Cleaner steel with synchrotron X-ray microtomography

Steel cleanliness is essential for strength, toughness, weldability, crack resistance, corrosion resistance, and fatigue resistance. Steel producer SSAB, the metal research institute SWERIM and the Research Institutes of Sweden (RISE) used synchrotron X-ray micro TomographyA non-destructive technique used for constructing three-dimensional images from two-dimensional projections to characterise the internal structures of materials.More info to explore ways to detect foreign particles in predefined steel. 

Steel is one of the world’s most important construction and engineering materials. It is used everywhere in our society. Iron is the base metal of steel, and it can be recycled repeatedly without losing its mechanical properties.  

Foreign particles, called inclusions, can reduce the mechanical properties of the steel. These inclusions are made up of substances such as oxides, sulphides, and carbon, which can enter the steel in the early stages of steel production and reduce the material’s mechanical properties.  

New insights into inclusions in steel 

Synchrotron X-ray micro Tomography offers unique opportunities to detect and quantify inclusions in steel due to its high coherence and spatial resolution. The technique is non-destructive and can provide a three-dimensional model and quantitative results of the inclusions’ size, shape, and volume fraction.  

The technique is particularly useful for detection when inclusions are few or when there is a varying orientation or shape of inclusion particles and irregularities in the inter-particle connectivity.

It is also less time-consuming as the scan time of the steel samples is shorter – about 15 minutes – compared to several hours for standard and complementary two-dimensional techniques such as scanning electron microscopy and light optical microscopy. 

Detecting tiny inclusions at high precision 

The team from SSAB, SWERIM, and RISE performed their experiment at the P07 High Energy Materials Science (HEMS) beamline of Helmholtz-Zentrum Hereon at the PETRA III synchrotron in Hamburg, Germany. Twelve samples of low carbon steel containing various predefined amount inclusions were prepared as rods of 10 mm in height and 1-2 mm in diameter. 

The experiment results show that it is possible to detect tiny inclusions down to a size of <2.4 μm. The data obtained with synchrotron X-ray micro TomographyA non-destructive technique used for constructing three-dimensional images from two-dimensional projections to characterise the internal structures of materials.More info were analysed using a data analysis pipeline for image processing and analysis. They revealed the actual size distribution of individual inclusions in the samples. 

Important findings for the metal industry 

The collaboration has yielded more knowledge about inclusions in steel, which can improve methods for detecting and quantifying inclusions in 3D. In time, this can support the production of cleaner and thus more durable steel.

The experiment has also shown that synchrotron X-ray micro TomographyA non-destructive technique used for constructing three-dimensional images from two-dimensional projections to characterise the internal structures of materials.More info can be used as a precise three-dimensional assessment technique, especially since it could be offered as a standardised and routinely used technique combined with optimised workflows for image analysis for industrial inclusion assessments at a synchrotron beamline. 

“This study has allowed us to develop and test a semi-automatic script to perform combined image processing and image analysis on large amounts of data on a large-scale computer cluster. The script has since been reused in other industry-relevant studies to analyse similar metallic materials with different defects inside. 

With an optimised workflow and tools from sample preparation, to routine scanning, to standardised scripts for image analysis, we have with our case study opened up the possibility of offering the metal industry routine scans of metallic materials on a synchrotron beamline for TomographyA non-destructive technique used for constructing three-dimensional images from two-dimensional projections to characterise the internal structures of materials.More info.”  

Emanuel Larsson, researcher, Solid Mechanics Department, LTH, Lund University