Introduction to Bimetal Screws
Bimetal screws are specialized fasteners manufactured using two different metals combined into a single screw to achieve superior performance. Typically, the screw point is made from alloy steel or Martenstic stainless steel for high strength after heat treatment, while the body or critical corrosion-resistant part is made from stainless steel. This combination provides both mechanical strength and excellent resistance to rust and corrosion.
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Construction of Bimetal Screws
A bimetal screw generally consists of:
■ Hardened point: SCM435, SS420
Provides high tensile strength and hardness for drilling and fastening applications.
■ Stainless steel head: SS304, SS316
Offers corrosion resistance, especially in outdoor, marine environments or corrosion atmosphere.
The two metals are metallurgically bonded during manufacturing, creating a durable and reliable fastener.
Material chemical composition (%)
| Material | C | Mn | P | S | Si | Cr | Ni | Mo |
|---|---|---|---|---|---|---|---|---|
| Max. | Max. | Max. | Max. | Max. | Max. | Max. | Max. | |
| SUS304 | 0.08 | 2 | 0.045 | 0.03 | 1.00 | 18.0-20.0 | 8.0-11.0 | — |
| SUS316 | 0.08 | 2 | 0.045 | 0.03 | 1.00 | 16.0-18.0 | 10.0-14.0 | 2.0-3.0 |
| SS420 | 0.16-0.25 | 1.0 | 0.04 | 0.03 | 1.00 | 12.0-14.0 | 0.60 | — |
| SCM435 | 0.33-0.38 | 0.6-0.9 | 0.03 | 0.03 | 0.15-0.35 | 0.90-1.20 | 0.25 | 0.15-0.30 |
Features of Bimetal Screws
■ High corrosion resistance
■ Superior drilling capability
■ Excellent mechanical strength
■ Long service life
■ Suitable for harsh environmental conditions
■ Reduced risk of screw breakage during installation
Advantages
■ Combines the strength of carbon steel with the corrosion resistance of stainless steel.
■ Performs better than ordinary stainless steel screws in hard materials.
■ Ideal for outdoor and industrial applications.
■ Reduces maintenance and replacement costs.
Applications
Bimetal screws are widely used in:
■ Roofing and cladding systems
■ HVAC installations
■ Marine structures
■ Solar panel mounting systems
■ Metal-to-metal fastening
■ Construction and infrastructure projects
Common Types
■ Self-drilling bimetal screws
■ Roofing bimetal screws
■ Self-tapping bimetal screws
■ Concrete anchor bimetal screws
■ Double end bimetal screws
Conclusion
Bimetal screws are high-performance fasteners designed for demanding environments where both strength and corrosion resistance are essential. Their unique dual-metal construction makes them highly reliable for industrial, construction, and outdoor applications.
Reasons for Recommendation:
■ No case-hardening for keeping anti-corrosion high. Surface hardness 350HV
■ Tensile stress value over A2 [700N/mm]
■ welding strength over the breaking values for torsion and tensile of part B
■ Case-hardening for over 600HV
Inspection Procedures for Bimetal Screws
Incoming & Warehouse Initial Inspection
1.Digital Vernier Caliper
● Test Item: Key Dimension Measurement
● Purpose: Measures critical dimensions (diameter, length, thread size) to verify compliance with drawings and tolerances.
2.Precision Electronic Balance
● Test Item: Weight / Indirect Coating Thickness
● Purpose: Precisely weighs fasteners, or calculates coating thickness via the gravimetric method.
3.Micro Vickers Hardness Tester (with Computer)
● Test Item: Surface / Case Hardness
● Purpose: Tests the hardness of the material surface, thread profile, or heat-treated carburized layer.
Inspection after Surface Treatment (Galvanizing, Coating & Anti-corrosion Finishing Completed)
1.Coating Thickness Gauge
● Test Item: Coating / Plating Thickness
● Purpose: Directly measures the thickness of protective coatings on fasteners.
2.Salt Spray Test Chamber
● Test Item: Corrosion Resistance
● Purpose: Simulates salt spray environment to test the anti-corrosion performance of fastener coatings or stainless steel materials.
Sampling Inspection for Mechanical Properties (Finished Product Strength & Torque Performance)
1.Universal Testing Machine (Tensile Tester)
● Test Item: Tensile Strength, Yield Strength, Breaking Load
● Purpose: Tests the mechanical performance of fasteners under tension to ensure structural integrity.
2.Screw Torsion Testing Machine
● Test Item: Torsional Strength / Breaking Torque
● Purpose: Applies torsional force to test the fracture torque of screws and ensure they do not fail under specified torque.
3.Dial Torque Wrench
● Test Item: Torque Calibration / Tightening Torque Verification
● Purpose: Calibrates torque tools or verifies the installation torque of fasteners.
Full On-line Inspection & Final Delivery Inspection (Sorting of Mass Finished Products & Foreign Matter Detection)
1.CCD Vision Sorting Machine
● Test Item: Full Appearance & Dimensional Inspection
● Purpose: Automatically detects defects (cracks, burrs, dimensional errors) and sorts out non-conforming fasteners.
2.Automatic Screw Optical Sorting Machine
● Test Item: Multi-dimensional Defect & Quality Control
● Purpose: Performs high-speed, multi-parameter inspection of fasteners for defects, mixed parts, and dimensional accuracy.
3.In-line Metal Detector / Production Line Inspection System
● Test Item: Foreign Metal Contamination & Mixed Parts Detection
● Purpose: Automatically inspects for metal impurities or mixed parts on the production line.
FASTO INDUSTRIAL CO., LTD.
Q1 What is a bimetal screw and how is it made?
A cold-forged stainless steel body friction-welded to a hardened carbon steel drill tip, then thread-rolled, induction-hardened, and surface-treated — fully in-house at Fasto (IATF 16949).
Q2 Bimetal vs. full stainless self-drilling screws for outdoor use?
Yes — bimetal tips (HRC 52–60) drill ≥2mm steel in one step; full stainless requires pre-drilling beyond thin sheet. Both offer equal stainless body corrosion resistance.
Q3 What drives bimetal self-drilling screw pricing?
Stainless grade, drill point size, coating, washer type, dimensions, certifications, volume, and customization. Fasto: factory-direct pricing, free samples.
Q4 Service life in coastal/marine environments?
C1–C2 inland: 304 ≥25, 316 ≥30 yrs. C3 coastal: 304 15–25, 316 25–30 yrs. C4–C5-M: 304 not recommended, 316 10–25 yrs. Inspect carbon steel tip in extreme marine conditions.
Q5 Main application scenarios?
Metal roofing, solar PV, light steel framing, HVAC, curtain walls, truck/trailer bodies — multiple head types and three material combos (304/A2, 316/A4, 304 + alloy steel).
Q6 How to select by substrate thickness?
Match drill point #1–#5 to thickness, observe 80% capacity rule, derate ~30% for stainless substrates, and ensure ≥3 full thread engagements.
Q7 Worth the premium over coated carbon steel?
Yes for 10+ yr outdoor projects — permanent stainless corrosion resistance, near-zero maintenance. Coated carbon steel risks rust at installation damage points.
Q8 Fasto's advantages as a bimetal screw manufacturer?
Since 2005: full vertical integration, on-site lab, IATF 16949, global certs, 24/7 support, fast delivery, free OEM design — all from one factory.
Q9 Core difference: 304 vs. 316 stainless?
316 (A4) adds 2–3% molybdenum for chloride pitting resistance in marine zones, ~40–60% costlier than 304 (A2) for inland low-salt use.
Q10 Correct installation to prevent breakage/stripping?
Use torque-controlled screw gun, stay perpendicular, ease pressure after penetration, stop at moderate washer squeeze. Pre-drill thick/hard substrates. Impact driver OK on large diameters with caution.