Overview
ISO 4991:2015 - "Steel castings for pressure purposes" - is an international standard that specifies requirements for steel castings used in pressure-containing parts. It covers both castings intended for components governed by pressure vessel codes and those not subject to codes. The standard defines accepted steel grades, chemical composition (Table 1), and mechanical properties (Tables 2–5), and includes guidance on heat treatment, welding, inspection and testing, and traceability.
Key technical topics and requirements
- Material grades and composition: Chemical composition ranges for unalloyed and alloyed steel castings are given in Table 1. Common ISO cast grades (e.g., GP240GH, G20Mn5, GX15CrMo5) are listed with compositional limits.
- Mechanical properties: Tensile, proof-stress and impact requirements are specified in Tables 2–5. Mechanical testing is performed on test pieces from representative test blocks.
- Heat treatment: Types of heat treatment are defined (symbols such as +N = normalized, +QT = quenched & tempered, +AT = solution annealed). Purchasers must state required heat treatment when multiple options exist.
- Welding: Welding is permitted (unless otherwise agreed) and shall comply with ISO 11970; Annex B provides welding conditions.
- Inspection, testing and NDT: Inspection rules include sample formation, lot definition and non‑destructive testing (NDT). Purchasers must specify NDT methods, areas to examine and acceptance criteria per ISO 4990.
- Marking and traceability: Castings must be marked for material identification and heat/serial numbers traceable to heats; manufacturer identification is also required.
- Supplementary requirements and annexes: Optional supplementary requirements (Annex A), informative welding and creep property guidance (Annexes B and C), and a helpful Annex D mapping ISO grades to similar UNS cast grades.
Practical applications and users
ISO 4991 is used by:
- Foundries and casting manufacturers producing pressure-containing components
- Pressure vessel and boiler manufacturers specifying cast parts
- Design engineers and procurement professionals ensuring material fitness-for-service
- Welding engineers and fabricators applying repair or joining procedures
- Quality and inspection teams performing mechanical tests and NDT
The standard helps ensure safety, repeatability and regulatory compliance when specifying steel castings for boilers, pressure vessels, valves, pumps and other pressurized equipment.
Related standards (normative and referenced)
- ISO 4990 - General technical delivery requirements for steel castings
- ISO 4885 - Heat treatment vocabulary
- ISO 11970 - Welding procedures for steel castings
- ISO 6892-1 / ISO 6892-2 - Tensile testing (room and elevated temperatures)
- ISO 148-1 - Charpy impact testing
- ISO 13520 - Ferrite determination in stainless castings
Keywords: ISO 4991, steel castings, pressure purposes, pressure vessel, chemical composition, mechanical properties, heat treatment, welding, inspection, non-destructive testing, UNS cast grades.
