ISO 3506-4:2009

Overview

ISO 3506-4:2009 specifies the mechanical properties of corrosion-resistant stainless steel tapping screws (tapping screws = thread-forming screws) made from austenitic, martensitic and ferritic stainless steels. The standard covers tapping screws with thread sizes ST2.2 up to and including ST8 (per ISO 1478) and applies to tests performed at ambient temperatures between 10 °C and 35 °C (properties can vary outside this range). ISO 3506-4 is Part 4 of the ISO 3506 series on stainless steel fasteners.

Key Topics

• Scope and applicability: Tapping screws (thread-forming) manufactured from A, C, F steel groups; excludes screws with special properties (e.g., weldability).
• Designation system: Material-grade and hardness-class designation (e.g., A4-25H). Low‑carbon variants may be marked with “L”; passivated screws with “P”.
• Hardness classes: Defined hardness classes (20H, 25H, 30H, 40H) with minimum Vickers hardness values (HV min. 200, 250, 300, 400).
• Chemical composition: Allowed chemical ranges for grades A2, A3, A4, A5 (austenitic), C1, C3 (martensitic), and F1 (ferritic) are listed (see Table 2 in the standard).
• Mechanical performance requirements: Surface hardness (martensitic), core hardness (austenitic/ferritic), torsional strength, and thread-forming capability.
• Test methods: Vickers hardness testing (ISO 6507-1), torsional and drive tests, and procedures for acceptance testing.
• Marking and packaging: Manufacturer identification, optional part marking, and mandatory package labelling (including lot number and grade/hardness symbol).
• Finish and corrosion treatment: Screws supplied clean and bright; passivation recommended/per ISO 16048 for improved corrosion resistance.
• Informative annexes: Steel group descriptions, cold-heading materials, chloride-resistance guidance, intergranular corrosion vs. carbon content, and magnetic properties after cold working.

Applications

• Specification and quality control for tapped fastener production and procurement.
• Design and engineering guidance where stainless steel tapping screws are used to form threads in materials such as aluminum or thin sheet without thread deformation or breakage.
• Use by fastener manufacturers, OEMs, procurement/specification engineers, test laboratories, and quality assurance teams in industries like construction, automotive, electronics, marine and machinery where corrosion resistance and assembly performance are critical.

Related standards

• ISO 1478 (tapping screw threads)
• ISO 16048 (passivation of stainless-steel fasteners)
• ISO 3651‑1 / ISO 3651‑2 (intergranular corrosion tests)
• ISO 6507‑1 (Vickers hardness test)
• ISO 16426 (fastener quality assurance)
• Other parts of ISO 3506 (Parts 1–3 for bolts, nuts, set screws)

Keywords: ISO 3506-4, tapping screws, stainless steel fasteners, mechanical properties, Vickers hardness, thread forming, passivation, austenitic martensitic ferritic.