Guide to Stainless Steel Fasteners
Are you working in an caustic environment where corrosion is important in material selection? Are you concerned about rust forming on the structural parts and fasteners you use due to a damp or moist environment? If so, fasteners made of stainless steel may be the answer to your problem. But did you know that there are many types and grades to choose from?
Stainless steel grade selection is important because the grade determines the properties of the material and, of course, the performance of the product in a given environment. Knowledge of the properties is important in determining the most appropriate grade to specify.
This article aims to give you a basic understanding of stainless steel to help you in determining the most appropriate material for your situation.
What is Stainless Steel?
Stainless Steel is a low carbon alloy with at least 10.5% chromium, as well as other alloying elements such as Nickel and Molybdenum which enhance the anti-corrosive properties of the material.
At or above this level of chromium, a layer of complex chrome-oxide forms on the surface which prevents oxygen atoms from penetrating the steel, thus preventing the formation of Iron Oxide (rust).
The chrome-oxide layer is what makes the steel “stainless” or stain less.
Note that stainless does not mean that minor surface rust may not appear. It is quite common, particularly in harsh environments, to see “tea stains” on the surface of stainless steel, where a small amount of surface rust has formed. Critically, as mentioned earlier, the important thing is that the rust does not penetrate the body of the fastener, thus maintaining its integrity.
Why use Stainless Steel Fasteners?
People choose stainless steel where corrosion, temperature and strength are of concern. Materials made of stainless steel have a mirror-like finish.
Some general characteristics of stainless steel include:
- Higher work hardening rate
- Higher ductility
- Higher strength & Hardness
- Higher corrosion resistance
- Higher cryogenic toughness
- Lower magnetic response (for Austenitic steels only).
What are the types of Stainless Steel Fasteners?
Stainless Steel comes in various types, based on the ratio of the alloying elements and on the molecular structure of the material.
The four fundamental types of stainless steel are:
- Ferritic Stainless Steels
- Martensitic Stainless Steels
- Austenitic Stainless Steels
- Duplex (Austenite-Ferrite) Stainless Steels
For more information on the family of stainless steels, refer to The Atlas Steels Technical Handbook of Stainless Steels.
There are 2 grades of Austenitic Stainless Steel – 200 & 300 series. Grade determines the quality, durability and temperature resistance of the stainless steel. Different grades have different ratio of Chromium, Nickel, Molybdenum and Carbon.
The 300 series is the most common. Specifically, the most common grades are 304 and 316. These are also referred to as A2 & A4 grades, respectively. 316 grade is also referred to, in the vernacular, as “marine grade stainless steel”, although “marine grade” is not a recognized industry standard so it would be incorrect to specify “marine grade”.
304 Stainless Steel
Grade 304 (UNS 30400) is the most common and widely used of in the 300 series. It has 18-20% Chromium and 8-12% Nickel. It is sometimes called the standard 18/8 austenitic stainless steel.
It is resistant to oxidation and corrosion, which is required in food environments and when aesthetics are important. It provides ease of fabrication and cleaning, prevention of product contamination and offers various finishes and appearances. It is used in corrosion resistant electrical enclosures, auto molding and trim, wheel covers, kitchen equipment, hose clamps, exhaust manifolds, stainless steel hardware, storage tanks, pressure vessels and piping.
316 Stainless Steel
On the other hand, Grade 316 (UNS 31600) has the addition of 2-3% Molybdenum. The Chromium content is lowered to 16-18% , and the Nickel content is raised to 10-14%. This change in the Chromium/ Nickel ratio, and the addition of Molybdenum significantly increases the metal’s resistance to “salt” corrosion.
It is a heat-resistant stainless steel with superior corrosion resistance as compared to other chromium-nickel steels when exposed to many types of solutions such as sea water and brine solutions. It has a greater resistance to chemical attack than Grade 304.
Grade 316 is durable, easy to fabricate, clean, weld and finish. It is considerably more resistant to solutions of sulfuric acid, chlorides, bromides, iodides and fatty acids at high temperatures. It is often used in more corrosive environments where the material will be exposed to chemical, solvent or salt water. Grade 316 Stainless Steel makes it the preferred material for marine construction.
It may cost a little more but could save you in the long run, especially if your fasteners and structures are going to be used outdoors.
304 vs 316 Stainless Steel Fasteners
If you work in an environment where there is exposure to chlorine in the atmosphere and other corrosive substances in the environment, choose 316 Stainless Steel.
Elsewhere, you may opt for 304 since it also serves its purpose of protecting your fasteners from corrosion.