Selecting the right stainless steel pipe grade is not always straightforward. A specification such as “304 or equivalent” may look simple, but the actual choice depends on factors such as corrosion exposure, welding requirements, operating temperature and service conditions. In some applications, 304 provides everything required, while others may need the additional chloride resistance of 316 or the high-temperature performance of 321.
Choosing a grade that is more expensive than necessary can increase material costs, while selecting a grade that does not match the service conditions can lead to premature corrosion, weld-related problems or reduced service life. Finding the necessary information in one place can be a challenge instead of sifting through a range of technical data sheets.
In the sections below, we discuss the common grades of stainless steel pipe namely 304, 304L, 316, 316L, 321 and 347 on the basis of their chemical composition, mechanical properties, corrosion resistance, and applicability in real life in the following paragraphs.
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What Do the Numbers and Letters Actually Mean?
The term “L” in alloys such as 304L and 316L indicates the quality of carbon in these alloys, which is restricted to a maximum of 0.035%, in comparison to 0.08% carbon in standard grades. This is important due to the possibility of carbon interacting with chromium during the welding process to produce chromium carbide, causing a decrease in the chromium content in the welded areas and making the weld more prone to corrosion. By reducing the upper percentage limit of carbon in the L-grade alloys, it is possible to significantly minimise the potential for the formation of carbide. Therefore, this is why grades 304L and 316L should be used for applications that require welding and expose the material to a possibly corrosive environment without post-weld heat treatment of the material.
321 and 347 solve the same sensitisation problem a different way: instead of lowering carbon, they add a stabilising element, titanium in 321, niobium in 347, that reacts with carbon preferentially, forming stable titanium or niobium carbides instead of chromium carbides. Chromium stays in solution, protecting the steel, even after prolonged exposure to the 425–870°C range where sensitisation is most aggressive. That's why 321 and 347 are the standard choice for high-temperature service with heavy welding, such as exhaust systems and high-temperature process piping, rather than the L-grades, which aren't rated for sustained high-temperature exposure in the same way.
Chemical Composition of 304, 316, 321 and 347
Composition ranges below follow ASTM A312 for seamless and welded austenitic stainless steel pipe. Phosphorus (max 0.045%) and sulphur (max 0.030%) limits apply to all six grades.
| Grade | C (max %) | Cr (%) | Ni (%) | Mn (max %) | Si (max %) | Mo (%) | Other |
| 304 | 0.08 | 18.0–20.0 | 8.0–10.5 | 2.00 | 1.00 | ||
| 304L | 0.035 | 18.0–20.0 | 8.0–13.0 | 2.00 | 1.00 | ||
| 316 | 0.08 | 16.0–18.0 | 10.0–14.0 | 2.00 | 1.00 | 2.00–3.00 | |
| 316L | 0.035 | 16.0–18.0 | 10.0–14.0 | 2.00 | 1.00 | 2.00–3.00 | |
| 321 | 0.08 | 17.0–19.0 | 9.0–12.0 | 2.00 | 1.00 | Ti ≥ 5×C | |
| 347 | 0.08 | 17.0–19.0 | 9.0–13.0 | 2.00 | 1.00 | Nb ≥ 10×C |
Mechanical Properties Comparison
Minimum mechanical properties per ASTM A312, plus an approximate Pitting Resistance Equivalent Number (PREN) for each grade; more on what that means below.
| Grade | Tensile Strength (min) | Yield Strength (min) | Elongation (min) | Approx. PREN |
| 304 | 515 MPa | 205 MPa | 35% | 18 |
| 304L | 485 MPa | 170 MPa | 35% | 18 |
| 316 | 515 MPa | 205 MPa | 35% | 24 |
| 316L | 485 MPa | 170 MPa | 35% | 24 |
| 321 | 515 MPa | 205 MPa | 35% | 18 |
| 347 | 515 MPa | 205 MPa | 35% | 18 |
Corrosion Resistance and PREN: Why 316 Costs More
PREN (Pitting Resistance Equivalent Number) is a quick way to rank how resistant a stainless steel is to pitting and crevice corrosion, calculated from its chromium, molybdenum and nitrogen content — higher is better. Looking at the table above, 304, 304L, 321 and 347 all sit around a PREN of 18, while 316 and 316L jump to roughly 24. That jump comes almost entirely from the 2–3% molybdenum in 316/316L, which is also exactly why 316 pipe costs more than 304 — you're paying for molybdenum's ability to resist pitting from chlorides, the kind of attack you get from seawater, de-icing salts, or chloride-bearing process chemicals. If your application never sees chlorides, that extra resistance (and extra cost) usually isn't buying you anything.
Which Stainless Steel Pipe Grade Should You Choose?
| Grade | Best For | Typical Industries |
| 304 | General-purpose corrosion resistance, food and water contact | Food & beverage, architecture, general fabrication |
| 304L | Heavily welded assemblies where weld-decay resistance matters | Storage tanks, pressure vessels, welded piping systems |
| 316 | Chloride exposure, mild chemical contact | Marine equipment, chemical processing, offshore |
| 316L | Welded fabrication in chloride or chemical environments | Pharmaceutical, dairy processing, marine welding |
| 321 | Sustained high temperature with intermittent welding | Exhaust systems, furnace parts, aerospace ducting |
| 347 | High temperature with sustained creep/pressure loading | Petrochemical piping, high-temp process piping |
304 vs 316: The Most Common Comparison
This is the single question we get asked more than any other, so it's worth its own section rather than burying it in a table. 304 is the general-purpose workhorse: good corrosion resistance, widely stocked, lower cost, and more than adequate for food contact, architectural work, and dry or mildly wet environments. 316 adds molybdenum, which specifically improves resistance to chloride-induced pitting and crevice corrosion, the kind of attack you get in marine air, seawater contact, or chemical processing involving chlorides. If your pipe will never see salt air, seawater, or chloride-bearing chemicals, 304 is very likely all you need. If it will, the extra cost of 316 is what prevents a pitting failure years down the line.
Grade Availability at Metinox Overseas
All six grades compared above are manufactured and stocked by Metinox Overseas. For full specifications, available sizes, and current stock, visit the individual grade pages:
Standards Referenced
- ASTM A312: seamless, welded, and heavily cold-worked austenitic stainless steel pipe, the primary composition and mechanical property standard used above.
- ASME SA312: the ASME boiler and pressure vessel code equivalent of ASTM A312, used for pressure-retaining applications.
- EN 10217-7: the European standard for welded austenitic stainless steel pressure pipe, referenced for grades sold into European specifications.
Frequently Asked Questions
316 contains 2–3% molybdenum, providing better chloride pitting and crevice corrosion resistance than 304.
Use L grades for welded applications where corrosion resistance is required without post-weld heat treatment. Their lower carbon content reduces sensitisation risk.
321 is titanium-stabilised, helping prevent chromium carbide formation during prolonged exposure to approximately 425–870°C.
321 uses titanium for stabilisation, while 347 uses niobium. Both are designed to resist high-temperature sensitisation.
Among the six grades compared, 316 and 316L have the highest PREN at approximately 24, providing better chloride pitting and crevice corrosion resistance.
Yes. The additional molybdenum in 316 increases raw material cost and typically results in a higher pipe price.
Grade refers to a specific standardised alloy composition (such as 316), while type usually refers to a broader family (such as austenitic). The two terms are often used loosely but grade is the more precise specification.
Yes, 304 performs well outdoors in most dry to moderately wet climates. In coastal or heavily salted environments, 316 is generally the safer choice due to its added chloride resistance.
321 is titanium stabilised for sustained high temperature service, while 304 is not stabilised and is not rated for the same prolonged high temperature exposure without risking sensitisation.
No, pressure rating depends on both the grade's mechanical properties and the pipe's wall thickness and schedule, not on grade alone. Confirm the specific schedule against ASME B31.3 for your application.
Why Source Stainless Steel Pipe From Metinox Overseas
Metinox Overseas has manufactured and exported ASTM/ASME-certified stainless steel pipe for more than 45 years, with an ISO 9001:2008 certified manufacturing process and a Mill Test Certificate supplied with every order. Our range spans all six grades compared in this chart: 304, 304L, 316, 316L, 321 and 347, with export experience across multiple international markets and industries including petrochemical, oil and gas, power generation and heavy engineering.
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