GB/T 3098.7-2000: Self-Tapping Screw Fastener Properties

Schema dell'articolo

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.

Materiali

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.

Proprietà meccaniche e prestazionali

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.

Heat Treatment

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.

Durezza

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.

Profondità della cassa

Case depth must comply with Table 4, providing sufficient hardened layer for torque and wear performance without excessive brittleness.

Resistenza alla torsione

Torsional strength per Table 3; failure must not occur in clamped threads. This test simulates real-world installation stresses.

Solidità della testa

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.

Resistenza alla fragilità da idrogeno

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.

Resistenza alla trazione

For screws ≥12 mm or ≥3d, tensile test by agreement; reference values in Table 3 guide expected performance.

Metodi di prova

Core Hardness Test

Measure at half-radius on cross-section away from end, through minor diameter, per GB/T 4340.1. This verifies internal toughness.

Surface Hardness Test

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.

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.

Retempering Test

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.

chiavi dinamometriche

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.

Marcatura

Marking Code

Viti autofilettanti cementate e temprate contrassegnate con “-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.

Domande frequenti

1. 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.
2. 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.
3. 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.
4. 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.
5. In che modo la profondità della cassa influisce sulle prestazioni della vite?
   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.
6. 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.