Makale Özeti
This article provides a structured overview of the GB/T 3098.7-2000 standard, optimized for clarity and practical application in mechanical engineering. The outline ensures comprehensive coverage of key aspects:
- Introduction: Scope and significance of the standard.
- Materials: Chemical composition and manufacturing guidelines.
- Mechanical and Performance Properties: Detailed requirements including hardness, torque, and more.
- Test Methods: Procedures for verifying compliance.
- Torque Wrenches: Specifications for testing equipment.
- Marking: Identification and labeling requirements.
- FAQ: Common questions and expert insights.
Introduction to GB/T 3098.7-2000 Standard
The GB/T 3098.7-2000 standard specifies the mechanical properties of self-tapping screws used in fasteners. It applies to screws manufactured from case-hardening steel, ensuring reliability in applications requiring high performance under mechanical stress. This standard is essential for industries such as automotive, construction, and machinery, where self-tapping screws provide secure fastening without pre-drilled threads.
Key benefits include standardized testing for durability, resistance to failure, and compatibility with various materials. Adhering to this standard minimizes risks like hydrogen embrittlement and ensures consistent quality in production. For optimal use, engineers should integrate these specifications into design and quality control processes.
Malzemeler
Self-tapping screws shall be manufactured by cold heading from case-hardening steel. The chemical composition provided in Table 1 is for guidance only, allowing flexibility while maintaining core properties. If boron content is controlled by adding titanium and/or aluminum to render it ineffective, the boron may reach up to 0.005%.
Guidance on material selection:
- Ensure steel meets carbon and manganese limits to achieve desired hardenability.
- Avoid impurities that could compromise case hardening.
- Verify composition through ladle and check analyses for batch consistency.
Table 1: Chemical Composition
| Analysis | Composition Limits, % | |
|---|---|---|
| Carbon | Manganese | |
| Ladle | 0.15~0.25 | 0.70~1.65 |
| Check | 0.13~0.27 | 0.64~1.71 |
Mechanical and Performance Properties
The standard outlines mechanical and performance requirements, with test methods referenced in Table 2. Screws must undergo case hardening and tempering at a minimum temperature of 340°C to meet these criteria.
Table 2: Mechanical and Performance Properties Items
| Property Item | Technical Requirement (Clause or Table) | Test Method (Clause) |
|---|---|---|
| Çekirdek Sertliği | 4.3 | 5.1 |
| Yüzey Sertliği | 4.3 | 5.2 |
| Dava Derinliği | 4.4, Table 4 | 5.3 |
| Burulma Dayanımı | 4.5, Table 3 | 5.4 |
| Head Soundness | 4.6 | 5.5 |
| Drive Test | 4.7, Table 3 | 5.6 |
| Hidrojen Kırılganlığı | 4.8 | 5.7 |
| Core Hardness After Retempering | 4.9 | 5.8 |
| Tensile Strength | 4.10, Table 3 | 5.9 |
Isıl İşlem
Finished screws require case hardening and tempering at no less than 340°C, complying with all properties in Table 3. This process enhances surface hardness while maintaining core toughness, crucial for torque resistance.
Table 3: Mechanical and Performance Requirements
| Nominal Thread Diameter (mm) | Minimum Torsional Strength (N·m) | Maximum Drive Torque (N·m) | Minimum Tensile Strength (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 |
Hardness
Core hardness shall be 290~370 HV10, with minimum surface hardness of 450 HV0.3. These values ensure balance between ductility and wear resistance, vital for self-tapping applications.
Dava Derinliği
Case depth must comply with Table 4, providing sufficient hardened layer for torque and wear performance without excessive brittleness.
Table 4: Case Depth
| Nominal Thread Diameter (mm) | Case Depth (mm) | |
|---|---|---|
| Min | Maksimum | |
| 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 |
Burulma Dayanımı
Torsional strength per Table 3; failure must not occur in clamped threads. This test simulates real-world installation stresses.
Head Soundness
No cracks at head-shank junction when support face deforms to 7° angle. Test qualifies even if failure occurs at first thread, provided head remains intact.
Thread Forming Ability
Screws must form mating internal threads without permanent deformation, with drive torque not exceeding Table 3 values. Formed threads shall accept GB/T 197 6h tolerance external threads and sustain grade 8 loads per GB/T 3098.2.
Hydrogen Embrittlement Resistance
Electroplated screws require process review per GB/T 3098.17 to control hydrogen embrittlement. Post-plating, perform hydrogen expulsion per GB/T 5267. Non-electrolytic zinc flake coatings per ISO 10683 are recommended.
Core Hardness After Retempering
Hardness drop after retempering shall not exceed 20 HV, ensuring stability in service.
Tensile Strength
For screws ≥12 mm or ≥3d, tensile test by agreement; reference values in Table 3 guide expected performance.
Test Yöntemleri
Çekirdek Sertlik Testi
Measure at half-radius on cross-section away from end, through minor diameter, per GB/T 4340.1. This verifies internal toughness.
Yüzey Sertliği Testi
Routine: At end, shank, or head per GB/T 4340.1 after coating removal. Arbitration: Vickers microhardness HV0.1 on profile ≥4 mm diameter, ≥0.05 mm from edge; negotiate for <4 mm.
Case Depth Test
Distancefrom surface to point where hardness = core +30 HV0.3; arbitration uses HV0.3 microhardness on prepared metallographic sample.
Torsional Strength Test
Clamp specimen with ≥2 full threads in fixture, ≥2 exposed; apply torque until failure, record value per Table 3.
Head Soundness Test
Insert into wedge with hole diameter = nominal +0.05 mm (≤M6) or +0.1 mm (>M6~M12); apply axial load to 7° deformation. Not for countersunk heads. Use hammer strikes if needed.
Drive Test
Drive into test plate (low-carbon steel, 140~180 HV30, thickness = nominal diameter, hole per Table 5) until ≥1 thread protrudes. Initial axial force ≤50 N (≤M5) or ≤100 N (>M5); speed ≤30 r/min for arbitration. Add lubricant if needed.
Table 5: Test Plate Thickness and Hole Diameters
| 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) | Maksimum | 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 for rolled steel plates.
Hydrogen Embrittlement Test
Per GB/T 3098.17, using parallel support method to assess process control.
Yeniden Isıl İşlem Testi
At 330°C for 1h; average of three core hardness points differs by ≤20 HV pre/post. For arbitration only.
Tensile Test
Clamp with ≥6 threads exposed; apply axial load ≤25 mm/min until failure. Failure in shank or threads, not head junction; self-centering grips required.
Torque Wrenches
For torsional and drive tests, error ≤±3% of specified torque. Equivalent powered devices allowed; manual for arbitration. Calibration ensures accurate assessment of screw performance under load.
İşaretleme
Marking Code
Case-hardened and tempered self-tapping screws marked with “-O-”.
Identification
Indented or raised marking required for ≥5 mm nominal diameter hex or hexalobular heads, preferably on head. By agreement for other types.
Manufacturer’s Identification Mark
Mandatory for all marked products, ensuring traceability and quality assurance.
SSS
- What is the purpose of case hardening in self-tapping screws?
Case hardening provides a hard surface for thread forming and wear resistance while keeping the core ductile to prevent brittle failure under torque or tension, as specified in clauses 4.2 and 4.3. - How can hydrogen embrittlement be mitigated in electroplated screws?
Implement process controls per GB/T 3098.17, perform post-plating hydrogen expulsion per GB/T 5267, and consider non-electrolytic coatings like zinc flake per ISO 10683 to reduce risk. - What if a screw fails the torsional strength test in the clamped area?
Failure in the clamped threads invalidates the test; ensure proper fixturing with at least two full threads clamped and exposed, as per 5.4, to accurately assess material strength. - Are tensile tests mandatory for all self-tapping screws?
No, only for lengths ≥12 mm or ≥3d by supplier-buyer agreement; Table 3 values are references, emphasizing torsional and drive properties as primary metrics. - How does case depth affect screw performance?
Insufficient depth may lead to premature wear or failure in thread forming, while excessive depth increases brittleness; adhere to Table 4 limits for balanced properties in applications like steel assemblies. - What equipment is recommended for arbitration tests?
Use manual torque wrenches with ±3% accuracy for torsional and drive tests, and Vickers microhardness for surface and case depth arbitration to ensure unbiased, precise results.
