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