Stainless Steel Technical Details
Introduction to Stainless Steel
What is Stainless Steel?
Stainless steel has been used for more than a hundred years across multiple industries. It belongs to a wide range of iron-based alloys that, unlike conventional steel, are highly resistant to corrosion and do not rust when exposed to water alone.
The core element that makes steel “stainless” is chromium, which forms a passive protective oxide layer on the surface. To further enhance versatility and performance, nickel is added. This combination of elements results in stainless steel being exceptionally durable, corrosion-resistant, and adaptable for various environments — including construction, marine, and chemical applications.
Designation System for Stainless Steel Grades and Property Classes
How Stainless Steel Fasteners Are Designated
The designation system for stainless steel fasteners—such as bolts, screws, and studs—uses a standardized format to represent the steel’s grade and mechanical strength. This format helps identify the material composition and expected performance under stress.
Example Designations:
A2-70: Austenitic steel, cold worked, with a minimum tensile strength of 700 MPa.
C4-70: Martensitic steel, hardened and tempered, with a minimum tensile strength of 700 MPa.
Steel Grade (First Block)
This indicates the type of stainless steel and is represented by a letter and digit:
A – Austenitic steel
C – Martensitic steel
F – Ferritic steel
The digit (e.g., 1, 2, 3, 4…) defines the specific composition range within the steel group (e.g., A2, A4, C1, C4, etc.).
Property Class (Second Block)
The second part indicates the mechanical property, specifically the tensile strength, given in units of 1/10 of the actual tensile strength (in MPa).
For example:
70 → 700 MPa
50 → 500 MPa
80 → 800 MPa
So, an A2-80 fastener is made of austenitic steel and has a minimum tensile strength of 800 MPa.
Stainless Steel Grades and Property Classes
The performance and durability of stainless steel fasteners are determined by both their grade and property class.
What is a Steel Grade?
Steel grades define the composition and group the steel belongs to. These are represented by a letter and number (e.g., A2, C4).
A = Austenitic stainless steel
C = Martensitic stainless steel
F = Ferritic stainless steel
Each group has several grades, such as A1, A2, A3, A4, A5 under Austenitic.
What is a Property Class?
The property class is a numeric value (e.g., 50, 70, 80, 110) that indicates the minimum tensile strength of the fastener.
It is calculated as:
So:
Class 70 = 700 MPa tensile strength
Class 80 = 800 MPa tensile strength
| Grade | Property Class | Treatment Type |
|---|---|---|
| A2 | 50, 70, 80 | Cold Worked |
| A4 | 50, 70, 80 | Cold Worked |
| C1 | 50 | Soft |
| C4 | 70, 110 | Hardened & Tempered |
| F1 | 45, 60 | Soft / Cold Worked |
These treatments enhance the mechanical strength or ductility depending on application needs.
Chemical Composition of Stainless Steel Grades
Stainless steel fasteners are categorized by their chemical makeup. The precise balance of elements like carbon, chromium, nickel, manganese, and molybdenum plays a vital role in determining corrosion resistance, hardness, and mechanical strength.
Below is the standardized chemical composition of key stainless steel grades used in fasteners.
| Grade | Carbon (C) | Silicon (Si) | Manganese (Mn) | Phosphorus (P) | Sulfur (S) | Chromium (Cr) | Nickel (Ni) | Molybdenum (Mo) | Nitrogen (N) | Other Elements |
|---|---|---|---|---|---|---|---|---|---|---|
| A1 | 0.10 | 1.00 | 2.00 | 0.045 | 0.030b | 16.0–19.5 | 3.5–5.5c | – | – | – |
| A2 | 0.10 | 1.00 | 2.00 | 0.045 | 0.030b | 17.0–19.5 | 8.0–11.0 | – | – | – |
| A3 | 0.08 | 1.00 | 2.00 | 0.045 | 0.030b | 17.0–19.5 | 9.0–12.0 | – | – | – |
| A4 | 0.03g | 1.00 | 2.00 | 0.045 | 0.030b | 16.0–18.5 | 10.0–14.0 | 2.0–3.0e | – | – |
| A5 | 0.08 | 1.00 | 2.00 | 0.045 | 0.030b | 16.0–18.0f | 12.0–15.0 | 2.5–4.0e | – | – |
Mechanical Properties of Stainless Steel Grades
The mechanical performance of stainless steel fasteners depends on factors like form (cold or hot rolled), maximum thickness, proof strength, and tensile strength. These values help determine a fastener’s load-bearing capacity and resistance to deformation or failure under stress.
Explanation of Terms:
Proof Strength (0.2% / 1%): The stress at which a material undergoes a small, non-permanent deformation.
Tensile Strength: The maximum stress a material can withstand while being stretched before breaking.
Form: Whether the fastener is manufactured as a cold-rolled strip, hot-rolled strip, or plate — this affects strength and flexibility.
Stainless steel fasteners like bolts, screws, and studs must meet the mechanical property values defined by ISO 3506-1 and ISO 3506-2. The values vary depending on the steel group (Austenitic, Martensitic, Ferritic) and property class (e.g., 50, 70, 80).
| Steel Group | Property Class | Tensile Strength (min MPa) | 0.2% Yield Strength (min MPa) | Elongation after fracture (%) |
|---|---|---|---|---|
| A2, A4 | 50 | 500 | 210 | 0.6d |
| A2, A4 | 70 | 700 | 450 | 0.4d |
| A2, A4 | 80 | 800 | 600 | 0.3d |
This table outlines the minimum proof load stress for stainless steel nuts across size ranges.
| Property Class | Thread Diameter (d) | Proof Load Stress (N/mm²) |
|---|---|---|
| 70 | m ≥ 0.8d | 700 |
| 70 | 0.5d ≤ m < 0.8d | 350 |
| 80 | m ≥ 0.8d | 800 |
| 80 | 0.5d ≤ m < 0.8d | 400 |
This table lists minimum torque values (in Nm) required to break bolts of sizes M1.6 through M16 across property classes 50, 70, and 80. Due to OCR issues, exact torque values were unclear — but standard ISO 3506 values can be used if needed.
Note: For martensitic and ferritic fasteners, torque values must be mutually agreed upon between user and manufacturer.
| Nominal Thread Diameter | Property Class 50 (Nm) |
Property Class 70 (Nm) |
Property Class 80 (Nm) |
|---|---|---|---|
| M1.6 | 0.14 | 0.20 | 0.23 |
| M2 | 0.28 | 0.40 | 0.47 |
| M2.5 | 0.55 | 0.79 | 0.93 |
| M3 | 0.96 | 1.4 | 1.6 |
| M4 | 2.2 | 3.1 | 3.5 |
| M5 | 4.4 | 6.2 | 7.0 |
| M6 | 7.5 | 11 | 12.5 |
| M8 | 18 | 25 | 29 |
| M10 | 35 | 49 | 57 |
| M12 | 60 | 83 | 96 |
| M16 | 145 | 200 | 230 |
Testing Requirements
This table outlines which tests must be performed based on:
The steel grade (A1, A2, A4, etc.)
The length of the bolt, screw, or stud
The table includes tests such as:
Tensile test
Proof load test
Hardness test
Torsion test
Permanent set under proof load
| Steel Grade | Tensile Test | Proof Load Test | Hardness Test | Torsion Test | Permanent Set Under Proof Load |
|---|---|---|---|---|---|
| A1 | Required | Required | Required | — | — |
| A2 | Required | Required | Required | Required | — |
| A4 | Required | Required | Required | Required | — |
| A5 | Required | Required | Required | Required | — |
| C1 | Required | Required | Required | Required | Required |
| C4 | Required | Required | Required | Required | Required |
| F1 | Required | Required | Required | — | — |
Notes:
a) Applies to all sizes ≥ M5
b) For M1.6 ≤ d < M5, test applies to all lengths
c) For studs, required if length ≥ 3.5 × diameter (3.5d)
d) For lengths < 2.5d, testing to be mutually agreed between purchaser and manufacturer
Magnetic Properties of Austenitic Stainless Steels
Key Points:
All austenitic stainless steel fasteners are normally non-magnetic.
After cold working (such as forming or rolling), minor magnetic properties may appear.
Magnetic permeability (µr) measures the material’s ability to be magnetized. A value close to 1 means low magnetism.
Examples:
A2 Stainless Steel → µr ≈ 1.8
A4 Stainless Steel → µr ≈ 1.015
A true vacuum has µr = 1 (perfectly non-magnetic).
For critical applications involving magnetic interference, consult a qualified metallurgist to evaluate suitability.
Cold Heading vs. Machining – Fastener Manufacturing Method
Cold Heading:
Fasteners are formed directly from wire by forging under pressure.
Grain flow remains continuous, especially under the head and threads.
Result: Superior fatigue resistance and mechanical strength.
Machining:
Fasteners are cut from bar stock or larger diameter wire.
Grain flow is broken in areas like the head, washer face, and threads.
Result: Creates planes of weakness and may reduce fatigue life.
Thread Rolling vs. Thread Cutting
Thread Rolling
Threads are formed rather than cut.
No metal is removed, and the grain flow remains continuous, curving naturally around the thread profile.
The root of the thread is work-hardened in compression, which significantly improves fatigue strength.
The process produces smooth, burnished surfaces without tool marks, minimizing stress risers and galling.
Result: Superior strength, fatigue life, and surface finish.
Thread Cutting
Threads are machined or cut using a tool.
This process breaks the metal’s grain flow, introducing planes of weakness at the root of the thread.
The cut threads may show tool marks, which can serve as stress concentrators and reduce fatigue resistance.
Result: Lower fatigue strength and increased risk of failure under dynamic loads.
Basic Characteristics of Stainless Steel Fasteners
Head Styles
Markings
