Bolts as fasteners are classified by their material, thread pitch and a number of other criteria. These specifications allow engineers and manufacturers to employ the correct fastener and eliminate many of the problems that might otherwise arise such as lack of strength, corrosion or excessive cost. A4-70 bolts are one type of bolt widely used in industry. They are available in a number of sizes, each with a specific strength.
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The "A4" in A4-70 indicates the steel the bolt is made from. A4 steel is a stainless steel with a complex composition. It contains .08-percent carbon, 1-percent silicon, 2-percent manganese, .05-percent phosphorous, .03-percent sulfur, 16-to-18.5-percent chromium, 2-to-3-percent molybdenum, and 10-to-14.4-percent nickel. These elements make A4-70 bolts very resistant to corrosion and less reactive to other materials than a typical carbon or tool steel.
A4-70 bolts are available in a variety of metric sizes from M5 to M30. These sizes designate the diameter of the actual bolt. Within each diameter, bolts are available in different lengths, although not every diameter is available in every length. The smallest-diameter bolt, M5, is available in sizes from 12 mm to 50 mm, while the largest-diameter bold is available in lengths from 70 mm to 200 mm.
The mechanical specifications such as tensile strength and yield strength differ slightly depending on the size of the bolt. However, the minimum tensile strength for A4-70 bolts is 800 MPa, while the minimum yield stress of the bolt is 600 MPa. These bolts can also be purchased in different strength classes for very stringent applications, and the actual tensile strength will vary from 500 to 1,000 MPa depending on the strength class.
- JAM Building Products: http://www.astm.org/Standards/F738M.htm
- RoyMech: Tensile and Proof Stress Of Metric Bolts and Screws. (Stainless Steel )
- Stainless Steel Information Center: Stainless Steel Fasteners
- ASTM F738M - 02(2008) Standard Specification for Stainless Steel Metric Bolts, Screws, and Studs
Writer, photographer and world traveler James Croxon is a jack of all trades. He began writing in 1998 for the University of Michigan's "The Michigan Times." His work has appeared in the "Toronto Sun" and on defenselink.com and globalsecurity.org. Croxon has a bachelor's degree in English from the American Military University.