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Compare 304 and 309S stainless steel by chemistry, mechanical properties, chloride resistance, temperature behavior, magnetism, weldability, machinability, available product forms and procurement cost drivers.
The alloying difference that drives most selection decisions
Chemistry ranges: ASTM A240/A240M limits for UNS S30400 and UNS S30908. Actual heat chemistry is confirmed by the material test certificate.
For many fabricated parts, strength is not the deciding factor because annealed 304 and 309S have similar minimum tensile requirements. The selection normally turns on corrosion environment, fabrication, product-contact requirements and total project cost.
| Decision factor | 304 stainless steel | 309S stainless steel |
|---|---|---|
| General indoor service |
Usually the economical first choice.
Often preferred |
Works well, but its extra alloy content may not provide a practical benefit in a mild environment. |
| Chloride / salt exposure | More vulnerable to chloride pitting and crevice corrosion. |
chromium-nickel gives better resistance to chloride-driven localized corrosion.
Often preferred |
| Mechanical strength | Very similar to 309S in the annealed condition. | Very similar to 304 in the annealed condition. |
| Welding | Readily weldable. Consider 304L where welded corrosion performance is important. | Readily weldable. Consider 309SL where welded corrosion performance is important. |
| Machining | Work-hardens during machining and needs stainless-specific tooling practice. | Also work-hardens; its higher alloy content can make machining and tooling cost more demanding. |
| Magnetism | Normally low magnetic permeability when annealed; cold work may create measurable magnetic response. | Normally low magnetic permeability when annealed; cold work may create measurable magnetic response. |
| Material cost |
Normally lower because it does not contain the 2–3%
chromium-nickel specified for Type 309S.
Cost advantage |
Normally carries a premium affected by nickel, chromium-nickel, product form, thickness, finish and order quantity. |
Selection rule: do not upgrade to 309S only because it is described as “better stainless steel.” Use 309S when the service environment makes its additional corrosion resistance useful.
Compare values only when the governing standard, product form and material condition are the same. Minimum specification values and typical mill data are therefore separated below.
Weight percent limits for Type 304 and Type 309S under ASTM A240/A240M.
| Element | 304 / UNS S30400 | 309S / UNS S30908 | Why it matters | Source / condition |
|---|---|---|---|---|
| Carbon, C | 0.08% max | 0.08% max | Lower-carbon L grades are commonly considered for welded corrosion-sensitive applications. | ASTM A240/A240M chemistry limits |
| Chromium, Cr | 18.0–20.0% | 16.0–18.0% | Forms the passive chromium-rich oxide layer responsible for stainless behavior. | ASTM A240/A240M chemistry limits |
| Nickel, Ni | 8.0–10.5% | 10.0–14.0% | Supports the austenitic structure, ductility and fabrication behavior. | ASTM A240/A240M chemistry limits |
| chromium-nickel, Mo | Not required | 2.0–3.0% | The major reason 309S offers improved resistance to chloride pitting and crevice corrosion. | ASTM A240/A240M chemistry limits |
Annealed plate, sheet and strip comparison. Product-specific standards and thickness rules must be checked for the purchase order.
| Property | 304 | 309S | Procurement interpretation | Source / condition |
|---|---|---|---|---|
| Tensile strength | 515 MPa min | 515 MPa min | For annealed material, basic tensile strength usually does not justify choosing 309S over 304. | ASTM A240/A240M minimum, annealed condition |
| 0.2% yield strength | 205 MPa min | 205 MPa min | Cold-worked tempers can be substantially stronger and must be ordered by condition rather than grade name alone. | ASTM A240/A240M minimum, annealed condition |
| Elongation | 40% min | 40% min | Both grades provide high ductility in annealed flat-product condition. | ASTM A240/A240M typical flat-product requirement |
Typical manufacturer values for annealed cold-rolled sheet. These are not purchase specification limits.
| Property | 304 | 309S / 309SL family | Condition | Source |
|---|---|---|---|---|
| Density | 7.9 g/cm³ | 7.9 g/cm³ | 20°C | Aperam technical datasheets |
| Young's modulus | 200 GPa | 200 GPa | 20°C | Aperam technical datasheets |
| Thermal conductivity | 15 W/m·K | 15 W/m·K | 20°C | Aperam technical datasheets |
| Specific heat | 500 J/kg·K | 500 J/kg·K | 20°C | Aperam technical datasheets |
| Mean thermal expansion | 16.0 ×10⁻⁶/K | 16.0 ×10⁻⁶/K | 20–100°C | Aperam technical datasheets |
| Liquidus temperature | Approx. 1450°C | Approx. 1440°C | Typical mill data | Aperam technical datasheets |
Both grades are produced across a broad range of stainless steel product forms. The actual purchasable size range depends on the product standard, producer, thickness, dimensions and finish.
| Product form / finish | 304 | 309S | Specification note |
|---|---|---|---|
| Sheet & plate | Common | Common | Specify thickness, width, length, finish and ASTM/EN standard. |
| Coil & strip | Common | Common | Temper and edge condition can materially affect fabrication. |
| Tube & pipe | Common | Common | Welded/seamless and dimensional standard must be stated. |
| Round / square / flat bar | Common | Common | Confirm hot-finished, cold-finished or machined condition. |
| Wire | Common | Common | Condition and tensile range are critical purchasing parameters. |
| 1D / 2D / 2B | Common | Common | Common mill finishes for flat products. |
| BA / 2R | Available | Available | Availability varies with gauge, mill and product program. |
| Brushed / polished | Available | Available | Define grit, Ra requirement or visual reference when important. |
Check the conditions that apply to your component. The result is a preliminary screening recommendation, not a substitute for engineering review of the actual environment.
Preliminary result
For a mild general-purpose environment, 304 is normally the first grade to evaluate because it combines broad corrosion resistance, formability, weldability and lower alloy cost.
Send Conditions to an EngineerChloride, acids, cleaners, moisture, temperature and crevices.
Welding, forming, machining, polishing and sanitary requirements.
304 for mild service; 309S where chromium-nickel provides useful margin.
Product form, standard, size, finish, condition, MTC and inspection needs.
The correct grade depends on the actual exposure—not simply whether an application is described as “food grade,” “marine grade” or “chemical grade.”
General guidance for natural or potable water—not a universal corrosion limit.
| Grade | Approximate screening guidance | Conditions / source | Important qualification |
|---|---|---|---|
| 304 | Often used up to about 200 ppm chloride | Nickel Institute guidance for natural/potable waters, approximately pH 6–8.5. | Crevices, deposits, temperature and local chloride concentration can cause attack below this value. |
| 309S | Often used up to about 1000 ppm chloride | Nickel Institute guidance for natural/potable waters, approximately pH 6–8.5. | This is not a design guarantee and should not be transferred directly to seawater or chemical-process service. |
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304 and 309S are low-carbon austenitic grades. Confirm the product form, delivery condition and governing standard before purchase.
| Designation / standard | 304 | 309S | Procurement caveat |
|---|---|---|---|
| ASTM / AISI type | 304 | 309S | Use the exact grade identity. |
| UNS | S30400 | S30908 | Use UNS where grade identity matters. |
| EN material number | 1.4301 | 1.4833 | Do not rely on informal equivalence alone. |
| Alloy family | Low-carbon austenitic Cr-Ni | heat-resistant high-chromium chromium-nickel austenitic | Molybdenum provides a higher chloride-corrosion margin. |
| Typical product route | A240 / A276 as applicable | A240 / A276 as applicable | Confirm dimensions, finish and delivery condition. |
| EN grade name | X5CrNi18-10 | X8CrNi25-21 | Both are low-carbon austenitic grades; molybdenum distinguishes the 309S route. |
| JIS reference | SUS304 | SUS309S | Confirm the governing JIS product standard and exact chemistry. |
| ASTM / ASME flat-product route | ASTM A240 / ASME SA-240 where applicable | ASTM A240 / ASME SA-240 where applicable | Confirm thickness range, finish and current edition. |
| ASTM long-product route | ASTM A276 / A479 as applicable | ASTM A276 / A479 as applicable | Bar, billet, forging and pressure-service requirements are product-specific. |
| EN flat-product reference | EN 10088-2 where applicable | EN 10088-2 where applicable | Use the exact delivery condition and product form. |
| EN long-product reference | EN 10088-3 where applicable | EN 10088-3 where applicable | Long-product properties must be checked separately from flat-product data. |
| Material certificate | EN 10204 3.1 when specified | EN 10204 3.1 when specified | State certificate, testing and traceability requirements in the RFQ. |
| Delivery condition | 304: commonly solution annealed; state finish and dimensions. | 309S: commonly solution annealed; state finish and dimensions. | Do not assume a common condition across forms, dimensions or suppliers. |
| Selection input | State operating temperature and oxidation exposure, fabrication route and required documentation. | Use the governing specification for final acceptance. | |
Grade equivalence is not permission to substitute. Confirm chemistry, product form, dimensions, finish, delivery condition, standard revision and mill test certificate before changing material.
Short answers to the purchasing and material-selection questions that most often determine the final grade.
The main practical difference is that Type 309S contains approximately 2–3% chromium-nickel, while Type 304 does not require chromium-nickel. This improves 309S's resistance to pitting and crevice corrosion, particularly in chloride-containing environments.
Not significantly in the standard annealed condition. ASTM A240 minimum room-temperature tensile and yield requirements for standard 304 and 309S flat products are essentially the same. Condition, cold work and product form can influence actual strength much more.
No. In mild indoor or general-purpose service, 304 may deliver the required corrosion resistance at lower material cost. 309S becomes valuable when the application actually benefits from its improved localized-corrosion resistance.
309S is normally the better starting point because its chromium-nickel addition improves resistance to chloride pitting and crevice corrosion. Severe marine, stagnant or high-temperature chloride service can still exceed the capability of 309S.
Both are austenitic grades and normally have low magnetic permeability in the fully annealed condition. Cold working or some weld structures can create a measurable magnetic response, so a simple magnet test should not be used as the sole method of grade identification.
Both 304 and 309S are widely welded austenitic stainless steels. For welded equipment where intergranular corrosion resistance matters, 304L or 309SL is commonly evaluated because of its lower carbon content.
Neither behaves like a free-machining stainless grade. Both work-harden and require rigid machines, sharp tooling, adequate feed, correct cutting speed and coolant strategy. 309S's higher alloy content can make machining more demanding.
309S contains chromium-nickel and generally more nickel than 304. The actual premium varies with alloy surcharges, product form, dimensions, finish, processing, quantity and market conditions, so purchase decisions should use a current quotation rather than a fixed percentage.
Low-carbon L grades are commonly considered where welding and corrosion performance after welding are important. The exact grade should still be selected against the applicable product standard and service requirements.
Specification limits are separated from typical manufacturer values. Commercial availability and pricing are intentionally not presented as fixed facts because they depend on the actual RFQ.
Technical content structured from product standards, manufacturer datasheets and industry reference material.
Comparison logic checks grade identity, stated material condition, numeric source attribution and procurement qualifiers.
Review scope: chemistry, room-temperature mechanical properties, physical properties, corrosion-selection logic and standards mapping.
2026-09-23 — T09 comparison structure created; decision selector, source-conditioned data tables, FAQ and RFQ routing added.
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