ISO 6157-2:1995

Overview

ISO 6157-2:1995 - “Fasteners - Surface discontinuities - Part 2: Nuts” defines acceptable limits and inspection practices for surface defects on nuts. It covers nuts with nominal thread diameters 5 mm to 39 mm, product grades A and B, and all mechanical property classes referenced in ISO 898-2 and ISO 898-6 (unless otherwise agreed). The standard ensures that visible surface discontinuities do not impair the mechanical or functional properties required by related fastener standards.

Key topics and technical requirements

• Types of surface discontinuities covered: cracks (quench, forging, inclusion, locking element, washer-retainer), shear bursts, bursts, seams, folds, voids, tool marks and general damages.
• Quantitative limits and definitions:
  • Cracks: e.g., forging cracks - max two that extend across the bearing face, neither deeper than 0.05·d; no crack shall extend into the tapped hole beyond the first full thread; cracks in the first full thread ≤ 0.5·H1 (H1 = 0.541·P).
  • Seams: permitted up to a depth of 0.05·d.
  • Voids: depth h < 0.02·d (max 0.25 mm); combined void area on bearing face ≤ 5% for d < 24 mm and ≤ 10% for d ≥ 24 mm.
  • Bursts / shear bursts: geometric width limits at wrenching flat intersections and flange periphery (e.g., width at top/bottom flat intersection ≤ 0.25 + 0.02·s mm; flange bursts width ≤ 0.08·d_f).
  • Tool marks: bearing surface roughness limited to 3.2 µm (measured per ISO 468); other tool marks permitted.
  • Damages: allowed unless they demonstrably impair usability.
• Inspection & evaluation procedures:
  • Sampling per ISO 3269.
  • Routine: visual inspection (x10 magnification recommended).
  • Non‑destructive testing options: visual magnification, magnetic or eddy‑current methods.
  • Destructive/referee tests: ISO 10484 (widening test) and optionally ISO 10485 (cone proof load).
  • Lots are rejected if discontinuities exceed limits or if destructive tests fail.

Applications and users

• Who uses it: fastener manufacturers, quality/inspection labs, procurement/specification engineers, and purchasers involved in mechanical assemblies.
• Practical uses: production acceptance criteria, incoming inspection of nuts, supplier contracts, quality control procedures, and dispute resolution (referee testing).
• Sectors benefiting: manufacturing, automotive, heavy machinery, construction and any industry requiring reliable threaded connections and documented surface‑quality controls.

Related standards

• ISO 6157-1, ISO 6157-3 (other parts of the series)
• ISO 898-2, ISO 898-6 (mechanical properties of nuts)
• ISO 2320, ISO 3269, ISO 468, ISO 10484, ISO 10485

Keywords: ISO 6157-2, surface discontinuities, nuts, fasteners, inspection, ISO 898, non‑destructive testing, voids, seams, cracks, quality control.