Outline of the Standard
This standard specifies the mechanical properties and test methods for self-drilling tapping screws. Below is a structured outline to guide through the key sections:
- Technical Requirements: Covers materials, metallurgical properties, and mechanical performance criteria.
- Test Methods: Details procedures for metallurgical and mechanical testing.
- Torque Wrench Specifications: Requirements for tools used in torque testing.
- Soalan Lazim: Common questions and professional insights.
Technical Requirements
The standard outlines precise requirements for self-drilling tapping screws to ensure reliability in fastening applications. These screws are designed for drilling and tapping in a single operation, commonly used in construction, automotive, and machinery industries.
Materials
Self-drilling tapping screws must be manufactured from case-hardening steel or heat-treated steel. This selection ensures the necessary balance between ductility in the core and hardness on the surface, critical for drilling through materials like steel or aluminum without pre-drilling.
Metallurgical Properties
Metallurgical properties are essential for the screw’s performance under stress. Proper heat treatment prevents failures such as cracking or brittleness.
Surface Hardness
After heat treatment, the surface hardness of self-drilling tapping screws shall be at least 530 HV 0.3. This high surface hardness enables effective drilling and tapping, reducing wear on the screw tip during installation.
Core Hardness
The core hardness after treatment is specified as follows:
- 320 HV 5 to 400 HV 5 for thread sizes ≤ ST 4.2.
- 320 HV 10 to 400 HV 10 for thread sizes > ST 4.2.
A recommended minimum tempering temperature is 330°C. Avoid the tempering range of 275°C to 315°C to minimize the risk of tempered martensite embrittlement, which could lead to premature failure under load.
Case Depth
The case depth must comply with the values in Table 1. This depth ensures sufficient hardened layer for drilling while maintaining core toughness.
| Saiz Benang | Min (mm) | Max (mm) |
|---|---|---|
| ST 2.9 and ST 3.5 | 0.05 | 0.18 |
| ST 4.2 to ST 5.5 | 0.10 | 0.23 |
| ST 6.3 | 0.15 | 0.28 |
Microstructure
In the heat-treated microstructure, no banded ferrite shall appear between the surface hardened layer and the core. This ensures uniform strength and prevents weak zones that could cause shear failure.
Hydrogen Embrittlement
Electroplated self-drilling tapping screws are at risk of hydrogen embrittlement-induced fracture. Manufacturers and platers must implement measures, including testing per GB/T 3098.17, to control this risk. Additionally, consider the hydrogen embrittlement relief requirements in GB/T 5267.1 for electroplated fasteners to enhance long-term durability.
Mechanical Properties
Mechanical properties define the screw’s ability to perform under operational conditions, including drilling, tapping, and load-bearing.
Drilling Performance
The drilling portion of the screw must drill a prefabricated hole suitable for extruding mating internal threads under the test conditions specified in section 4.2.1. This ensures efficient installation without additional tools.
Thread Forming Performance
In the prefabricated hole drilled per 3.3.1, the screw shall extrude mating internal threads without deformation when screwed into the test plate as per 4.2.1.1. This property is vital for secure fastening in thin materials.
Torsional Strength
When tested per 4.2.3, the torsional strength shall ensure the failure torque equals or exceeds the values in Table 4. High torsional strength prevents breakage during tightening.
Test Methods
Standardized test methods verify compliance with the requirements, providing reproducible results for quality assurance.
Metallurgical Performance Tests
Surface Hardness Test
Conduct per GB/T 4340.1. Indentations should be on flat surfaces, preferably the screw head, to accurately measure the hardened layer.
Core Hardness Test
Perform per GB/T 4340.1 on a transverse microsection to assess internal toughness.
Case Depth Measurement
Measure using a microscope on a longitudinal microsection at the flank midway between crest and root, or at the root for screws ≤ ST 4.2. For arbitration, use micro-Vickers hardness with 300 g force on the thread profile, calculating from the point exceeding core hardness by 30 HV.
Microstructure Test
Conduct per relevant metallographic inspection standards to confirm absence of defects.
Mechanical Performance Tests
Drilling and Tapping Test
Test Apparatus
Refer to Figure 1 for an example setup. Test plates are made from low-carbon steel (≤ 0.23% carbon) with hardness 110 HV 30 to 165 HV 30 per GB/T 4340.1. Plate thickness per Table 2.
| Saiz Benang | Test Plate Thickness (mm) | Axial Force (N) | Max Screwing Time (s) | Screw Speed (rpm) |
|---|---|---|---|---|
| ST 2.9 | 0.7 + 0.7 = 1.4 | 150 | 3 | 1800–2500 |
| ST 3.5 | 1 + 1 = 2 | 150 | 4 | 1800–2500 |
| ST 4.2 | 1.5 + 1.5 = 3 | 250 | 5 | 1800–2500 |
| ST 4.8 | 2 + 2 = 4 | 250 | 7 | 1800–2500 |
| ST 5.5 | 2 + 3 = 5 | 350 | 11 | 1000–1800 |
| ST 6.3 | 2 + 3 = 5 | 350 | 13 | 1000–1800 |
Test plate thickness may consist of two steel plates. These values are for acceptance inspection only.
Test Procedure
Screw the coated or uncoated specimen into the test plate until one full thread passes through. Apply axial force and speed per Table 2 throughout drilling and tapping.
Drilling Inspection
By agreement, perform drilling inspection using test plates per 4.2.1.1 with thickness per Table 3. Pre-punch a locating point. After drilling through, the maximum hole size shall not exceed Table 3 limits.
| Saiz Benang | Plate Thickness (mm) | Min Hole Diameter (mm) | Max Hole Diameter (mm) |
|---|---|---|---|
| ST 2.9 | 1 | 2.2 | 2.5 |
| ST 3.5 | 1 | 2.7 | 3 |
| ST 4.2 | 2 | 3.2 | 3.6 |
| ST 4.8 | 2 | 3.7 | 4.2 |
| ST 5.5 | 2 | 4.2 | 4.8 |
| ST 6.3 | 2 | 4.8 | 5.4 |
The fixture in Figure 2 supplements Figure 1. Sleeve inner diameter is about 0.25 mm larger than thread major diameter. Sleeve length allows drill point extension. Axial forces per Table 2 guide installation; exceeding may cause fracture or overheating.
Torque Test
Clamp the screw in a matching thread split die or device without damaging the clamped portion. Refer to Figure 3 for setup. After clamping, at least two full threads protrude, and at least two full threads (excluding drill point) are secured. For short screws, clamp the entire thread without head clamping force.
Apply torque using a calibrated device until fracture. The screw must meet minimum failure torques in Table 4.
| Saiz Benang | Min Failure Torque (Nm) |
|---|---|
| ST 2.9 | 1.5 |
| ST 3.5 | 2.8 |
| ST 4.2 | 4.7 |
| ST 4.8 | 6.9 |
| ST 5.5 | 10.4 |
| ST 6.3 | 16.9 |
Torque Wrench
Torque wrenches for testing must have a measurement error within ±3% of the specified torque value. Power devices with equivalent accuracy and torque display may be used. For arbitration tests, employ manual torque wrenches to ensure precision and reproducibility.
Soalan Lazim
- What materials are recommended for self-drilling tapping screws under this standard?
Case-hardening steel or heat-treated steel, providing the required surface hardness for drilling and core ductility for strength. - How is hydrogen embrittlement managed in electroplated screws?
Through measures like baking post-plating and testing per GB/T 3098.17, alongside considerations from GB/T 5267.1 to minimize fracture risks. - What is the significance of avoiding the 275–315°C tempering range?
This range can induce tempered martensite embrittlement, leading to brittle failure; tempering at ≥330°C ensures better toughness. - How should core hardness be tested for different screw sizes?
Use HV 5 for ≤ ST 4.2 and HV 10 for > ST 4.2 on transverse microsections per GB/T 4340.1 for accurate internal property assessment. - What axial forces are applied during drilling tests, and why?
Forces range from 150 N to 350 N per thread size (Table 2) to simulate installation conditions, preventing overload that could damage the drill point. - Why is torsional strength testing crucial?
It verifies the screw can withstand installation torques without breaking, ensuring reliability in applications with high tightening requirements per Table 4.
