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High strength bolts are commonly used in the construction industry, and the term refers to heat treated bolts that deliver to applicable specifications. Most often these bolts are quenched and tempered to increase the strength, but your fastener distributor may warn you that galvanization and high strength bolts do not always work well together. Issues With the Galvanization Process Hot-dipped galvanized fasteners are immersed in a bath of acid before the galvanization process begins. While pickling in this acid, bolts that have been quenched and tempered to boost strength could experience hydrogen embrittlement. This process may result in lower ductility and fastener failure, the very things a high strength bolt should help you to avoid. ASTM standards deal with this issue by stating that hydrogen embrittlement could occur on steel exceeding 150ksi in tensile strength. Most high strength bolts exceed this level, and are at risk of this issue. Another ASTM spec states that any high strength fastener that exceeds 33 HRC in hardness could be degraded during the galvanization process through internal hydrogen embrittlement. Some common grades of high strength bolts that should not be put through the hot-dipped galvanization process include: * ASTM A354 grade BD * ASTM A490 * SAE J429 grade 8 (medium carbon alloy steel) Some grades of bolts can be galvanized without fear of hydrogen embrittlement. Ask your fastener distributor about galvanized high strength bolts such as: * ASTM A325 * ASTM A449 * ASTM A193 grade B7 * ASTM F1554 grade 105 * SAE J429 grade 5 (medium carbon steel) How Heat Comes Into Play Heat also has the potential to degrade the strength of fasteners. Your fastener distributor should know that welding a high strength fastener is not recommended, according to the American Institute of Steel Construction Manual. This standard may lead you to believe that any amount of heat, such as the heat used in hot-dipped galvanization, should be avoided. But there is more to consider. Hot bending is another process that should be avoided when working with high strength fasteners. ASTM specs note that hot bending processes should only use levels of heat within 100 degrees Fahrenheit of the heat used in tempering to avoid weakening the fastener. If galvanization also uses heat approaching or exceeding that level, the same result may occur. The difference between heat in hot bending and heat in the galvanization process is vital. During the heat immersion, fasteners are in a 840 degree bath for a few minutes, but the fastener will not likely heat up to that level. In hot bending, a specific point on the fastener must heat up to facilitate the bending process. This area may be weakened during the process. If your high strength bolts are tempered at levels lower than 840 degrees, hot-dipped galvanization is not a good idea. If those temperature levels are much higher than those used in galvanization, you may not need to worry about weakening the fastener through galvanization. Your fastener distributor has a wide range of high strength fasteners available, both galvanized and non-galvanized. Find out which fastener will meet your specifications in terms of strength, while performing above your expectations in terms of durability. Mike Moore is published on more than 300 websites. He writes about industrial fasteners and building, commercial building, and home building projects . He is published on various website including http://www.melfast.com
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