Views: 3 Author: Monica Publish Time: 2026-04-27 Origin: Site
ASTM A312 covers both seamless and welded austenitic stainless steel pipe intended for high-temperature and general corrosive service. ASTM A358 covers electric-fusion-welded (EFW) austenitic stainless steel pipe specifically designed for high-pressure, high-temperature service—usually in larger diameters with more rigorous weld examination requirements.
When choosing stainless steel pipe for industrial applications, two ASTM standards dominate procurement lists: ASTM A312 and ASTM A358. Both cover austenitic stainless steel pipes used in pressure service. Yet they differ substantially in manufacturing method, weld inspection requirements, applicable size range, and intended service conditions. This article provides a clear, side-by-side comparison table for you.
ASTM International (formerly the American Society for Testing and Materials) publishes technical standards used worldwide in engineering, manufacturing, and procurement. Within its piping standards family, A312 and A358 both fall under the umbrella of austenitic stainless steel pipe—meaning pipe made from iron-chromium-nickel alloys with a face-centered cubic crystal structure that delivers excellent corrosion resistance and toughness at both cryogenic and elevated temperatures. To understand why two standards exist, it helps to first understand what makes them different at a fundamental level: the manufacturing process and the inspection.
ASTM A312 is one of the most widely used stainless steel pipe standards in the world. It covers pipe intended for high-temperature and general corrosive service and includes three manufacturing routes:
• Seamless pipe — produced by hot extrusion or rotary piercing of a solid billet
• Welded pipe — produced by forming strip or plate and longitudinally welding the seam (ERW or EFW)
• Heavily Cold Worked (HCW) pipe — a special category that achieves enhanced mechanical properties through cold reduction after welding
ASTM A312 encompasses a broad range of austenitic grades, including:
• TP304 / TP304L — the workhorse grades; 18% chromium, 8% nickel
• TP316 / TP316L — with 2–3% molybdenum for enhanced chloride resistance
• TP321 — titanium-stabilized; excellent for high-temperature service
• TP347 — niobium-stabilized; preferred for nuclear and high-temperature applications
• TP904L, TP310S, and others for specialized corrosive environments
Dual certification (e.g., TP304/304L) is common and accepted under A312, provided the material meets the chemical and mechanical requirements of both grades.
For welded pipe under A312, a hydrostatic test is standard. Nondestructive examination (NDE) — typically ultrasonic testing (UT) or radiography (RT) — can substitute for the hydrostatic test, but full radiographic examination of all welds is not universally mandatory for all product grades. The standard provides supplementary requirements (e.g., S1 through S9) that purchasers may invoke to add extra examination obligations.
• Food and beverage processing lines
• Pharmaceutical manufacturing (sanitary piping)
• Chemical and petrochemical plants
• Power generation (steam and condensate systems)
• Water treatment and desalination
• Pulp and paper industry
ASTM A358 — Standard Specification for Electric-Fusion-Welded Austenitic Chromium-Nickel Stainless Steel Pipe for High-Temperature Service and General Applications
ASTM A358 covers pipe manufactured exclusively by the Electric Fusion Welding (EFW) process — a technique where plate or strip is formed into a cylinder and the longitudinal seam is joined by an electric arc welding process using a filler metal.
The standard is designed for large-diameter, high-pressure, and high-temperature service, typically in sizes where seamless production is impractical or cost-prohibitive. A358 pipe is manufactured in NPS 2" through NPS 30" (and larger by agreement), though it is most commonly specified for NPS 16" and above where seamless pipe is unavailable or excessively expensive.
One of the most important — and often misunderstood — features of ASTM A358 is its five-class system, which defines the level of weld examination and quality assurance applied to the pipe:
Class | Weld Examination | Typical Use Case |
|---|---|---|
Class 1 | 100% radiographic examination (RT) | High-pressure, critical service — maximum quality assurance |
Class 2 | Spot radiographic examination | Moderate-pressure service with selective inspection |
Class 3 | Hydrostatic test only | Lower-pressure general service |
Class 4 | No RT; NDE per purchaser | Per specific agreement between manufacturer and buyer |
Class 5 | 100% ultrasonic testing (UT) | Alternative to Class 1 when RT is impractical |
Class 1 — requiring full radiographic examination of every weld — is the most commonly specified class in critical applications such as power generation and heavy chemical processing.
ASTM A358 covers the same suite of austenitic grades as A312, including TP304, TP304L, TP316, TP316L, TP321, TP347, and several others. Dual-certified material is also common.
• Steam supply and extraction lines in power plants
• Large-bore process piping in oil refineries and petrochemical plants
• High-pressure water injection systems
• Heat recovery steam generators (HRSGs)
• Large-diameter headers and main distribution lines
The table below summarizes the most important differences and similarities between the two standards:
Feature | ASTM A312 | ASTM A358 |
|---|---|---|
Manufacturing Process | Seamless or Welded (EFW/ERW) | Electric Fusion Welded (EFW) Only |
Wall Thickness (NPS ≤ 8") | As specified; no radiography for seamless | Class 1 requires full radiography |
Applicable Size Range | NPS 1/8" to 30" and larger | NPS 16" and larger (typically) |
Pressure Rating | Standard service pressure | Higher pressure capability (Class 1) |
Testing Requirements | Hydrostatic or NDE per grade | Mandatory RT or UT on all welds (Class 1) |
Typical Applications | General piping, food, pharma, chemical | Large-diameter high-pressure systems |
Common Grades | TP304, TP316, TP321, TP347 | 304/304L, 316/316L, 321, 347 |
Dual Certification | Common (e.g., 304/304L) | Common (e.g., 304/304L) |
ASME Code Acceptance | Yes (B31.3, Section I, VIII) | Yes (B31.1, B31.3, Section I, VIII) |
Cost (General) | Lower to moderate | Moderate to higher (larger, more testing) |
Manufacturing Process
This is the most fundamental distinction. ASTM A312 permits seamless, welded, and HCW pipe — giving engineers and fabricators maximum flexibility in sourcing. ASTM A358 covers only EFW pipe. EFW involves welding with filler metal, making it distinct from the higher-heat ERW (Electric Resistance Welded) process and enabling production of heavier wall thicknesses and larger diameters.
In practical terms, if a project calls for NPS 6" seamless pipe, A312 is the applicable standard. If it calls for NPS 24" welded pipe for a high-pressure steam header, A358 (Class 1) is the specification to invoke.
Weld Examination Requirements
A312 welded pipe requires either a hydrostatic test or, as an alternative, NDE per specified supplementary requirements. The baseline does not mandate 100% weld radiography unless the purchaser invokes supplementary requirements.
A358, by contrast, is built around a tiered weld quality system. Class 1 mandates 100% RT of the entire weld length — a rigorous requirement that provides a high level of assurance against internal weld defects such as porosity, lack of fusion, and inclusions.
This difference is critical when the pipe is destined for high-pressure steam or aggressive chemical service where a weld defect could have catastrophic consequences.
Applicable Diameter Range
A312 is available in sizes from NPS 1/8" to NPS 30" (and beyond for some welded grades). Seamless A312 pipe is readily available up to around NPS 12" to NPS 14"; larger seamless diameters become increasingly expensive and may require extended lead times. A358 is primarily a large-diameter standard. While it technically begins at NPS 2", it is almost exclusively used for NPS 16" and above. Below that range, A312 or A409 (for large-diameter, lighter-wall pipe) are more common choices.
Pressure and Temperature Rating Implications
Both standards produce pipe acceptable for pressure service under ASME B31.1 (Power Piping) and B31.3 (Process Piping). However, the enhanced weld examination of A358 Class 1 qualifies the pipe for a weld joint efficiency factor (E) of 1.0 under ASME Section I and Section VIII, effectively allowing the pipe wall to be designed at full material strength — no penalty for the weld seam. A312 welded pipe with spot or no radiography typically uses a lower joint efficiency factor, which means slightly thicker walls may be required to achieve the same pressure rating.
Choose ASTM A312 when: the pipe diameter is NPS 14" or smaller; seamless pipe is preferred or required; the service is general corrosive, food-grade, pharmaceutical, or moderate-pressure; budget constraints favor a lower-cost standard; or the project code allows welded pipe without 100% RT.
Choose ASTM A358 when: the pipe diameter is NPS 16" or larger; the service involves high-pressure steam or aggressive process fluids; the piping code requires a weld joint efficiency factor of 1.0; maximum quality assurance on the weld seam is required (invoke Class 1); or the design operates at elevated temperatures where any weld defect poses an unacceptable risk.
Always consult the applicable piping code (ASME B31.1, B31.3, etc.) and your engineering specification before finalizing the standard. In some projects, both standards may appear — A312 for small-bore branch connections and A358 for the large-bore main header, for example.
Both ASTM A312 and A358 require mill test reports (MTRs) certified to the standard. For ASTM A358 Class 1 pipe, the MTR must include radiographic examination results in addition to chemical and mechanical test data. When reviewing MTRs:
• Confirm the heat number, pipe size, wall thickness, and schedule match the purchase order • Verify the chemical composition falls within the specified limits for the declared grade
• Check that tensile and yield strength values meet or exceed minimum requirements
• For A358 Class 1: confirm that 100% RT was performed and accepted per the standard
• Look for the certifying inspector's signature and the mill's accreditation body
Counterfeit and mislabeled pipe is a real concern in global markets. Engage certified, reputable manufacturers and, when warranted, retain a third-party inspection agency to witness testing at the mill.
Can ASTM A312 and A358 pipe be used interchangeably?
Not without engineering review. While both standards cover austenitic stainless steel pipe, the manufacturing and inspection requirements differ. A358 Class 1 pipe meets more stringent quality criteria than baseline A312 welded pipe. Substituting A312 welded pipe for A358 Class 1 in a high-pressure application may not satisfy the applicable piping code.
Is seamless pipe better than EFW pipe?
Not categorically. Seamless pipe has no longitudinal weld seam, which eliminates a potential point of failure. However, EFW pipe produced under A358 Class 1 — with 100% radiographic examination — provides a high level of weld integrity assurance. For large diameters, A358 EFW pipe is often the only practical option. The choice depends on size, service conditions, and code requirements rather than a blanket quality judgment.
What is the difference between ERW and EFW welded pipe?
ERW (Electric Resistance Welded) pipe is made by passing a high-frequency electric current through the strip edges; the heat of resistance fuses them together without filler metal. EFW (Electric Fusion Welded) pipe uses an arc welding process with filler metal, producing a weld that can be deposited in multiple passes for heavier walls. A358 specifies EFW exclusively; A312 welded pipe can be either ERW or EFW.
Do both standards apply to nickel alloys?
Neither A312 nor A358 applies to nickel alloys such as Alloy 625, Alloy 825, or Hastelloy C-276. Those materials are covered by separate ASTM standards (e.g., ASTM B444 for nickel alloy seamless pipe). However, the selection logic between seamless versus EFW pipe, and between baseline versus radiographically examined pipe, follows similar engineering principles for nickel alloy piping.
What schedule (wall thickness) options are available under each standard?
ASTM A312 pipe is available in a wide range of schedules from SCH 5S through SCH XXS. ASTM A358 pipe is typically produced in standard wall schedules and heavier; the specific schedules available depend on the manufacturer's capability for a given diameter. For very large diameters, custom wall thicknesses may be manufactured to order.
ASTM A312 and ASTM A358 are complementary, not competing, standards. A312 is the versatile, widely available specification that covers seamless and welded austenitic stainless steel pipe for the broadest range of applications. A358 is the large-diameter, high-integrity EFW standard — purpose-built for situations where a weld defect cannot be tolerated and where the pipe diameter exceeds practical seamless production limits.
Making the right choice lies in understanding four factors: the pipe diameter, the manufacturing method, the weld examination level demanded by the piping code and service conditions, and the pressure and temperature design parameters.
Our company specializes in stainless steel and nickel alloy products manufactured to ASTM A312, A358, and related standards. Contact us for material selection assistance, technical datasheets, and competitive pricing on stocked and custom-manufactured pipe.
References and Standards
ASTM A312 / A312M — Standard Specification for Seamless, Welded, and Heavily Cold Worked Austenitic Stainless Steel Pipes
ASTM A358 / A358M — Standard Specification for Electric-Fusion-Welded Austenitic Chromium-Nickel Stainless Steel Pipe for High-Temperature Service and General Applications
ASME B31.1 — Power Piping
ASME B31.3 — Process Piping
ASME Boiler and Pressure Vessel Code, Section I (Power Boilers) and Section VIII (Pressure Vessels)
ASME B36.19M — Stainless Steel Pipe (dimensions and schedules)
About the Author This article was prepared by the technical content team of a manufacturer specializing in stainless steel and nickel alloy pipe, fittings, and flanges. Our engineers hold certifications in materials science and piping system design and draw on decades of practical experience in supplying products to the power, chemical, and oil and gas industries worldwide.
