SMALL ANGLE X-RAY SCATTERING UNIT FOR ANALYSIS OF SECONDARY NANOPHASES AND PORES IN BAINITE STEEL

The work presents the results of studies aimed at analyzing the morphology and dimensional distribution of cementites and pores/microcracks in bainite steel used as tubing couplings during fluid transportation. For the first time, the method of small-angle X-ray scattering (SAXS) has demonstrated the possibility of analyzing not only secondary phases (particles, precipitates), but also shows the potential to expand the limits of applicability of this method for analyzing the morphology and dimensional distribution of pores/microcracks that originated both during casting and heat treatment and during subsequent operation of the tubing. The SAXS approach was adapted, taking into account the shape of cementites/pores, the relationship and ordering of cementites/pores in three-dimensional space, the factor of scale and absorption, analyzed volume and background radiation when processing scattering curves. The analysis of the scattering curves obtained on the transmission  revealed cylindrical cementites with a diameter of 26 nm and a length of 180 nm, as well as manganese sulfides of an elliptical shape. Analysis of scattering curves measured in reflection mode has been shown to detect cylindrical nanopores/microcracks 36 nm in diameter and 170 nm in length, as well as spherical macropores 700 nm in diameter. To validate the obtained data, microstructure studies were carried out by scanning and transmission electron microscopy methods, which showed a convincing convergence of results between them. Nature and causes of identified pores in bainite steel are discussed.