Overview
ISO/TR 20491:2019 - Fasteners - Fundamentals of hydrogen embrittlement in steel fasteners is a Technical Report that consolidates current knowledge about hydrogen embrittlement (HE) and translates it into practical, industry-relevant know‑how for steel fasteners. Rather than setting mandatory requirements, the report explains mechanisms, terminology and risk factors in a clear, applied way. It focuses especially on high‑strength steel fasteners (tensile strength > 1 000 MPa) used in safety‑critical applications where HE can cause unexpected brittle failure.
Key topics
The report covers the following technical topics and guidance:
- Terminology and symbols - clear definitions for HE, internal hydrogen embrittlement (IHE), environmental hydrogen embrittlement (EHE), hydrogen diffusion/effusion, threshold stress, etc.
- Mechanisms of damage - how atomic hydrogen diffuses to high‑stress locations, produces hydrogen‑assisted cracking (HAC/HIC), and transitions ductile steel to brittle fracture (theories such as decohesion and HELP are discussed).
- Fracture morphology and crack‑tip conditions - recognition of brittle vs ductile features and factors that concentrate stress at crack initiation points.
- Root causes and triggers - material susceptibility, defects, tensile and residual stresses, and hydrogen sources (internal from manufacture and external/environmental).
- Manufacturing and surface processes - implications for case‑hardening, hot‑dip galvanizing, electroplating, thermal treatments, thread rolling after heat treatment, and stress relief.
- Mitigation and testing - guidance on baking, stress relief procedures, and HE test methods for assessing susceptibility and defining hydrogen embrittlement threshold stress.
- Practical measurement concepts - methodology for measuring HE threshold stress and discussion of traps, diffusion and effusion phenomena.
Note: ISO/TR 20491 is informative (no normative references) and synthesizes existing research into actionable guidance for the fastener supply chain.
Applications and users
ISO/TR 20491 is valuable to:
- Fastener manufacturers - to identify process steps that introduce or remove hydrogen and to design heat‑treatment, plating and baking practices.
- Steel mills and material suppliers - for material selection and understanding susceptibility linked to microstructure and strength levels.
- Coaters and platers - to balance corrosion protection with HE risk (e.g., galvanizing vs cathodic hydrogen evolution).
- Design, quality and maintenance engineers - for joint design, risk assessment, inspection, and specifying appropriate testing or baking protocols.
- Regulatory bodies and standards committees - as a technical reference to clarify terminology and align industry practices.
Related standards
ISO/TR 20491 references terminology and source material from standards such as ISO 1891‑2 and technical documents (examples in the report include ASTM definitions). It should be used alongside product standards and test methods for fasteners and coatings when developing specifications and QA procedures.
Keywords: ISO/TR 20491:2019, hydrogen embrittlement, steel fasteners, hydrogen assisted cracking, internal hydrogen embrittlement, environmental hydrogen embrittlement, baking, galvanizing, case‑hardening, HE threshold stress.