STAINLESS STEELS USED IN FASTENERS
The term “stainless steel” refers to a group of steels that are resistant to corrosion due to the addition of high percentages of chromium to their chemistry. The term “stainless” was originally used to describe these materials because of their resistance to staining when used for food processing equipment. Standards steels would become discolored (start to corrode) when used in food processing applications where stainless steel would not become discolored (or corrode) when it comes in contact with food. The properties of stainless steel where found to be useful for fasteners exposed to saltwater and many different types of chemicals.
Classification of Stainless Steel
There are two main groups of stainless steels that are used in the manufacture of
fasteners in the United States.
1. 300 Series
Ex. 301, 302, 303, 304, 310, 316, 347
2. 400 Series
Ex. 403, 405, 410, 420
The 300 series materials also contain high percentages of nickel in addition to chromium. This gives them better corrosion resistance than the 400 series. The 400 series can be heat treated to make them stronger than the 300 series. The 300 series can not be heat treated, The only way to increase their strength is by “cold working”. This refers to the process of deforming the material.
300 Series stainless steel is not very strong. Its tensile strength is approximately 80,000 psi (approximately as strong as a Grade 2) and its yield strength is about 50% of it tensile strength. 400 Series stainless can be heat treated up to a tensile strength of 125,000 psi (approximately as strong as a Grade 5)
Stainless steels do not have “grades” associated with them. They are referred to as “Types”, example: Type 304 stainless or Type 316 stainless.
A sub group of the 300 series stainless is called “18-8” stainless. This expression refers to the amount of chromium andnickel that is present in the steel: 18% Chrome and 8% Nickel. Examples of 18-8 stainless are 301, 302, 303, 304.
The two most popular stainless steels used in fastener manufacture are Type 304 and Type 316. Type 316 stainless steel is considered to provide the best corrosion protection of the stainless steels that are used to manufacture fasteners. This stainless contains more chrome and nickel than the 18-8 types and provides better corrosion resistance when used n seawater and against many different types of chemicals.
Cap Screws are typically manufactured from Type 304, 316, 410 or 420 stainless.
Nuts are typically manufactured from type 303, 304, 316 or 410 stainless.
Washers are typically made from 302, 303, 304, 316 or 410 stainless.
Sheet Metal Screws are typically made from Type 304, 316 or 410 stainless.
There are other more exotic types of materials used in the manufacture of cap screws that are classified as “stainless steel” such as Monel, Hastelloy, 17-4ph but these materials are less common.
In European standards the two main types of stainless they designate are A2 and A4. The A2 is approximately equivalent to our Type 304 (or 18-8) and the A4 is approximately equivalent to Type 316.