Guide to Stainless Steel Fasteners
Are you working in a 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, then fasteners made of stainless steel may be the answer to your problem. But did you know that there are many grades and types to choose from?
We hope this article gives you a basic understanding of stainless steel and what's the best choice for your intended use, especially when it comes to fasteners.
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".
Does stainless steel rust?
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. But what is critical is that the rust does not penetrate the body of the fastener, thus maintaining its integrity.
What are the types of stainless steel?
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:
- Martensitic stainless steel
- Austenitic stainless steel
- Ferritic stainless steel
- Duplex (austenite-ferrite) stainless steel
For more information on the family of stainless steel, please refer to The Atlas Steels Technical Handbook of Stainless Steels.
What about stainless steel grades?
The grade determines the quality, durability, quality, temperature resistance and other essential characteristics of the material. Different grades have different ratios of chromium, nickel, molybdenum and carbon.
Stainless steel grades tell you how the product will perform in any given environment, hence it's very important to know which grade is appropriate for your intended use.
The typical grades you'll encounter in your search are the 200 and 300 series. The 300 series is the most common. Specifically, 304 and 316 (also known as A2 and A4 grades, respectively).
304 stainless steel
Grade 304 (UNS 30400) is the most common and widely used of 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, and are best used in food preparation and processing environments, and when aesthetics are important. This is the one you'll commonly see on stainless steel hardware in your household and workshop.
- Ease of fabrication
- Ease of cleaning
- Helps prevent product contamination
- Offered in various finishes and appearances
Commonly used in:
- Auto mouldings, accent trims, intake and exhaust manifolds, wheel covers etc
- Corrosion-resistant electrical enclosures
- Hose clamps
- Kitchen equipment
- Pressure vessels
- Storage tanks
316 stainless steel
Grade 316 (UNS 31600) is also referred to, in the vernacular, as “marine grade stainless steel”, although the term is not a recognized industry standard. It 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.
This makes it a very heat-resistant stainless steel with superior corrosion resistance as compared to other chromium-nickel steels when exposed to many types of solutions such as seawater and brine. It has a greater resistance to chemical attacks than Grade 304.
- Ease of fabrication
- Ease of cleaning
- More resistant to common solutions (sulfuric acid, chlorides, bromides, iodides and fatty acids at high temperatures)
- More protected seawater and brine
Best used in:
- Marine construction
- More corrosive environments where the material will be exposed to chemicals and solvents
Grade 316 may cost a little more, but it translates to more savings in the long run, especially if your fasteners and structures are going to be used outdoors.
Why use stainless steel fasteners?
If you're concerned about corrosion, temperature and strength are of concern. Some even go for it just for the appearance -- materials made of stainless steel have a mirror-like finish. Who wants to use rusty screws, bolts and nuts, right?
You'll want to use stainless steel fasteners if these characteristics* matter to you:
- Higher work hardening rate
- Higher ductility
- Higher durability
- Higher hardness
- Higher corrosion resistance
- Higher cryogenic toughness
- Lower magnetic response (austenitic steels)
*Versus other common materials
The most common grade is 304 stainless steel, but there will be certain situations where 316 is your best option.
If you work in an environment where there is exposure to chlorine in the atmosphere and other corrosive substances, choose 316 stainless steel. Elsewhere, you may opt for 304 since it also serves the purpose of basic corrosion protection.