API RP 934-A-2025 PDF

Full title and description

API RP 934-A:2025 — Materials and Fabrication of 2¼Cr-1Mo, 2¼Cr-1Mo-¼V, 3Cr-1Mo, and 3Cr-1Mo-¼V Steel Heavy-wall Pressure Vessels for High-temperature, High-pressure Hydrogen Service. This Recommended Practice provides materials, fabrication, welding, heat treatment, testing, inspection, and documentation guidance for new heavy‑wall Cr‑Mo and Cr‑Mo‑V steel pressure vessels intended for high‑temperature, high‑pressure hydrogen-containing service in petroleum refining, petrochemical and chemical facilities.

Abstract

This fourth‑edition RP specifies requirements for heavy‑wall (shell thickness ≥ 4 in / 100 mm) pressure vessels fabricated from 2¼Cr‑1Mo, 2¼Cr‑1Mo‑¼V, 3Cr‑1Mo and 3Cr‑1Mo‑¼V grades intended to operate at elevated temperatures (generally ≥ 650 °F / 345 °C) and high hydrogen pressures. Topics include material selection, acceptance chemistry limits, welding and filler‑metal guidance, post‑weld heat treatment, mechanical testing (including toughness and time‑dependent properties), non‑destructive examination, cladding/overlay considerations, and documentation consistent with applicable ASME Code requirements.

General information

• Status: Current (Fourth edition, active Recommended Practice).
• Publication date: January 2025 (published January 21, 2025 — Fourth edition).
• Publisher: American Petroleum Institute (API).
• ICS / categories: Equipment for petroleum and natural gas industries; iron and steel products (examples reported for earlier editions: ICS 75.180 and 77.140).
• Edition / version: Fourth edition (2025).
• Number of pages: 61 pages (PDF / printed formats).

Scope

This RP applies to new heavy‑wall pressure vessels (shell thickness of 4 in / 100 mm or greater) fabricated from the specified Cr‑Mo and Cr‑Mo‑V steels and intended for high‑temperature (commonly ≥ 650 °F / 345 °C up to the limits where ASME time‑dependent properties apply) and high‑pressure hydrogen‑containing service. It is intended for equipment designed, fabricated, certified and documented in accordance with ASME Section VIII, Division 2 (including relevant supplemental requirements for Cr‑Mo steels) and, where applicable, ASME Code Case 2151. Heat‑treatment, welding, filler material selection, NDE, mechanical testing (including time‑dependent properties and toughness), and documentation/traceability requirements are covered; some vessel types or configurations (e.g., multi‑layer cladding techniques or vessels outside the defined temperature/wall‑thickness ranges) are discussed for purchaser‑defined application but may be outside the RP’s normative scope.

Key topics and requirements

• Material classes and permissible grades: requirements for 2¼Cr‑1Mo, 2¼Cr‑1Mo‑¼V, 3Cr‑1Mo and 3Cr‑1Mo‑¼V steels (chemical limits, mechanical properties, and acceptance criteria).
• Definition of “heavy‑wall” (≥ 4 in / 100 mm) and guidance on applicability vs. thinner sections.
• Welding procedure qualification, filler‑metal selection, controls on trace/impurity elements (e.g., elements affecting weldability and hydrogen compatibility), and acceptance criteria.
• Post‑weld heat treatment and thermal processing to achieve required toughness and time‑dependent properties; references to ASME Section II‑D property thresholds.
• Mechanical testing requirements: tensile, impact/toughness, and, where applicable, time‑dependent (creep/rupture) considerations for elevated temperatures.
• Non‑destructive examination (NDE) and acceptance levels for welds and base material; requirements for inspection documentation and traceability.
• Cladding/overlay guidance (stainless overlays, roll‑bonded or explosion bonded cladding) and when cladding is considered within or outside scope.
• Compatibility with ASME Code (Section VIII Division 2) including application of Code Case 2151 and supplemental Cr‑Mo clauses.

Typical use and users

This RP is used by materials and mechanical engineers, pressure‑vessel designers, fabricators, welding supervisors, NDE and QA personnel, purchaser/specification writers, and plant inspection teams in refineries, petrochemical complexes, and chemical plants where hydrogen or hydrogen‑containing streams are processed at elevated temperatures and pressures. It supports procurement specifications, fabrication and qualification activities, and in‑service inspection planning for new heavy‑wall Cr‑Mo vessels.

Related standards

Key related documents and standards include ASME Boiler & Pressure Vessel Code (ASME Section VIII, Division 2 and ASME Section II‑D material tables), ASME Code Case 2151, API RP 934 series documents (e.g., 934‑C, 934‑E and other 934 family RPs addressing related alloys and inspection topics), API 941 (materials reliability in hydrogen service), and guidance on trace element analysis (see ISO/TR 22281 and ISO/IIW guidance referenced for weld filler limits). These documents are routinely used together when specifying, designing, fabricating and qualifying heavy‑wall Cr‑Mo pressure vessels for hydrogen service.

Keywords

API RP 934‑A, Cr‑Mo steels, 2¼Cr‑1Mo, 3Cr‑1Mo, heavy‑wall pressure vessel, hydrogen service, high temperature, welding, post‑weld heat treatment, ASME Section VIII Division 2, Code Case 2151, materials qualification, NDE, toughness, time‑dependent properties.

FAQ

Q: What is this standard?

A: API RP 934‑A:2025 is a Recommended Practice from the American Petroleum Institute that provides material and fabrication guidance for specified Cr‑Mo and Cr‑Mo‑V steel heavy‑wall pressure vessels intended for high‑temperature, high‑pressure hydrogen service (Fourth edition, 2025).

Q: What does it cover?

A: It covers material selection and acceptance, welding and filler‑metal guidance, heat treatment, mechanical testing (including toughness and time‑dependent properties), NDE and inspection requirements, cladding/overlay considerations, and documentation/traceability for new heavy‑wall pressure vessels designed per applicable ASME Code provisions.

Q: Who typically uses it?

A: Materials and mechanical engineers, fabricators, welding engineers and inspectors, QA/QC teams, purchaser/specification writers, and plant inspection/maintenance personnel in refining, petrochemical and chemical industries.

Q: Is it current or superseded?

A: It is current as the Fourth edition (2025) and replaces earlier editions (including the 2019 edition and its errata). Users should reference the 2025 edition for the latest guidance.

Q: Is it part of a series?

A: Yes — it is part of the API RP 934 family of documents addressing materials and fabrication for various Cr‑Mo alloy steels and related inspection/operation guidance (e.g., RP 934‑C, RP 934‑E and other related RPs and technical reports).

Q: What are the key keywords?

A: Cr‑Mo steels, heavy‑wall, high‑temperature, hydrogen service, pressure vessel materials, welding, post‑weld heat treatment, ASME Section VIII Division 2, Code Case 2151, NDE, toughness, time‑dependent properties.