Fasteners made from stainless steel are very popular in areas that are corrosive. These fasteners are also used in areas that need high temperature resistance. They cost more and are weaker than heat treated bolts but they are invaluable in certain environments.
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What makes a steel fastener stainless?
The stainless steel used in most fasteners is essentially low carbon steel, which contains chromium at 10% or more by weight. It is this addition of chromium that gives the steel its unique corrosion resisting properties.
The chromium content of the steel allows the formation of a rough, adherent, invisible, corrosion-resisting chromium oxide film on the steel surface. If damaged mechanically or chemically, this film is self-healing, providing that oxygen, even in very small amounts, is present. The corrosion resistance and other useful properties of the steel are enhanced by increased chromium content and the addition of other elements such as molybdenum, nickel and nitrogen.
Which stainless do you use in making stainless steel fasteners?
There are more than 100 grades of stainless steel. However, the entire group can be divided into five classes. The three classes of stainless steel used in Fastener manufacturing are reviewed below.
1. Austenitic
Austenitic refers to the 300 series of stainless steel fasteners, which were named for Sir Robert Williams Austen, an English metallurgist. This series of alloys are the most popular of the stainless steels, accounting for 85% of all the stainless steel fasteners made. Austenitic fasteners have the highest level of corrosion resistance in the stainless fastener families and for all practical purposes are non-magnetic.
These types of stainless steels contain high chromium and nickel content. Although usually non-magnetic, fasteners may exhibit some magnetism after coldworking. This family of stainless offers the best corrosion resistance. Included in this group are types: 302, 303, 304, 305, 316, 321, 384, & XM7. Austenitic stainless steels account for approximately 85% of all stainless steel fastener production.
2. Martensitic
This stainless steel contains no nickel, can be heat-treated and is magnetic. Its corrosion resistance is not as good as the Austenitic types. This family of stainless steel includes types: 410, 416 and 431 and account for approximately 10% of stainless steel fastener production. Drill Point (TEK) screws are usually made with this type of stainless so they can be heat-treated.
3. Ferritic
This stainless steel does not contain nickel and is not heat treatable. This type is magnetic and its corrosion resistance is not the same high degree as the Austenitic types. This family of stainless steel includes types: 430 and 430F and account for approximately 5 % of stainless steel fastener production.
What is an 18-8 stainless steel fastener?
The term 18-8 stainless steel fastener is used to describe a family of stainless steels that contains 18% chromium and 8% nickel in their formulas. This family includes types 302, 303, 304, and 305.
It became common for stainless steel fastener buyers to simply order, in lieu of a specific type, any stainless fastener with 18% chromium & 8% nickel in the �recipe�. This was due to the great deal of interchangeability of this group. So, the short hand term �18-8� was born. The Austenitic grades we now know collectively as 18-8 are by far the most popular of stainless steel fasteners. The 18-8 types of stainless steel are very corrosion resistant and have a minimum tensile strength similar to a Grade 2 bolt.
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