What Is the Difference Between ASTM A312 and ASTM A358?

ASTM A312 covers seamless and welded austenitic stainless steel pipe for general corrosive and high-temperature service, typically in sizes NPS 1/8 to NPS 30. ASTM A358 covers electric-fusion-welded (EFW) austenitic stainless steel pipe for high-pressure, high-temperature service, typically in larger diameters (NPS 8 to NPS 96).

The key difference: A312 includes seamless pipe for smaller diameters and higher pressures, while A358 focuses on welded pipe for large-diameter applications.

Key Takeaways

• ASTM A312 includes both seamless and welded pipe; ASTM A358 covers only electric-fusion-welded (EFW) pipe.

• A312 is typically used for NPS 1/8 to NPS 30; A358 is used for larger diameters, typically NPS 8 to NPS 96.

• A312 seamless pipe is preferred for high-pressure, small-diameter applications; A358 EFW pipe is preferred for large-diameter, high-temperature service.

• A358 requires 100% radiographic testing of welds (Class 1), providing superior weld quality assurance compared to A312 welded pipe.

• A358 pipe is generally 20-40% less expensive than equivalent seamless A312 pipe in large diameters.

• Both standards cover the same austenitic stainless steel grades (TP304, TP316, TP321, TP347, etc.) with identical chemical compositions.

• A312 is more widely available from stock; A358 is often produced to order with longer lead times.

• For LNG, nuclear, and petrochemical large-diameter piping, A358 is the industry standard; for refinery and chemical plant process piping, A312 is preferred.

What Is ASTM A312?

ASTM A312 is the most widely used standard for austenitic stainless steel pipe, covering both seamless and welded manufacturing methods for sizes from NPS 1/8 to NPS 30, primarily serving general corrosive and high-temperature applications in chemical plants, refineries, and process piping systems.

ASTM A312/A312M is the ASTM International standard specification for seamless, welded, and heavily cold-worked austenitic stainless steel pipe intended for high-temperature and general corrosive service. First published in 1948 and regularly updated (current edition: A312/A312M-23), this standard is the backbone of stainless steel pipe procurement across multiple industries.

Key characteristics of ASTM A312:

• Manufacturing Methods: Includes seamless (SMLS), electric-resistance-welded (ERW), and submerged-arc-welded (SAW) pipe in a single standard.

• Size Range: Typically NPS 1/8 (DN6) to NPS 30 (DN750), with larger sizes available by agreement.

• Grades Covered: TP304, TP304L, TP304H, TP316, TP316L, TP316H, TP317, TP317L, TP321, TP321H, TP347, TP347H, TP310S, and duplex grades like S31803 and S32750.

• Manufacturing Quality: Seamless pipe is produced by piercing solid billets; welded pipe is produced by rolling plate and welding the seam.

• Testing Requirements: Hydrostatic testing, nondestructive electric testing, and dimensional inspection per ASTM A999.

• Typical Applications: Process piping in refineries, chemical plants, food processing, pharmaceutical facilities, and general corrosive service.

Why ASTM A312 Is So Widely Used:

• Availability: A312 pipe is stocked by distributors worldwide in standard sizes and grades, enabling fast procurement.

• Versatility: The standard covers both seamless and welded pipe, allowing engineers to select the most appropriate manufacturing method for each application.

• Cost-Effectiveness: For smaller diameters (NPS 8 and below), seamless A312 pipe is competitively priced and widely available.

• Code Compliance: A312 is referenced in ASME B31.3 (Process Piping), ASME B31.1 (Power Piping), and most international piping codes.

What Is ASTM A358?

ASTM A358 is the specialized standard for electric-fusion-welded (EFW) austenitic stainless steel pipe in large diameters (NPS 8 to NPS 96), featuring mandatory radiographic examination and strict weld quality control for high-pressure, high-temperature applications such as LNG plants, nuclear facilities, and petrochemical heat exchangers.

ASTM A358/A358M is the ASTM International standard specification for electric-fusion-welded (EFW) austenitic chromium-nickel stainless steel pipe intended for high-pressure and high-temperature service. First published in 1976 (current edition: A358/A358M-23), this standard fills a critical gap: providing a specification for large-diameter welded stainless steel pipe where seamless manufacturing is impractical or prohibitively expensive.

Key characteristics of ASTM A358:

• Manufacturing Method: Electric fusion welding (EFW) only, which includes submerged-arc welding (SAW), gas tungsten arc welding (GTAW), and gas metal arc welding (GMAW) processes.

• Size Range: Typically NPS 8 (DN200) to NPS 96 (DN2400), with wall thicknesses from 3.0mm to 50mm depending on diameter.

• Grades Covered: TP304, TP304L, TP304H, TP316, TP316L, TP316H, TP321, TP321H, TP347, TP347H, and TP310S.

• Weld Classes: Five classes (1 through 5) defining the extent of radiographic examination, from 100% RT (Class 1) to no RT (Class 5).

• Weld Quality: All welds are 100% radiographed in Class 1, ensuring the highest weld quality for critical applications.

• Typical Applications: LNG plant piping, nuclear power piping, petrochemical heat exchanger shells, large-diameter process piping, and power plant piping.

Why ASTM A358 Exists as a Separate Standard:

• Large-Diameter Need: Seamless pipe above NPS 24 is extremely expensive and difficult to produce. A358 provides a standardized specification for welded alternatives.

• Weld Quality Control: A358 includes specific requirements for weld examination that are more rigorous than A312 welded pipe, ensuring reliability in critical service.

• Plate Material Traceability: A358 requires that plate material used for pipe manufacturing meets ASTM A240 specifications, ensuring consistent chemistry and mechanical properties.

• Pressure Vessel Code Alignment: A358 pipe is commonly used in ASME Section VIII (Pressure Vessels) and Section III (Nuclear) applications.

ASTM A312 vs ASTM A358: 20-Point Comparison Table

The comparison table below provides a comprehensive side-by-side analysis of 20 key parameters, showing that while both standards cover similar stainless steel grades, they differ significantly in manufacturing method, size range, weld testing, cost, and typical applications.

Manufacturing Process Differences

ASTM A312 seamless pipe is produced by piercing solid stainless steel billets, while ASTM A358 pipe is manufactured by rolling stainless steel plate into cylindrical form and welding the longitudinal seam using electric fusion welding, with mandatory radiographic inspection of the weld.

Understanding the manufacturing process is essential for material selection, as it directly affects pipe quality, cost, and lead time.

ASTM A312 Seamless Pipe Manufacturing

• Step 1 - Billet Preparation: Stainless steel billets are cut to length, inspected, and heated to approximately 1,200 degrees C.

• Step 2 - Piercing: The heated billet is pierced by a mandrel in a rotary piercing mill (Mannesmann process), creating a hollow shell.

• Step 3 - Rolling: The hollow shell is rolled through a series of rolls to reduce wall thickness and increase length.

• Step 4 - Sizing: The pipe passes through sizing rolls to achieve the final outside diameter within ASTM A999 tolerances.

• Step 5 - Heat Treatment: Solution annealing at 1,040-1,150 degrees C (depending on grade) followed by rapid water quenching.

• Step 6 - Straightening and Cutting: The pipe is straightened, cut to length, and inspected.

ASTM A358 EFW Pipe Manufacturing

• Step 1 - Plate Selection: Stainless steel plate certified to ASTM A240 is selected, with chemistry and mechanical properties verified.

• Step 2 - Forming: The plate is rolled (via plate rolling machine or press brake) into a cylindrical shape with precise edge alignment.

• Step 3 - Welding: The longitudinal seam is welded using submerged-arc welding (SAW), gas tungsten arc welding (GTAW), or gas metal arc welding (GMAW). For thick walls, multiple passes are used.

• Step 4 - Weld Inspection: 100% radiographic testing (RT) for Class 1 pipe; partial RT for Classes 2-4; no RT for Class 5.

• Step 5 - Heat Treatment: Solution annealing at grade-specified temperature, ensuring full carbide dissolution and corrosion resistance restoration.

• Step 6 - Straightening and Finishing: The pipe is straightened, ends are beveled, and final

  dimensional inspection is performed.

Key Manufacturing Differences Summary:

• A312 seamless pipe has no weld seam, eliminating potential weak points for corrosion or fatigue.

• A358 EFW pipe has a longitudinal weld seam that is 100% radiographed (Class 1), providing documented weld quality.

• A312 seamless is limited by piercing mill capacity (typically NPS 24 maximum for seamless).

• A358 can produce pipe up to NPS 96 or larger, limited only by plate rolling capacity.

• A358 welding consumes additional time and requires qualified weld procedures (WPS/PQR per ASME IX).

Which One Should You Choose?

The choice between ASTM A312 and ASTM A358 depends primarily on pipe diameter, pressure requirements, and application type, with A312 preferred for small-diameter process piping and A358 preferred for large-diameter high-temperature service.

 

When to Choose ASTM A312

Choose ASTM A312 when you need small-to-medium diameter pipe (NPS 8 or below), highest pressure integrity (seamless), fast procurement from stock, or compliance with food/pharmaceutical industry standards requiring seamless pipe.

ASTM A312 is the preferred choice for the following applications and industries:

Industries That Typically Use ASTM A312

• Refineries: Process piping for crude oil, intermediate products, and finished fuels. Typical sizes NPS 2 to NPS 12, grades TP316L and TP304L.

• Chemical Plants: Corrosive fluid handling, acid lines, and solvent piping. Typical grades TP316L, TP317L, and TP321.

• Food and Beverage Processing: Sanitary piping systems requiring smooth internal surfaces. Typical grade TP304L and TP316L seamless.

• Pharmaceutical Industry: High-purity piping systems requiring seamless construction for cleanliness and regulatory compliance (FDA, ASME-BPE).

• Pulp and Paper: Process piping for chemical recovery and bleaching. Typical grades TP316L and TP317L.

• Water Treatment: Potable water and wastewater piping. Typical grade TP304L.

Specific Scenarios Where A312 Is the Better Choice:

• Diameter Below NPS 8: Seamless A312 pipe is readily available and competitively priced.

• High-Pressure Service: Seamless A312 pipe has no weld seam, making it the preferred choice for high-pressure applications.

• Cyclic Service: For piping subjected to thermal cycling or vibration, seamless A312 pipe has better fatigue resistance.

• Fast Delivery Needed: A312 pipe in standard sizes and grades is widely stocked by distributors, enabling quick procurement.

• Sanitary Requirements: Food, beverage, and pharmaceutical applications often require seamless pipe for cleanability.

When to Choose ASTM A358

Choose ASTM A358 when you need large-diameter pipe (NPS 12 and above), welded construction with documented quality (100% RT), cost-effective solutions for high-temperature service, or compliance with nuclear/LNG project specifications.

ASTM A358 is the preferred choice for the following applications and industries:

Industries That Typically Use ASTM A358

• LNG Plants: Large-diameter cryogenic piping for liquefied natural gas facilities. Typical sizes NPS 16 to NPS 48, grade TP304L.

• Nuclear Power: Safety-related piping in nuclear power plants requiring Class 1 (100% RT) weld quality. Typical grade TP304L and TP316L.

• Petrochemical: Heat exchanger shells and large-diameter process piping. Typical sizes NPS 24 to NPS 60, grade TP316L.

• Power Generation: Main steam and feedwater piping in thermal power plants. Typical sizes NPS 16 to NPS 36, grade TP304H.

• Offshore Platforms: Large-diameter seawater intake and firewater piping. Typical sizes NPS 24 to NPS 48, grade TP316L.

Specific Scenarios Where A358 Is the Better Choice:

• Diameter Above NPS 12: Seamless pipe becomes prohibitively expensive or unavailable. A358 EFW pipe is the economical solution.

• Documented Weld Quality: A358 Class 1 requires 100% radiographic testing, providing quality assurance for critical applications.

• High-Temperature Service: A358 pipe is specifically designed for high-temperature and high-pressure service with appropriate grade selection.

• Custom Wall Thickness: A358 can be manufactured to custom wall thicknesses by selecting appropriate plate, providing design flexibility.

• Long Pipe Lengths: A358 pipe can be manufactured in custom lengths (up to 12m or longer), reducing field welds and installation time.

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.

Common Grades in Both Standards

Both ASTM A312 and ASTM A358 cover the same core austenitic stainless steel grades (TP304, TP316, TP321, TP347, TP310S) with identical chemical composition requirements per ASTM A240, meaning material properties are equivalent regardless of which manufacturing standard is selected.

Table: Common Grades and Their Applications

 

Important Note on Grade Selection:

• "L" Grades (TP304L, TP316L): Low carbon versions (0.03% max) prevent sensitization during welding. Preferred for welded construction in both A312 and A358.

• "H" Grades (TP304H, TP316H): Higher carbon (0.04-0.10%) for improved elevated-temperature strength. Common in A358 for power plant applications.

• Stabilized Grades (TP321, TP347): Titanium or niobium additions prevent carbide precipitation. Excellent for welded A358 pipe in high-temperature service.

• Chemical Composition: Identical whether produced to A312 or A358, as both reference ASTM A240 for material chemistry.

Testing and Inspection Requirements

ASTM A358 requires more rigorous weld inspection (mandatory radiographic testing for Classes 1-4) compared to ASTM A312 (where NDE is optional), making A358 the preferred choice when documented weld quality is essential for critical applications.

Table: Testing Requirements Comparison

 

A358 Weld Classes Explained:

• Class 1: 100% radiographic examination of all welds. Highest quality assurance. Used for nuclear, LNG, and critical petrochemical applications.

• Class 2: Spot radiographic examination (minimum 1 plate per lot). Moderate quality assurance. Used for general high-temperature service.

• Class 3: Random radiographic examination. Basic quality assurance. Used for non-critical applications.

• Class 4: Radiographic examination only when specified by purchaser. Used for low-pressure applications.

• Class 5: No radiographic examination required. Used for non-pressure applications or where RT is not practical.

Cost Comparison

ASTM A358 EFW pipe is typically 20-40% less expensive than equivalent seamless ASTM A312 pipe in large diameters (NPS 12 and above), while small-diameter A312 seamless pipe is more cost-effective than attempting to produce small A358 welded pipe.

Table: Relative Cost Comparison (TP316L, per meter)

 

Cost Factors to Consider Beyond Material Price:

• Lead Time: A312 standard sizes are stocked (2-4 week delivery); A358 is often made-to-order (8-16 week lead time).

• Welding Costs: A358 pipe requires field welding of circumferential welds; qualified welders and WPS are required.

• Inspection Costs: A358 Class 1 requires 100% RT, adding inspection cost but providing quality assurance.

• Logistics: Large-diameter A358 pipe requires special transport arrangements, adding to delivered cost.

• Total Installed Cost: In large diameters, A358 EFW pipe (including welding and inspection) is typically 30-50% less than equivalent seamless A312.

Expert Recommendations

Based on JN Alloys experience supplying stainless steel pipe to projects across 40+ countries, the selection between A312 and A358 should be driven by application requirements, not by assumption or habit, with A312 preferred for process piping below NPS 24 and A358 preferred for large-diameter and high-temperature applications.

Expert Recommendation 1: For process piping below NPS 24:

"For process piping below NPS 24, ASTM A312 is generally the preferred specification due to its wider availability, lower cost for seamless construction, and simpler procurement process. Seamless A312 pipe eliminates weld seam concerns and is suitable for most refinery, chemical, and food industry applications."

Expert Recommendation 2: For large-diameter piping:

"For large-diameter piping in petrochemical or power generation projects, ASTM A358 often provides a more practical solution because it is manufactured from plate using electric fusion welding, which is more economical than seamless production. Specify Class 1 for critical service to ensure 100% radiographic weld quality."

Expert Recommendation 3: For LNG and cryogenic service:

"For LNG and cryogenic applications, ASTM A358 TP304L or TP316L Class 1 is the industry standard. The 100% RT requirement ensures weld integrity at cryogenic temperatures, and the EFW manufacturing process provides consistent plate material properties verified through A240 certification. Always specify impact testing at the minimum design temperature."

Expert Recommendation 4: For high-temperature service:

"For high-temperature applications above 540 degrees C, specify stabilized grades (TP321H or TP347H) in either A312 or A358. The stabilization prevents sensitization during welding and service, maintaining corrosion resistance. For welded construction, A358 with GTAW root pass and SAW fill passes provides excellent high-temperature performance."

Common Mistakes When Selecting A312 vs A358

The most common mistakes include assuming A358 is simply a substitute for A312 (it is not, due to weld seam considerations), ignoring weld class requirements when specifying A358, and over-specifying seamless A312 for large diameters where A358 would be more economical without compromising quality.

Mistake 1: Assuming A358 is a direct substitute for A312

• Reality: A358 pipe has a longitudinal weld seam, which may affect fatigue life, corrosion resistance at the weld, and inspection requirements.

• Correct Approach: Evaluate whether the application allows welded pipe. If seamless is required for fatigue, cleanliness, or corrosion reasons, A312 seamless is the only option.

Mistake 2: Not specifying weld class for A358

• Reality: A358 has 5 weld classes with vastly different inspection levels. Simply ordering "A358" without specifying the class can result in receiving Class 5 pipe with no radiographic testing.

• Correct Approach: Always specify the weld class (e.g., "ASTM A358 TP316L Class 1") and require 100% RT for critical applications.

Mistake 3: Over-specifying seamless A312 for large diameters

• Reality: Seamless A312 pipe above NPS 12 is expensive and may have long lead times. A358 Class 1 with 100% RT can provide equivalent quality at 30-50% lower cost.

• Correct Approach: Evaluate A358 Class 1 for diameters above NPS 12 unless the application specifically requires seamless construction.

Mistake 4: Ignoring heat treatment requirements

• Reality: Both A312 and A358 require solution annealing per the grade specification. Skipping or incorrectly performing heat treatment can result in sensitization and premature corrosion failure.

• Correct Approach: Always specify solution annealed condition and request heat treatment certificates with material test reports.

Mistake 5: Not requiring PMI verification

• Reality: Grade mix-ups are common, especially between TP304 and TP316. Without PMI, incorrect material may be installed, leading to premature corrosion failure.

• Correct Approach: Require Positive Material Identification on 100% of pipes and fittings before installation.

Frequently Asked Questions

Q1: Is ASTM A358 stronger than ASTM A312?

No. The base material strength is identical because both standards use the same stainless steel grades (TP304, TP316, etc.) with the same chemical composition per ASTM A240. However, seamless A312 pipe may have slightly better fatigue resistance because it has no weld seam. A358 Class 1 with 100% RT provides documented weld quality that can match seamless pipe performance for most applications.

Q2: Can ASTM A358 replace ASTM A312?

It depends on the application. A358 can replace A312 welded pipe in many applications, but it cannot replace A312 seamless pipe where seamless construction is specifically required (e.g., food/pharmaceutical sanitary piping, high-fatigue service, or client specifications requiring seamless). Always check the project specification and applicable code (ASME B31.3, B31.1) before substituting.

Q3: Is ASTM A358 seamless?

No. ASTM A358 covers only electric-fusion-welded (EFW) pipe. It does not include seamless pipe. If seamless pipe is required, you must use ASTM A312 (or ASTM A376 for high-temperature service, or ASTM A790 for duplex grades).

Q4: Is ASTM A312 always seamless?

No. ASTM A312 includes three manufacturing types: seamless, welded (ERW or SAW), and heavily cold-worked. The standard covers both seamless and welded pipe. When ordering A312, you must specify the manufacturing type (seamless, ERW, or SAW).

Q5: Which is cheaper, ASTM A312 or ASTM A358?

For small diameters (NPS 8 and below), A312 seamless pipe is typically cheaper because it is mass-produced. For large diameters (NPS 12 and above), A358 EFW pipe is 20-40% cheaper than seamless A312 because welding plate is far more economical than piercing large billets. For medium diameters (NPS 8-12), costs are comparable and depend on market conditions.

Q6: Which has better corrosion resistance, ASTM A312 or ASTM A358?

The base metal corrosion resistance is identical because both use the same grades. However, A312 seamless pipe has no weld seam, which eliminates potential weld-zone corrosion. A358 EFW pipe has a weld seam that is solution-annealed after welding to restore corrosion resistance. In highly corrosive service, seamless A312 is preferred; in moderate service, A358 Class 1 is perfectly adequate.

Q7: Can ASTM A358 be used for pressure piping?

Yes. ASTM A358 is specifically designed for high-pressure and high-temperature service. It is referenced in ASME B31.3 (Process Piping) and ASME B31.1 (Power Piping). A358 Class 1 with 100% RT is suitable for pressure piping in petrochemical, power generation, and nuclear applications. Always verify that the specific grade and class meet your design code requirements.

Q8: What sizes does ASTM A358 cover?

ASTM A358 typically covers NPS 8 (DN200) to NPS 96 (DN2400), with wall thicknesses from 3.0mm to 50mm. While smaller sizes are technically possible, they are rarely produced because A312 is more economical for small diameters. For sizes above NPS 96, custom manufacturing arrangements can be made with the pipe mill.

Q9: Does ASTM A358 require RT (radiographic) testing?

It depends on the class. A358 Class 1 requires 100% radiographic testing of all welds. Class 2 requires spot RT. Class 3 requires random RT. Class 4 requires RT only when specified by the purchaser. Class 5 requires no RT. For critical applications, always specify Class 1 to ensure full weld quality documentation.

Q10: Is ASTM A358 suitable for LNG (cryogenic) service?

Yes. ASTM A358 TP304L and TP316L Class 1 are widely used in LNG plants worldwide for cryogenic piping at temperatures as low as -196 degrees C. The 100% RT requirement ensures weld integrity at cryogenic temperatures. Impact testing (Charpy V-notch) at the minimum design temperature must be specified and verified.

Q11: What is the difference between A358 Class 1 and Class 5?

Class 1 requires 100% radiographic examination of all longitudinal welds, providing the highest level of weld quality assurance. Class 5 requires no radiographic examination at all. Class 1 is used for critical pressure piping in petrochemical, nuclear, and LNG applications. Class 5 is used for non-critical or non-pressure applications where weld quality documentation is not required.

Q12: Can I use A312 welded pipe instead of A358?

Technically yes, but A312 welded pipe does not have the same weld examination requirements as A358. A312 welded pipe (ERW or SAW) does not require mandatory RT. If you need documented weld quality (100% RT), specify A358 Class 1 rather than A312 welded. If weld quality is not a concern, A312 welded may be acceptable.

Q13: What is the maximum temperature for A312 and A358 pipe?

The maximum temperature depends on the grade, not the standard. TP304H and TP316H can be used up to 760 degrees C in ASME B31.1 service. TP310S can be used up to 1,100 degrees C. TP321H and TP347H are used up to 815 degrees C. Both A312 and A358 can be produced in any of these grades with the same temperature limits.

Q14: Does A358 pipe need post-weld heat treatment (PWHT)?

No. Austenitic stainless steel pipe (both A312 and A358) does not typically require PWHT after welding. Instead, solution annealing at the grade-specified temperature is used to restore corrosion resistance in the weld zone. However, for field welds, solution annealing is usually not practical, and the as-welded condition is generally acceptable for "L" grades (TP304L, TP316L) due to their low carbon content.

Q15: How do I specify A358 pipe correctly when ordering?

Specify: (1) Standard: ASTM A358/A358M, (2) Grade: TP316L, (3) Class: 1 (for 100% RT), (4) Size: NPS 24, (5) Wall thickness: Sch 40S or custom, (6) Length: 6m random, (7) End: beveled for welding, (8) Heat treatment: solution annealed, (9) Material test reports: EN 10204 3.1, (10) PMI: required. Example: "ASTM A358 TP316L Class 1, NPS 24, Sch 40S, 6m random length, BE, solution annealed, MTR 3.1, PMI required."