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Metallography of Iron-Nickel Meteorites

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. Displays of meteorites in museums generally consist of large, solid chunks of iron meteorites and of etched slices, as shown in Figures 2 to 6. These slices are ground smooth and then etched with a strong acid solution that brings out the growth structure. The octahedrites are commonly exhibited in this manner because they undergo a solid-state phase transformation where the kamacite (ferrite) nucleates and grows along the octahedral planes of the parent taenite (austenite) phase producing a beautiful etched pattern.

Microstructure of Isothermally-Treated Steels

The microstructure of iron-based alloys is very complicated, being influenced by composition, homogeneity, processing and section size. Microstructures in coarse-grained steels are much easier to observe than in fine-grained steels. Of course, steels are normally made with a fine grain size for best mechanical properties.

In general, it is easiest to identify heat-treated structures after transformation and before tempering. But, in most applications, hardened steels must be tempered and they are usually examined in this condition.  If a mixed microstructure of bainite and martensite is formed during quenching, these constituents will become more difficult to identify reliably as the tempering temperature given the product increases towards the lower critical temperature.

Conducting the Failure Examination

Failures in metallic components may be caused by any of the following factors or combinations of factors: Design shortcomings, imperfections due to faulty processing or fabrication, overloading and other service abuses, improper maintenance and repair and environmental factors.

Not all failures are catastrophic. Many failures involve a gradual degradation of properties or excessive deformation or wear until the component is no longer functional. Failures due to wear or general corrosive attack usually are not spectacular failures, but account for tremendous material losses and downtime every year. Of course, early failures of the spectacular catastrophic order capture the most attention-and rightly so. Nevertheless, all failures deserve the attention of the investigator because they reduce production efficiency, waste critical materials, and increase costs. In some instances, they cause considerable damage or personal injury. Finally, failures can result in costly litigations.

Identifying the Cause of Tool and Die Failure

Steels used for tools and dies differ from most other steels in several aspects. First, they are used in the manufacture of other products by a variety of forming processes. Second, tools and dies are generally used at a higher hardness than most other steel products; 58 to 68 Rockwell C is a typical range. Dies for plastic molding or hot working are usually used a at lower hardness, typically from 30 to 55 Rockwell C.

These high hardness values are required to resist anticipated service stresses and to provide wear resistance. However, the steels must also be tough enough to accommodate service stresses and strains without cracking. Premature failure caused by cracking must be avoided, or at least minimized, to maintain minimum manufacturing costs. Unexpected tool and die failure can shut down a manufacturing line and disrupt production scheduling. Tools and dies must also be produced with the proper size and shape after hardening so that excessive finishing work is not required. Heat-treatment distortion must be controlled, and surface chemistries must not be altered. Because of the careful balance that must be maintained in heat treatment, control of the heat-treatment process is one of the most critical steps in producing successful tools and dies. In addition to controlling the heat-treatment process, tool and die design and steel selection are integral factors in achieving tool and die integrity. 

Metallographic Examination of Medical Implants

Medical technology has developed many new devices that can be implanted into humans (in-vivo) to repair, assist or take the place of diseased or defective bones, arteries and even organs. The materials used for these devices have evolved steadily over the past fifty years with titanium and cobalt-based alloys replacing stainless steels. Metallographic examination has become an indispensable tool in the testing, quality control, failure studies and post-mortem analyses of these devices. This paper presents techniques and results for examination of titanium-based acetabular cups and Co-Cr-Mo femoral hip stems and knees. These implants have porous metallic coatings on one side to enhance bone/metal interface adhesion by in-growth of bone into the porous coatings. 

What is a Normal, Uni-Modal Grain Size Distribution?

ASTM Test Method E 112 says it covers test methods to determine the average grain size of specimens with a uni-modal distribution of grain areas, diameters or intercept lengths. It says that these distributions are approximately log-normal. But, it does not describe how one can determine if their specimen’s grain size distribution is a uni-modal normal (Gaussian) distribution. ASTM E 1181, Standard Test Methods for Characterizing Duplex Grain Sizes, says it covers test methods to characterize grain size in products with any other distribution (other than a “single log-normal distribution of grain sizes”). But, the only example given in Appendix X2 shows the percentage of the number of intercept measurements in 38 length classes from 0 to 1 to 37 to 38 mm. Thirty eight classes is far too many to properly reveal the grain size distribution. This procedure reveals a log-normal distribution but it is not in terms of ASTM grain size numbers, which makes it less useful. 

Avoid Microindentation Hardness Testing at Low Loads!

For many years, ASTM E384 has stated that the load range for microindentation hardness testing with both Knoop and Vickers indenters is 1 to 1000 gf. But, it also states that tests that produce a Knoop long diagonal or a Vickers mean diagonal of < 20 gf should be avoided as the precision in measuring such small indents is poor. The standard recommends using loads no lower than 25 gf. This article shows that the Knoop test exhibits better measurement precision at loads of 200 gf and below because the long diagonal is 2.7 times greater in length than the Vickers mean diagonal length for the same specimen tested at identical loads. Knoop, however, does not produce constant hardness values as the load changes, which is a major problem. If one can use a 100X objective with a numerical aperture of 0.95 – while obtaining adequate image contrast – indents as small as 14.7 μm in length could be measured. But, the challenge is to obtain acceptable image contrast at 1000X magnification so that the indent tips can be clearly seen. Realistically, a minimum diagonal length of 20 μm is a better target. 

Image Caption: Aluminum brass, Cu-22% Zn – 2% Al that was solution annealed at 850C (producing a nice coarse grain size – average of ASTM 3.3). The specimen was etched with Beraha’s PbS tint etched and viewed with Nomarski DIC to bring out the deformation around the Vickers indents. The hardness was 57 HV – very soft!  A 500 gf load was used.  And it was taken at 100X.

Very Low Loads in Micro-indentation Tests Must Be Avoided

For many years, ASTM E384 has stated that test forces from 1 to 1000 gf can be used to determine the Vickers or Knoop micro-indentation hardness. But, is it realistic to consider using very low test forces when the indents are measured with a light optical microscope? ASTM E92 is being resurrected and changed to cover all test loads, micro- and macro-loads, from 1 gf to 120,000 gf. Most micro-indentation hardness testers manufactured over the last 50 years or more have provided the user with a 10X objective used to find the area of interest for testing and one measurement objective, 40X magnification being the most common. A few testers have offered 50X or 60X objectives to measure the indents. It is rare to find a tester with a multiple objective (and indenter) turret, such as the DuraScan system which has ports for 2 indenters and 4 objectives. But, even with the highest quality 100X objective, indents smaller than ~15 μm in length cannot be measured with adequate precision for realistic work. The ASTM standards should eliminate recommendation of use of test loads <50 gf for Vickers and <20 gf for Knoop.

In both ASTM E384 (Micro-indentation Hardness Test Standard) and the proposed revision and re-instatement of E92 (to cover both Macro- and Micro-Loads for Vickers and Knoop), test forces below 25 gf for both Vickers and Knoop testing are listed as permissible for use. The proposed E92 lists test forces for Vickers macro-testing up to 120 kgf , although no machine built in some time has provided forces above 50 kg. The original Vickers testers made in England did use test forces up to 120 kgf, but that is a historical fact, irrelevant today.

Using the Control Chart Approach to Evaluate Hardness Tester Performance

wrought-7-Mo wsThe control chart data analysis approach is an ideal method to evaluate the quality of test data using a specific tester, such as a microindentation hardness tester, over a period of usage time. The method described in ASTM E2554 was used for this work.  This analysis is done by plotting a means and a standard deviation control chart of the weekly/periodic verification data obtained with certified hardness test blocks at a specific test force and a specific hardness level. The method is illustrated using a Knoop hardness test block certified at a test force of 500 gf and with a long diagonal length of 116.18 μm (527.1 HK). From this data, one can easily calculate the uncertainty of the measurements.

The Control Chart method described in ASTM E2554, and discussed by Neil Ullman, is an ideal procedure for evaluating the performance of hardness testers as it will detect any abnormalities that may occur with usage time. The control chart concept was first developed by Walter Shewhart in 1931 to define the state of statistical control and to detect random or special problems. In 1933, ASTM Committee E1 produced STP 15, “ASTM Manual on Presentation of Data.” This was supplemented in 1935 with “Presentation ± Limits of Uncertainty of an Observed Average,” the first use of the term “uncertainty” in statistical analysis of test data. Today, additional information on control charts is provided by E2587.

Image caption: Microstructure of wrought 7-Mo Plus duplex stainless steel (Fe – <0.03% C – 27.5% Cr – 4.2% Ni – 1.75% Mo – 0.25% N) electrolytically etched with aqueous 20% NaOH (3 V dc, 5 s) to color the ferrite blue. There is some light yellow-tan coloring of the austenite. The average Knoop hardness of the austenite was 361.8 HK and that of the ferrite was 263.5 HK.  Magnification bar is 20 µm in length. 7-Mo Plus is a registered trademark of Carpenter Technology Corp., Reading, Pennsylvania.

Measurement of the Volume Fraction of Beta Phase in Naval Brass

Measurement of the amount of phases or constituents in metals and alloys is probably the most commonly performed quantitative microstructural test. The amount present is usually referred to as the volume fraction, although it is rarely expressed as a fraction but usually as a percentage. The volume fraction, or VV, in stereological terms, is the volume per unit volume of the phase or constituent. However, there is no simple direct way to measure the volume fraction. Instead we measure the area fraction, AA, a lineal fraction, LL, or a point fraction, PP, which can be measured and correlate with the volume fraction: VV = AA = LL = PP (1).

Areal analysis was first described by Delesse, a French geologist, in 1848. As the minerals were rather coarse in size, he could measure the area fraction of the grains of interest compared to the total two-dimensional area. As microstructures are rather fine in size, this is not a simple method to perform manually. Delesse suggested that a linear ratio of dimensions could also be used, but he thought that the accuracy would not be as good and did not try to develop a lineal analysis method. Rosiwal, a German geologist, was the first to publish a lineal fraction method in 1898 to assess the volume fraction. The point counting method to assess the volume fraction came much later and was proposed by Thompson in 1933, by Glagolev in 1933 and by Chalkey in 1943 – each working in different countries and different fields of science.