bolt grades and coatings

Bolt Grades & Bolt Coatings

Here’s an in-depth guide to the most common grades of fasteners under the BSI (British Standard Institute) system. Grade 4.6, Grade 8.8, and grade 10.9. 

The grade of steel fasteners refers to the quality and strength of the steel used in its manufacture, and there are several systems used to grade steel. In the USA they use the Society of Automotive Engineers (SAE) and the American Society for Testing and Materials (ASTM). In the UK, we tend to refer to the British Standard or the ISO.



The British Standard Institute of Steel Grading

The BSI is the oldest standardisation and can be used across 172 countries, although it is mostly used in the UK. There are two systems of grading in the UK, one for metric and the other imperial. The Unified National Thread (UNT) system grades bolts on a scale from 1 to 5 for imperial bolts, with 5 being the strongest. The BSI system is used for metric bolts and ranges from 4 to 10.

Grade 4.6 steel: Grade 4.6 steel is characterised by its low carbon content, contributing to relatively lower strength levels. With a minimum tensile strength of 400 MPa and a minimum yield strength of 240 MPa, this type of steel is considered suitable for a variety of general applications. Nuts and bolts crafted from grade 4.6 steel are particularly well-suited for scenarios where only a modest amount of strength is necessary. The low carbon steel content makes it a practical choice for common applications that do not demand exceptionally high strength requirements.

Grade 8.8 steel: These types of fasteners are made from medium carbon steel and have a tensile strength of 800 MPa (megapascals). This medium strength steel has a medium quantity of carbon steel and is more suitable for projects where more strength is needed than grade 4.6 fasteners. This is the most common grade used for bolts and fasteners and some of Fixabolt’s biggest sellers.

Grade 10.9 steel: Fasteners made from a grade 10.9 consist of a higher strength alloy steel and which has a tensile strength of 1000 MPa.

The highest, most common strength carbon steel available. This grade of steel is required for high strength and reliability projects.

In addition to the more common grades of steel listed above, there are some more specialised grades of steel for specific fastener needs available under the BSI (British Standards Institute) system.

Grade 5.6 steel: This grade of steel is perfect for applications that need a medium strength, medium carbon steel content, but with a lower level of hardness, allowing for the steel to be deformed and manipulated during installation.

Grade 5.8 steel: Once again, a medium carbon steel with medium strength, but with increased ductility. Commonly used in manufacturing and engineering, ductility can be extremely important. It is the ability of steel to sustain deformation under tensile stress before failure. Situations where this would be required is in cases of extreme weather and heat fluctuations.

Grade 12.9 steel: This grade of steel is usually used in situations that require a high strength, high carbon steel, such as military and aerospace projects. It is a hard and strong steel.

The Society of Automotive Engineers Grading System

As with the British version, the SAE system, grades steel fasteners based on the strength and hardness of the steel being used.

Here’s a list of the most common grades of SAE steel fasteners.

  • Grade 2 steel: refers to a low carbon steel with low strength. Best suited for general applications where low to moderate strength is required.
  • Grade 5 steel: Medium strength, medium carbon steel. Once again, most suited to medium strength and hardness applications.
  • Grade 8 steel: Suitable for situations that require a high strength and hardness of steel.

The American Society for Testing & Materials Grading System

Similar to the above, the ASTM grading system is based on chemical composition and mechanical properties of the steel.

  • Grade A steel: Low strength, low carbon steel.
  • Grade B steel: Medium strength, medium carbon steel.
  • Grade C steel: High strength, high carbon steel.

Bolt Coatings

Bolt grades refer to the strength of the steel and are important considerations when selecting fasteners for your application, whilst the coating can provide extra protection, especially against corrosion.

There are are a variety of coatings suitable for steel, with the most common being zinc plating (BZP), and black oxide.

Plating steel with zinc is a popular choice when it comes to general purpose bolts and fastenings. The zinc plating provides protection against corrosion and give a shiny finish, which makes them ideal in decorative situations.

However, the main difference between plain stainless steel bolts and zinc plated bolts is the amount of protection they offer against corrosion, which makes stainless steel more expensive than BZP.

What is Black Oxide?

Another presentation of steel is black oxide, or black passivation, a chemical process of coating that is generally applied to stainless steel to protect from corrosion and give them a black finish. It also makes the objects, such as black oxide bolts, harder and more resistant to wear, and quite often is used in engineering applications.

It’s a clever process of dipping the steel in a solution of chemicals at a high temperature, the steel bolts are then rinsed and oiled to provide a solid strong finish.

Utilising a supplementary post-treatment following the application of a black oxide finish is a prevalent practice aimed at bolstering the anti-corrosion capabilities of black oxide coatings. In its standalone form, black oxide provides a modest level of defence against corrosion.

However, through the meticulous application of a well-suited post-treatment, one can achieve optimal results by allowing thorough absorption of the supplementary coating into the pores of the black oxide finish.

This not only elevates the metal's resistance to corrosion, but also imparts a more profound and enduring black appearance. Furthermore, a judiciously chosen post-treatment contributes to heightened abrasion resistance in scenarios involving the mating of parts, enhancing the overall longevity and functionality of the black oxide-coated components.

What is 316 and 304 Stainless Steel?

The difference between 316 and 304 stainless steel is the level of corrosive resistance within each grade. 316 stainless steel is a higher grade of stainless steel, which is often known as marine grade (A4) stainless steel. 304 grade steel (A2), is more commonly used for everyday objects as it has a mix of 18% chromium and 8% nickel, giving it the title of stainless steel.


As you see, using the wrong grade of steel in engineering will result in failure of a fastener or part, which could lead to disastrous effects. So take time choosing the right grade of steel for your bolts and make sure it is relevant to your desired outcome. Generally speaking, A2 stainless steel and bright zinc coated bolts are more than adequate for most DIY and trade projects. Our most popular bolt sizes are the M6 and M8 full thread bolts in bright zinc plate.

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