A procurement enquiry may describe a component as “Grade 350”, while a supplier offers a bolt in “Class 4.8”. Because both designations contain numbers associated with strength, it is tempting to compare them directly or treat one as an alternative to the other.
That would be a mistake.
AS/NZS 3678 Grade 350 and ISO 898-1 Property Class 4.8 belong to different classification systems and apply to different products.
- Grade 350 classifies structural steel plate.
- Property Class 4.8 classifies the mechanical performance of a finished bolt, screw or stud.
They may appear in the same connection, but they do not perform the same function and cannot be substituted on the basis of tensile or yield strength alone.
1. What is AS/NZS 3678 Grade 350?
AS/NZS 3678 covers hot-rolled structural steel plates, floorplates and slabs. Grade 350 therefore identifies the material properties of a structural steel product before it is fabricated into a component such as a plate washer, bracket, baseplate or connection plate.
BlueScope describes its AS/NZS 3678-350 product as a high-strength structural steel plate with good weldability and formability. The mechanical requirements vary with plate thickness.
For example, a 10 mm Grade 350 plate has the following minimum properties:
| Property | AS/NZS 3678 Grade 350, 10 mm plate |
|---|---|
| Minimum yield strength | 360 MPa |
| Minimum tensile strength | 450 MPa |
| Minimum elongation | 20% |
| Product form | Structural plate |
| Governing standard | AS/NZS 3678 |
The grade name is nominal. It does not mean that every thickness has exactly 350 MPa minimum yield strength. In the BlueScope product range, the minimum yield strength is:
| Plate thickness | Minimum yield strength |
|---|---|
| 5–12 mm | 360 MPa |
| Over 12–20 mm | 350 MPa |
| Over 20–80 mm | 340 MPa |
| Over 80–100 mm | 330 MPa |
This thickness dependency is important when reviewing drawings, material certificates and proposed substitutions.
Grade 350 tells procurement what structural plate material must be purchased. It does not define:
- bolt thread dimensions;
- fastener proof load;
- head soundness;
- thread decarburisation;
- fastener marking;
- nut compatibility; or
- the tensile capacity of a finished threaded component.
It is therefore a material grade, not a complete fastener specification.
2. What is ISO 898-1 Property Class 4.8?
ISO 898-1 specifies mechanical and physical properties for finished carbon-steel and alloy-steel bolts, screws and studs.
The current ISO 898-1:2013 edition applies to qualifying metric fasteners with coarse threads from M1.6 to M39 and fine-pitch threads from M8 × 1 to M39 × 3. ISO also states that the standard does not establish requirements for weldability, corrosion resistance, shear strength, torque–clamp-force performance or fatigue resistance. Those requirements must be addressed separately when relevant.
The designation 4.8 is decoded as follows:
- The first number, 4, gives a nominal tensile strength of
4 × 100 = 400 MPa. - The second number, 0.8, represents the nominal ratio between the yield-related strength and nominal tensile strength.
- The nominal yield-related value is therefore
400 × 0.8 = 320 MPa.
However, these decoded numbers are designation values. They are not a complete statement of the finished fastener’s acceptance requirements.
For Class 4.8, published ISO 898-1 engineering tables show:
| Property | ISO 898-1 Class 4.8 |
|---|---|
| Nominal tensile strength used in designation | 400 MPa |
| Minimum tensile strength | 420 MPa |
| Nominal proof stress | 310 MPa |
| Nominal yield-related designation value | 320 MPa |
| Product form | Finished bolts, screws and studs |
| Governing standard | ISO 898-1 |
ISO 898-1 also specifies requirements covering proof load, full-size tensile testing, hardness, head soundness, surface integrity and other characteristics of the finished fastener.
This distinction is important: a Class 4.8 bolt is not accepted merely because its raw steel has a yield strength above 320 MPa. The completed fastener must comply with the applicable finished-product tests.
3. Side-by-side comparison
The following table is useful for understanding the classifications, but it should not be used as a substitution table.
| Characteristic | AS/NZS 3678 Grade 350 | ISO 898-1 Class 4.8 |
|---|---|---|
| What it classifies | Structural steel plate | Finished bolt, screw or stud |
| Governing standard | AS/NZS 3678 | ISO 898-1 |
| Typical component | Plate washer, baseplate, bracket | Bolt, screw, stud |
| Strength basis | Plate test specimen | Finished-fastener tests |
| Yield-related value | Depends on plate thickness | Nominal designation value of 320 MPa |
| Tensile strength | 450 MPa minimum | 400 MPa nominal; 420 MPa minimum |
| Ductility assessment | Plate elongation test | Fastener-specific ductility tests |
| Weldability | Structural plate is supplied for weldable applications | Not specified by ISO 898-1 |
| Thread requirements | None | Metric thread and tolerance requirements apply |
| Proof-load requirements | None as a fastener | Specified for the finished fastener |
| Certification focus | Steel heat and plate traceability | Production lot and finished-fastener conformity |
Numerically, a 10 mm Grade 350 plate has higher minimum yield and tensile values than the familiar nominal values associated with Class 4.8. That does not mean a component made from Grade 350 plate is automatically a Class 4.8 fastener.
The tests, manufacturing processes and component geometries are different.
4. Why Grade 350 cannot automatically replace Class 4.8
The standards certify different things
A Grade 350 material certificate establishes that a batch of structural plate complies with AS/NZS 3678.
A Class 4.8 certificate establishes that finished fasteners comply with ISO 898-1 or another specified fastener standard.
A plate certificate cannot replace a fastener test certificate, even where the listed tensile and yield values appear similar.
Threading changes the component
A bolt does not carry tensile load across its full nominal diameter. Its tensile capacity is normally evaluated using the thread stress area.
Manufacturing can also affect the properties of the finished component through:
- cold forming;
- hot forging;
- thread rolling;
- thread cutting;
- straightening;
- heat treatment;
- surface treatment; and
- local stress concentrations.
The properties of the original steel product are therefore only one part of fastener performance.
Fastener tests extend beyond tensile strength
ISO 898-1 conformity can involve requirements for:
- minimum ultimate tensile load;
- proof load;
- hardness;
- head soundness;
- surface integrity;
- decarburisation;
- thread geometry; and
- elongation or ductility.
An AS/NZS 3678 plate certificate does not address these finished-fastener characteristics.
Component capacity depends on geometry
A 100 × 100 × 10 mm Grade 350 plate washer and an M36 Class 4.8 threaded component may be installed in the same connection.
The plate washer may be evaluated for:
- bending;
- bearing;
- deformation around the hole; and
- load distribution into the supporting structure.
The threaded component may be evaluated for:
- tensile load;
- proof load;
- thread stripping;
- preload;
- fatigue; and
- interaction with the nut.
Comparing only their material strength values ignores how the components actually carry load.
5. When Grade 350 should be specified
Grade 350 is appropriate when the drawing or bill of materials requires a fabricated structural steel component, for example:
- square or circular plate washers;
- anchor plates;
- baseplates;
- splice plates;
- brackets;
- cleats;
- stiffeners; or
- other profiled plate components.
A procurement description should identify more than “Grade 350”. It should normally include:
- the governing standard, such as AS/NZS 3678;
- plate thickness;
- finished dimensions;
- hole diameter and tolerance;
- edge condition;
- impact grade where required;
- coating or surface protection;
- material certificate requirements; and
- heat-number traceability.
An example description could be:
Plate washer, 100 × 100 × 10 mm, 40 mm diameter central hole, material AS/NZS 3678 Grade 350, hot-dip galvanised after fabrication, material test certificate and heat-number traceability required.
The standard designation is particularly important because “Grade 350” by itself can be ambiguous. Other Australian steel-product standards also contain 350-grade designations for different product forms.
6. When Class 4.8 should be specified
Class 4.8 should be purchased when it is the property class nominated by the engineer, drawing, product specification or approved bill of materials.
It may be suitable for general-purpose joints where the calculated loading, tightening method and service conditions are consistent with its mechanical properties.
A complete fastener description should include:
- product type;
- dimensional standard;
- diameter and length;
- thread pitch;
- thread tolerance;
- property class;
- governing mechanical-property standard;
- surface finish;
- coating standard and thickness;
- nut and washer requirements;
- marking requirements;
- inspection certificate; and
- lot traceability.
For example:
M20 × 80 hexagon head bolt to ISO 4014, coarse thread, thread tolerance 6g, Property Class 4.8 to ISO 898-1, zinc plated, matching nut and washers as specified, certificate of conformity required.
Specifying only “M20 bolt, mild steel” would not provide adequate control over the mechanical performance of the finished fastener.
7. Should procurement upgrade Class 4.8 to Class 8.8?
Not without approval.
It may appear safe to replace a Class 4.8 bolt with a stronger Class 8.8 bolt, but a higher property class is not automatically an acceptable substitution.
Changing the property class can affect:
- required tightening torque;
- achievable preload;
- joint stiffness;
- nut compatibility;
- ductility;
- failure mode;
- coating risk;
- installation procedure; and
- compliance with the engineer’s design.
The correct procurement approach is to submit the proposed substitution to the responsible engineer or technical authority.
The same principle applies in reverse: a supplier must not downgrade an 8.8 requirement to 4.8 because the components have the same dimensions.
8. Common procurement scenarios
| Drawing or RFQ requirement | Correct procurement response |
|---|---|
| “100 × 100 × 10 plate washer, Grade 350” | Source AS/NZS 3678 Grade 350 plate, fabricate the washer and obtain plate traceability |
| “M20 bolt, Class 4.8” | Source a finished fastener complying with ISO 898-1 and the applicable dimensional standard |
| “Bolt manufactured from Grade 350” | Query the requirement; Grade 350 plate certification does not define finished-bolt performance |
| “Grade 350 or Class 4.8” | Reject the ambiguity and request identification of the component and governing standard |
| “Class 4.8 minimum” | Obtain engineering approval before offering a higher property class |
| Supplier offers a Grade 350 material certificate for a Class 4.8 bolt | Do not accept the certificate as proof of ISO 898-1 conformity |
| Custom threaded component is machined from structural steel | Confirm the approved material route and require finished-component testing where specified |
9. Can Grade 350 steel be used to manufacture a bolt?
It may be physically possible to machine a threaded component from suitable structural steel, but this does not automatically make it an ISO 898-1 Class 4.8 bolt.
Several questions must first be resolved:
- Does the drawing specifically permit that material?
- Is AS/NZS 3678 the correct raw-material standard for the supplied product form?
- Is the fastener geometry within the scope of ISO 898-1?
- Will the finished fastener be proof-load and tensile tested?
- Are the thread, hardness, surface-integrity and marking requirements satisfied?
- Will the engineer accept machined rather than forged or rolled manufacture?
- What certificate is required for the completed fastener?
Unless these questions are answered, procurement should not infer fastener compliance from the raw material certificate.
Conclusion
AS/NZS 3678 Grade 350 and ISO 898-1 Property Class 4.8 are not competing steel grades.
Grade 350 specifies structural plate material. Class 4.8 specifies the mechanical performance of a finished fastener.
They can be used together in the same connection:
- the plate washer or anchor plate may be Grade 350;
- the bolt, screw or stud may be Class 4.8, 8.8 or another class selected by the designer.
The procurement rule is straightforward:
Purchase each component against the standard that governs that component. Never approve a substitution merely because two strength values appear similar.
For a fabricated plate component, verify the steel grade, thickness and material traceability. For a fastener, verify the dimensional standard, property class, proof-load requirements, coating, marking and finished-product certification.
The numbers may look comparable, but the certificates are not interchangeable.