Analysis of Non-Metallic Inclusions in Steel with Brightfield Microscopy and Image Analysis

Non-metallic inclusions are can contaminate steel during manufacturing. They have a different chemical composition, causing varying mechanical properties in the steel.

These inclusions often have a great impact on steel properties such as corrosion resistance, machinability, toughness and formability. Hence, it can be said that fewer inclusions directly correlates to higher quality steel. This emphasizes the importance of studying the non-metallic inclusions of steel during quality control.

ASTM E45 is the most common standard for analyzing non-metallic inclusions in North and South America. The "Chart Comparison" method is one of the widely used practices in quality-control laboratories, where the operators carry out a visual estimation of the type and severity of the inclusion.

However, with the advent of digital imaging, samples can now be analyzed for inclusions at a resolution of 1.0µm/pixel or more, using a 10x objective lens. Digital image analysis is now widely preferred, because of its exceptional repeatability and accuracy.

This article describes a typical equipment configuration from Olympus for analyzing non-metallic inclusions in steel using digital image-analysis.

Overview of Digital Image-Analysis

Non-metallic inclusions can be observed on the steel sample surface with a compound microscope under brightfield conditions. Inclusions appear dark on the bright, highly reflective background of steel due to their high contrast ratio, and hence they are easy to observe. However, morphological parameter differs between inclusion types such as silicate and globular oxide.

With the continuous development of advanced image-analysis software, any user can operate a fully integrated digital solution to rate non-metallic inclusions in steel, based on the requirements of any laboratory environment. Designed according to the ASTM E45 standard, microscopy-based image-analysis solutions enable the user to precisely and repeatedly rate inclusions in steel, with minimal training.

Configuration of Olympus Digital Image-Analysis System

A typical digital image analysis system from Olympus consists of an inverted microscope placed over the mechanical stage to hold the flat, polished sample. A 10x metallurgical objective lens provides required objective magnification for non-metallic inclusions rating.

Another important feature of the system is the microscope-specific CCD or CMOS digital camera with a calibrated digital pixel size of 1µm per pixel as per ASTM E45 standard. A manual or motorized XY scanning stage manipulates the sample and position at the region of interest based on the analysis and observation. In addition, a PC and high-resolution monitor is employed to meet the minimum system requirements of the camera and image-analysis software.

Non-metallic inclusions in steel can be precisely, repeatedly and easily detected using the image-analysis software. This software package also includes optional add-on modules to achieve results that comply with ASTM E45, as well as various International standards. Figure 1 shows the schematic of Olympus digital image analysis system.

Schematic of Olympus digital image analysis system

Figure 1. Schematic of Olympus digital image analysis system

Method of Using Olympus Digital Image Analysis System

The following steps describe the procedure involved in operating the Olympus Digital Image Analysis system:

  • Using the 10x objective lens under reflected-light, brightfield conditions, maneuver the sample on the scanning stage to visualize the region of interest to analyze the inclusions. While using a motorized stage, the software can be programmed to allow the stage to scan the required region.
  • With the help of the image-analysis software, capture the digital image
  • Set the grayscale threshold values within the software to characterize all inclusions and differentiate between oxides and sulfides. This enables the image-analysis software to differentiate between two types.
  • Analyze the image, and the software automatically rates the inclusions in compliance with the chosen standard.
  • A report containing the analysis results such as inclusion images and all relevant data, is automatically generated with respect to a user's pre-defined template.

Figure 2 shows the report generated by the digital analysis system.

Report generated by the digital analysis system

Figure 2. Report generated by the digital analysis system

Conclusion

Non-metallic inclusions can be repeatedly and accurately rated using image-analysis software owing to the high-contrast ratio between non-metallic inclusions in steel and their metallic background.

Most of the software packages comply with ASTM E45 and a wide-range of international standards, ensuring easy implementation. They also provide automatically generated reports of analysis data, and an integrated database for storing and quick-and-easy searching of images and related data.

This information has been sourced, reviewed and adapted from materials provided by Olympus Corporation of the Americas Scientific Solutions Group.

For more information on this source, please visit Olympus Corporation of the Americas Scientific Solutions Group.

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