What is hex bolts grade 12.9?

Grade 12.9 hex bolts are the highest strength class for metric steel bolts, made from alloy steel and quench-tempered to achieve their exceptional hardness . They are designed for the most demanding high-stress applications .

Here are the critical mechanical properties you need to know :

· Tensile Strength: ≥ 1220 MPa (the maximum stress before breaking)
· Yield Strength: ≥ 1100 MPa (the stress point where permanent deformation begins)
· Proof Strength: 970 MPa
· Core Hardness: Typically 39-44 HRC (Rockwell C)

· Material & Marking: Made from alloy steel (e.g., chromium molybdenum steel ), identified by a "12.9" head marking .
· Application: Used in automotive engines, heavy machinery, and structural connections where maximum strength is vital .
· Warning: Grade 12.9 bolts are susceptible to brittle failure and stress corrosion cracking, so they are not suitable for high-vibration or corrosive environments unless specifically designed for it . 

Producing Grade 12.9 hex bolts is a multi-step precision manufacturing process. It relies on specific alloy steels and carefully controlled heat treatment to achieve the required ultra-high strength (over 1220 MPa) without making the bolts brittle .

Here is the step-by-step breakdown of the production process:

1. Material Selection

The foundation is high-quality alloy steel. Common choices include:

· SCM435 / SCM440 (Chromium-Molybdenum steels) 
· 42CrMo 
· Advanced options: For even higher performance, micro-alloyed steels with Vanadium (V) or Niobium (Nb) are used to improve resistance to delayed fracture .

2. Forming the Head (Cold or Hot Forging)

The bolt head is formed from steel wire:

· Cold Heading: Used for most high-volume production. Wire is fed into a machine at room temperature and struck by dies to form the head .
· Hot Forging: Used for very large bolts or complex shapes. The steel is heated until red-hot and then pressed into shape .

3. Thread Rolling

Threads are rolled, not cut. The bolt is squeezed between two hard dies to displace the material and form the thread. This work-hardens the thread root, significantly improving fatigue resistance .

4. Heat Treatment (Quenching and Tempering)

This is the most critical step for achieving Grade 12.9 properties. It consists of two stages:

· Quenching (Hardening): The bolt is heated to a specific temperature (typically 880-940°C ) to transform its structure to austenite. It is then rapidly cooled (quenched) in oil or a specialized polymer solution . This locks the structure into a very hard but brittle form called martensite.
· Tempering: The hardened bolt is re-heated to a lower temperature (typically 500-600°C ). This step reduces brittleness, relieves internal stresses, and develops the final mechanical properties (strength, toughness, and hardness of 39-44 HRC) .

5. Surface Finishing

A protective coating is applied to prevent corrosion. Common finishes include black oxide or zinc plating .