All You Need to Know About 304 Stainless Steel

304 stainless steel - UNS S30400, EN 1.4301, SUS304, the classic 18/8 austenitic - is the default stainless grade for more applications than any other. This article consolidates what engineers, buyers and fabricators need to know: composition, mechanical properties, corrosion limits, temperature behaviour and fabrication practice.

Chemical Composition and the Role of Each Element

Composition ranges per ASTM A240 for UNS S30400 (wt%):

Element	Range (wt%)
Carbon, max	0.08
Manganese, max	2.00
Phosphorus, max	0.045
Sulfur, max	0.030
Silicon, max	0.75
Chromium	18.00-20.00
Nickel	8.00-10.50
Nitrogen, max	0.10

Chromium is the corrosion engine: it forms the self-healing chromium-oxide film that makes the steel stainless. Nickel stabilizes the austenitic structure, which gives 304 its toughness at low temperature and its formability at room temperature. Carbon adds strength but also carries the sensitization risk discussed below, which is why low-carbon variants exist. Manganese and silicon serve mainly as deoxidizers and residual-strength contributors.

Mechanical Properties at Room Temperature

Annealed minimums per ASTM A240:

Property	Value
Tensile strength, min	515 MPa
Yield strength (0.2% offset), min	205 MPa
Elongation in 50 mm, min	40%
Brinell hardness, max	201 HBW

Cold-rolled strip and sheet can be supplied in higher-strength tempers per ASTM A666, with tensile strength up to roughly 860 MPa in the extra-high-strength temper, at the cost of reduced elongation. Most architectural and equipment applications use the annealed condition.

Corrosion Resistance in Common Environments

The chromium-oxide film makes 304 resistant to atmospheric corrosion, fresh water, most foodstuffs, neutral salt solutions in many conditions, and a wide range of mild chemicals. It is the standard grade for kitchens, food plants and architectural exteriors. The film does fail in specific conditions: high chloride concentrations cause pitting and crevice corrosion (which is why coastal hardware often moves to 316), and strong acids such as hydrochloric or hot dilute sulfuric acid attack it. Above roughly 60 °C in chloride-bearing media, 304 is also susceptible to chloride stress-corrosion cracking. For these duties, molybdenum-bearing or higher-alloyed grades are required.

Temperature Performance: From Cryogenic to Hot

Because 304 is fully austenitic, it has no ductile-to-brittle transition and keeps excellent toughness down to -196 °C, making it a standard material for cryogenic vessels and LNG equipment. At the hot end, 304 is suitable for continuous service up to about 800 °C and intermittent service to roughly 870 °C, above which oxidation and strength loss accelerate. Between about 425 °C and 815 °C, long exposure can precipitate chromium carbides at grain boundaries (sensitization), which lowers corrosion resistance in subsequent service; where the part must be welded and then exposed to corrosive media, 304L or a stabilized grade is specified instead.

Fabrication: Machining, Forming and Welding

304 work-hardens quickly. Cold forming such as deep drawing or severe bending may require intermediate annealing, and springback must be allowed for. Machining needs sharp tools, positive feed and generous coolant because the work-hardened layer dulls tools fast; free-machining grades such as 303 exist for screw-machine work where corrosion loads are mild. Welding is straightforward with all common processes, but for thicker sections use low-carbon filler such as 308L to keep the weld metal and heat-affected zone resistant to sensitization, and keep the interpass temperature moderate. The grade is also easy to polish, brush and passivate, which is why it dominates architectural and food-contact finishes.

Common Applications

304 is specified for food-processing and kitchen equipment, brewery and dairy tanks, building facades, handrails, cladding and roofing details, medical instruments and hospital furniture, automotive trim and fasteners, chemical storage tanks for mild media, cryogenic vessels, and domestic sinks and cookware. When in doubt about an application, 304 is usually the first material evaluated because it balances cost, corrosion resistance, formability and availability better than any other stainless grade.

Frequently Asked Questions

Is 304 stainless steel magnetic?

In the annealed condition 304 is essentially non-magnetic. Cold working can transform part of the structure to martensite and make it weakly magnetic; the effect is normal and does not indicate a quality problem.

What is the maximum working temperature of 304?

Continuous service is commonly rated to about 800 °C and intermittent service to about 870 °C. Above that, oxidation and loss of strength require heat-resistant grades such as 309S or 310S.

Does 304 rust?

Under normal conditions, no - the chromium-oxide film protects it. It can show surface staining or pitting in high-chloride, acidic or abrasive environments, or when contaminated by carbon steel dust, which is why fabrication areas must keep 304 separate from carbon steel tooling.

Can 304 be used for food contact?

Yes. 304 is a widely accepted food-contact grade for processing equipment, tanks and utensils. For the most corrosive food-processing duties, 316 is often preferred for its higher resistance to chlorides and cleaning chemicals.

What is the difference between 304 and 316?

316 adds 2.00-3.00% molybdenum, which markedly improves pitting and crevice resistance in chloride environments such as seawater and de-icing salts. 304 covers most general, food and architectural service at lower cost.

How do I weld 304 without losing corrosion resistance?

Use low-carbon filler metal such as 308L, keep heat input moderate, and avoid excessive interpass temperature. For service in corrosive media after welding, specify 304L so the heat-affected zone does not sensitize.