12.9 Grade Steel vs Stainless Steel

12.9 Grade Steel vs Stainless Steel

In this article, we take a look at the considerations when comparing two fundamentally different types of bolts - namely, 12.9 grade alloy steel and stainless steel. If you're interested in whether or not stainless steel bolts are interchangeable with high-tensile fasteners, this is the article for you. We look at this issue, as well as the key distinctions between these types of fasteners.

Metric Vs Imperial Note

Just a reminder that different countries and industries use different systems (or standards) to describe fasteners. The main systems used are metric and imperial. These systems use different terminology and measurement methods - it's important to know which system you're using in order to correctly compare fastener capabilities.

Some of the key differences to be aware of include:

Measured In:
Metric: Mpa (Megapascals)
Imperial: PSI (Pounds Per Square Inch)

Grade Names:
Metric: Grades
Imperial: Product Class

Metric: Range: Product Class 4.6 for Mild Steel through to Class 12.9 for High Tensile Steel
Imperial: Range: Grade 5 for Mild Steel through to Grade 10 for High Tensile Steel

Thread Pitches:
Imperial: Unified National Fine Thread (UNF) and Unified National Course Thread (UNC)
Metric: Metric Course Thread (M) and Metric Fine Thread (MF) 

If you're looking to substitute a fastener, you will need to identify the standard used by the original fastener (metric or imperial) in order to make an accurate comparison.

Can You Substitute a Stainless Steel Fastener for a High-Tensile Fastener?

It depends. 'General' stainless steel fasteners such as A2 (304) or A4 70/80 (316) are for construction and are produced within the range of classes 6-8. These fasteners are unsuitable for high tensile applications - they don't have the required tensile strength. In fact, they may come in at less than half the tensile strength of a high-tensile fastener at the top of the scale (product class 12.9, metric). If you substitute a general stainless steel fastener for a high tensile fastener, it will not be able to tolerate the required forces. 

However, there are also exceptions to this rule - niche high-tensile fasteners which are also made from stainless steel. While these are available, there are some limitations. They will come at a higher cost associated with the merging of these two attributes (corrosion resistance and high tensile capability). This puts them 'outside' of the general fastener landscape. Additionally due to relatively limited production, these fasteners may not be available in the appropriate style of the required application. For example, some vehicle bolts require allen key heads, whereas the product may only be available in hex heads.

Why Would Someone Want to Substitute Stainless Steel for High Tensile?

Stainless steel fasteners have two attributes which make them popular - enhanced corrosion resistance and the stainless steel look. These are both desirable attributes, which is possibly what leads people to wonder whether they can be applied in a high-tensile fastener application. However, if the fasteners doesn't have sufficient strength, the integrity of the application will be compromised by such inclusions.

The Problem with Using Inadequate Fasteners

Substituting high-tensile fasteners with general stainless steel fasteners can create a range of problems. For example, in automobile construction, high-tensile fasteners are used for all structural fastening. Replacing them with an inadequate substitute will compromise the vehicle - not a great idea!

In Conclusion

In conclusion, when looking to replace an existing fastener with a new one, it's essential to meet the required mechanical properties of the original fastener as described by the manufacturer and local codes. Just because a bolt is the same size doesn't make it a match.

High tensile fasteners are specifically designed to meet the needs of high tensile applications, so swapping them out for an inferior (from the point of view of tensile strength) stainless steel fastener can only compromise the vehicle. If you're interested in learning more or ordering high-tensile or stainless steel fasteners, please contact our team.