steels
  • Precipitation strengthening
  • Matrix dissolution
  • Rietveld refinement

• Graduation date

  Fall 2009

• Type of Item

  Thesis

• Degree

  Doctor of Philosophy

• DOI

  https://doi.org/10.7939/R37425

• 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.