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Nonmetallic Inclusion Analysis in Steel


Nonmetallic Inclusion Analysis in Steel

Background

Nonmetallic inclusions are compound materials embedded inside steel during the manufacturing process. Inclusions have a different chemical origin and give different mechanical properties to steel, such as formability, toughness, machinability, and corrosion resistance. As a general rule, the fewer or less severe the inclusions, the higher the quality of the steel. Therefore, analyzing and documenting nonmetallic inclusions is important for quality control.

Challenge

In North and South America, ASTM E45 is the dominant standard for nonmetallic-inclusion analysis in steel. Quality- control laboratories used, and continue to use, the ASTM chart comparison method to analyze inclusions. With this method, operators perform a visual estimate 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 the ASTM E45 standard was that inclusion ratings be performed with a compound microscope at 100× total magnification (10× objective lens and 10× eyepieces). However, this standard was updated to accommodate the growing trend in digital image analysis, stating that a resolution of 1.0 μm/pixel or better is required when using a 10× objective lens. Digital image analysis is now the preferred methodology because of its superior accuracy and repeatability, helping eliminate the risk of subjectivity introduced by the human eye. Furthermore, documenting the analysis results is seamlessly integrated into the workflow. How can one successfully implement a digital imaging solution for nonmetallic-inclusion analysis in steel while conforming to the ASTM E45 standard?

Solution

After proper sample preparation, nonmetallic inclusions can be observed directly on the surface of a given steel sample using a compound microscope (either an upright or inverted model) that is 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).

Nonmetallic Inclusion Analysis in Steel

Thanks to the continuous development of modern image-analysis software, any user can start working immediately with a fully integrated digital solution for rating nonmetallic inclusions in steel that specifically caters to the requirements of a laboratory environment. Designed with ASTM E45 in mind (as well as other international standards), microscopy-based image-analysis solutions enable any user to accurately and repeatedly rate inclusions in steel, with minimal training.

Configuration

A typical equipment configuration for analyzing nonmetallic inclusions in steel through digital image analysis consists of the following components.

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 the user maneuvers the scanning stage.

Materials-Science-Specific Image-Analysis Software:

Because of their inherent high contrast level as well as their unique morphological parameters, nonmetallic inclusions in steel can be accurately, repeatedly, and easily detected using image-analysis software. Many materials-science-specific image-analysis software packages offer optional add-on modules that enable users to obtain results in compliance with ASTM E45, as well as various international standards.

Typical equipment configuration: inverted metallurgical microscope, 10× metallurgical objective lens, and a high-resolution microscope camera
Typical equipment configuration: inverted metallurgical microscope, 10× metallurgical objective lens, and a high-resolution microscope camera
Typical equipment configuration: inverted metallurgical microscope, 10× metallurgical objective lens, and a high-resolution microscope camera


Typical equipment configuration: inverted metallurgical microscope, 10× metallurgical objective lens, and a high-resolution microscope camera

10× Metallurgical Objective Lens:

The required objective magnification for nonmetallic-inclusion rating.

CCD or CMOS Digital Microscope Camera:

The most important specification to consider when choosing a camera for inclusions rating is the pixel size. According to the ASTM E45 standard, a calibrated digital pixel size of 1.0 μm/pixel or better is required. For example, a digital camera with an uncalibrated, actual pixel size of 6.3 μm or finer is required when using a 10× objective lens and 0.63× 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 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 automatic recognition helps eliminate the risk of manually entering the incorrect lens magnification in the software.

A manual or motorized XY scanning stage is required to manipulate the sample and position the area of interest for observation and analysis. However, since ASTM E45 states that an area of at least 160 mm2 must be scanned for analysis, it is recommended to use a motorized stage so that you can program it to scan an area that conforms with the standard.

The PC that you choose should meet the minimum system requirements of the camera and image-analysis software. A high-resolution monitor is also required.

Procedure

  1. Select the 10× objective lens and then, under reflected-light, brightfield conditions, maneuver the sample on the XY stage to view the area of interest, which contains the inclusions to be analyzed. Or, if you are using a motorized stage, program the software so that the stage will scan the required area of interest (at least 160 mm2 in the case of ASTM E45).
  2. Capture the digital image via the image-analysis software.
  3. Within the inclusion-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.
  4. The software analyzes the image and rates the inclusions’ compliance with the chosen standard.

    Typical equipment configuration: inverted metallurgical microscope, 10× metallurgical objective lens, and a high-resolution microscope camera
    Typical equipment configuration: inverted metallurgical microscope, 10× metallurgical objective lens, and a high-resolution microscope camera
  5. Based on a user’s predefined template, a report is automatically generated incorporating the analysis results, supporting inclusion images, and all relevant data.

Summary

Due to the inherent high contrast ratio between nonmetallic inclusions in steel and their metallic background, nonmetallic 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 nonmetallic 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.

References
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

Olympus IMS

Products used for this application


MPLFLN-BD

The MPLFLN-BD lens has semi apochromat color correction and is suitable for the widest range of applications. Especially designed for darkfield observation and the examination of scratches or etchings on polished surfaces.

GX53NEW

The GX53 inverted microscope features exceptional image clarity and excellent resolution at high magnifications. With accessories including a coded revolving nosepiece and software, the microscope's modular design makes it easy to customize for your requirements.

OLYMPUS StreamNEW

OLYMPUS Stream image analysis software offers fast, efficient inspection workflows for all process steps for image acquisition, quantitative measurements and image analysis, reporting, and advanced materials science inspections tasks.
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