304, 304L, 316 and 316L: designation systems and procurement basics

304, 304L, 316 and 316L are among the most familiar stainless-steel names in industry. They are also frequently ordered too casually.

A buyer may say “316L plate” and assume the description is complete. It is not.

The shorthand identifies a material family, but the purchase still needs a product standard, product form, dimensional requirements, delivery condition, surface condition, inspection certificate and any project-specific testing.

The common designation map

A practical starting point is:

These are common cross-designations, not permission to ignore the applicable product standard. EN and ASTM/UNS systems do not always have a perfect one-to-one relationship across every product form and specification.

For the general approach, see How to compare steel grades across standards.

What does the “L” mean?

L means low carbon.

Lower carbon reduces the risk of chromium-carbide precipitation and the associated loss of resistance to intergranular corrosion after thermal exposure such as welding.

That is why 304L and 316L are widely used in welded fabrication.

The L designation does not mean:

• low alloy;
• low strength;
• low corrosion resistance;
• a different stainless-steel family.

It is primarily a carbon-control designation within the corresponding 304 or 316 family.

What is the main difference between 304 and 316 families?

The 316 family contains significant molybdenum, while standard 304 does not.

That molybdenum addition improves resistance to localized corrosion such as pitting in many chloride-containing environments. It is one reason 316/316L is commonly selected for more corrosive service.

But “316L is better than 304L” is not a complete material-selection rule. Actual corrosion performance depends on environment, temperature, chloride concentration, crevices, surface condition, fabrication, cleaning and other factors.

For a composition-based screening index, see PREN explained for stainless and duplex steels.

304L does not always mean only 1.4307

In European material lists you may encounter more than one low-carbon Cr-Ni grade associated commercially with the 304L family. Likewise, 316L may appear alongside related EN grades with different composition limits.

For procurement, do not replace the specified EN material number with a generic “304L” or “316L” unless the purchaser accepts that change.

The safest order description identifies both the applicable standard and the exact grade/material number required by the project.

Dual certification is common — but it has to be real

A heat may sometimes satisfy both a normal-carbon and low-carbon grade, depending on the relevant chemistry and mechanical-property requirements. That can lead to dual certification such as 304/304L or 316/316L.

But a distributor cannot create legitimate dual certification merely by adding another grade name to the paperwork.

The material and test results have to satisfy the requirements being declared.

The underlying principle of dual certification should be treated separately: one product can only be certified to multiple designations when the applicable requirements are actually satisfied.

What should a stainless RFQ actually say?

A stronger RFQ identifies at least:

• product form: plate, sheet, coil, bar, pipe, fitting, etc.;
• product standard;
• exact grade/material designation;
• dimensions and tolerances;
• quantity and lengths/sheet sizes;
• delivery/heat-treatment condition;
• surface finish where relevant;
• inspection document, commonly EN 10204 3.1 where required;
• PMI or additional testing if required by the project;
• corrosion or sour-service requirements where applicable;
• marking and traceability requirements.

For heat treatment, see Solution annealing of stainless and duplex steel.

316Ti is another route to sensitization resistance

A stabilized grade such as 316Ti/1.4571 uses titanium rather than relying only on a very low carbon content.

That is a related but different metallurgical approach. See 1.4571 (316Ti) vs 316L.

MetalMate takeaway

304/304L/316/316L are useful family names, not complete purchase specifications. The L means low carbon; the 316 family adds molybdenum; and the exact EN/UNS/product-standard combination still has to be stated and verified on the MTC.

Related Knowledge

• How to compare steel grades across standards
• 1.4571 (316Ti) vs 316L
• PREN explained
• Solution annealing of stainless and duplex steel

References

• Outokumpu, Austenitic stainless steel grades — 304/4301 and 304L/4307: https://www.outokumpu.com/en/products/stainless-steel-types/austenitic-stainless-steel
• Outokumpu, Supra range — 316/4401 and 316L/4404: https://www.outokumpu.com/en/products/product-ranges/supra
• Outokumpu, Core range — common 304 and 304L cross-designations: https://www.outokumpu.com/core