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Distributor's Link Magazine Spring 2019 / Vol 42 No2

114 THE DISTRIBUTOR’S

114 THE DISTRIBUTOR’S LINK ROB LaPOINE METALLOGRAPHY FOR FASTENERS from page 30 FIGURE 2. MACROSTRUCTURE DETAIL (1 X MAGNIFICATION) The etchant is heated, and the specimen is etched, usually between 20-60 minutes. Mounting the specimen in a plastic housing (Figure 3) aids the process of getting a flat and smooth surface using grinding and polishing equipment. This is particularly necessary for microstructure exams where the grain is observed at higher magnifications. Microstructure, as the name suggests, is a view of the small structure in the metal. Examining the microstructure reveals the grain size and boundaries, but also can show grain flow that is not easy to see unless magnified. For FIGURE 3. MOUNTED CROSSexample, seeing the flow SECTION OF A SOCKET HEAD CAP SCREW MADE FROM A286 AND pattern created from the ETCHED WITH FRY’S ETCHANT (ASTM E407 # 79) TO REVEAL thread forming process THE GRAIN STRUCTURE. in Figure 4 would be very difficult without magnification. Microstructure observations are usually done in the range of 50-500 X magnification. Fastener procurement specifications such as NAS 4003 require microstructure exams which contain observations of grain flow around the threads and head, grain size, deleterious effects of processes such as overheating, grinding burns and any other material processing defects such as bursts or voids. Microstructure exams require a much finer grind and polish process than macrostructure. Typically requiring 4-7 different abrasives and finishing off with a 1-micron diamond slurry against a felt pad for the final polish. Once the specimen is polished to a mirror finish, it is etched to reveal the grain structure and then viewed under a microscope. At 100 X, the grain flow from head and thread forming processes are clearly visible as seen in Figure 4. Grain size can be determined in a few ways. The simplest way is to make a comparison to a standard. To make this comparison most direct, the standard is printed on a on a piece of glass and used as a reticle (Figure 5) in the eyepiece of the microscope. This way, the observer can see the actual grains and the standard simultaneously to make the most accurate comparison. FIGURE 4. MICROSCOPIC IMAGE OF THREAD ROOT IN A286 SOCKET Many specifications HEAD CAP SCREW. FRY’S ETCHANT, require consistency of 100 X MAGNIFICATION grain size, allowing only a small fraction of grains to be larger than a certain value. Other conditions that can be observed are banding (where grains segregate into bands) and grain boundary conditions. Fasteners made from alloy steel can suffer from an effect known as decarburization. Decarburization is a FIGURE 5. GRAIN SIZE RETICLE condition where carbon is SHOWING ASTM NUMBERS 1-8. removed from the material during the heat-treating process. This affects the outside surface of the fastener and has a large effect where the material is thinnest. The removal of carbon makes the material softer and typically weakens the thread area, reducing the holding strength of the fastener. Decarburization can be observed on a properly polished and etched specimen as a lighter shade of color toward the outside edge of the thread (Figure 6). Once observed, the depth of decarburization can be measured to see if it conforms with the limits set by the manufacturing or procurement specification. The opposite effect, known as carburization, is also possible. This appears as a darker band toward the outside edge and causes the surface material to be harder than the underlying material. CONTINUED ON PAGE 115

THE DISTRIBUTOR’S LINK 115 ROB LaPOINE METALLOGRAPHY FOR FASTENERS from page 114 Carburizing is often used purposely as a method of case hardening, where the material is harder on the outside than in the interior. This FIGURE 6. MICROSCOPIC VIEW OF process is typical for THE EDGE OF A THREAD SHOWING DECARBURIZATION AS THE LIGHTER self-drilling screws COLORED EDGE. 100 X MAGNIFICATION which require a hard cutting-surface and a more ductile interior to absorb the energy of stretching and twisting. Certain mechanical properties such as hardness can also be determined with metallographic techniques. Microscopic hardness, known as microhardness, is a technique which uses a microscope to measure the small indentation made in the polished surface of metal. Vickers and Knoop hardness scales are used for microhardness. Both scales use a diamond to make the indentation but have different shaped indenters. The Vickers indentation is measured across each diagonal of the indentation to determine the hardness of the material. Figure 7 shows a microscopic image of a Vickers indentation with a measurement being taken across one of the diagonals. Metallography provides us with critical insights into the materials and processes that are used to produce the products the industry makes, sells and uses. This science is critical to understanding FIGURE 7. VICKERS MICROHARDNESS the structure of the INDENTATION SHOWING FILER LIKES FOR material itself as well MEASUREMENT. 500 X MAGNIFICATION. as the effects the processes have on that material in making the things that are essential for building necessary and useful tools, hardware and machines. ROB LaPOINTE / AIM TESTING LABORATORY

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