Precipitation Analysis in Microalloyed X70 Steels and Heat Treated L80 and T95 Steels

Abstract

Microalloyed steels and heat treated steels are widely used in the pipeline industry. Microalloyed steels are a type of high strength low alloy steels containing small additions of C, N, Nb, Ti and other elements in amounts less than 0.1 wt%. They may also contain additions of other elements in amounts exceeding 0.1 wt%, such as Mo or Cr. Precipitation of secondary phases has a significant contribution to the overall steel properties and is dependent on steel composition and processing conditions. Changing the steel composition or processing conditions noticeably affects precipitation and therefore affects steel properties. For example, recently thicker wall pipes are being produced. It is legitimate to wonder how this change in design affects precipitation. Because of the small sizes and low volume fractions, conventional techniques, such as electron microscopy, are not suitable to characterize precipitates in steels. Alternative techniques, such as matrix dissolution methods combined with quantitative X-ray diffraction (QXRD) analysis, have to be used. These techniques allow for collection of a statistically significant amount of precipitates. In this thesis, scanning electron microscopy (SEM) is used to characterize 2 X70 microalloyed steels and 3 heat treated steels. Matrix dissolution techniques are used and improved to extract precipitates from the steels. QXRD analysis is done on the extracted precipitates to quantify the relative abundance of precipitates, compositions and size distributions. Energy-dispersive X-ray spectroscopy (EDX) analysis is also done to confirm the QXRD analysis. In addition, a model of NbC precipitation during laminar cooling is developed to predict volume fraction and particle size distribution of nanoscale precipitates. The effect of X70 pipe wall thickness on precipitation is discussed and the experimental results are compared to NbC precipitation simulation results. The volume fraction of large precipitates (1 to 4 microns) appears to be twice as high in thick walled X70 steel than the thinner X70 steel. The HCl solution was not successful for extracting Cr and most of the Mo based precipitates for the heat treated steels. Meanwhile, the mathematical model of NbC precipitates showed reasonable trends with the nanoprecipitates (<5 nm) extracted in the microalloyed steels.