GB/T3098.7-2000 specifies the mechanical properties of self-tapping screws made from carburized steel. This standard ensures that these fasteners meet rigorous performance criteria for applications requiring reliable threading and strength. Below, we detail the key aspects of materials, mechanical properties, test methods, and related requirements.
Matériels
Self-tapping screws shall be manufactured from carburized steel via cold heading. The chemical composition provided in Table 1 serves as a guideline.
| Analysis | Carbon (%) | Manganese (%) |
|---|---|---|
| Ladle | 0.15~0.25 | 0.70~1.65 |
| Check | 0.13~0.27 | 0.64~1.71 |
Note: Boron content may reach 0.005% if controlled by additions of titanium and/or aluminum to render it ineffective.
Mechanical and Performance Properties
4.1 Properties and Test Methods
The mechanical and performance properties, along with their requirements and test methods, are outlined in Table 2.
| Property | Requirement (Clause or Table) | Test Method (Clause) |
|---|---|---|
| Core Hardness | 4.3 | 5.1 |
| Surface Hardness | 4.3 | 5.2 |
| Case Depth | 4.4, Table 4 | 5.3 |
| Torsional Strength | 4.5, Table 3 | 5.4 |
| Head Soundness | 4.6 | 5.5 |
| Driveability | 4.7, Table 3 | 5.6 |
| Hydrogen Embrittlement Resistance | 4.8 | 5.7 |
| Core Hardness after Retempering | 4.9 | 5.8 |
| Tensile Strength | 4.10, Table 3 | 5.9 |
4.2 Heat Treatment
Finished screws shall undergo case hardening and tempering. The minimum tempering temperature is 340°C, and they must comply with the mechanical and performance requirements in Table 3.
| Nominal Thread Diameter (mm) | Minimum Torsional Strength (N·m) | Maximum Drive Torque (N·m) | Minimum Tensile Load (Reference) (N) |
|---|---|---|---|
| 2 | 0.5 | 0.3 | 1940 |
| 2.5 | 1.2 | 0.6 | 3150 |
| 3 | 2.1 | 1.1 | 4680 |
| 3.5 | 3.4 | 1.7 | 6300 |
| 4 | 4.9 | 2.5 | 8170 |
| 5 | 10 | 5 | 13200 |
| 6 | 17 | 8.5 | 18700 |
| 8 | 42 | 21 | 34000 |
| 10 | 85 | 43 | 53900 |
| 12 | 150 | 75 | 78400 |
4.3 Hardness
Core hardness shall be 290~370 HV10, and minimum surface hardness shall be 450 HV0.3.
4.4 Case Depth
The case depth shall comply with Table 4.
| Nominal Thread Diameter (mm) | Minimum (mm) | Maximum (mm) |
|---|---|---|
| 2, 2.5 | 0.04 | 0.12 |
| 3, 3.5 | 0.05 | 0.18 |
| 4, 5 | 0.1 | 0.25 |
| 6, 8 | 0.15 | 0.28 |
| 10, 12 | 0.15 | 0.32 |
4.5 Torsional Strength
When tested per 5.4, the torsional strength shall meet Table 3. Fracture shall not occur in the clamped thread portion.
4.6 Head Soundness
When tested per 5.5, no cracks shall appear at the head-shank junction when the head bearing surface deforms permanently to a 7° angle with the plane perpendicular to the screw axis. The test is passed if the head does not break off, even if fracture occurs at the first thread.
4.7 Thread Forming Ability
Screws without permanent thread deformation (checked under 10x magnification) shall form mating internal threads in the test plate per 5.6. Drive torque shall not exceed Table 3 values. The formed internal thread shall accept an external thread per GB/T 197 with 6h tolerance and withstand proof loads for property class 8 per GB/T 3098.2.
4.8 Hydrogen Embrittlement Resistance
Self-tapping screws, especially electroplated ones, are susceptible to hydrogen embrittlement. Processes shall be checked per GB/T 3098.17 (parallel bearing surface method) to control hydrogen-related risks. If embrittlement is detected, processes must be improved. Electroplated screws shall undergo hydrogen expulsion per GB/T 5267.
Note: Non-electrolytic zinc flake coatings per ISO 10683 are recommended.
4.9 Core Hardness after Retempering
When tested per 5.8, the core hardness reduction after retempering shall not exceed 20 HV.
4.10 Tensile Strength
For screws with length ≥12 mm or ≥3d, tensile testing may be agreed upon by supplier and purchaser. Note: Table 3 tensile loads are for reference only.
Méthodes d'essai
5.1 Core Hardness Test
Core hardness shall be measured at half the radius on a cross-section away from the end and through the minor diameter, per GB/T 4340.1.
5.2 Surface Hardness Test
Routine testing may be at the end, shank, or head (if geometry allows). Testing per GB/T 4340.1 after coating removal. For arbitration, use Vickers microhardness (HV0.1) on profiles ≥0.05 mm from edge for diameters ≥4 mm; negotiate for <4 mm.
5.3 Case Depth Test
Case depth is the perpendicular distance from surface to point where hardness is core hardness +30 HV0.3. For arbitration, use microhardness (HV0.3).
5.4 Torsional Strength Test
Clamp screw with at least two full threads in fixture and two exposed. Apply torque until failure; value shall meet Table 3.
5.5 Head Soundness Test
Insert screw into wedge with hole diameter = nominal +0.05 mm (≤M6) or +0.1 mm (>M6~M12). Apply axial load until 7° deformation. Not for countersunk screws.
5.6 Driveability Test
Drive screw into test plate (low-carbon steel, 140~180 HV30, thickness = nominal diameter, hole per Table 5) until one thread protrudes. Initial axial force: ≤50 N (≤M5), ≤100 N (>M5). Speed ≤30 r/min for arbitration. Maximum torque is drive torque; lubricants allowed.
| Nominal Thread Diameter (mm) | 2 | 2.5 | 3 | 3.5 | 4 | 5 | 6 | 8 | 10 | 12 | |
|---|---|---|---|---|---|---|---|---|---|---|---|
| Thickness (mm) | 2 | 2.5 | 3 | 3.5 | 4 | 5 | 6 | 8 | 10 | 12 | |
| Hole Diameter (mm) | Max | 1.825 | 2.275 | 2.775 | 3.18 | 3.68 | 4.53 | 5.43 | 7.336 | 9.236 | 11.143 |
| Min | 1.8 | 2.25 | 2.75 | 3.15 | 3.65 | 4.5 | 5.4 | 7.3 | 9.2 | 11.1 | |
Note: Test plate thickness tolerance per GB/T 709.
5.7 Hydrogen Embrittlement Test
Per GB/T 3098.17.
5.8 Retempering Test
Difference in average core hardness (three points) before/after retempering (330°C, 1h) ≤20 HV. For arbitration only.
5.9 Tensile Test
Clamp screw with ≥6 threads exposed. Apply axial load ≤25 mm/min until fracture, which shall be in shank or thread, not head junction.
Torque Wrenches
Torque wrenches for torsional and driveability tests shall have measurement error within ±3% of specified value. Manual wrenches for arbitration.
Marking
7.1 Marking Symbol
Case-hardened and tempered self-tapping screws marked with “-O-”.
7.2 Identification
Case-hardened screws shall bear recessed or raised marking per 7.1. Mandatory for hex or hexalobular heads ≥5 mm, preferably on head. Other types by agreement.
7.3 Manufacturer’s Mark
Manufacturer’s trademark or identification mandatory on all marked products.
FAQ
- What is the minimum tempering temperature for self-tapping screws per this standard?
- The minimum tempering temperature is 340°C to ensure compliance with mechanical properties like hardness and strength.
- How is hydrogen embrittlement addressed in electroplated screws?
- Processes are checked per GB/T 3098.17, and hydrogen expulsion is required per GB/T 5267. Non-electrolytic coatings like zinc flake per ISO 10683 are recommended to minimize risk.
- What are the core and surface hardness requirements?
- Core hardness must be 290~370 HV10, and surface hardness at least 450 HV0.3, ensuring durability and thread-forming capability.
- For which screw sizes is tensile testing optional?
- Tensile testing is optional by agreement for screws with length ≥12 mm or ≥3 times nominal diameter, with reference loads in Table 3.
- What marking is required for case-hardened self-tapping screws?
- The symbol “-O-” must be applied, along with the manufacturer’s trademark, especially on heads of diameters ≥5 mm for hex types.
- How is case depth measured and what are the limits for a 4 mm screw?
- Measured per 5.3 using HV0.3; for 4 mm, minimum 0.1 mm and maximum 0.25 mm to balance hardness and toughness.
