ISO 3506-4:2025

Overview

ISO 3506-4:2025 - Fasteners - Mechanical properties of corrosion‑resistant stainless steel fasteners - Part 4: Tapping screws with specified grades and hardness classes specifies the mechanical and physical requirements for corrosion‑resistant tapping screws made from austenitic, martensitic, ferritic and duplex stainless steels. The standard covers grade and hardness designation, material requirements, mechanical performance (hardness, torsional strength, thread forming ability), inspection, test methods and marking/labelling. It applies to tapping screws with threads ST2,2 to ST8 (ISO 1478) and excludes tapping screws with special properties (for example weldability).

Key Topics and Requirements

• Material classes and designation: Defines a two‑block designation system (stainless steel grade - hardness class), examples include common formats like “A4‑25H” (and “A4L‑25H” for low‑carbon austenitic steels).
• Stainless steel families: Covers austenitic, martensitic, ferritic and duplex grades (newer additions include grades A8 and duplex D2–D8 for certain hardness classes).
• Hardness classes and heat treatment: Specifies surface and core hardness criteria (including surface hardness for martensitic grades) and heat‑treatment requirements where applicable.
• Mechanical and functional properties: Requirements for core hardness, thread forming ability, torsional strength and drive performance to ensure safe installation and in‑service behavior.
• Testing and inspection: Employs standardized tests (Vickers hardness per ISO 6507‑1, torsional and drive tests) with provisions for manufacturer, supplier and purchaser inspections and delivery of test results.
• Temperature limits and warning: Tapping screws are tested at ambient 10 °C to 35 °C and intended for use in the service range –20 °C to +150 °C. The standard warns that properties may not be retained outside these temperatures and places responsibility on users to verify suitability.
• Threads & exclusions: Applies to threads ST2,2–ST8 (ISO 1478) and explicitly excludes screws with special properties (e.g., weldability).

Practical Applications and Who Uses This Standard

ISO 3506-4:2025 is essential for:

• Fastener manufacturers - to specify production, heat treatment and testing regimes for stainless steel tapping screws.
• Design and mechanical engineers - selecting appropriate stainless steel grades and hardness classes for assemblies in construction, marine, automotive, HVAC, electronics and industrial equipment.
• Procurement and quality assurance teams - creating specifications, acceptance criteria and inspection plans.
• Test laboratories and certification bodies - performing hardness, torsional and drive tests and issuing conformity documentation.

Use this standard when specifying corrosion‑resistant tapping screws where mechanical integrity, corrosion performance and consistent labelling/traceability are required.

Related Standards

• ISO 3506‑6 - General rules for selection of stainless steels and nickel alloys for fasteners
• ISO 1478 - Tapping screws thread
• ISO 6507‑1 - Vickers hardness test (test method)
• ISO 16228 - Types of inspection documents
• ISO 1891‑4 - Fasteners - Vocabulary (inspection/delivery)

Keywords: ISO 3506-4:2025, tapping screws, stainless steel fasteners, hardness classes, stainless steel grades, corrosion‑resistant fasteners, Vickers hardness, torsional strength.