1. What is the application and the function for the fastener? 

Will the fastener be used in a military, automotive, marine, or food service product? Will the fastener be used for a drilling machine, a metal cutting machine, or a robotic machine? Does the fastener secure the flanges of a valve? The application determines the environment that the fastener must withstand. Depending on the application, a fastener can be subject to extreme temperatures, high pressure, shock, and vibration. Understanding the purpose for the fastener will dictate many of its characteristics and properties. 

2 What material should be used so the fastener can support the load? 

The strength required for the bolt is the main determinant for selection of material. Most fasteners consist of carbon steel. The majority of fasteners are made from low to medium carbon and carbon alloy steel. Grade 2 fasteners are a standard hardware grade. A Grade 2 fastener is the most common grade of steel and the least expensive. Select Grade 2 bolts where high strength Is not a concern. Handrails and pipe hangers are examples of applications in which Grade 2 fasteners provide sufficient strength. Grade 2 derives its strength from cold hardening of the fastener during forming. Many applications will require a fastener that requires a strength greater than a Grade 2 steel. In order to achieve a greater strength, the fastener will require heat treatment consisting of neutral harden, quench, and temper operations. Typical higher-strength fastener grades are 5 and 8. Fasteners made to Grade 5 are typically made from medium carbon steel. For applications requiring greater strength than a Grade 5 fastener, a grade 8 fastener is recommended. Grade 8 fasteners are typically made from medium carbon alloy steel that has been heat treated for greater hardness than the Grade 5 steel. Be cautious when selecting a fastener with a high tensile strength. Fasteners with higher hardness and tensile strength tend to be more brittle. Stainless steel has the typical tensile strength equivalent to grade 5 heat treated steel. It is not the strongest grade of steel, but it is corrosion resistant; and, as a result, stainless steel is a higher cost material. Stainless steel fasteners are necessary in food production, water treatment facilities, and in factories that use corrosive chemicals. Two other materials that have excellent corrosion resistance are brass and bronze and are used primarily in the marine industry. Brass and bronze also have the benefit of being a visually-appealing fastener; but, their strength is generally equivalent to an un-hardened Grade 2 fastener.