What Is a Fastener Material?
There is much consideration when choosing the ideal type and material for fasteners. For example, the material chosen should generally be reliable, cost-effective, and maintain durability in the environment.
This often means the corrosion resistance of the fastener material. The corrosion resistance of a particular fastener will also depend on the materials involved because the uneven metal that is in contact with each other may cause galvanic corrosion. The most common materials for mechanical fasteners, such as bolts and screws, are steel.
Different grade of steels is available, depending on the strength required. If rust or rust is a concern, stainless steel, brass, or titanium may be used. Surface coatings, such as bright zinc plating, are also often used to prevent corrosion.
Not all fasteners are made equally, and the project you have on hand will determine not only the size, type, and length of the ideal fastener but also the material your fastener should be made from that best for your results.
Stainless Steel of Fastener Material:
Stainless steel is mainly used for long-lasting applications due to its corrosion-resistant nature and durability. Scratching or bouncing the metal will not cause corrosion to the surface as corrosion resistance is present within the metal itself.
Stainless is a soft metal due to its low carbon content, so most stainless steel bolts are cold-formed and are not heat-treated or hardened. Stainless bolts tend to be slightly magnetic due to cold formation and threading; some fastener will be more magnetic than other depending on size & how fast the cold forming process is.
Stainless fasteners usually have a clean silver color, which makes them common in finishing and decorative applications as well. Stainless steels should never be used with aluminum, with galvanic corrosion likely to occur. Stainless steel will not rust due to scratches, making a thin layer of chromium an invisible protective layer.
This thin layer will reconstruct itself in the presence of oxygen. Note: If you are not in an oxygen-rich environment, it will not take long to rebuild or rebuild the material. This will leave it open for possible corrosion. Stainless steel can be divided into three different types: austenitic, martensitic, and ferritic.
#1. Austenitic Stainless Steel
Austenitic stainless has the highest degrees of corrosion resistance of the three types. This type of stainless includes these grade: 302, 303, 304, 304l, 316, 32, 347, & 348. They also have a tensile strength between 80,000 – 150,000 psi. (between 15% -20% chromium, between 5% -19% nickels)
#2. Martensitic Stainless Steel
Martensitic stainless steel is considered magnetic steel. It can be heat-treated to increase its hardness & is not recommended for welding. These types of stainless include 410, 416, 420, and 431. They have a tensile strength between 180,000 and 250,000 psi. (between 12% -18% chromium)
#3. Ferritic Stainless Steel
The tensile strengths of ferritic stainless steels are 65,000 – 87,000 psi. Although it is still resistant to corrosion, it is not recommended for areas where corrosion is likely. This material cannot be heats treated. Due to the formings process, it is magnetic & not suitable for welding. The ferritic grades of stainless include 430 and 430f. (between 15% -18% chromium)
This is the most commons stainless designation for hardware. For information on the properties of 18-8 stainless steel materials, see our Material Grade Identification and Properties Chart. 18-8 refers to any stainless steel consisting of approximately 18% chromium and 8% nickel.
More expensive than 18-8. Highly corrosion-resistant grade of stainless steel. Ideal in saltwater & chlorine environments.
The Stainless alloy is harder than 18-8 stainless steel but not resistant to corrosion.
The Steel of Fastener Material:
Steel is the most common fastener material. Plain steel fasteners with various surface treatments such as zinc plating, galvanization, and chrome plating are also available. Steel fasteners are typically available in 4 grades: grade 2, grade 5, grade 8, and alloy steel. Several other grades exist but are often underutilized. Grades 2, 5, and 8 are usually plated with a slight blue-ish or yellow zinc coating or galvanized to resist corrosion.
#1. Determining Bolt Grade
Note that in addition to grade marking, many bolts also have a manufacturer’s mark. The bolts are usually marked on the head depending on what grade of bolt they show. See our Material Grade Identification and Properties Chart for a list of the most common grade marks.
- Grade 2:- Grade 2 is standard hardware-grade steel. It is the most commons grade of steel fastener and the least expensive. The grade 2 bolt has no head marking, except for a possible manufacturer’s mark.
- Grade 5 / Grade F:- Grade F is approximately equivalent to Grade 5. Grade F nuts are used with grade 5 bolts. Grade 5 bolts are hardened to increase strength and are the most common bolts found in automotive applications. Grade 5 bolts have three evenly radial lines on the head.
- Grade 8 / Grade G:- Grade G is approximately equivalent to Grade 8. Grade G nuts with grade 8 grade are used. Grade 8 bolts are more rigid than grade 5 bolts. They are thus robust and are used in demanding applications such as automotive suspensions. Grade 8 bolts have six radial lines evenly over the head.
#2. Alloy Steel
Alloy steel is the most common material manufactured in fasteners. Alloy steel fastener is often treated, coated, or plated with zinc for corrosion resistance. Alloy steel is used for the hot-dipped galvanized process, which is treated in a molten zinc bath that creates a tightly bonded alloy finish. Alloy steels are most often available in grade A, grade C, grade 2, grade 5, & grade 8. Untreated alloy steel fasteners are black.
#3. Silicon Bronze
Silicon bronze often referred to as bronze, is an alloy made of mostly copper and tin with small amounts of silicon. Bronze is mainly used in marine environments. Bronze is similar to coppers in color and is sometimes seen in fine woodwork, where it is used for its appearance. The main drawbacks of bronzes are their high cost. They were preferred over stainless in wooden boat constructions & re-fastening due to their superior corrosion resistance and over brass due to their high strength.
Brass is primarily an alloy of coppers & zinc. Brass is highly corrosion resistant & electrically conductive. However, its use as fasteners is somewhat limited due to its relative softness. It is mainly used for its appearance.
Coatings of Fastener Material:
#1. Zinc Plating
Many steel fasteners are electrically coated with zinc for superior corrosion resistance. A fastener that has been zincing plated has a shiny, silver, or golden appearance, respectively known as clear or yellow zinc. They are quite corrosive resistant but will rust if the coating is destroyed or exposed to marine environments.
#2. Hot Dip Galvanizing
Hot dipped galvanized fasteners are often used outdoors, especially in coastal environments. Galvanizing is another coating that uses a layer of zinc. Hot-dip galvanizing puts the thickest coating possible on the metal, resulting in better corrosion resistance. The hot-dipped galvanized bolt is not compatible with other nuts due to the thickness of the coating. The galvanized nuts are slightly enlarged compared to other nuts to accommodate this coating.
The fasteners are chrome plated and polished for appearance. Chrome plating offers corrosion resistance similar to zinc plating. The main drawback of polished chrome is its high cost. If more corrosion resistance is required, the stainless steel can be chrome-plated, allowing the chrome to penetrate to prevent any corrosion.