What grade screw is the strongest?
The world of bolts and fasteners can be confusing, but getting this right is pretty straightforward. The strength of any screw depends on its 'tensile strength' or the ability of the screw to withstand maximum stress without breaking.
Based on their tensile strength, a system of bolt grade is developed. A typical hardware place in North America will use the SAE American Grades system. As per this system, Grade 9 is the strongest bolt grade which has a minimum tensile strength of 180,000 psi (pound-force per square inch). They can be identified easily with nine raised dashes on the screw head and are made of quenched and tempered, medium-carbon alloy steel.
However, for residential and personal experimental use, you may rarely use this bolt grade. Let's explore the key different grades and their common applications.
Grade 2
A grade 2 bolt is distinguishable by its complete lack of head markings and has a minimum tensile strength of 60,000 to 74,000 psi. They are made of low or medium-carbon steel, making them cost-effective. Coatings of Chrome, Nickel, or yellow Zinc may also be present on them to lend a corrosion resistance ability. Due to their low load capacity, they are used in non-critical applications such as prototype design and maintenance. The nominal size range of the grade 2 bolt is ¼ inch to 4 inches.
Grade 5
Manufactured using medium carbon alloy steel and hardened for increased strength and durability, Grade 5 bolts are easily identifiable by three radial lines and offer a tensile strength ranging from at least 105,000 to 120,000 psi. These bolts are commonly used in automotive and OEM applications or in situations where medium strength is required. The most frequently used sizes for Grade 5 bolts range from ¼ inch to 1½ inches.
Grade A325
With A325 embossed on head markings, these grades of bolts have a minimum tensile strength of 120,000 psi. A325 bolts must have a heavy hex cap screw to tolerate high pressure in structural work such as heavy construction projects. Made of Carbon or alloy steel with or without boron, A325 bolts cover sizes from ½ to 1½ inches in diameter.
Grade 8
Grade 8 is one of the strongest SAE-grade screws in the field, identifiable by six raised dashes or dots on their screw head. They have more than double the tensile strength of a Grade 2 screw due to repeated quenching and tempering of medium carbon steel. With a minimum tensile strength of 150,000, will often find grade 8 bolts in demanding applications, such as in automotive suspensions, industrial machinery, motors, pumps, valves, tanks, and salt water applications.
Grade 9
They are structural bolts, also known as grade 9 hex cap screws, are one the strongest structural bolts that can be used today. While the typical grade 8 bolt has a tensile strength of 150,000 PSI, a grade 9 bolt has a tensile strength of 180,000 PSI. Grade 9 bolts are commonly used in heavy-duty applications and heavy machinery like bulldozers. They are considerably stronger than grade 8 bolts and are also more corrosion-resistant due to the presence of yellow zinc chromate in their composition.
Generally, the SAE grading system only goes up to grade 8; grade 9 bolts are, without a doubt, the stronger and most durable structural bolts.
While you decide on the right bolt grade for your projects, it's also critical to keep the thread length in mind. They should be typically 1-1.5 times the diameter. The higher the grade of the screw, the shorter the length needed for it to perform well.
Grade 2 or grade 5 are common grades of screws suitable for residential applications such as building decks and woodwork. Selecting the right fastener grade depends on how strong you want it to be. Ultimately, the bolt that you select will depend on how strong it needs to be. The more demanding the application, the higher the grade that will be required.
