Stainless steel 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 stainless, 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.
There are more than 100 grades of stainless steel fasteners. However, the entire group can be divided into five classes. The three classes used in fastener manufacturing are reviewed below.
Austenitic refers to the 300 series of stainless steel fasteners, which were named for Sir Robert Williams Austen, and 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 nonmagnetic, fasteners may exhibit some magnetism after cold working. This family of stainless offers the best corrosion resistance. Included in this group are types: 302, 303, 304, 305, 316, 321, 384, and XM7.
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.
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.
18-8 Fasteners – The Most Common
The term 18-8 stainless steel 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 fastener buyer to simply order, in lieu of a specific type, any stainless fastener with 18% chromium and 8% Nickel in the “recipe”. This was due to a great deal of interchangeability of this group. So, the shorthand 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.
18-8 fasteners also offer meaningful strength at elevated temperatures, so it is common to see stainless steel being used in high heat applications – even where corrosion is not a major concern.
316 Fasteners – More Corrosion Resistant Around Salts
Another common austenitic stainless steel used to fastener manufacturing is type 316. This type uses additional amounts of nickel and molybdenum for increased resistance to chlorides and sulfates (salts, salt water). 316 stainless fasteners are also often used in food processing, marinas, photographic works, chemical refineries, ink, textile, bleach, and rubber manufacturing.
The FDA will often require 316 hardware be used on food handling equipment. Under some conditions, meat blood and even items such as mayonnaise will pit 18-8 fasteners. When in doubt you need to check with your local FDA representative.
A 316 stainless steel fastener will cost more than an 18-8 fastener of the same size but will have the same strength. The 316 stainless fasteners are just more corrosion resistant in certain environments.
A2 is the ISO (International Standards Organization) designation for a stainless steel comparable to 18-8. If you see a nut marked A2 you know immediately that is not only stainless steel but also that it is metric.
A4 is the ISO designation for stainless steel comparable to 316.
These bolts will commonly be marked A2-70 and A4-70. The 70 denoting that they were cold formed.
Stainless steel fasteners in the early days were not marked. The first marking were the 90-degree opposing lines as shown below.
Now, you will most often see these fasteners marked with ASTM F593 head marks. These are F593C and F593D for 18-8 and F593G and F593H for 316.
Under the A193 specifications (often found around oil fields) you may see the use of B8 for 304 and B8A for 316.
Under the unified numbering systems, you will see bolts marked as the specific series such as S 30400 for a 304 series bolt.