Characterization of thermomechanical rolled x70 grade pipeline steels by ebsd and tem techniques

There are limited studies on structure-property relationship of API X70 steels used under low temperature and high pressure atmosphere, especially pipelines. Therefore, properties of material were measured and microstructure was investigated via EBSD and TEM techniques in order to reveal structure-property relationship in this study. Firstly; suitable sample preparation and analysis parameters were determined for Al and steel based alloys in order to obtain high quality diffraction pattern. Accordingly, (i) it was seen that combined mechanical + electrolitic polishing is the most suitable method for these materials. (ii) After studying the effect of accelerating voltage on the pattern quality and reliability of the EBSD data and sample thickness it was found better that the lower accelerating voltage with thin sections gave the more reliable data. (iii) According to the t-EBSD results of fine milled and electron transparent sample, data was at all magnifications compared to the conventional EBSD. Next, EBSD and TEM analysis of X70 Pipeline Steels were carried out by conditions determined at the first stage. In accordance with obtained crystallographic and microstructural results; it was concluded that (iv) microstructural constituents could be distinguished and quantified according to the band contrast raised as a result of dislocation density. In this regard, while tensile properties decrease with decreasing dislocation density, impact properties increase. (v) Hardness, tensile and impact properties were correlated with the grain size, texture and grain boundary characteristics. In conclusion, it was seen that the sample having smallest grain size, random texture, smaller LAGB and less Σ3 grain boundaries has lower tensile but higher impact properties. (vi) In addition, Mn, Ti, Nb and S based precipitates were obtained by STEM and t-EBSD analysis. Keywords: EBSD, TEM, API X70, band contrast, CSL, t-EBSD