Overview
ISO 4948-2:1981 - "Steels - Classification - Part 2" defines how unalloyed and alloy steels are classified by their main quality classes and by prioritized property or application characteristics. This part complements ISO 4948‑1 (classification by chemical composition) by prescribing quality class definitions (base, quality, special) and the key material characteristics used for designation and selection.
Key topics and requirements
- Main quality classes
- Base steel (B): steels with no special production requirements.
- Unalloyed quality steel (Q): production requires additional care (e.g., reduced S/P, improved surface).
- Unalloyed special steel (S): steels needing production control comparable to classical special steels (e.g., controlled hardenability, inclusion control).
- Equivalent classes exist for alloyed steels: alloyed quality and alloyed special steels.
- Main characteristics used with priority
- Mechanical and heat‑treatment properties (tensile/ yield strength, hardness, hardenability, surface hardness after hardening).
- Impact properties in quenched/tempered conditions.
- Chemical limitations (e.g., carbon or residual element limits).
- Surface discontinuities, internal homogeneity and non‑metallic inclusion limits.
- Electrical or magnetic properties when specified.
- Quantitative guidance
- The standard provides example mechanical criteria for base steels (for 3–16 mm thickness) - tensile, yield, elongation, bend radius, impact energy and maximum Rockwell hardness - and numerical alloy limits for high‑yield alloyed quality steels (e.g., Cr ≈ 0.9%, Cu ≈ 0.06%, Mn ≈ 1.80%, Mo ≈ 0.10%, Ni ≈ 0.9%, and microalloy limits such as Nb, Ti, V ≈ 0.08–0.12%).
- A practical rule: when an alloying element is specified only by a maximum value, classification can use 70% of that maximum for class assignment; combinations of elements are assessed jointly (see table rules).
Applications and users
- Used by steel producers to define manufacturing and quality controls tied to product classes.
- Employed by specifiers, standards writers and materials engineers to select steel grades for applications where production cleanliness, mechanical performance or magnetic/electrical properties are priorities.
- Relevant for industries such as construction, pressure vessels, pipelines, automotive, shipbuilding, wire/fastener manufacture and tooling, where clear classification supports procurement, testing and conformity.
- Useful for testing laboratories and QA teams to interpret designation systems and acceptance criteria tied to quality classes.
Related standards
Keywords: ISO 4948-2:1981, steel classification, unalloyed steels, alloy steels, main quality classes, main characteristics, chemical composition, steel standards.