Wednesday, July 25, 2012

Selecting the best material for your fastner application

Materials

Stainless Steel
Stainless steel is an alloy of low carbon steel and chromium for enhanced corrosion characteristics. Stainless steel is highly corrosion resistant for the price and because the anti-corrosive properties are inherent to the metal, it will not lose this resistance if scratched during installation or use.
It is a common misconception that stainless steel is stronger than regular steel. In fact, due to the low carbon content, stainless steel cannot be hardened. Therefore when compared with regular steel it is slightly stronger than an un-hardened (grade 2) steel fastener but significantly weaker than hardened steel fasteners.
Stainless steel is also much less magnetic than regular steel fasteners though some grades will be slightly magnetic.
18-8 Stainless
18-8 refers to any stainless steel containing approximately 18% chromium and 8% nickel. This is the most common stainless designation for hardware. For information on 18-8 stainless steel material properties see our Material Grade Identification and Properties Chart.
Steel
Steel is the most common fastener material. Steel fasteners are available plain as well as with various surface treatments such as zinc plating, galvanization, and chrome plating.
Steel fasteners are commonly available in 4 grades. Many other grades exist but are used far less often. The most common grades are Grade 2, Grade 5, Grade 8, and Alloy Steel. Grade 2, 5, and 8 are usually plated with a silver or yellow zinc coating or galvanized to resist corrosion.
Determining Bolt Grade
Bolts of different grades are marked on the head to show what grade bolt they are.
Grade 2
Grade 2 is a standard hardware grade steel. This is the most common grade of steel fastener and is the least expensive. Grade 2 bolts have no head marking (sometimes a manufacturer mark is present).
Grade 5
Grade 5 bolts are hardened to increase strength and are the most common bolts found in automotive applications. Grade 5 bolts have 3 evenly spaced radial lines on the head.
Grade 8 
Grade 8 bolts have been hardened more than grade 5 bolts. Thus they are stronger and are used in demanding applications such as automotive suspensions. Grade 8 bolts have 6 evenly spaced radial lines on the head.
Bumax88
Special 316L stainless steel bolt equivlent to a hardered carbon steel bold SAE J429  Grade 5 bolt while retaining or exceeding all characteristics of 316L stainless steel
Bumax109
Special 316L stainless steel bolt equivlent to a hardered carbon steel bold SAE J429  Grade 8 bolt while retaining or exceeding all characteristics of 316L stainless steel
Alloy Steel
Alloy steel bolts are made from a high strength steel alloy and are further heat treated. Alloy steel bolts are typically not plated resulting in a dull black finish. Alloy steel bolts are extremely strong but very brittle.


Silicon Bronze
Silicon bronze, often referred to simply as bronze, is an alloy made mostly of copper and tin with a small amount of silicon. Bronze is used primarily in marine environments. It is preferred over stainless in wooden boat construction and re-fastening due to its superior corrosion resistance, and over brass due to its higher strength. Bronze is similar to copper in color and is also sometimes seen in fine woodworking where it is used for its appearance. The main drawback of bronze is its high cost.
Brass
Brass is an alloy of primarily copper and zinc. Brass is highly corrosion resistant and electrically conductive. However, its use as a fastener is somewhat limited due to its relative softness. It is used primarily for its appearance.
Aluminum
Aluminum is a light, soft, corrosion resistant metal. Like stainless steel, aluminum's corrosion resistance is inherent to the material. Therefore scratches and nicks will not effect the corrosion resistance.
Fasteners are made from a variety of aluminum alloys with elements such as manganese, silicon, iron, magnesium, zinc, copper, and silicon being added to increase strength and melting point.
Rivets are often made from aluminum alloys in the 5000 series which uses magnesium as the primary alloying element.

Coatings

Zinc Plating
Many steel fasteners are electro-plated with zinc for better corrosion resistance. Fasteners that have been zinc plated have a shiny silver or golden appearance referred to as clear or yellow zinc respectively. They are fairly corrosion resistant but will rust if the coating is destroyed or if exposed to a marine environment.
Hot Dip Galvanizing
Galvanizing is another coating involving the application of a layer of zinc. Hot dipped galvanizing puts the thickest possible coating on the metal resulting in superior corrosion resistance. Due to the thickness of the coating hot dipped galvanized bolts are not compatible with other nuts. Galvanized nuts are tapped slightly larger than other nuts to accommodate this coating. 
Hot dipped galvanized fasteners are frequently seen in coastal environments.
Chrome
Chrome is used in plating fasteners for its appearance. It provides similar corrosion resistance to zinc plating. The main drawback of chrome is the extremely high cost. If more corrosion resistance is required stainless steel may be chrome plated, preventing any corrosion should the chrome be penetrated.

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