George F. Vander Voort

Computer-aided microstructural analysis of specialty steels

Abstract Computers are utilized in a variety of microstructural characterization functions—some are quite common, others are not. In this paper, three case studies of detailed specialty steel image analysis programs performed at Carpenter Technology Corporation will be reviewed. All involve computer-controlled automatic image analysis, computer databases, and computer-generated graphical analysis. The first case study concerns development and application of an image analysis program to assess clustering of particles, mainly primary MC-type carbides, in Alloy 718, a nickel-based superalloy. Image amendment procedures were utilized to join together closely spaced particles. The second study concerns the evaluation of processing conditions that convert M2C to MC in M-50 bearing steel. Billet carbide distribution ratings are improved by this, but grinding is impaired. The third study covers a comparison of sulfide inclusions in free-machining stainless steels as a function of composition.

Anisotropy estimation properties for microstructural models

The paper concerns the evaluation of anisotropy of planar fibre systems. A general method of the estimation error quantification and a test of anisotropy are suggested based on the Prokhorov distance between the theoretical and estimated rose of directions. By means of simulations, the exact distribution of the test statistics can be obtained for various microstructural models, given test systems, and estimation methods. The approach is demonstrated on real data from material research.

Phase Identification by Selective Etching

The Identification of phases or constituents in metals and alloys is a common metallographic problem. Metallographers can often predict which phases are likely to be present in a given alloy from a knowledge of its sample chemical composition, processing history, previously published information on its characteristics, or by reference to phase diagrams. Guided by such information, the metallographer examines the microstructure to identify the phases present. Even when one is possessed of such information, however, the task is often far from trivial, even for well-documented alloy systems. Only after the phases have been identified should a quantification of the amount and morphological characteristics of the phases be attempted.

Influence of magnification on feature-specific image analysis measurements

Abstract Feature-specific measurements of test circles of known size were conducted at different magnifications using a stored image of the detected circles (256 × 256 picture points within the guard frame). Measurement of the diameter and perimeter of calibrated test circles with different objectives has demonstrated that the diameter and perimeter can be measured within an accuracy of ±2% of the true value if the particular circle is magnified to such an extent that it is 15 mm or greater in diameter on the screen. Measurement of the areas of these circles has demonstrated that the area can be measured within an accuracy of ±4% of the true value if the circle is magnified to such an extent that it is 20 mm or greater in diameter on the screen. Repeat measurements have shown that precisions greater than these (±2% for the diameter and perimeter and ±4% for the area) should not be expected. Although a 2 % error in the diameter corresponds to a 4 percent error in the area, the minimum critical sizes for the circle on the screen were not the same. The effect of erosion and dilation procedures varied substantially with magnification. As expected, erosions and dilations have a greater influence on the measured area than the measured diameter or perimeter. The magnitude of the shift in area, diameter, and perimeter was greater for erosions than for dilations. Measurements of the area and lengths of a large number of similar-sized squares revealed the same trends regarding the influence of the size of the square on the screen and the measurement accuracy. While the mean values for the area and length were essentially identical using × 16, × 32 and × 50 objectives, accurate definition of the dispersion of the area and length data required use of the × 50 objective for the length and either the × 32 or × 50 objective for the area, with preference for the latter.

Metallographic assessment of the thermal exposure to the Three Mile Island Unit 2 reactor lower head

Abstract The accident at Three Mile Island Unit No. 2 (TMI-2) on March 28, 1979 was the worst nuclear accident in US history. One of the tasks of the international TMI-2 Vessel Investigation Project (VIP) was to assess the integrity of the vessel. By January 1990, it was possible electrochemically to machine coupons from the lower head by using a specially designed tool. These specimens were taken from regions believed to have experienced the highest temperatures, based on the location of the core debris. The specimens contained the ER308L stainless-steel cladding and the A533 grade-B plate material to a depth of about midwall. The microstructures of these specimens were compared to that of specimens cut from the lower head of the Midland, MI, reactor vessel, made from the same grade and thickness but never placed in service. These specimens were subjected to known thermal treatments between 800 and 1100 °C for periods of 1 to 100 minutes. Because the initial comparison work was qualitative in nature, the writer volunteered to quantify microstructural parameters in the control specimens and in those from TMI-2. Furthermore, selective etchants were utilized to better discriminate desired microstructural features, particularly in the cladding. Hardness traverses were conducted using Vickers microindentations, which have revealed more information than the original bulk hardness traverses. Microprobe analysis that has been conducted on a few specimens has revealed new details about the cladding, interface, and heat-affected zone. This is a progress report on the quantification of changes in both the degree of carbide precipitation and delta-ferrite content and shape in the cladding as a function of temperature and time to refine the estimates of the maximum temperatures experienced.