First, a key point: **In the vast majority of cases, high-strength bolt fractures are not simply due to “insufficient strength causing the bolt to snap.” The primary causes to investigate are hydrogen embrittlement, stress concentration, heat treatment defects, excessive assembly loads, fatigue, and material defects. When a bolt head cracks, the crack often originates at the transition radius between the head and shank, at the location of markings or engravings, or on the head’s bearing surface. **

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> High-strength bolts (Grades 8.8/10.9/12.9) have high strength but lower ductility, making them more sensitive to defects, stress, hydrogen, and manufacturing processes; they are more prone to brittle fracture than standard Grade 4.8 bolts.

## I. Stress Concentration Fracture (Most Common; High-Incidence Areas on Bolt Heads)

Typical crack initiation points: **the R fillet at the junction of the head and shank, the location of the head marking, and the edge of the lower bearing surface of the head**

1. **Transition fillet R is too small / rough machining with tool marks**

The transition zone between the bolt head and shank is a natural stress peak area. Standards require a sufficient radius; if the manufacturer’s turning or upsetting processes result in an R that is too small, or if burrs, folds, or scratches are present, the stress concentration factor will multiply during tightening. Since high-strength materials inherently have poor ductility, cracks will initiate directly from these areas, manifesting as circumferential cracking of the head or head detachment.

2. **Head markings (stamped grade and logo) pressed too deeply**

Many 10.9 and 12.9 bolts fracture precisely at the stamped mark location: the stamping process creates microcracks in the surface layer combined with stress concentration; these defects persist after heat treatment and cause the bolt to split open at the stamped mark when subjected to load.

3. **Uneven head contact surface with dents or indentations**

Localized point contact occurs on the contact surface, concentrating the tightening force into a small area and causing the head to crack under pressure.

## II. Hydrogen Embrittlement Fracture (The Number One Killer of High-Strength Bolts; Be Especially Vigilant with 10.9 and Higher Grades)

✅ Typical Characteristics: **Fracture occurs without obvious plastic deformation; the fracture surface is smooth and brittle. Fracture does not necessarily occur immediately upon tightening; delayed fracture may occur hours to days after assembly; Most commonly occurs in high-strength bolts that have undergone electroplating (galvanizing, colored zinc), acid pickling, or blackening pretreatments**

Mechanism:

During electroplating and acid pickling, hydrogen ions penetrate the steel; the high-strength martensitic microstructure (Grades 10.9, 12.9) traps hydrogen atoms, which accumulate under tensile stress, reducing atomic bonding strength and leading to gradual crack propagation.

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> Key Point: **For bolts of Grade 10.9 or higher, hydrogen removal baking is mandatory after galvanizing; if the manufacturer omits this step or the baking temperature/duration is insufficient, the risk of hydrogen embrittlement is extremely high.**

> Hydrogen embrittlement fractures mostly occur at the R position of the head and shank, appearing externally as a crack in the bolt head.

Distinction: Hydrogen embrittlement is a form of **delayed brittle fracture**; it does not result in immediate breakage upon tightening.

## III. Heat Treatment Defects (Inherent Issues Within the Material)

The mechanical properties of high-strength bolts rely entirely on quenching and tempering:

1. **Quenching Cracks**: Excessively rapid cooling during quenching, poor quenching hardenability of the steel, or severe banding in the raw material can lead to the formation of microscopic cracks invisible to the naked eye during the heat treatment stage; these cracks then propagate and rupture under the stress of subsequent tightening. Cracks often appear at the head.

2. **Insufficient Tempering (Tempering Brittleness)**: If the tempering temperature is too low, the hardness exceeds the specified limit (e.g., Grade 12.9 hardening to HRC 42 or higher), causing the material’s toughness to drop sharply and become brittle;

3. **Overheating / Over-burning**: When the head is hot-forged, if the heating temperature is too high, it results in coarse grain structure and a loss of toughness, causing the bolt to crack under even slight stress;

4. **Decarburization**: Decarburization of the head surface results in insufficient strength in the surface layer, creating a source of surface cracks.

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> Key inspection points: Hardness testing, metallographic examination, and SEM scanning of fracture surfaces to examine grain structure and decarburized layers.

## IV. Assembly-Related Causes (Overload, Off-Center Loading, Incorrect Tightening Procedures)

1. **Excessive Torque, Tightening Beyond Yield Strength* *

Tightening excessively with a torque wrench beyond specifications subjects the bolt to tensile forces exceeding its yield strength; high-strength bolts have virtually no margin for error after yielding and will fracture immediately.

Note: Many high-strength bolts are installed based on **preload control**, not by tightening to feel; deviations in the torque coefficient or improper lubrication can both cause the actual preload to exceed specifications.

2. **Eccentric Tightening and Off-Center Loading**

When contact surfaces are not parallel, washers are skewed, or hole positions are misaligned, the bolt head is subjected to additional bending stress; the combination of bending and axial tensile forces causes the stress at the R-point of the head to far exceed that of simple tensile stress, making it extremely prone to cracking. This is very common in flange and bracket assemblies.

3. **Repeated Tightening and Frequent Disassembly/Reassembly**

High-strength bolts undergo plastic deformation after preloading. Many 10.9/12.9 bolts **are not permitted for reuse**; retightening will cause them to fracture immediately.

## V. Raw Material and Forging Defects (Cold Heading Defects)

The bolt head is formed by cold heading:

- The raw material bar contains inclusions, delamination, shrinkage cavities, and subsurface gas bubbles;

- Poor metal flow during cold heading results in **head folding (heading creases)**—this defect occurs at the base of the head and shank, where a closed crack forms internally; it may not be visible externally but will open up under load;

- Excessive sulfur and phosphorus content in the raw material leads to hot brittleness or cold brittleness.

## VI. Fatigue Fracture (Alternating Load Conditions)

If equipment is subject to vibration or repeatedly varying loads:

Cracks slowly propagate from stress concentration points; they are invisible in the early stages but cause the head to suddenly break after a certain number of cycles. Fatigue fracture surfaces exhibit characteristic shell-like arcs. This is very common in high-strength bolts used in wind turbines and vibrating equipment.

## VII. Corrosion-Induced Cracking (Stress Corrosion Cracking, SCC)

In humid, chloride-ion (coastal, salt spray), or acidic/alkaline environments, the combined effect of tensile stress and corrosive media leads to stress corrosion cracking; once the galvanized coating is damaged and the base metal comes into contact with the corrosive medium, the risk for Grade 10.9 bolts increases significantly.

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> Difference from hydrogen embrittlement: Hydrogen embrittlement results from hydrogen permeation during electroplating; stress corrosion stems from corrosion in the service environment.

# ✅ Quick On-Site Preliminary Assessment Methods (Practical)

1. Examine deformation: If the fracture surface is bright and smooth with no elongation → brittle fracture (primarily suspect hydrogen embrittlement, heat treatment, or stress concentration); if there is obvious necking or elongation → overload tensile failure;

2. Examine the timing of failure: Failed after being installed and left for some time → Strong suspicion of hydrogen embrittlement; Failed the moment it was tightened → Excessive torque, R-defects, or off-center loading; Failed gradually under vibration → Fatigue;

3. Examine crack location: Crack originates at the stamp mark → Stress concentration from the stamp mark; originates at the fillet between the head and shank → Insufficient R-radius / cold heading folding / high-risk area for hydrogen embrittlement;

4. Trace the manufacturing process: Was galvanizing or acid pickling performed? Was hydrogen removal carried out? Is the bolt grade ≥10.9?

# ✅ Summary of Corresponding Improvement Measures

1. Drawings & Production: Increase the transition radius (R) between the head and shank; use a shallow stamping depth and avoid high-stress areas; control the cold heading process to eliminate folding;

2. Surface Treatment: For Grade 10.9 and above, perform **hydrogen removal baking at 200–230°C** after galvanizing; prioritize low-hydrogen processes such as Dacromet or thermal zinc diffusion as alternatives to electrogalvanizing;

3. Heat Treatment: Strictly control the hardness range to avoid excessive hardness; perform metallographic inspections for decarburization, grain structure, and quenching cracks;

4. Assembly: Calibrate torque, control preload, ensure parallel contact surfaces, and avoid off-center loading; perform torque coefficient testing on critical connections; do not reuse high-strength bolts in accordance with specifications;

5. Inspection: Conduct random incoming inspections for hardness, metallographic examination, hydrogen embrittlement verification (delayed fracture test), and non-destructive testing (magnetic particle testing, MT, suitable for detecting near-surface cracks on bolt heads).