Rating chart clarifies flake length in gray iron.
However, the AFS chart was never intended as a standard. Rather, it is a visualization of actual microstructures, showing an average graphite size representative of the artist's plates of graphite sizes in ASTM A247. These photomicrographs achieve that objective.
Recent AFS-sponsored research has increased the emphasis on flake length in relation to tensile strength. Many people are taking a closer look at A247, attempting to carry the standard to its quantitative limits. The ASTM also is assessing its standards for graphite shape and size. Some background is helpful to understand the nature of the differences between the AFS chart and the ASTM A247 standards.
Section 3.1 of ASTM A247 clearly states: "The reference diagrams included in this standard form the basis for classification. Characteristic features of graphite particles are designated by numerals and letters. Type, distribution and size of observed graphite are compared with the idealized microstructures in the standard charts and rated as closely as possible to equal or similar microstructures in the charts."
The standard goes on to state in 4.1 that "the comparison of observed graphite particles with the structures shown in the charts only gives descriptive information on the type, distribution and size of the graphite in the sample being evaluated. It does not indicate, except in a very broad way, the origin of the graphite or the suitability of the iron-carbon alloy for a particular service."
Section 1.2 adds that "reference standards included in this method are in no way to be construed as specifications. In an appropriate specification for a specific material where graphite microstructure is an important consideration, this method may be used as a reference to define concisely the graphite microstructure required."
Table 1 Graphite Fake Length, (mm) Flake Size No. "Old" A247-47 "Current" A247-67 1 |is greater than 100~ 128 max 2 100 max 64 max 3 50 max 32 max 4 24 max 16 max 5 12.5 max 8 max 6 6.3 max 4 max 7 3.2 max 2 max 8 1.6 max 1 max
Finally, Section 1.3 states that "these standards are offered primarily to permit accurate reporting of microstructures of cast irons and to facilitate the comparison of reports by different laboratories or investigators."
Problems occur in the portion of the standard that refers to the specific length of a single graphite flake. To fully understand the standard, one should examine the effort to develop a quantitative approach to what was a subjective standard in 1947. That standard used actual photomicrographs.
To quantify the 1967 standard, the ASTM committee used the work of Wyman and Moore in the paper, "Quantitative Metallographic Evaluation of Graphitic Microstructures."
The revision not only eliminated the use of actual photomicrographs but substituted artistic representations of the graphite flake sizes and revised the chart to specify the maximum flake size as shown in Table 1.
Wyman's and Moore's work shortened the specified maximum length of the graphite flake for each class. The standard states the "carefully calibrated, ocular scales may be used to measure the serpentine length of flakes or the diameter of nodules." It refers to "length being the linear distance between the two most remote points in the |graphite~ particle," certainly not its serpentine length.
Industry primarily has been using the portion of the standard that discusses visual comparison to the artist's representation of flake size. Apparently, almost no one has been measuring the length of a single flake and using that single measurement to characterize the entire structure, let alone measuring the serpentine length. ASTM is reviewing its graphite flake standard A247.
Current AFS-sponsored research indicates the importance of graphite flake length and its relationship to tensile strength. The present AFS chart has been revised to state that flake size is for visual reference only. Under each graphite photomicrograph, the longest length (the linear distance between the two most remote particle points) will be printed. The size classification is based on the original work by Wyman and Moore, and meets the intent of ASTM A247-88.
A replacement paste-on strip for the 1986 "Gray Iron Microstructure Rating Chart" is available free from AFS. The strip states: "This chart was developed for use as a visual reference for rating gray iron microstructures. There are individual flakes in some of the size classifications that do not conform to the maximum size referred to in Table 1 of ASTM A247."
In summary, there must be a clear understanding between the foundry and customer on interpreting ASTM A247.
A new chart with the changes is available from AFS for $15 for corporate members, $22.50 for individual members and $30 for nonmembers.