Nonmetallic inclusions are compound materials embedded within steel during the manufacturing process.
Inclusions have a different chemical origin and give various mechanical properties to steel, such as corrosion resistance, machinability, toughness, and formability.
As a general rule, the fewer or less severe the inclusions, the higher the steel quality. Consequently, analyzing and documenting nonmetallic inclusions is crucial for steel quality control.
Meeting the ASTM E45 Standard
ASTM E45 is an international standard test method for determining the inclusion content in steel. Quality-control laboratories continue to use the ASTM chart comparison method to analyze inclusions.
With this technique, operators compare a live microscope image to a micrograph chart, often displayed on the wall near the microscope, to acquire a visual estimate of the inclusion severity and type.
Before the introduction of digital imaging, the magnification criteria in the ASTM E45 standard specified that inclusion ratings should be performed using a compound microscope with a 100X total magnification (10X objective lens and 10X eyepieces).
This standard was later revised for digital image analysis, specifying that a resolution of 1.0 μm/pixel or better is necessary when using a 10X objective lens.
Digital image analysis is now the preferred method due to its repeatability and accuracy, helping users eliminate the risk of subjectivity introduced by the human eye. Another benefit is that documentation is seamlessly integrated into the workflow.
Read on to learn how to analyze nonmetallic inclusions using digital imaging software.
Rating Nonmetallic Inclusions Using Image Analysis Software
After sample prep, inspectors can view nonmetallic inclusions directly on a steel sample’s surface by using an inverted or upright compound microscope that can image under reflected-light, brightfield conditions.
Inclusions are easy to observe and measure because of their high contrast ratio—inclusions look dark against the highly reflective, bright background of steel.
Morphological parameters vary between the inclusion types (e.g., silicate and globular oxide) as do their grayscale values (e.g., sulfide and alumina).
With modern image analysis solutions, users of all skill levels can rate nonmetallic inclusions in steel with accuracy and repeatability while meeting industry and laboratory standards.
Standard Configuration for Nonmetallic Inclusion Analysis
The standard configuration for analyzing nonmetallic inclusions in steel includes the following components:
- Inverted Metallurgical Microscope
An inverted microscope is typically preferred over an upright model, since the polished, flat sample lays flat on the mechanical stage, enabling consistent focus as the user controls the scanning stage.
- Image Analysis Software for Materials Science
Nonmetallic inclusions in steel can be easily, accurately, and repeatedly identified using image analysis software due to their inherently high contrast level and unique morphological parameters.
Many image analysis software packages for materials science provide optional add-on modules that enable users to capture results compliant with ASTM E45 and other international standards.
- 10X Metallurgical Objective Lens
This is the objective magnification required for rating nonmetallic inclusions.
- CCD or CMOS Microscope Digital Camera
When selecting a camera for rating nonmetallic inclusions, the most important specification to consider is the pixel size. A calibrated digital pixel size of 1.0 μm per pixel or better is required according to the ASTM E45 standard.
Typical equipment configuration: inverted metallurgical microscope, 10× metallurgical objective lens, and a high-resolution microscope camera.
For instance, a digital camera with an uncalibrated, actual pixel size of 6.3 μm or finer is required when using a 10X objective lens and 0.63X camera adaptor.
Calibrated digital resolution = (Actual pixel size) ÷ (Objective lens mag.) ÷ (Camera adaptor mag.)
Calibrated digital resolution = (6.3 μm) ÷ (10) ÷ (0.63)
Calibrated digital resolution = 1 μm per pixel
A coded manual or motorized revolving objective nosepiece is also recommended. The image analysis software must be able to automatically read the objective lens’ magnification at all times.
This helps ensure the highest level of measurement accuracy, because automatic recognition eliminates the risk of manually entering the wrong lens magnification in the software.
In addition, a motorized or manual XY scanning stage is required to control the sample and position the area of interest for observation and analysis.
Since ASTM E45 outlines that an area of at least 160 mm2 must be scanned for analysis, a motorized stage is recommended so the user can program it to scan an area that adheres to the standard.
The inspectors’ PC should also meet the minimum system requirements of the image analysis software and camera. A high-resolution monitor is required.
Procedure for Nonmetallic Inclusion Analysis
1. Attach the 10X objective lens to the microscope. Under reflected-light, brightfield conditions, manipulate the sample on the XY stage to observe the area of interest where the inclusions are located. For users with a motorized stage, program the software so that the stage scans the area of interest (at least 160 mm2 to meet ASTM E45 standards).
2. Capture the digital image using the image analysis software.
3. In the image analysis software, assign the grayscale threshold values to define all inclusions, and differentiate between oxides and sulfides, respectively. This will enable the image analysis software to distinguish between both types.
4. The software analyzes the image and rates the compliance of the inclusions to the required standard.
5. The software automatically generates a report based on the predefined template. The report includes the analysis results, supporting inclusion images, and all relevant data.
Summary
Due to the inherently high contrast ratio between nonmetallic inclusions in steel and their metallic background, nonmetallic inclusions can be analyzed with repeatability and accuracy using an image analysis software for materials science.
Image analysis solutions like Evident Stream™ software adhere to ASTM E45 and other international standards, require minimal user training, and produce instant reports based on the analysis data.
When evaluating a potential solution, it is important to work directly with a microscope manufacturer that has materials science expertise, as they can help with every step of the process, from equipment selection to full deployment.
References and Further Reading
- 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
This information has been sourced, reviewed and adapted from materials provided by Evident Corporation - Industrial Microscopy.
For more information on this source, please visit Evident Corporation - Industrial Microscopy