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Distributor's Link Magazine Spring 2019 / Vol 42 No2

30 THE DISTRIBUTOR’S

30 THE DISTRIBUTOR’S LINK Rob LaPointe AIM TESTING LABORATORY Rob LaPointe is a noted authority in materials and fastener technology. With extensive experience in the management and science of materials testing laboratories combined with master’s degrees in physics and education, he excels at bringing solutions to the client. Working specifically in the fastener testing industry, he has developed expertise in mechanical, nondestructive, metallurgical and chemical testing. With a background of 20 years in physics education, Rob is effective at communicating complex ideas in a simple and understandable manner, communicating well with clients enabling them to make informed decisions about their products and business. METALLOGRAPHY FOR FASTENERS Modern fasteners have significant materials science behind them and are used to assemble nearly all of our technology from bridges and infrastructure to aircraft and deep-space probes. Technical applications of fasteners demand that they be produced and tested to high quality standards to ensure they function as they’re intended. Critical application fasteners undergo many tests to verify that they have the performance characteristics needed for the job. Most people with some fastener industry knowledge are familiar with some of the tests that are required on today’s products such as tensile and hardness, but the more critical applications, such as aerospace, require significantly more poking and prodding to see if they’ll make the grade. Product specifications written for critical application fasteners usually include required metallographic tests such as macrostructure and microstructure. If the fastener is made from alloy steel, decarburization testing is often required. These tests, as well as many others, are classified as metallographic testing. Metallography is the science of understanding the structure and properties of metal as well as the tests that insure this structure and performance. Metallographic tests generally require microscopic observation and are often documented with high resolution photography. Sometimes confused with metallurgy which is the science of crafting alloys to achieve desired properties, metallography is the study of the products that metallurgical scientists produce to confirm the materials can do what they’re designed to do. This article will highlight the most-often required metallographic tests that support the quality of the products the industry provides. TECHNICAL ARTICLE Macrostructure is the most fundamental test for determining the quality of metal the mill is producing and for determining the quality of cold or hot forming processes. Acceptable grain flow ensures that the forming process didn’t compromise the native strength of the metal. Macrostructure is typically viewed at low magnification. Usually between 1 X and 10 X. Macrostructure exams are routinely done by the metal producer in the mill to insure the quality of the melt and ingot or the continuously-cast forming process. These exams reveal issues with the material such as inclusions (unwanted material mixed in with the metal) or discontinuities such as voids or laps. For finished products, testing standards for highstrength fasteners often require a macrostructure exam to determine that acceptable grain flow is maintained during the FIGURE 1. HOT FORMED BOLT upsetting process of SHOWING GRAIN FLOW. HCL + H 2 O AT 140°F FOR 45 MIN head forming. Figures 1 (1 X MAGNIFICATION). and 2 show the grain flow distortion created by a hot forming process. Macrostructure exams for a product, such as a socket-head cap screw, require a longitudinal (along the long axis) cross-section showing the areas of interest. Specifically, the head and thread sections need to be examined to see that the grain flow in the regions upset by the forming process. This requires the fastener to be cut, sanded smooth (sometimes polished) and etched to reveal the grain. A mixture of 50 % hydrochloric acid (HCl) and water (H ² O) is often the etchant of choice. CONTINUED ON PAGE 114

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