A Practical Engineering Guide to Correct Fastener Selection in Australia
Bolts are one of the most common engineered components on any project — and also one of the most misunderstood.
In drawings they appear as a simple note:
M16 – 8.8 – GALV
Yet behind that small call-out sits structural capacity, fatigue life, corrosion resistance, inspection compliance, and legal responsibility.
Many engineering failures do not occur because a beam was undersized or a calculation was incorrect.
They occur because the wrong fastener type was selected for the application.
This article explains:
- Bolt and nut property classes
- Where each class should be used
- Carbon steel vs stainless steel
- Coatings and environment suitability
- Structural vs mechanical bolting
- Australian Standards governing fasteners
- How to review and challenge incorrect selections — especially when mentoring graduate engineers
1. The Three Different Worlds of Bolting
Most confusion exists because people think a bolt is simply a stronger or weaker version of the same item.
In reality, bolts exist in three different engineering systems:
| System | Purpose | Governing Standards |
|---|---|---|
| General Mechanical Fastening | Holding components together | ISO / AS 1110 / AS 4291 |
| Structural Bolting | Load transfer between steel members | AS/NZS 1252 / AS 4100 |
| Corrosion Resistant Fastening | Survive environment | Stainless / coatings standards |
Using a bolt from the wrong system often creates hidden failures.
2. Bolt Property Classes (Metric)
Metric bolts are marked with numbers such as 4.6, 8.8, 10.9, 12.9
These numbers define material strength.
What the Numbers Mean
First number → Ultimate tensile strength (×100 MPa)
Second number → Yield ratio
Example:
8.8 bolt
800 MPa tensile strength
Yields at 80% = 640 MPa
Typical Bolt Classes and Their Uses
| Class | Strength Level | Typical Applications |
|---|---|---|
| 4.6 | Low | Light brackets, sheet metal |
| 4.8 | Low–medium | General hardware |
| 5.8 | Medium | Automotive covers |
| 6.8 | Medium | Machinery guards |
| 8.8 | High tensile | General engineering & structural connections |
| 9.8 | Higher tensile | Automotive mechanical |
| 10.9 | Very high tensile | Mining equipment, heavy plant |
| 12.9 | Ultra high tensile | Tooling, precision machinery |
Important Engineering Concept
A stronger bolt is not always better.
Higher strength bolts:
- are less ductile
- tolerate less misalignment
- fatigue faster in bending
Many failures occur when 12.9 bolts are used where 8.8 bolts were intended.
3. Nut Property Classes
Nuts are graded differently.
They must match the bolt strength.
| Nut Class | Suitable Bolt |
|---|---|
| 4 | 4.6 |
| 5 | 5.8 |
| 6 | 6.8 |
| 8 | 8.8 |
| 9 | 9.8 |
| 10 | 10.9 |
| 12 | 12.9 |
Critical Rule
Nut class must be equal or higher than bolt class first number
If not, the joint will strip before correct preload is reached.
4. Carbon Steel vs Stainless Steel
Many installations choose stainless assuming it is “better”.
It is not stronger — it is more corrosion resistant.
Mechanical Comparison
| Property | High Tensile Carbon Steel | Stainless Steel |
|---|---|---|
| Strength | High | Medium |
| Fatigue resistance | Good | Lower |
| Vibration resistance | Good | Poorer |
| Corrosion resistance | Depends on coating | Excellent |
| Galling risk | Very low | High |
| Torque capacity | High | Limited |
Stainless Grades
| Grade | Equivalent Strength | Typical Use |
|---|---|---|
| A2-50 | ~5.8 | General hardware |
| A2-70 | ~7.0 | Outdoor equipment |
| A4-80 | ~8.8 tensile | Marine / chemical |
Important
Stainless steel often fails in structural joints due to:
- lower yield strength
- thread galling
- relaxation under load
5. Coatings and Environment Suitability
Carbon steel requires corrosion protection.
| Coating | Environment |
|---|---|
| Black oxide | Indoor machinery |
| Zinc plated | Indoor dry |
| Zinc passivate | Workshop conditions |
| Hot dip galvanised | Outdoor structural |
| Mechanical galvanised | Structural bolting |
| Dacromet / Geomet | Mining & heavy corrosion |
Engineering Impact of Coatings
Coatings change friction.
Friction changes preload.
Therefore torque charts must match coating type.
Incorrect torque values are one of the most common installation errors.
6. Structural Bolting vs Mechanical Bolting
These must never be confused.
Mechanical Bolting
Purpose: hold parts together
Failure mode: loosening
Structural Bolting
Purpose: transfer load through friction or bearing
Failure mode: structural collapse
Structural bolts require:
- certified assemblies
- controlled tightening method
- inspection records
General hardware bolts must never be substituted.
7. Storage and Handling Requirements
Fasteners can degrade before use.
Problems Caused by Poor Storage
- Coating breakdown
- Hydrogen embrittlement risk
- Rust under galvanising
- Lost certification traceability
- Incorrect torque performance
Recommended Storage Practices
Environment
Dry
Covered
Off concrete
Stable temperature
Handling
Keep manufacturer packaging
Do not mix batches
Record heat numbers
Stainless Steel
Must be isolated from carbon steel contamination.
Carbon particles embed → rust later appears
8. Australian Standards for Fasteners
Below is a consolidated list relevant to Australian engineering practice.
Mechanical Properties
AS/NZS 4291.1 — Mechanical properties of bolts, screws and studs
AS/NZS 4291.2 — Mechanical properties of nuts
ISO 898-1 / ISO 898-2 — Referenced strength properties
ISO 3506 — Stainless steel fasteners
Dimensions & Threads
AS 1110 — Metric hex bolts & screws
AS 1111 — Metric fasteners
AS 1112 — Hexagon nuts
AS 1275 — Metric screw threads
AS 1721 — General purpose metric threads
Structural Bolting
AS/NZS 1252 — High strength structural bolting assemblies
AS 4100 — Steel structures design
AS/NZS 5131 — Structural steel fabrication & erection
Corrosion Protection
AS/NZS 1214 — Galvanised coatings on threaded fasteners
AS/NZS 4680 — Hot dip galvanising
AS 2312.2 — Corrosion protection guide
AS 1897 — Electroplated coatings
Locking and Reliability
AS 4145.2 — Locking devices for fasteners
9. Mentoring the Graduate Engineer
What To Do When the Selection Is Wrong
One of the responsibilities of senior engineers is not just checking work — but teaching judgement.
A graduate will often select bolts by:
- copying an old drawing
- choosing stainless for safety
- choosing highest strength available
- assuming galvanised means structural
Rather than correcting immediately, guide the reasoning.
Questions That Help Them Learn
Instead of saying “that is wrong”, ask:
What load path is the bolt carrying?
Is it clamping, locating, or supporting?
What failure mode are we preventing?
Slip, fatigue, shear, corrosion, loosening?
Is the environment or the force governing selection?
Does the standard require a certified assembly?
What inspection method applies?
The Goal
Teach that engineering is not selecting a stronger component —
it is selecting the correct component for the failure mode.
Conclusion
Fasteners are engineered components.
Correct selection depends on understanding:
- strength class
- application type
- environment
- installation method
- applicable standards
Most bolted joint failures occur not from calculation error, but from incorrect assumptions about what the bolt is meant to do.
Engineering quality is achieved when design intent matches real behaviour.
