STRENGTH
It is a unique service application in which the fastener does not support or transmit some form of externally applied load. When fastener strength is the single design consideration, there is no need to look beyond steel. Steel is by far the most economical and versatile material choice. In fact, raw material costs are such that materials other than steel should only be considered when some special requirement of the application, other than or In combination with strength, dictates their selection.
Low carbon steel fasteners have minimum specified tensile strengths of 60,000 psi; medium carbon heat treated fasteners 120,000 psi; low alloy steel 150,000 psi and higher; and some of the super alloys have strengths of 260,000 psi and even higher. Fastener strengths exceeding 400,000 psi are technologically possible although designing with and using such fasteners is no game for the amateur. In most engineering applications, other than aerospace, seldom is there any need to consider fasteners with tensile strengths over 180,000 psi.
Each of the three basic types of stainless steel ? austenitic, ferritic and martensitlc ? have distinctly different strength properties. Austenitic grades ? for example, 303, 304, 305, 316 and 321 ? are non-heat treatable. However, the strengths of fasteners of these materials can be Improved through cold working and strain hardening techniques. Generally, solution annealed fasteners of these grades have tensile strengths of 75,000 psi; cold worked fasteners have up to 90,000 psi; and when strain hardened, strengths to 125,000 psi are possible dependent on fastener size. The ferritic grades, such as 430 and 430F, do not respond to heat treatment and have tensile strengths of only about 70,000 psi. Fasteners of martensitic stainless steels ? for example, 410, 416 and 431 ? are heat treatable and have tensile strengths as high as 180,000 psi.
