High-strength bolts play an essential role in manufacturing. Here, we explain their types, advantages and disadvantages, and the critical points you should consider when selecting them.

What Are High-Strength Bolts?

High-strength bolts are bolts that, as the name suggests, have higher strength than standard bolts. Common commercially available bolts have strength grades ranging from 4.8 to 12.9, with higher numbers indicating higher strength. For example, a grade 4.8 bolt has a tensile strength of 400 N/mm², while a grade 12.9 bolt has a tensile strength of 1,200 N/mm².

Benefits of High-Strength Bolts

• Improved fastening force and prevention of loosening
High clamping force and precise threading help prevent loosening.
•More compact and lightweight designs
Because of their high strength, smaller bolt sizes can be used to achieve the same fastening force, contributing to overall product compactness and weight reduction.
•High reliability and safety
Able to withstand large loads and impacts, improving the sealing and rigidity of connections.

Grade 12.9 Hex Bolts and Grade 14.9 Hex Socket Cap Bolts

Grade 12.9 Hex Bolts and Grade 14.9 Hex Socket Cap Bolts

• Grade 12.9 bolts
Extremely strong and used in fields requiring high safety standards, such as aerospace, automotive racing, industrial machinery, construction, high-speed rail, engineering projects, and military applications. Commonly made from SCM (chromium-molybdenum) steel and often feature a hex socket. While most hex bolts are up to grade 10.9, some sizes are available in grade 12.9.

• Grade 14.9 ultra-high-strength hex socket cap bolts
Also known as “super high-strength bolts,” these are developed collaboratively by Japanese manufacturers with advanced technology. They have a tensile strength of 1,400 N/mm². Hex socket bolts have smaller heads and require less tool clearance, making them suitable for tight spaces and compact designs.

Market Situation and Customized Production of High-Strength Bolts

Traditionally, large-diameter hex bolts and hex socket head bolts (cap bolts) exceeding M20 were mostly classified as strength grade 10.9. As a result, large-diameter bolts with strength grade 12.9 were treated as special-order products, leading to higher costs.

However, in recent years, more manufacturers have begun producing strength grade 12.9 bolts in larger nominal diameters such as M22, M24, and M30. The main reasons why most commercially available large-diameter bolts are limited to grade 10.9 include the difficulty and higher cost of achieving higher strength in larger sizes (due to heat treatment and cold working), as well as the increased importance of managing hydrogen embrittlement risks when post-processing with plating.

Electroplating Must Not Be Applied to High-Strength Bolts!

One of the most important precautions when selecting high-strength bolts is whether electroplating is permissible. As a general rule, bolts of strength grade 12.9 and above should not be electroplated.

What is Hydrogen Embrittlement?
Hydrogen embrittlement is a phenomenon where steel absorbs hydrogen during the manufacturing process or surface treatment, leading to reduced toughness (ductility) and increased brittleness. It has been reported to occur more frequently in high-strength steels (high-tensile steels).

What is Delayed Fracture?
Delayed fracture refers to the sudden, unpredictable failure of a bolt under a constant tensile load due to hydrogen embrittlement. Even if no issues occur at the time of tightening, the bolt may break several hours or even days later, making it extremely dangerous.

While bolt failures can result from various causes such as insufficient strength or excessive tightening, selecting and using high-strength bolts appropriately enables more compact, lightweight designs while ensuring high safety. However, the risk of delayed fracture due to hydrogen embrittlement must always be considered.


If you have concerns about adopting high-strength bolts, or if you require bolts of even greater strength, please contact Hanshin Neji (Thailand). We provide fastening solutions tailored to your application and environment, supporting safe and reliable manufacturing.