Stainless Steel Fasteners—Specifying the Proper Grade from buzai232's blog

Stainless Steel Fasteners-Specifying the Proper Grade

Stainless steel fasteners are specified for construction applications primarily for their resistance to corrosion, but are often misunderstood with reference to material compatibility, performance and availability.Get more news about stainless steel screws fasteners,you can vist our website!

Many people may recognize a fastener made from stainless steel, but find it difficult to determine what grade is needed and how it may perform in the application.

What is Stainless Steel?
Typically, stainless steel is defined as a family of iron-based alloys that contain a minimum of approximately 12% chromium. It's primarily this chromium that gives steel its corrosion resistance properties when exposed to weather, much like pure chromium plate used on automobile trim. The chromium reacts with oxygen and forms chromium oxide, which is very thin passive film that is invisible, self-forming and self-healing. Other elements are added in various amounts that will enhance corrosion resistance, workability and increase strength. Generally, stainless grades that have higher chromium content will be more resistant to red rust (iron oxide).

What Grade of Stainless Do I Need?
The two most popular types of stainless steel fasteners are 18-8 (300 series) and 400 series. Of these two, the 18-8 series provides the best corrosion resistance and ductility. This stainless family consisting of type 302, 304, 305 and 316, is one of the most misunderstood. Each grade offers almost identical corrosion protection, but varies in performance with reference to installation and resistance to chemical attack. These grades cannot be heat treated and rely on cold-working for additional strength.
One of the more popular grades is type 302, and is typically used in sheet metal and machine screws. Although this grade provides exceptional corrosion resistance, it does not work-harden enough to produce the thread hardness required to minimize thread rollover caused by tapping high-tensile or thick steel.

The same problem occurs when using type 305. To reduce the amount of thread failure, a special modified 304 stainless steel material was developed in the early 1970s primarily for use in the metal building industry. This special grade of 304 stainless is more durable, and characteristically work-hardens during the fastener's fabrication, thereby producing tougher threads. It offers virtually identical corrosion protection and strength as 302 and 305, and it becomes slightly magnetic during the cold-working process.

304 stainless steel fasteners are readily available in self-tapping screws. As with any self-tapping fastener, using the proper hole size is critical to optimize fastener performance and minimize thread rollover. They are typically plated to provide additional lubricity for tapping, and reduce thread failure. An installation speed of 1,000 rpm or less is required to preserve thread integrity and holding power.Self-drilling fasteners are also available in 304 and can have a harden carbon steel drill tip welded to the screw shank in order to drill steel. These fasteners are widely used in aluminum framing and a variety of panel applications. They are available in a wide range of sizes and head styles. Once again, slower speed screw-guns are required to minimize thread rollover.

Another grade of 300 series stainless steel is 316, and was originally developed for sulfurous acid applications. This grade has a higher molybdenum content, which provides additional corrosion protection in highly corrosive applications and works well in chloride type environments. This grade is primarily available in self-tapping screws.

For less severe applications, 410 stainless fasteners are often specified. They are available in self-tapping and self-drilling styles and come in a variety of diameters and lengths. 410 stainless fasteners provide good corrosion resistance in mild atmospheres like light industrial environments. In some applications, 410 may show signs of red rust due to its lower chromium content (compared to 18-8 grades), and will continue to rust although at a much slower rate than carbon steel. As previously mentioned, it's the chromium in the form of chromium oxide that protects the surface and provides the corrosion resistance in normal atmospheres.