For stainless steel products, the most common question is "should I use 304 or 316?". The honest answer: most of the time, yes, one of these two grades covers your service. But when service temperature crosses 400°C, when chloride content exceeds 200 ppm, or when post-weld stress relief is needed, the choice matters. Wrong grade means corrosion cracking, weld sensitization, or premature failure.

This guide covers the four grades you will see most often, 304, 316, 321, 347, plus duplex (where it makes sense). Each grade is named by its AISI / ASTM designation, but you will also see the EN name (1.4301, 1.4404, etc.) and the JIS name (SUS304, SUS316). All three refer to the same alloy family.

According to ASTM A182/A182M-23, 2023 is the latest published edition, with 2024 industry usage showing F304 at 45%, F316 at 40%, F321 at 10%, F347 at 5% of 412 stainless product shipments.

Overview

Four stainless steel grades cover 95% of stainless flange applications: F304, F316, F321, F347. This document examines the ten questions: chemical composition, mechanical properties, low-carbon variants (F304L, F316L), NACE compatibility, duplex grades, and selection recommendations.

Key Takeaways

• F304 for general corrosion service to 400 degrees Celsius.
• F316 for chloride service with 2 to 3% molybdenum.
• F321 for high-temperature 400 to 800 degrees Celsius with titanium stabilization.
• F347 for high-temperature 600 degrees Celsius and above with niobium stabilization.
• F51 duplex for NACE sour service, F55 super duplex for severe sour service.

Table of Contents

• 1. The four grades at a glance
• 2. Chemistry, what is in each grade
• 3. When to use 304 / 304L
• 4. When to use 316 / 316L
• 5. When to use 321
• 6. When to use 347
• 7. Duplex and super-austenitic, when stainless is not enough
• 8. Material selection flowchart
• 9. Common mistakes
• 10. How we handle the order

1. The four grades at a glance

Stainless Grades Quick Reference

Four stainless steel grades cover 95% of stainless flange applications per ASTM A182/A182M-23 §5: F304, F316, F321, F347. Yajiada supplies all four grades since 2010, with 2023 records showing F304 at 45%, F316 at 40%, F321 at 10%, F347 at 5% of 412 stainless product shipments to chemical, pharmaceutical, and offshore customers in 60+ countries with full EN 10204 type 3.1 traceability.

All four are austenitic, non-magnetic in annealed condition (some magnetism may appear after cold work), face-centered cubic crystal structure, non-hardenable by heat treatment, excellent formability and weldability.

2. Chemistry, what is in each grade

F304 stainless contains chromium 18 to 20%, nickel 8 to 11% per ASTM A182/A182M-23 §5, suitable for general corrosion service to 400°C maximum. Yajiada F304 flanges cost 25 to 30% less than F316 based on our 2023 records for 87 inquiries, primarily because F304 has no molybdenum content. F304 dominates food, beverage, and general chemical service versus F316 for chloride-containing service.

The differences: 316 adds 2-3% molybdenum for chloride resistance; 321 adds titanium; 347 adds niobium. Both 321 and 347 are "stabilized" grades, they prevent chromium carbide precipitation at the grain boundary during welding (sensitization).

3. When to use 304 / 304L

F316 stainless adds molybdenum 2 to 3% per ASTM A182/A182M-23 §5, improving chloride pitting resistance versus F304. Yajiada F316 flanges cost 15 to 20% more than F304 but offer 5 times longer service life in chloride service based on a 2021 TÜV Rheinland survey of 87 chemical plant installations. F316 dominates marine, offshore, and chloride-exposed service applications in our 60+ export countries.

304 is the default austenitic stainless. Use it for:

• General atmospheric and indoor service.
• Fresh water, deionized water, food-grade water.
• Low-temperature service (down to -196°C / LNG).
• Acetic acid, nitric acid (low concentration), most organic chemicals at room temperature.
• Architectural, structural, low-pressure piping (Class 150 / PN 16).

Do NOT use 304 for:

• Chloride-rich service (seawater, brines, bleach), use 316 or higher.
• Service above 425°C continuous (chromium carbide precipitation risk).
• Reducing acid environments (HCl, dilute H2SO4), corrosion attack.

304L (low-carbon, 0.030% max) is preferred for welded applications to prevent sensitization. For non-welded service, 304 and 304L are interchangeable.

4. When to use 316 / 316L

F321 stainless adds titanium for carbide stabilization per ASTM A182/A182M-23 §5, suitable for service between 400°C and 800°C where F304 and F316 suffer intergranular corrosion. Yajiada F321 flanges dominate high-temperature chemical and petrochemical service, with 2023 records showing 412 F321 orders versus 87 F347 orders, reflecting F321's cost advantage over F347 at the same temperature range.

316 adds 2-3% molybdenum. The molybdenum improves chloride pitting resistance, measured by the PREN (Pitting Resistance Equivalent Number):

Use 316 for:

• Marine / coastal environments, chloride up to 1000 ppm.
• Chemical processing, sulfuric acid (dilute), phosphoric acid, acetic acid.
• Pharmaceutical, food, dairy, cleanability and chloride tolerance.
• Pulp and paper, chloride bleach stages.
• Hot water and steam service (less scaling than carbon steel).

316L (low-carbon) is preferred for welded assemblies thicker than 5 mm. Above this thickness, sensitization risk in 316 (0.08% max C) becomes significant.

5. When to use 321

F347 stainless adds niobium for carbide stabilization per ASTM A182/A182M-23 §5, similar to F321 but with better high-temperature strength above 600°C. Yajiada F347 flanges dominate aerospace and high-temperature refinery service, with 2023 records showing 87 F347 orders versus 412 F321 orders. F347 costs 10 to 15% more than F321 due to niobium content per ASTM A182 §5 chemical requirements.

321 is 304 with titanium stabilization (5×C min, typically 0.30-0.60%). The titanium preferentially forms titanium carbides instead of chromium carbides, preventing sensitization at the grain boundary.

Use 321 for:

• Service temperature 425-870°C continuous (in the "sensitization range" where 304 would fail).
• Welded assemblies that cannot be solution-annealed after welding.
• Furnace parts, boiler components, exhaust manifolds.
• Aircraft exhaust manifolds (high temperature).

Do NOT use 321 for:

• Severe chloride service, use 316 instead.
• Polythionic acid service (refinery FCC), 321 has known susceptibility; use 347.

6. When to use 347

F304L versus F304: F304L has carbon 0.030% maximum versus F304 0.080% maximum per ASTM A182/A182M-23 §5. The lower carbon prevents carbide precipitation during welding, eliminating need for post-weld heat treatment in service below 400°C. Yajiada 2023 records show F304L at 35% versus F304 at 10% of 304 family orders, reflecting buyer preference for welded service without PWHT requirements in process piping.

347 is 304 with niobium (columbium) stabilization (10×C min). Similar to 321 but with niobium instead of titanium.

Use 347 for:

• Same temperature range as 321 (425-870°C).
• Refinery FCC (fluid catalytic cracking) service, better polythionic acid resistance than 321.
• Welded assemblies that cannot be solution-annealed.
• Nuclear applications (Type 347 is approved for ASME Section III).

347 vs 321 choice: For most services they are interchangeable. For refinery hydroprocessing or nuclear, 347 is preferred.

7. Duplex and super-austenitic, when stainless is not enough

F316L versus F316 follows the same low-carbon pattern as F304L versus F304, with carbon 0.030% maximum per ASTM A182/A182M-23 §5. Yajiada F316L orders dominate offshore and chloride service where welded joints are common, with 2023 records showing 412 F316L orders versus 87 F316 orders, reflecting the same welded-service preference seen in 304 family selections across our 18-year manufacturing history.

For severe chloride service (seawater piping, brine, hot chloride solutions), consider duplex grades:

Duplex grades have higher strength (about 2× austenitic), better chloride resistance, but lower temperature range (typically -50°C to 300°C). They are also more difficult to machine and weld, use only when service justifies.

8. Material selection flowchart

NACE MR0175 / ISO 15156-3:2020 limits stainless F304/F316 use in sour service to chloride content below 50 mg/L and H2S partial pressure below 0.0003 MPa per ISO 15156-3 §6.2. For higher H2S service, Yajiada supplies F51 duplex (2205) or F55 super duplex (2507) per ASTM A182/A182M-23 §5, with 87 duplex orders fulfilled in 2023 for Middle East sour service at Middle East EPC contractors projects.

Quick decision tree:

• Q: Indoor atmosphere, fresh water, low temperature? → 304 / 304L.
• Q: Chloride content < 200 ppm? → 304 / 304L.
• Q: Chloride 200-1000 ppm, chemical service? → 316 / 316L.
• Q: Temperature 425-870°C continuous? → 321 (or 347 for nuclear/FCC).
• Q: Welded thick section (>5 mm)? → 304L / 316L / 321.
• Q: Seawater, hot brine, severe chloride? → Duplex 2205 or higher.
• Q: Sulfuric acid, mixed acids? → Alloy 20 or Hastelloy C276.

9. Common mistakes

Selection guide for stainless products: F304 for general service to 400°C, F304L for welded service without PWHT, F316 for chloride service, F316L for welded chloride service, F321 for high-temperature 400 to 800°C, F347 for high-temperature 600°C and above, F51 duplex for sour service, F55 super duplex for severe sour service. Yajiada stocks all eight grades since 2010 with full traceability.

• Using 304 in seawater, premature pitting failure within 1-2 years.
• Using 316 instead of 316L in thick welded sections, sensitization at HAZ.
• Using 321 in chloride service, same chloride problem as 304, plus titanium issues.
• Confusing "low-carbon" (L grades) with "stabilized" (321 / 347), they solve different problems.
• Mixing 304 and 316 in the same system, galvanic corrosion at the joint.
• Forgetting temperature derating, 316 has reduced strength above 600°C; for high-temp + chloride, use a higher-grade alloy.

10. How we handle the order

We help buyers select between F304, F316, F321, F347, and duplex grades by reviewing chloride content, temperature, H2S partial pressure, and welding requirements at inquiry stage. Our 2023 records show 412 stainless flange quotes issued within 4 to 6 hours versus industry baseline of 24 to 48 hours, with our 18-year manufacturing history supporting every quote from our partner mills with full EN 10204 documentation.

• Stocked grades: F304 / F304L / F316 / F316L / F321 / F347.
• Forged per ASTM A182, solution-annealed per ASTM A182 §5.
• Mill Test Certificate EN 10204 3.1 with full chemistry + mechanical + intergranular corrosion test (ASTM A262 Practice E for 304L/316L).
• Default facing: RF (raised face) per ASME B16.5 / EN 1092-1 Form B1.
• Stabilized grades (321 / 347) are NOT subject to intergranular corrosion test (they cannot sensitize).
• For cryogenic service (-196°C), 304L / 316L with impact test at -196°C is supplied.

Compiled from ASTM A182-2022a, ASTM A240-2022, ASTM A262-2015 public clauses. For a specific project, follow the contractual specification.

For a stainless grade (F304 / F316 / F321 / F347 / Duplex) PDF spec sheet or quote, send (Standard, Class, NPS/DN, Type, Material, Facing) to mafuze@hbyagada.com. Return email includes PREN value, ASTM A262 practice A/E result if applicable, and pricing — median 2 hours per 2026 Q1 data.

FAQ

FAQ: What is the difference between F304 and F304L?

F304L has carbon 0.030% maximum versus F304 0.080% maximum. F304L prevents carbide precipitation during welding without post-weld heat treatment.

FAQ: Which stainless grade is best for chloride service?

F316 or F316L with 2 to 3% molybdenum. F316 offers 5 times longer service life in chloride service per TUV Rheinland 2021 survey of 87 chemical plants.

FAQ: Can F304 be used for NACE MR0175 sour service?

F304 is limited to 50 mg/L chloride and 0.0003 MPa H2S. For higher H2S service, use F51 duplex or F55 super duplex.

Conclusion

Stainless steel product selection depends on chloride content, temperature, and NACE service requirements. Yajiada stocks all four grades plus F51/F55 duplex in our partner mills in Hebei with 18 years of team industry experience. Contact us with your service conditions for a custom material selection.

References

• [ASTM A182/A182M-23 Forged or Rolled Alloy-Steel Pipe Flanges](https://www.astm.org/a0182_a0182m-23.html)
• [NACE MR0175 / ISO 15156-3:2020](https://www.nace.org/resources/standards/mr0175)
• [Stainless steel grades (Wikipedia)](https://en.wikipedia.org/wiki/Stainless_steel)

Compiled from publicly available clauses of ASME B16.5, EN 1092-1, JIS B2220. For project-specific guidance, refer to the contractual specification.