Non-Metallic Inclusion Analysis in Steel
Non-metallic inclusions are compound materials embedded inside steel during the manufacturing process. Inclusions have a different chemical origin and give different mechanical properties to steel. Inclusions have influence on several properties of steel, such as formability, toughness, machinability and corrosion resistance. As a general rule, the fewer or less severe the inclusions, the higher quality steel. Therefore, analyzing and documenting non-metallic inclusions is important for quality control.
In North and South America, ASTM E45 is the dominant standard1 for non-metallic inclusion analysis in steel. In previous times, and even still in practice today, quality-control laboratories would analyze inclusions via the "Chart Comparison" method. Here, operators perform a visual estimation of the inclusion type and severity by comparing a live image under an optical microscope to a micrograph chart, often posted on the wall near the microscope.
Prior to the advent of digital imaging, the magnification criteria specified by ASTM E45 standard was that Inclusion ratings shall be performed with a compound microscope at 100x total magnification (10x objective lens and 10x eyepieces). However, this standard was recently updated to accommodate the growing trend in digital image-analysis, stating that a resolution of 1.0μm / pixel or better is required when using a 10x objective lens. Digital image analysis is now the preferred methodology because of its superior accuracy and repeatability, eliminating potential for subjectivity introduced by the human eye. Further, documenting the analysis results is seamlessly integrated into the workflow. How can one successfully implement a digital imaging solution for non-metallic inclusions analysis in steel, while conforming to ASTM E45 standard?
After proper sample preparation, non-metallic inclusions can be observed directly on the surface of a given steel sample using a compound microscope (either an upright or inverted model), capable of reflected-light, brightfield conditions. Inclusions are easy to observe and analyze due to their high contrast ratio; inclusions appear dark on the bright, highly reflective background of steel. Morphological parameters differ between inclusion types (i.e. Globular Oxide and Silicate) as do their grayscale values (i.e. Alumina and Sulfide).
D) Globular Oxide
Thanks to the continuous development of modern image-analysis software, any user can hit the ground running with a fully integrated digital solution for rating non-metallic inclusions in steel, specifically catered to the requirements of any laboratory environment. Designed with ASTM E45 in mind (as well as other international standards), microscopy-based image-analysis solutions allow any user to accurately and repeatedly rate inclusions in steel, with minimal training.
A typical equipment configuration for analyzing non-metallic inclusions in steel via digital image-analysis consists of:
Inverted Metallurgical Microscope:
An inverted microscope is typically preferred over an upright model because the flat, polished sample lays flat on the mechanical stage, ensuring consistent focus as one maneuvers the scanning stage.
Material-Science Specific Image-Analysis Software:
Due to their strong inherent level of contrast as well as unique morphological parameters, non-metallic inclusions in steel can be accurately, repeatedly and easily detected via image-analysis software. Many Material-Science specific image-analysis software packages offer optional add-on modules that allow users to obtain results in compliance with ASTM E45, as well as various International standards.
Typical equipment configuration: Inverted Metallurigacal Microscope, 10x Metallurgical Objective Lens, Microscope-Specific High-Resolution Digital Camera.
10x Metallurgical Objective Lens:
The required objective magnification for non-metallic inclusions rating.
Microscope-Specific CCD or CMOS Digital Camera:
The most important specification to consider when choosing a camera for inclusions rating is the pixel size. According to ASTM E45 standard, a calibrated digital pixel size of 1.0μm / pixel or better is required. As an example, a digital camera with an uncalibrated, actual pixel size of 6.3μm or finer is required when using a 10x objective lens and .63x camera adapter.
Calibrated Digital Resolution = (Actual Pixel Size) ÷ (Objective Lens Mag) ÷ (Camera Adapter Mag)
Calibrated Digital Resolution = (6.3μm) ÷ (10) ÷ (.63)
Calibrated Digital Resolution = 1μm per pixel
A coded manual or motorized revolving objective nosepiece is recommended. The chosen image-analysis software should be capable of automatically reading the objective lens magnification at all times. This ensures the highest level of measurement accuracy as the potential of manually entering the incorrect objective lens magnification into the software is eliminated.
A manual or motorized XY scanning stage is required to manipulate the sample and position at the area of interest for observation and analysis. However, since ASTM E45 states that an area of 160mm2 shall be scanned for analysis, the integration of a motorized stage programmed to scan this specific area is recommended.
A PC and high-resolution monitor meeting the minimum system requirements of the camera and image-analysis software.
- Choosing the 10x objective lens, under reflected-light, brightfield conditions, maneuver the sample on the XY stage to view the area of interest, in which the inclusions are to be analyzed. Or, if using a motorized stage, program the software so that the stage will scan the required area of interest (160mm2 in the case of ASTM E45).
- Capture the digital image via the image-analysis software.
- Within the inclusions-rating software, set the grayscale threshold values to define all inclusions, and also differentiate between oxides and sulfides, respectively. This will allow the image-analysis software to differentiate between the two types.
- The image is analyzed and the inclusions are automatically rated in compliance with the chosen standard.
- Based on a user's pre-defined template, a report is automatically generated incorporating the analysis results, supporting inclusion images and all relevant data.
Due to the inherent high-contrast ratio between non-metallic inclusions in steel and their metallic background, non-metallic inclusions can be rated accurately and repeatedly using material-science specific image-analysis software. Many software packages are designed to comply with ASTM E45 and a wide-range of international standards, and can be implemented with minimal efforts. Going beyond the scope of the analysis, many software packages additionally provide the ability to automatically generate reports based on the analysis data, and even go so far as providing an integrated database for archiving and quick-and-easy searching of images and related data. When considering a solution for non-metallic inclusions rating, working directly with a material-science specific microscope manufacturer is of utmost importance, as they can assist you in every step of this process, from equipment selection to full deployment.
Carmo Pelliciari, Dr. Eng., Metallurgical Consultant
American Society for Testing and Materials (ASTM) E45-11 Standard
ASTM International, 100 Barr Harbor Drive, PO Box C700, West Conshohocken, PA, 19428-2959 USA