Know Your Stainless Steel
Stainless steel is an iron-based alloy containing 10.5 percent chromium and a variety of other alloys including nickel, manganese, molybdenum, sulfur, selenium and titanium. The chromium is principally responsible for heat and corrosion resistance. While some of the other alloys also help prevent corrosion, they principally add strength to the bolt and make fabrication easier.
Most bolts, except those that are highly specialized, are identified by a numbering system instituted by the American Iron and Steel Institute (AISI) reflecting their composition and thus potential uses, such as 200, 300 or 400 series. They are further identified by a Unified Numbering System (UNS). For example, a 304 AISI bolt would be numbered S30400 under the UNS.
There are five basic bolt classifications, each reflecting its metallurgical structure that determines its corrosion resistance, hardness and ease of fabrication. Austenitic stainless steels are comprised of chromium, nickel and manganese and identified in the 200 and 300 series. They are cold hardened and non-magnetic. A common example is type 304, which is 18 percent chromium and 8 percent nickel. In construction nomenclature it would be referred to as a 304 18-8 bolt. Ferritic stainless steel are 100 percent chromium, slightly hardened by cold treatment and magnetic. An example would be in the 430 series. Martensitic stainless steels are entirely chromium and are heat hardened only. All are magnetic and categorized in the 410 group.
Precipitation-hardened stainless steels are made through a process of low-temperature aging and cold working and are known only by their UNS numbers. They are formed entirely in a hardened state, avoiding any distortions that can occur with heat hardening. Duplex stainless steels are half austenitic and half ferritic, which gives them both strength and design versatility.
Strength and Corrosion
Nickel is the alloy that adds the greatest amount of strength to a bolt. A bolt (or other fastener) has greater tensile strength the more nickel it contains. Carbon and nitrogen help strengthen a bolt when heat treated. The addition of aluminum, titanium and columbium also increase strength through heat treatment. Good examples of this strength are bolts UNS numbers S13800, S15500, S17400 and 17700, which are used extensively in the aerospace industry and for large-diameter bolts in civil engineering projects, like bridge building. Chromium is what gives stainless steel the stainless part of its name. Bolts containing 12 percent or more chromium generally will not rust or oxidize except under extreme conditions. Typically, the 300 series bolts are used when corrosion prevention is a high priority.