Over the past forty-plus years, steelmakers have introduced improved practices for reducing the inclusion content of steels. The success of these practice changes can be monitored in a variety of ways. Chemical analysis of the bulk sulfur and oxygen contents provides a relatively simple means to assess the impact of these changes. However, microscopical test methods are still needed to assess the nature of the inclusions present.

Traditional chart-based measurement methods have wide acceptability, and their data are well understood by both purchaser and producer for heat acceptance purposes. These methods do have inherent weaknesses which limit their usefulness for quality control, SPC, and database applications. Image analysis-based chart measurements are an improvement over manually generated chart ratings, but the data still does not lend itself to databases and statistical comparisons. ASTM Committee E-4 on Metallography has developed a stereologically-based image analysis standard test method, E 1245, which provides the necessary data in a form which is easily databased and analyzed statistically. The presentation will describe E 1245 and show how data from different heats and melting practices can be compared statistically to ascertain valid test differences. By George Vander Voort

Meteorites have fascinated mankind for centuries. Indeed, more than two dozen meteorites have been venerated by Indian tribes, aborigines, Arabs and other ancient peoples. The study of meteorites is part of the overall study of the origin of our solar system. There was a recent meteor explosion over the city of Chelyabinsk with up to 1000 injuries. Think what the damage would have been like if it hit a major city. Some asteroids are exceptionally large, and when they strike earth, they can make an immense crater. Some of these, as in Figure 1, are in arid climates and can be seen today. Such an impact near the Yucatan Peninsula has been claimed to have caused the extinction of dinosaurs.

There are three basic types of meteorites: stones, stoney-irons and iron. The classification of meteorites is a complex subject. For the iron meteorites, classification is based upon chemical composition, macrostructure and microstructure. Basically, iron meteorites “fall” (no pun intended!) into three categories – hexahedrites, octahedrites and ataxites. Some, however, do not fully fit the requirements of these groups and are classed as anomalous. By George Vander Voort

Aside from the well-known grain size measurement techniques using either the planimetric methods of Jeffries or Saltykov, or the intercept method of Heyn, Hilliard and Abrams, one can measure the grain size through a count of grain-boundary triple point intersections within a known area through the use of Euler’s law. This technique has rarely been used but it should be possible to do such a count by image analysis. In general, measurements based on point counts (0 dimensional) are less subject to errors than lineal measurements (one-dimensional) which are less subject to error than areal measurements (two-dimensional). By George Vander Voort

If a specimen has been cold worked, or it did not recrystallize after hot working, the grains will not be equiaxed and extra care must be taken when assessing the specimen’s grain size. Always test a longitudinally oriented plane first to determine if the grains are, or are not, equiaxed.

A low-carbon sheet steel was tested in the as-received condition (reportedly annealed), and after cold reductions in thickness of 12, 30 and 70%. Above 70% reduction, it can be quite difficult to reveal the ferrite grain boundaries well enough to get a precise measurement. Measurements were made on the three principal planes using the Jeffries planimetric method, the Abrams three-circle intercept method and the intercept method using directed parallel test lines. By George Vander Voort

The two previous articles covered methods for measuring grain size that have been incorporated into ASTM E112 for many years. The Jeffries planimetric method was introduced into standard E2 in 1917 – Committee E-4’s first standard. Zay Jeffries was a founding member of the committee and had published several articles about the method, which he learned from his PhD advisor, Albert Sauveur, the dean of American metallographers.

This method is precise, but a bit slow for production work because the grains must be marked off as they are counted manually. The method, however, can be modified for image analysis work. The second method was the Heyn intercept method, which was developed in Germany in 1903 and was mentioned briefly in ASTM E2, but not described in detail, when published in 1917. The intercept method was later modified by John Hilliard and then by Halle Abrams. The Abrams three-circle intercept method is used in production work as the intercepts (or intersections) do not need to be marked off on a template when counted. But, the writer recently has introduced the Saltykov rectangle to E112 as it can yield accurate grain size measurements down to fewer counts per field than the other two methods. As with the Jeffries method, the Saltykov method does require marking of the grains for accurate counting, although it, too, can be used by image analysis. By George Vander Voort

When ASTM standard E 2 was published in 1917, ASTM Committee E-4 on Metallography’s first standard, it described the planimetric method for measuring grain size based upon publications by Zay Jeffries, a founding member of E4; but, E 2 only briefly mentioned the intercept method developed in Germany in an appendix at the end of the standard.

The intercept method suggested by Heyn in 1903 [1] is considerably faster to perform manually which has made it popular, despite the fact that there is no direct mathematical connection between the mean lineal intercept length and G. Both straight lines and circles have been used as templates, plus other shapes. By George Vander Voort

When ASTM standard E 2 was published in 1917, ASTM Committee E-4 on Metallography’s first standard, it described the planimetric method for measuring grain size based upon publications by Zay Jeffries, a founding member of E-4.

Jeffries was a graduate student under the famous Harvard professor, Albert Sauveur. Sauveur published a paper in 1894 where he defined grain structures in terms of the number of grains per square mm at 1X. But, he did not develop details on his method. This method is more tedious to use than the Heyn intercept method because a count of the grains must be made by physically marking the grains as they are counted, when done manually. Experiments were conducted to determine the influence of the number of grains counted per grid application using the Jeffries planimetric procedure of ASTM E 112 with a single test circle of varying size. Results show that this is a viable test method and produced good data down to relatively low count numbers per grid application. Bias was not observed at low counts, only data scatter. By George Vander Voort

Grain size is probably the most frequent microstructural measurement due to its influence on properties and behavior/service performance. Grain size can be determined by several methods. Chart comparison ratings are probably the most often performed, as this method is fast and simple. But its accuracy is at best ± 1 G value.

An ASTM E-4 interlaboratory round robin test using Plate I of ASTM E 112 showed that chart ratings were biased with the rating being ½ to 1 G value coarser than the actual measured grain size. Similar studies have not been conducted with Plates II or III. Actual measurements of grain size are done by either the planimetric or the intercept methods, as defined in E 112. These are unbiased methods, as long as the grain boundaries were properly delineated by the etchant. Experience has shown that measuring the grain size of BCC metals is much easier than measuring the grain size of FCC metals and alloys that contain annealing twins. ASTM E 112 has two comparison charts for such metals; Plate II for specimens that exhibit a so-called “flat” etch appearance and Plate III for those that exhibit a grain contrast etchant response. Plate III was developed using copper specimens and the images are at 75X, while the other E 112 charts are at 100X. To further confuse the issue, Plate III expresses grain size in terms of d, the average grain diameter, calculated by taking the square root of the average grain area (which is the reciprocal of the number of grains per mm²), rather than as an ASTM grain size number, G. By George Vander Voort

Cold working is well known to change the properties of metals and alloys. Deformation increases the strength of metals but usually reduces it toughness and leads to anisotropy of properties, that is, directionality. Hot working also produces similar affects, the microstructural results after hot work with low finishing temperatures may appear to be the same as from cold working.

Hot rolling of shapes, plate or bar, for example, elongates the nonmetallic inclusions in the deformation direction, which will reduce the isotropy of mechanical properties. Hot working can also lead to segregation being elongated parallel to the deformation direction, which also reduces isotropy. Reducing the finishing temperature, that is, the temperature of the steel at the last deformation pass, will promote “banding” – parallel alignment of the constituents into layers, such as alternate bands of ferrite and pearlite. This also promotes anisotropy of mechanical properties, chiefly toughness and ductility. Strength is not usually affected to a significant degree by banding, compared to toughness and ductility. By George Vander Voort

Relatively few metallographers work with precious metals, other than those used in electronic devices.  Precious metals are very soft and ductile, deform and smear easily, and are quite challenging to prepare. Pure gold is very soft and the most malleable metal known. Alloys, which are more commonly encountered, are harder and somewhat easier to prepare.

Gold is difficult to etch. Silver is very soft and ductile and prone to surface damage from deformation. Embedding of abrasives is a common problem with both gold and silver and their alloys. Iridium is much harder and more easily prepared. Osmium is rarely encountered in its pure form; even its alloys are infrequent subjects for metallographers. Damaged surface layers are easily produced and grinding and polishing rates are low. It is quite difficult to prepare. Palladium is malleable and not as difficult to prepare as most of the precious metals. Platinum is soft and malleable. Its alloys are more commonly encountered. Abrasive embedment is a problem with Pt and its alloys. Rhodium is a hard metal and is relatively easy to prepare. Rh is sensitive to surface damage in sectioning and grinding. Ruthenium is a hard, brittle metal that is not too difficult to prepare. By George Vander Voort