Heat Treatment of High‑Strength Steel Fasteners (8.8, 10.9, 12.9)
Strength classes such as 8.8, 10.9 and 12.9 are achieved not only through chemical composition but through controlled heat treatment. This process transforms the steel’s microstructure to deliver the required balance of strength, hardness and ductility.
Most high‑strength fasteners are formed in a soft condition and then hardened and tempered to reach their final mechanical properties. Without heat treatment, the steel would be too soft for structural or engineering applications.
The Heat Treatment Process
The process typically involves two key stages:
- Quenching – the fastener is heated to a high temperature and rapidly cooled. This forms a hard martensitic structure with very high strength but low ductility.
- Tempering – the quenched fastener is reheated to a lower temperature to restore ductility and toughness while maintaining high strength.
Heat Treatment by Strength Class
Medium‑carbon alloy steel, quenched and tempered to achieve a typical tensile strength of around 800 MPa. Offers a strong balance of strength and ductility for structural and general engineering use.
Higher alloy content (chromium, molybdenum) allows deeper hardening during quenching. Tempered to achieve higher tensile strength (approx. 1,000 MPa) with improved fatigue resistance.
High‑carbon, high‑alloy steel fully quenched and tempered to very high strength (approx. 1,200 MPa). Used in demanding mechanical and precision assemblies where maximum clamping force is required.
Understanding Strength Class Numbers (8.8, 10.9, 12.9)
The two numbers in a fastener strength class describe its tensile strength and yield strength after heat treatment:
-
First number × 100 = Tensile strength (MPa)
This is the maximum stress the fastener can withstand before breaking.
• 8.8 → ~800 MPa
• 10.9 → ~1,000 MPa
• 12.9 → ~1,200 MPa -
Second number ÷ 10 = Yield ratio
This defines how much of the tensile strength is available before permanent deformation begins.
• 8.8 → 0.8 (80% of tensile strength)
• 10.9 → 0.9 (90% of tensile strength)
• 12.9 → 0.9 (90% of tensile strength)
Note: Strength values are shown in MPa (megapascals), which is the standard engineering unit for stress. 1 MPa is equal to 1 N/mm², meaning one newton of force applied to one square millimetre of area.
Hydrogen Embrittlement Warning
High‑strength fasteners (especially 10.9 and 12.9) are vulnerable to hydrogen embrittlement when exposed to electroplating or acid pickling. Hydrogen can diffuse into the hardened steel and cause delayed brittle failure under load.
An explanation of hydrogen embrittlement and plating risks is provided on the Finishes & Coatings page.