ASME vs ASTM vs API: Understanding the Three Code Bodies

By WeldFabWorld June 22, 2026 21 min read
ASME vs ASTM vs API — The Three Code Bodies | WeldFabWorld

ASME vs ASTM vs API: Understanding the Three Code Bodies

ASME vs ASTM vs API confusion is extremely common among engineers, fabricators, and QA/QC personnel early in their careers, because all three acronyms show up on the same drawing, the same material certificate, and the same project specification, yet they answer three completely different questions. ASME tells you how a pressure-retaining component must be designed, fabricated, and qualified. ASTM tells you what the material itself is — its chemistry, mechanical properties, and how it must be tested. API tells you how an asset is operated, inspected, and maintained once it is in industrial service, and in some cases also provides original design rules for storage tanks and pipelines.

This guide untangles the three bodies one at a time, then shows exactly how their documents interlock on a real project — a pressure vessel built from a specific steel plate, fabricated under a construction code, and later inspected under an industry mechanical integrity program. Along the way you will see why a single line item like “SA-516 Grade 70” on a drawing is actually a handoff between all three organizations at once.

By the end, you will be able to look at any code reference on a drawing or specification and immediately identify whether it governs what the material is, how the component must be built, or how the asset must be run — the three distinct jobs that ASTM, ASME, and API each exist to do.

Three roles in one line each ASME writes construction and qualification codes for boilers, pressure vessels, and piping. ASTM writes material specifications and test methods used across nearly every engineering discipline. API writes petroleum and petrochemical industry standards covering both original design (tanks, pipelines) and in-service inspection and mechanical integrity (vessels, piping).

Quick Overview: Three Different Jobs

The fastest way to stop confusing the three bodies is to stop thinking of them as competitors and start thinking of them as specialists who each own one layer of the same stack. A finished, code-compliant pressure vessel did not come from just one of these organizations — it came from all three, layered on top of each other, each one solving a different engineering question.

AspectASMEASTMAPI
Full nameAmerican Society of Mechanical EngineersAmerican Society for Testing and Materials (now ASTM International)American Petroleum Institute
Core question answeredHow must this component be designed, built, and qualified?What is this material, and how is it tested?How is this asset operated, inspected, and maintained?
Primary document typeConstruction/qualification codes (Section VIII, B31.3, Section IX)Material specifications and test methods (A516, A370, E8)Industry recommended practices and standards (API 510, API 650)
Document scopeCross-industry, pressure equipment focusedCross-industry, virtually every material and product typePrimarily petroleum, petrochemical, and natural gas industries
Used independently?Rarely — usually requires ASTM materials and may layer with APIYES — can stand alone for general material supplyPartly — design standards stand alone, inspection standards build on ASME-built assets

What Is ASME — Design, Construction, and Qualification Codes

ASME, the American Society of Mechanical Engineers, publishes the Boiler and Pressure Vessel Code (BPVC) along with the B31 series of piping codes. These are construction codes: they define allowable stresses, design margins, required wall thickness calculations, fabrication tolerances, nondestructive examination extent, and — through ASME Section IX — how welding and brazing procedures and personnel must be qualified before they touch a pressure-retaining joint.

Crucially, ASME does not invent its own steel grades or write its own tensile-testing method from scratch. Section II Part D of the BPVC is essentially a giant cross-reference: it lists which material specifications are permitted for pressure-part construction and assigns each one an allowable stress at temperature. Almost every one of those listed specifications is, underneath an “SA-” prefix, an ASTM specification that ASME has formally adopted for use inside its own code.

Code sections most engineers touch first ASME Section VIII (Divisions 1 and 2) for pressure vessel design, Section I for power boilers, Section IX for welding/brazing qualification, and the B31 series (B31.1 power piping, B31.3 process piping) for piping design. Each of these references back to Section II for materials and Section V for nondestructive examination methods.

What Is ASTM — Material Specifications and Test Methods

ASTM International publishes thousands of voluntary consensus standards covering materials, products, systems, and services across nearly every industry, not just pressure equipment. For a welding engineer, the two categories that matter most are material specifications — documents like ASTM A516 (pressure vessel plates) or A106 (seamless carbon steel pipe) that define chemistry, mechanical property minimums, heat treatment condition, and required certification — and test methods, documents like ASTM E8 (tension testing of metallic materials) or A370 (mechanical testing of steel products) that define exactly how a property must be measured so results are comparable between labs.

ASTM specifications are written to be usable entirely on their own. A structural fabricator buying plate for a non-pressure application can specify “ASTM A516 Grade 70” and receive a fully defined, traceable material with no need to reference ASME or API at all. It is only when that same plate is destined for ASME-code construction that a second layer gets added.

The SA- vs A- distinction ASME republishes selected ASTM specifications under its own “SA-” designation specifically for use inside the BPVC, sometimes with small supplementary requirements added by the relevant ASME subcommittee. ASTM A516 and ASME SA-516 describe essentially the same plate grades, but Section VIII directly invokes only the SA version without requiring a Code Case for equivalence. The same logic applies to filler metals: AWS A5 specifications are republished by ASME under “SFA-” designations in Section II Part C.

What Is API — Industry Practice, Tanks, and Mechanical Integrity

The American Petroleum Institute represents the oil, gas, and petrochemical industry, and its standards fall into two distinct families that are easy to conflate. The first family is original design and construction standards for equipment specific to that industry — API 650 for welded steel atmospheric storage tanks and API 620 for large, low-pressure welded storage tanks are full design-and-build standards, playing a role similar to an ASME code but scoped to tankage rather than pressure vessels or boilers.

The second, arguably more widely cited, family is in-service inspection and mechanical integrity practice — API 510 for pressure vessel inspection, repair, alteration, and rerating, and API 570 for piping inspection, are not construction codes at all. They assume the vessel or pipe was already built correctly under an ASME code and instead define how that asset should be periodically inspected, how corrosion and remaining life should be calculated, and when a repair, rerate, or replacement decision is required years into its service life.

Common mistake Treating API 510 or API 570 as if they were design codes. They are not. A vessel built without proper ASME Section VIII design and fabrication cannot be made compliant simply by running an API 510 inspection program on it afterward. API mechanical integrity standards manage risk on an asset that was already correctly designed and built — they do not retroactively fix a design or fabrication gap.

How the Three Bodies Interlock on a Real Project

Consider a carbon steel pressure vessel fabricated for a refinery process unit. The plate is purchased to ASTM A516 Grade 70, certified with chemistry and tensile results traceable to ASTM A370 and E8 test methods. The vessel itself is designed and fabricated under ASME Section VIII Division 1, which accepts that plate because Section II Part D lists SA-516 Grade 70 with an assigned allowable stress at the design temperature. The welding procedures used to join the shell and heads are qualified under ASME Section IX. Once the vessel is installed and in service, the refinery’s mechanical integrity program applies API 510 to schedule periodic inspections, calculate corrosion rates, and determine remaining life — all without touching the original ASME design basis.

One Vessel, Three Code Bodies ASTM Plate material: ASTM A516 Gr.70 – chemistry, tensile, CVN test methods (A370, E8) Defines WHAT the material is ASME Section VIII Div 1 design + Section II Part D adopts SA-516 (allowable stress) Section IX qualifies the WPS/PQR/WPQ used to weld the vessel Defines HOW it must be designed, built, and welded API API 510 mechanical integrity program: inspection intervals, corrosion rate, remaining life Defines HOW the in-service asset is inspected and maintained
Figure 1: A single pressure vessel relies on ASTM for material definition, ASME for design and fabrication qualification, and API for in-service mechanical integrity, with each layer building on the one below it.

Worked Example: Tracing One Line Item Through All Three Bodies

STEP 1 — Material call-out on the drawing Shell plate: SA-516 Gr.70, normalized SA-516 = ASME’s adopted version of ASTM A516 for BPVC construction STEP 2 — Trace the base ASTM specification ASTM A516 defines: chemistry limits, tensile/yield minimums, CVN options, heat treatment Testing performed per ASTM A370 (mechanical testing) and ASTM E8 (tension test method) STEP 3 — Apply the ASME construction code ASME Section II Part D assigns allowable stress S at design temperature for SA-516 Gr.70 ASME Section VIII Div 1 uses S in shell thickness formula: t = PR / (SE – 0.6P) Result: code-compliant minimum shell thickness STEP 4 — Apply the in-service API standard API 510 schedules inspection interval based on calculated corrosion rate and remaining life Result: next inspection due date, independent of the original ASME design basis

Side-by-Side Comparison Table

FunctionASMEASTMAPI
Original design rulesCore functionNot applicableYes, for tanks/pipelines (API 650, 620)
Material chemistry/propertiesReferences ASTMCore functionReferences ASTM/ASME
Welding qualificationCore function (Section IX)Not applicableReferences ASME Section IX
In-service inspection/fitness-for-serviceLimited (in-service codes exist but narrower)Not applicableCore function (API 510, 570, 579)
Stands alone without the othersRarelyYESDepends on document
Engineering tip When you see a document reference on a drawing or specification, ask yourself which question it is answering. “What is this material made of and how was it tested?” points to ASTM. “How must this be designed, fabricated, and welded?” points to ASME. “How will this be operated, inspected, and kept fit for continued service?” points to API. Most project confusion disappears once you sort documents into those three buckets.

None of the three bodies has independent legal authority; all three are private-sector, consensus-based standards organizations. Their documents become enforceable only when adopted by a government regulation, a jurisdictional boiler and pressure vessel law, or a contract and project specification. In practice, the ASME BPVC has been adopted into law by most U.S. states and many countries worldwide, which is why pressure equipment compliance feels mandatory in a way that a general ASTM specification, used purely by contractual reference, may not. API standards typically gain enforceability through company mechanical integrity programs, insurance requirements, or, in some jurisdictions, regulatory adoption such as OSHA’s Process Safety Management standard referencing recognized and generally accepted good engineering practice.

Supporting note for QA/QC review When auditing a material certificate or fabrication package, confirm three things separately: the ASTM/SA material specification and grade match the design drawing, the ASME code edition and addenda used for design and welding qualification are current and correctly referenced, and, if the asset falls under an owner’s mechanical integrity program, that the relevant API inspection standard and interval have been correctly applied post-commissioning.

Frequently Asked Questions

Is ASME a law, like a government regulation?
Not by itself. ASME codes such as Section VIII are private-sector consensus standards developed by volunteer committees, not government regulations. They become legally enforceable only when a jurisdiction’s boiler and pressure vessel law, a state or national regulation, or a contract specifically adopts them by reference. In practice almost every U.S. state and many countries have adopted the ASME Boiler and Pressure Vessel Code into law, which is why compliance feels mandatory even though ASME itself has no police power.
Why does an ASME code call out an ASTM material instead of just describing the material itself?
Because ASTM specifications already define exactly how a material must be produced, tested, and certified, including chemistry ranges, mechanical property minimums, and required certification documents like a material test report. Rather than rewriting all of that detail, ASME Section II Part D simply lists which ASTM (or harmonized SA) specifications are permitted for pressure-retaining construction, then assigns each one allowable stresses at temperature. This avoids duplicating thousands of pages of material science inside a construction code.
What is the difference between an ASTM A-number and an ASME SA-number for the same material?
They are almost always technically identical specifications; ASME republishes selected ASTM specifications under its own SA designation specifically for use inside the Boiler and Pressure Vessel Code, sometimes with minor supplementary requirements added by the relevant ASME committee. For example, ASTM A516 and ASME SA-516 describe the same pressure vessel plate grades, but only the SA version is directly invoked by Section VIII without an ASME Code Case. Material certified only to the ASTM version may still need an equivalence review before acceptance on an ASME job.
Does API write design rules like ASME, or only inspection rules?
API publishes both, but its core identity is industry operating, inspection, and maintenance practice rather than original pressure-part design. Standards such as API 650 for atmospheric storage tanks and API 620 for low-pressure tanks are genuine design and construction standards, similar in role to an ASME code. Other widely used API documents, including API 510 for pressure vessel inspection and API 570 for piping inspection, govern in-service inspection intervals, fitness-for-service, and repair decisions rather than original fabrication design.
Can a fabricator use an ASTM specification on its own without referencing ASME or API?
Yes, for general commercial steel structures, machinery parts, or any application where no pressure-equipment or process-industry code is contractually invoked, an ASTM material specification can be the only governing document, since ASTM standards are written to be usable independently. It is only when the component is pressure-retaining, in regulated boiler or vessel service, or covered by an owner’s API-based mechanical integrity program that a second layer of ASME or API rules is layered on top of the base ASTM material specification.
Which document should a welding engineer check first when starting a new pressure vessel job?
Start with the project specification and the governing construction code, typically ASME Section VIII Division 1 or 2 for vessels, to confirm design rules, required material specifications, and welding qualification requirements under ASME Section IX. From there, check Section II Part D for the allowable ASTM/SA material specifications, and finally check whether the owner’s operating company layers any API mechanical integrity or inspection requirements, such as API 510, on top of the ASME-built vessel once it enters service.
Do ASTM specifications cover welding consumables too, or only base metals?
ASTM itself focuses primarily on base metal and general material test methods, while filler metal classification is handled mainly by AWS A5 specifications, which ASME republishes under its own SFA designation in Section II Part C in the same way it republishes ASTM base metal specs under SA designations. So for a complete pressure vessel job, an engineer typically references SA material specs for base metal and SFA specs for filler metal, both ultimately rooted in AWS or ASTM source documents. See our guide on welding consumable nomenclature for how filler metal classification works.
Why do API 510 and API 570 matter if the vessel or pipe was already built to ASME?
ASME Section VIII and B31.3 govern new construction design and fabrication, but they do not define how an in-service asset should be periodically inspected, how corrosion rates should be calculated, or when a repair or rerate is required years after installation. API 510 and API 570 fill exactly that gap for pressure vessels and piping respectively, giving plant owners a structured mechanical integrity program that works in conjunction with, not instead of, the original ASME construction code.

Recommended Reference Books

ASME Boiler and Pressure Vessel Code Section VIII Div. 1

The construction code that ultimately invokes ASTM/SA materials and ASME Section IX welding qualification.

View on Amazon

ASTM A370 — Mechanical Testing of Steel Products

The core ASTM test-method standard referenced by nearly every material specification used in pressure equipment.

View on Amazon

API 510 Pressure Vessel Inspector Certification Guide

A practical study and reference guide for API’s pressure vessel in-service inspection standard.

View on Amazon

Pressure Vessel Design Manual

A widely used engineering reference that ties ASME design rules to real ASTM/SA material selection decisions.

View on Amazon

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ASME Section IX Quiz Test your knowledge of WPS, PQR, and WPQ qualification under ASME. ASME Section VIII Div. 1 Quiz Pressure vessel design rules built on ASTM/SA material specifications. P-Number and F-Number Guide How ASME groups ASTM/SA base metals and AWS/SFA filler metals. Mechanical Testing for Welds Tension, bend, and impact testing rooted in ASTM test methods. UG-84 Charpy Impact Testing Toughness testing requirements from ASME Section VIII Division 1. Welding Consumable Nomenclature Decode AWS A5/SFA filler metal classifications used alongside ASTM base metals. Corrosion and Types Guide The degradation mechanisms API mechanical integrity programs are built to manage. Tube-to-Tubesheet Qualification A specialized ASME Section IX qualification scenario for heat exchangers.