304S Stainless Steel: Composition, Properties and Applications

What Is 304S Stainless Steel?

304S is the legacy British Standard label for a family of 18/8 austenitic stainless steels written in the BS 970 designation system. Grades such as 304S15 and 304S31 were the everyday specification for UK fabricators for decades, and the same chemistry is published today as EN 1.4301, X5CrNi18-10, AISI 304 and UNS S30400. In practical terms 304S describes a chromium-nickel austenitic steel with roughly 18% chromium and 8-10% nickel, low carbon, and a fully austenitic microstructure in the annealed condition. It cannot be hardened by heat treatment; strength is raised by cold working only.

The grade is selected for the combination of corrosion resistance, ductility and low-temperature toughness. That balance suits complex components produced by deep drawing, cold forming or welding, where a higher-carbon austenitic steel would risk sensitisation and intergranular attack in the heat-affected zone.

Chemical Composition and Applicable Standards

Composition is controlled to EN 10088-2 for European supply and to ASTM A240 for plate, sheet and strip. The table below lists the specification windows normally used for order review.

Element	EN 1.4301 / X5CrNi18-10	ASTM A240 Type 304 (S30400)	ASTM A240 Type 304L (S30403)
Carbon (C)	0.07 max	0.07 max	0.030 max
Silicon (Si)	0.75 max	0.75 max	0.75 max
Manganese (Mn)	2.00 max	2.00 max	2.00 max
Phosphorus (P)	0.045 max	0.045 max	0.045 max
Sulfur (S)	0.015 max	0.030 max	0.030 max
Chromium (Cr)	17.5-19.5	18.0-20.0	18.0-20.0
Nickel (Ni)	8.0-10.5	8.0-10.5	8.0-12.0
Nitrogen (N)	0.10 max	0.10 max	0.10 max

Values are percent by mass. A mill test certificate issued to EN 10204 3.1 is the normal route for verifying that a heat falls inside these windows.

Mechanical Properties

Property	Type 304 (S30400)	Type 304L (S30403)
Tensile strength	515 MPa min	485 MPa min
0.2% proof strength	205 MPa min	170 MPa min
Elongation in 50 mm	40% min	40% min
Modulus of elasticity	about 193-200 GPa	about 193-200 GPa
Structure	austenitic, non-magnetic annealed	austenitic, non-magnetic annealed

The high elongation is the reason the grade tolerates aggressive forming schedules. Deep-drawn parts and expanded tube ends can be produced without intermediate annealing far more readily than with ferritic or martensitic grades of similar strength.

304S and 304L Compared

Both grades belong to the same austenitic family, so the difference is narrower than buyers often expect. Four points drive the selection.

Carbon ceiling. 304S and standard 304 allow up to 0.07% carbon; 304L is capped at 0.030%.

Intergranular corrosion. The lower carbon content of 304L suppresses chromium carbide precipitation in the 425-815 C band, so welded assemblies can enter service without a post-weld solution anneal.

Strength. The slightly higher carbon of standard 304 gives marginally higher annealed yield and tensile values, which matters for thin-section stiffness.

Cost. 304L usually carries a small premium, although the gap narrows because both grades are priced off the same nickel and chromium indices and are frequently dual certified as 304/304L.

Selection rule: specify 304L for heavy-section welds and for post-weld service in aggressive media; specify 304 or 304S where forming dominates and the welds are thin.

Fabrication, Forming and Typical Applications

The alloy work hardens rapidly, so bending radii should be generous and cold reductions should be scheduled in stages with intermediate annealing where deformation exceeds roughly 15%. Welding is straightforward with 308L filler for 304-to-304 joints; 308L or 309L consumables handle dissimilar joints to 316L or to carbon steel. Heat input should be kept moderate and the 425-815 C sensitisation band avoided where the part will see corrosive service.

Typical uses include:

Food processing tanks, conveying pipelines, mixers and filling equipment, where the surface is non-toxic and resists organic acids.

Chemical storage vessels, process piping and heat exchanger tubing.

Sanitary ware, kitchen equipment and architectural cladding.

Low-temperature piping and cryogenic components, where austenitic toughness is retained.

Dairy, brewery and pharmaceutical utility skids built to hygienic finish requirements.

FAQ

Q: Is 304S the same as 304 stainless steel?
In practice yes. The 304S prefix comes from the BS 970 designation system, where 304S15 and 304S31 identified 18/8 grades of differing composition windows. That chemistry is standardised today as EN 1.4301 and ASTM A240 Type 304, so 304S and 304 are interchangeable for most engineering purposes.

Q: What is the difference between 304S and 304L?
Carbon content is decisive. 304S and 304 allow up to 0.07% carbon, while 304L is limited to 0.030%. The lower carbon in 304L prevents sensitisation after welding at the cost of slightly lower annealed strength.

Q: Is 304S stainless steel magnetic?
Annealed 304S is austenitic and effectively non-magnetic. Cold working such as deep drawing or severe bending can form strain-induced martensite and produce a weak magnetic response, which does not by itself indicate a material defect.

Q: Can 304S be welded to 316L?
Yes. Austenitic grades join to one another easily. Use 308L filler for 304-to-304 work and a 308L or 309L consumable for 304-to-316L joints, with controlled heat input to limit carbide precipitation.

Q: Which temperature band should be avoided during processing?
Avoid the 425-815 C sensitisation range for standard 304S when the component will see corrosive service. If hot forming or stress relief in that range cannot be avoided, specify 304L or a stabilised chemistry instead.

Q: Which surface finishes are available?
Common mill finishes are 2B, BA, No. 4 and No. 8 polished sheet; plate is supplied hot rolled, annealed and pickled. Finish choice affects cleanability and appearance rather than the corrosion performance of the alloy itself.