Stainless Steels are broadly defined as Iron alloys containing from 12 to 30% Chromium and from 0 to 20% Nickel. This analysis is further modified by additions of Carbon and other minor elements which contribute specific effects either to control mechanical properties or to improve corrosion resistance. The corrosion resistance of Stainless Steels is attributed to a surface phenomenon known as passivity. When oxygen comes in contact with the surface, it forms an invisible film which protects the underlying metal from rusting and corrosion under sever environment.
The family of Stainless Steels is divided into three general classifications:
“Austenitic” Stainless Steels make up the general group of the 18-8 (or 300) series. They are the Chromium-Nickel type containing upwards of 8% Nickel. They are not hardenable by heat treatment, non-magnetic for practical purposes and offer the greatest degree of corrosion resistance. “Martensitic” Stainless Steels contain from 12 to 20% Chromium. They are magnetic and hardenable. Type 410 and 416, common fastening alloys, are Martensitic Stainless Steels.
“Ferritic” alloys are also Chromium Stainless Steel alloys. They are magnetic and not hardenable by heat treatment. Type 430 is an example.
There are almost as many uses for Stainless Steel fastenings as there are problems of corrosion, temperature and strength. Because of its high tensile strength, corrosion resistant qualities and ability to attain a mirror-like finish, it is one of the most versatile of all metals.
Applications include its use in the petroleum, chemical, food, plumbing, transportation and oil equipment industries to mention just a few. Listed below are the types of Stainless Steel alloys which are most frequently used in the manufacture of fasteners.
Type 302: A general purpose 18-8 chromium-nickel stainless steel. It retains an untarnished surface under most atmospheric conditions and offers high strength at reasonably elevated temperatures.
Type 303: A free machining 18-8 chromium-nickel stainless steel with qualities similar to Type 302. Elements have been added to improve its machining characteristics.
Type 304: An 18-8 grade generally used for cold headed products. It is somewhat superior to Type 302 in corrosion resistance and is now the alloy used for many standard headed fasteners.
Type 310: A chromium-nickel stainless steel with a ratio of 24-26% chromium and 19-22% nickel. It offers the highest heat resisting qualities of any of the chromium-nickel grades.
Type 316: It differs from 304 mainly by its molybdenum content and has qualities which give it superior corrosion resistance to other chromium nickel steels when exposed to sea water and many types of chemical atmospheres. It is also a superior stainless steel for strength at high temperatures.
Type 321: Similar to the 302/304 group with the addition of Titanium, which aids in resisting intergranular corrosion when subject to operating or fabricating temperatures in the range of 800o F to 1650o F.
Type 410: A chromium alloy containing no nickel. It is a general purpose corrosion and heat resisting, hardenable chromium steel. It can be easily headed and has fair machining properties.
Type 416: Similar to Type 410, but has slightly more chromium and is considered a better machining grade than Type 410. It is used for fabricating studs, nuts and other machined products. It is hardenable.