24 The following material is intended to provide a brief overview of the metallurgical considerations that, daily, influence the design and production of the most reliable fasteners in motorsports. It is hoped that a simple understanding of the knowledge and commitment required to produce this reliability will make your future fastener decisions much, much easier. By Russell Sherman, PE 1. What is grain size and how important is it? Metals freeze from the liquid state during melting from many origins and each one of these origins grows until it bumps into another during freezing. Each of these is a grain and in castings, they are fairly large. Grains can be refined (made smaller); by first cold working and then by recrystallizing at high temperature. Alloy steels, like chrome moly, do not need any cold work; to do this – reheat treatment will refine the grain size. But austenitic steels and aluminum require cold work first. Grain size is very important for mechanical properties. High temperature creep properties are enhanced by large grains but good toughness and fatigue require fine grain size – the finer the better. All ARP bolts and studs are fine grain – usually ASTM 8 or finer. With 10 being the finest. 2. How do you get toughness vs. brittleness? With steels, as the strength goes up, the toughness decreases. At too high a strength, the metal tends to be brittle. And threads accentuate the brittleness. A tool steel which can be heat-treated to 350,000 psi, would be a disaster as a bolt because of the threads. 3. Define Rockwell as we use it. Why do we use the C scale? A man named Rockwell developed a means of measuring hardness of metals which was superior to other methods. A Rockwell hardness tester measures the depth of penetration into the metal when a load is applied. For hard materials, a diamond penetrator is used. For soft material, small balls are used – 1/16˝ or 1/8˝ diameter-and the machine measures the depth. We use the C scale for the 120,000 psi strength level and above. The C scale uses the greatest load – 150 Kg. The A scale uses only a 60 Kg. load but can be correlated with C. It is necessary to use the A scale for thin sheets because using the 150 Kg load would cause the diamond to penetrate almost all the way through. ARP engineers use “Scanning Electron Microscopic” inspection capable of detecting all elements in the periodic table with atomic numbers greater than 5 – permitting the acquisition of high resolution imaging. Metallurgy for the Non-Engineer FASTENER TECH
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