General Fastener Materials
Fastener and Fixing Materials
Extracted from Fastfix Direct Tools Fasteners and Fixings
Fasteners and fixings such as hexagon nuts, hexagon bolts, coach bolts, blind rivets, solid rivets, woodscrews, machine screw, nails etc, are manufactured in a wide range of materials from common steel to titanium, plastic and other exotic materials. Many materials are further separated into different grades to describe specific alloy mixtures, hardening processes, etc. In addition, some materials are available with a variety of coatings or plating to enhance the corrosion resistance, or appearance of the fastener.
Fastener material can be important when choosing a fastener due to differences between materials in strength, brittleness, corrosion resistance, galvanic corrosion properties, and of course cost.
When replacing fasteners, it is generally best to match what you are replacing. Replacing a bolt with a stronger one is not always safe. Harder bolts tend to be more brittle and may fail in specific applications. Also some equipment is designed so that the bolts will fail before more expensive or critical items are damaged. In some environments such as salt water galvanic corrosion must also be considered if changing fastener materials.
Stainless Steel (from a marine fastener perspective)
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 loose 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 or toughenede in the same way. Therefore when compared with regular steel they can be stronger than mild steel fastener but many are weaker than most hardened steel fasteners.
Stainless steel is also much less magnetic than regular steel fasteners though some grades will be slightly magnetic.
Good corrosion resistance is a feature of all stainless steels. Low alloy grades can resist corrosion in normal conditions. Higher alloys resist corrosion by most acids, alkaline solutions and chloride environments.
The corrosion resistance of stainless steels is due to their chromium content. In general, stainless steels contain a minimum of around 10.5% chromium. The chromium in the alloy forms a self-healing protective clear oxide layer that forms spontaneously in air. The self healing nature of the oxide layer means the corrosion resistance remains intact regardless of fabrication methods. Even if the material surface is cut or damaged, it will self heal and corrosion resistance will be maintained. The most commonly used grades are A2 stainless steel and A4 stainless steel, these are the two types offered by Fast Fix Direct Ltd.
A1 = a free-cutting quality, having a superior machine ability due to a higher phosphorus and sulphure percentage. As a consequence, however, the general corrosion resistance is decreased. This “automatic lathe” stainless steel is seldom used for mass production fasteners.
A2 = the most commonly used stainless steel grade - also called 18/8 (18% Cr, 8% Ni) - with outstanding corrosion resistance under normal atmospheric conditions, in wet surroundings, oxidizing and organic acids, many alkali and salt solutions. A2-0 bolts are nearly as strong as 8.8 heat treated fasteners.
A4 = the most corrosion resistant steel grade - also called “acid proof” - with an increased nickel percentage and addition of molybdenum. Better resistance to aggressive, corrosive environments such as sea climate (chlorides), industrial atmosphere (sulphure dioxide), oxidizing acids and there where pitting may occur.
Stainless Steel fasteners are the work horse for modern boatbuilding or exterior woodworking applications. They are perfect for marine use due to great corrosion resistance provide while remaining affordable. Hardened steel screws are more common in the woodworking environment but cannot be used in a corrosive environment. Stainless steel is available in a wide range of engineered grades, each with its own designation number and specific properties. Only a few grades of Stainless Steel are suitable for fasteners, those that can be cold headed easily and have adequate torsional and shear strengths. The 300 series stainless steels are the most popular.
The most commonly used grade is A2 (304), however A4 (316) is a major step up and recommended for use in highly corrosive environments, such as in contact with salt water. A4 (316) series stainless, also known as marine quality or food grade stainless steel, contains molybdenum, which significantly increases corrosion resistance and strength.
Use Stainless Steel screws cautiously below the waterline. Stainless Steel Screws cannot be in an anaerobic environment. If the screw is immersed in "still water" with no oxygen the corrosion-resistant film, chromium oxide, will not be allowed to form. Without the chromium oxide film the screw will suffer from galvanic corrosion and eventual failure.
When using stainless steel nuts and bolts especially in larger in size always remember to lubricate with an ant seize compound. This will prevent the nut from binding on the thread (known as galling or cold-welding).
Steel (carbon 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 in the Din standard are mild steel 4.6 to 5.6 grade, this equates to testing 460 to 560 NM tensile strength per square mm.
Grade 8.8 which is the metric standard high tensile bolt strength at 880 NM per square mm. The next grade being 10.9 which is often found in automotive applications where high loads are expected and 12.9 grade is the UK standard for socket headed cap screws.
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 and in pressure treated lumber where the chemicals in the lumber may corrode other fasteners.
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.
Cadmium was a traditional corrosion prevention plating treatment, though concerns and costs over environmental ccleanup by plating companies have reduced its prevalence. However, it is usual to chromate treat or 'passivate' the finish which in simple terms, completes the surface to neutralise and fill microscopic holes. This results in the well known shiny yellowish or satin brown/green effect (the latter on military steel hardware) Cadmium plating is mainly used to provide a barrier between dissimilar metals in contact, to prevent, or reduce the rate and effect of electrolytic or galvanic corrosion. As cadmium is soft and doesn't embrittle when plated it is ideal for threaded fasteners. It is usually passivated to further enhance the finish and maybe because cadmium is posionous.