ASTM A247 - Evaluating the Microstructure of Graphite in Iron Castings
Understanding ASTM A247 - how to evaluate the micro structure of graphite in iron castings
1. Scope
This test method outlines the procedure for classifying graphite in cast irons based on type, distribution, and size through visual comparison with reference photomicrographs. It is applicable to various iron-carbon alloys, including gray irons, malleable irons, compacted graphite irons, and ductile (nodular) irons.
2. Referenced Documents
The following ASTM standard is referenced in this test method:
ASTM E3 Guide for Preparation of Metallographic Specimens
3. Summary of Test Method
The test method relies on a set of reference photomicrographs that form the basis for graphite classification. By observing the characteristic features of graphite particle shape and distribution, the type, distribution, and size of graphite in the sample can be compared to the idealized microstructures in the standard charts. The classification is based on the closest match of observed graphite features to those in the charts.
4. Significance and Use
The test provides purely descriptive information on the type, distribution, and size of graphite in the cast iron sample being evaluated. It does not, however, indicate the origin of the graphite or the suitability of the iron-carbon alloy for specific applications. Instead, it aids in accurately reporting microstructures and facilitates comparison between reports from different laboratories or investigators.
5. Test Specimens or Samples
The preferred sample for testing is a section cut from an actual casting, agreed upon between the manufacturer and purchaser. In cases where this is impractical, a test lug or projection can be appended to the casting and used as the test sample. If both options are inconvenient, microstructural test coupons, representative of the castings poured, may be used.
6. Polishing
Standard metallographic procedures for grinding and polishing, as covered in ASTM Guide E3, are followed. Particular care is taken to retain graphite at the polished surface and avoid tearing or dragging it out. The use of diamond powder polishing compound in the final stages of polishing is recommended for effectively retaining the graphite.
7. Classification of Graphite Form Using Chart
The graphite form type chart is used as a reference standard, with the polished specimen observed under a microscope. The observed graphite forms are compared to type designations on the chart, and the percentages of each graphite type are estimated or counted. The types present in the sample are reported as percentages totaling 100. Different magnifications can be used to reveal the graphite form effectively.
Graphite Types:
Type I: Nodular shape, normal and usually desirable in ductile iron.
Type II: Slightly irregular nodules, little or no adverse effect on ductile iron properties.
Type III: Most common in malleable iron castings after annealing, may also contain Types I or II.
Type IV: Predominant in compacted graphite iron, evaluated by the percentage of Types I and II observed.
Type V: Spiky graphite form occasionally seen in ductile iron with Types I and II.
Type VI: Exploded nodule graphite form occasionally seen in ductile iron with Types I and II.
Type VII: Flake graphite form usually seen in gray iron.
8. Classification of Graphite Distribution Using Chart
The graphite distribution chart is primarily used to rate flake graphite (Type VII) distributions in gray cast iron. Malleable iron may exhibit a non-random distribution pattern that can be described using distributions B to E.
9. Classification of Graphite Size by Chart
Graphite particles are categorized into eight size classes. Size charts for flake graphite and nodular graphite facilitate comparisons. The maximum dimension of graphite particles for each size class is listed in Table 1. The specimen to be evaluated is captured at 100× magnification for direct comparison with the size classes. If a mixture of sizes is present in the same sample, the sizes may be reported as percentages of the total graphite area represented by the involved sizes.
10. Nodularity
Nodularity is expressed as a percentage, obtained by counting the nodular particles and reporting the results relative to the total amount of graphite in the microstructure. Ductile irons typically exhibit a nodularity between 80 to 100 %, gray irons have 0 % nodularity, and compacted graphite irons range from 0 to 20 %. The nodularity assessment should be based on the evaluation of multiple fields of view, and the location of measurement should be agreed upon between the manufacturer and purchaser.
11. Nodule Count
Nodule count is expressed as the number of graphite nodules per square millimeter and is performed at 100× magnification. High nodule counts generally indicate good metallurgical quality, but an optimum range exists for each section size of the casting. Counts exceeding this range may lead to a degradation of properties. The nodule count should be established between the manufacturer and purchaser.
Conclusion
In conclusion, this comprehensive test method for graphite classification in cast irons provides valuable information for researchers, manufacturers, and purchasers in assessing microstructures and ensuring material quality for specific applications. By adhering to the outlined test procedure, accurate and standardized reports can be generated, promoting consistency and comparability among different laboratories and investigators.