ASTM-A247:19 pdf free download – Standard Test Method for Evaluating the Microstructure of Graphite in lron Castings.
7. Classification of Graphite Form Using Chart
7.1 The graphite form type chart (Fig. I) is used as a reference standard by scanning the polished specimen under the microscope and noting the graphite forms in the micro- structure that more nearly correspond to type designations on the chart. The percentages of each graphite type are estimated by observation, or better, by counting the particles of each type. Types present in a sample are to be reported in percentages to total 100. Any convenient magnification that clearly reveals the graphite form may be used.
7.2 Type I graphite is nodular in shape and is the normal and usually desirable graphite form in ductile iron.
7.3 Type II graphite consists of slightly irregular nodules and has little or no adverse effect on properties of ductile iron.
7.4 Type III is the graphite form most often seen in malleable iron castings after annealing. Some malleable irons may also contain Type I or II graphite particles.
8. Classification of Graphite Distribution Using Chart
8. 1 The graphite distribution chart (Fig. 2) is useful principally in rating flake graphite, Type VII, distributions in gray cast iron. Graphite in malleable iron may occur in a nonrandom distribution pattern which the distributions B to E may be used to describe.
9. Classification of Graphite Size by Chart
9.1 This method categorizes the graphite particles by size into eight classes. Figs. 3-5 show size classes for flake graphite and nodular graphite to facilitate comparisons. The maximum dimension of the graphite particles for the various size classes arc listed in Table 1.
9.2 For direct comparison with the size classes in Figs. 3-5, the specimen to he evaluated shall be captured at the magnification of exactly lOOx. Usually visual comparison with the chart is adequate to define the size class. Where a mixture of one or more sizes occurs in the same sample. the sizes may he reported as percentages of the total graphite area represented by the sizes involved. It is a common practice in malleable irons to use nodule count per unit area instead of a comparison chart as given here. Nodule count, with known free carbon content, is a measure of calculated average nodule area.
10. Nodularity
10.1 Nodularity (Figs. 6 and 7) is expressed by counting the nodular particles and reporting the results as a percentage of the total amount of graphite present in the microstructure. Ductile irons typically exhibit a nodularity between 80 to 100 %, gray irons a nodularity of 0 %, and compacted graphite irons a nodularity of 0 to 20 %. Nodularity should he assessed by evaluating more than one field of view. The final report should state the number of fields of view or area of the sample evaluated. The location of the measurement should be agreed upon between the manufacturer and purchaser.
11. Nodule Count
11 . I Nodule count is expressed as the number of graphite nodules/mm2 and is performed at a magnification of lOOx. Generally, high nodule count indicates good metallurgical quality, but there is an optimum range of nodule count for each section size of casting. and nodule counts in excess of this range may result in a degradation of properties. Nodule count should be established between the manufacturer and the purchaser. Figs. 8-10 compare the nodule counts of ductile iron at 100 % nodularity, 90 % nodularity, and 80 % nodularity, respectively.