Introduction
This article details the mechanical performance standards for self-drilling tapping screws as outlined in GB/T 3098.11-2002. These standards ensure the reliability and functionality of self-drilling screws in fastening applications, covering materials, metallurgical properties, and mechanical performance. Self-drilling screws are designed to drill their own hole and form mating threads without pre-drilling, making them essential in construction, automotive, and manufacturing industries.
Technical Requirements
1.1 Materials
Self-drilling tapping screws shall be manufactured from carburizing steel or heat-treated steel to provide the necessary hardness and durability for drilling and tapping operations.
1.2 Metallurgical Properties
1.2.1 Surface Hardness
After heat treatment, the surface hardness of self-drilling tapping screws shall be at least 530 HV0.3.
1.2.2 Core Hardness
The core hardness after treatment shall be:
- 320 HV5 to 400 HV5 for thread sizes ≤ ST4.2;
- 320 HV10 to 400 HV10 for thread sizes > ST4.2.
The recommended minimum tempering temperature is 330°C. Tempering temperatures between 275°C and 315°C should be avoided to minimize the risk of tempered martensite embrittlement.
1.2.3 Carburized Layer Depth
The depth of the carburized layer shall comply with the values in Table 1.
| Thread Size | Minimum | Maximum |
|---|---|---|
| ST2.9 and ST3.5 | 0.05 | 0.18 |
| ST4.2 to ST5.5 | 0.10 | 0.23 |
| ST6.3 | 0.15 | 0.28 |
1.2.4 Microstructure
In the microstructure after heat treatment, no banded ferrite shall appear between the surface hardened layer and the core.
1.2.5 Hydrogen Embrittlement
Electroplated self-drilling tapping screws are at risk of fracture due to hydrogen embrittlement. Measures shall be taken by the manufacturer and/or plater, including testing per GB/T 3098.17, to control this risk. Requirements for eliminating hydrogen embrittlement in electroplated fasteners per GB/T 5267.1 shall also be considered.
1.3 Mechanical Properties
1.3.1 Drilling Performance
The drilling portion of the screw shall drill a prefabricated hole suitable for extruding mating internal threads under the test conditions specified in section 2.2.1.
1.3.2 Thread Forming Performance
In the prefabricated hole drilled per section 2.2.1, the self-drilling tapping screw shall extrude mating internal threads without deformation when screwed into the test plate specified in section 2.2.1.1.
1.3.3 Torsional Strength
When tested per section 2.2.3, the torsional strength shall ensure the failure torque is equal to or greater than the values in Table 4.
Test Methods
2.1 Metallurgical Property Tests
2.1.1 Surface Hardness Test
Surface hardness testing shall be conducted per GB/T 4340.1. Indentations should be made on flat surfaces, preferably on the screw head.
2.1.2 Core Hardness Test
Core hardness testing shall be conducted per GB/T 4340.1 on a transverse microsection.
2.1.3 Carburized Layer Depth Measurement
The carburized layer depth shall be measured using a microscope on a longitudinal microsection at the flank midway between crest and root, or at the root for screws ≤ ST4.2. For arbitration, use micro-Vickers hardness with a 300 g test force on the thread profile, calculating depth from the point exceeding core hardness by 30 HV.
2.1.4 Microstructure Test
Microstructure testing shall follow relevant metallographic inspection standards.
2.2 Mechanical Property Tests
2.2.1 Drilling and Tapping Test
2.2.1.1 Test Apparatus
The test plate shall be made of low-carbon steel with carbon content ≤ 0.23% and hardness 110 HV30 to 165 HV30 (per GB/T 4340.1). Plate thickness shall comply with Table 2. The test apparatus is exemplified in Figure 1 (not shown here; refer to standard for diagram).
| Thread Size | Test Plate Thickness (mm) | Axial Force (N) | Maximum Screwing Time (s) | Screw Speed Under Load (r/min) |
|---|---|---|---|---|
| ST2.9 | 0.7 + 0.7 = 1.4 | 150 | 3 | 1800–2500 |
| ST3.5 | 1 + 1 = 2 | 150 | 4 | 1800–2500 |
| ST4.2 | 1.5 + 1.5 = 3 | 250 | 5 | 1800–2500 |
| ST4.8 | 2 + 2 = 4 | 250 | 7 | 1800–2500 |
| ST5.5 | 2 + 3 = 5 | 350 | 11 | 1000–1800 |
| ST6.3 | 2 + 3 = 5 | 350 | 13 | 1000–1800 |
Note: Test plate thickness may consist of two steel plates. These values are for acceptance inspection only.
2.2.1.2 Test Procedure
Screw the coated or uncoated screw (as per application) into the test plate until one full thread passes through. Axial force and screw speed from Table 2 apply to both drilling and tapping.
2.2.2 Drilling Inspection
By agreement, drilling inspection may be performed. Test plate per 2.2.1.1, thickness per Table 3. Pre-punch a locating point. After drilling through, the maximum hole size shall not exceed Table 3 limits. The fixture in Figure 2 (not shown; refer to standard) supplements Figure 1, with sleeve inner diameter about 0.25 mm larger than thread major diameter. Sleeve length allows drill point extension. Axial forces from Table 2 guide installation; exceeding may cause drill point fracture or overheating.
| Thread Size | Plate Thickness | Minimum Hole Diameter | Maximum Hole Diameter |
|---|---|---|---|
| ST2.9 | 1 | 2.2 | 2.5 |
| ST3.5 | 1 | 2.7 | 3 |
| ST4.2 | 2 | 3.2 | 3.6 |
| ST4.8 | 2 | 3.7 | 4.2 |
| ST5.5 | 2 | 4.2 | 4.8 |
| ST6.3 | 2 | 4.8 | 5.4 |
2.2.3 Torque Test
Clamp the screw in a matching thread split die or device without damaging the clamped portion. The apparatus is exemplified in Figure 3 (not shown; refer to standard). After clamping, at least two full threads extend beyond the device, and at least two full threads (excluding drill point) are securely clamped. For short screws, clamp the entire thread without applying force to the head. Apply torque using a calibrated device until fracture. The screw shall meet the failure torque in Table 4 (units: N·m).
| Thread Size | Minimum |
|---|---|
| ST2.9 | 1.5 |
| ST3.5 | 2.8 |
| ST4.2 | 4.7 |
| ST4.8 | 6.9 |
| ST5.5 | 10.4 |
| ST6.3 | 16.9 |
For torque testing, the torque wrench measurement error shall be within ±3% of the specified value. A power device with equivalent accuracy may be used. For arbitration, use a manual torque wrench.
FAQ
- What materials are required for self-drilling tapping screws per GB/T 3098.11-2002?
- They must be made from carburizing steel or heat-treated steel to achieve the specified hardness and performance.
- How is hydrogen embrittlement managed in electroplated screws?
- Manufacturers and platers must implement measures, including testing per GB/T 3098.17 and considering GB/T 5267.1 for hydrogen removal, to prevent fracture risks.
- What is the minimum surface hardness for these screws?
- The surface hardness must be at least 530 HV0.3 after heat treatment.
- Why avoid certain tempering temperatures?
- Tempering between 275°C and 315°C increases the risk of tempered martensite embrittlement; a minimum of 330°C is recommended.
- What are the torsional strength requirements for ST4.8 screws?
- The minimum failure torque is 6.9 N·m when tested per the specified method.
- How is the carburized layer depth measured for arbitration?
- Use micro-Vickers hardness with 300 g force on the thread profile, starting from the point where hardness exceeds core by 30 HV.
