In the metallographic laboratory, the task of analyzing Cast Iron for Graphite nodularity, size, form and distribution parameters, as well as the Ferrite-to-Pearlite ratio, is of extreme importance from a quality-control perspective. As Cast Iron is utilized to manufacturer a wide-variety of precision products, the Automotive Industry is a prime example of how the quality-control of cast iron plays a vital role in the performance of sports cars. For example, engineers have
chosen molded ductile iron for the crankshaft of a British manufactured sports car because of its strength;
this material does not easily become fatigued. Further, the Graphite contained in the cast iron helps dampen vibration and suppress engine noise. Should this material not meet strict qualitycontrol specifications, the overall integrity of the crankshaft, as well as engine noise, could be compromised. Another example is Ford choosing ductile iron for the suspension arm of the Ford Mustang. Originally, Ford engineers had considered aluminum alloys, but were eventually rejected because their lower strength would make the suspension arm too large. Ductile iron was ultimately chosen due to its unique mechanical properties, in turn reducing noise and weight, while lowering cost. In this example, should the quality of this material not meet Ford's strict quality-control specifications, the car's suspension performance may be compromised.
Fierce competition, both on the racetrack and from competitive automobile manufacturers, leaves no choice but to take quality-control of these high-performance automotive components very seriously.
Graphite in Cast Iron at 100x Magnification
Some important characteristics often analyzed in the microstructure of cast iron are:
• Graphite Type (Form)
• Graphite Distribution
• Graphite Size
• Graphite Nodularity
• Percent Graphite
• Percent Ferrite-to-Pearlite
Although several international standards exist1, ASTM A247 is the dominant standard within North and South America used to evaluate the microstructure of graphite in cast iron. This standard presents three parameters: Graphite Type (also called Form), Graphite Distribution and Graphite Size. The graphite type, or form, varies from I to VII. Type I indicates nodular (Ductile - Graphite in form of nodules) Cast Iron, whereas Type VII indicates gray Cast Iron (Graphite in form of flakes). Graphite distribution varies from A to B and is mainly used to rate Type VII graphite. Size of Graphite varies from 1 to 8, whereas the lower number indicates the larger size.
Classification according to Form of Graphite gave origin to a term called "Nodularity", which indicates the percentage of Graphite in nodular form. For example, Cast Iron rated at 100% Nodularity contains all of its graphite in the Form of nodules (Form I Graphite), whereas 80% Nodularity means that the Cast Iron contains 80% of nodules and 20% of other form(s) of graphite.
Percent of Graphite, Ferrite and Pearlite indicate the amount that each structure represents of the total material. Determination of such parameters is a classic example of the evaluation of percent of area.
Historically, most quality-control laboratories analyze cast iron via the "Chart Comparison" method. Here, an operator performs a visual estimation of the parameters by comparing a live image under an optical microscope (typically at 100x magnification) to micrographs charts, often posted on the wall near the microscope. Because Cast Iron is typically analyzed according to several parameters, comparison to various charts can be time-consuming. Since the results are being interpreted by the operator, this methodology can produce inaccurate and unrepeatable results, often not reproducible between different operators. Also, quality-control technicians are required to manually enter their results into a computer-based spreadsheet or report providing another opportunity for errors.
The challenge presents itself for the modern metallurgical quality-control laboratory to implement a turn-key, fully automated analysis and documentation solution for Cast Iron, in full compliance with ASTM A247 or other international standards, while eliminating any potential inaccuracies and subjectivity.
Enter the modern digital metallurgical Quality Control laboratory. Thanks to advancements in material-science microscopy specific software, operators can leverage image-analysis to analyze Cast Iron, in compliance with ASTM A247 as well as a wide-variety of international standards. Within a few clicks of the mouse, an unetched sample can be completely analyzed for Graphite size, form, nodularity and distribution.
Detailed Graphite analysis results are obtained quickly and accurately via image-analysis software
After the Graphite analysis is complete, the image-analysis software will automatically calculate the percentage Graphite, or "Graphite fraction". This Graphite fraction will be used subsequently when analyzing the Pearlite-to-Ferrite ratio of an etched sample. The Graphite fraction is used to distinguish between Graphite and Pearlite, because their similar grayscale values make them indistinguishable from one another via image-analysis software.
Example of an etched sample where the Ferrite-to-Pearlite ratio can be calculated.
Once the analysis is complete, all data is appended into a results spreadsheet directly within the image-analysis software. Reports, containing relevant analysis data and associated images, can also be generated with the push of a button - all with minimal training.
A typical equipment configuration for analyzing Cast Iron 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.
Typical equipment configuration: Inverted Metallurigacal Microscope, 10x Metallurgical Objective Lens, Microscope-Specific High-Resolution Digital Camera.
Material-Science Specific Image-Analysis Software:
Material-Science microscope specific image-analysis software packages often offer optional add-on modules that allow users to analyze Cast Iron directly in compliance with ASTM A247, as well as various International standards.
10x Metallurgical Objective Lens:
The required objective magnification for Cast Iron analysis.
Microscope-Specific High-Resolution CCD or CMOS Digital Camera:
When considering a digital camera for Cast Iron analysis, more important than digital resolution is the pixel size, or resulting pixel density. To ensure enough pixels are provided to sample and digitally reconstruct the smallest detail, many microscopists follow "Nyquist Theorem", which states that 2 to 3 pixels are required to sample the smallest detail, or optical resolution. Considering that Cast Iron analysis will be always performed with a 10x objective lens (coupled with 10x eyepieces = 100x total magnification), the optical resolution of a typical mid-grade objective lens would be approximately 1.1μm. This means that the actual, calibrated pixel size must be smaller than 366nm (providing the required 3 pixels per smallest distinguishable feature). For example, a 5MP camera with 3.45μm pixel size will yield a calibrated pixel size of 345nm (dividing the actual pixel size by the 10x objective lens, using a 1x camera adapter). Dividing the lens resolution (1.1μm) by the calibrated pixel size (345nm) = 3.2. In this example, 3.2 pixels are present to sample the smallest distinguishable feature, meeting the Nyquist criteria of 2 to 3 pixels per distinguishable feature. Although this may sound confusing, a general rule of thumb is that most common material-science microscopy specific cameras 3MP or greater (considering the pixel size of most common CCD and CMOS sensors) are recommended for Cast Iron analysis.
Because Cast Iron analysis can be adequately performed in grayscale mode (where setting threshold parameters is simpler than color mode), the chosen camera should be capable of imaging in grayscale mode as opposed to color only. Also, choosing a camera that can achieve a fast refresh-rate in live mode will help when focusing or positioning the sample.
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.
A PC meeting the minimum system requirements of the camera and image-analysis software, and high-resolution monitor are required.
Choosing the 10x objective lens, under reflected-light, brightfield conditions, maneuver the sample on the XY stage to view the area of interest be analyzed.
Capture the digital image via the image-analysis software.
Note: Alternatively, many software platforms offer the ability to analyze a live image in addition to a captured image.
Set the grayscale threshold levels so that the Graphite is detected on the unetched sample. If required, modify the particle results (split, connect, draw or delete Graphite nodes). The percent Graphite, or Graphite fraction, is memorized and utilized in the subsequent Ferrite-to-Pearlite analysis.
Percentage Ferrite-to-Pearlite Analysis:
Set the grayscale threshold levels so that the Ferrite is detected. Since Graphite and Pearlite possess similar grayscale values, the percent Graphite from the Graphite analysis is considered. A morphological filter can be applied prior to the analysis to separate bright voids in the Pearlite from being erroneously detected as Ferrite.
The image is analyzed in compliance within the chosen standard. Resultant data is written into a spreadsheet directly within the image-analysis software.
Based on a user's pre-defined template, a report is automatically generated incorporating the analysis results, supporting Cast Iron images and relevant data.
Example of an automatically generated report including relevant images and analysis results
Unlike former techniques where operators performed a visual estimation of the Graphite size, nodularity, form or distribution manually by eye, modern material-science microscope specific image-analysis software allows the Graphite parameters to be calculated accurately and repeatedly, as human intervention is minimized. Many software packages are designed to comply with ASTM A247 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 offer the ability to automatically generate reports based on the analysis data, and even go so far as providing an integrated database solution for archiving and quick-and-easy searching of images and related data. When considering a turn-key solution for automatic Cast Iron analysis, working directly with an experienced 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.
Note: Nodularity in cast iron can also be measured ultrasonically, without need for surface preparation. Further details can be found here.
Carmo Pelliciari, Dr. Eng., Metallurgical Consultant
Miguel Angel Yescas-Gonzalez and H. K. D. H. Bhadeshia, University of Cambridge
American Society for Testing and Materials (ASTM) A247 Standard
ASTM International, 100 Barr Harbor Drive, PO Box C700, West Conshohocken, PA, 19428-2959 USA