Quantitative analysis of Nb in solid solution in low carbon steels by atom probe tomography and inductively coupled plasma mass spectroscopy

The influence of solute drag effects on austenite grain growth in microalloyed steels has been overshadowed by the well-established phenomenon of Zener pinning associated with a dispersion of fine precipitates. Enhanced toughness in the weld heat-affected zone (HAZ) of high Nb steels suggests that microalloying effects are still operative at peak temperatures even beyond known precipitate stability. As the cornerstone for unveiling the solute drag effect, the solute concentration in the matrix should be accurately measured. In this work, we compared inductively coupled plasma mass spectroscopy (ICP-MS) and atom probe tomography (APT) in determining the extent of Nb dissolution as a function of increasing temperature in two commercial American Petroleum Institute steel grades with different levels of Nb and marked variations in weld HAZ toughness. Both techniques demonstrated that the majority of Nb exists in solid solution at the peak temperatures. APT provided a more consistent determination of soluble Nb, in a better agreement with thermodynamic calculations, whilst ICP-MS underestimated the concentration of Nb in the matrix. This work provides valuable insights in determining soluble Nb concentration in steels.