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Stainless steel grade comparison

303 vs 304 Stainless Steel

Compare 303 and 304 stainless steel by machinability, sulfur content, corrosion resistance, welding, formability, mechanical properties, product form and purchasing requirements.

Quick answer: choose 303 when high-volume machining, chip breaking and machining productivity control the component. Choose 304 when corrosion resistance, welding, forming and broad product availability are more important.
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Free machining vs general-purpose fabrication

303
EN 1.4305 · UNS S30300
  • Structure Austenitic
  • Nominal Cr* 17.5%
  • Nominal Ni* 9%
  • Nominal S* 0.20%
  • Machining Primary advantage
  • Welding Special control
304
EN 1.4301 · UNS S30400
  • Structure Austenitic
  • Typical Cr 18.1%
  • Typical Ni 8.1%
  • Sulfur Standard low level
  • Fabrication Broad capability
  • Welding Good

*303 nominal chemistry shown from a current manufacturer free-machining hollow-bar reference. 304 values are typical manufacturer values. Use the governing standard and MTC for contractual acceptance.

M03 · Decision summary

303 or 304 stainless steel: which one should you specify?

The deciding factor is usually manufacturing. 303 modifies the conventional Cr-Ni austenitic composition with sulfur to improve machining behavior. 304 sacrifices some machining efficiency in exchange for better corrosion, forming and welding performance.

Decision first: start with 303 for high-volume turned, drilled or threaded parts where machining time and chip control dominate. Start with 304 when the component will be welded, deeply formed, exposed to a more demanding corrosive environment, or purchased in sheet, coil or plate.
Choose 303

Machining productivity controls the part

Best for CNC-turned components, shafts, bushings, threaded parts, fittings and complex machined geometry.
Not recommended for Highly corrosive chloride service, major welded assemblies or severe forming.
Key trade-off Sulfur improves chip breaking and machinability but reduces corrosion, welding and forming margin compared with 304.
Alternative 304 when fabrication or corrosion becomes more important, or 316-family material where chloride corrosion controls.
Choose 304

Corrosion, forming and welding control the design

Best for Sheet-metal fabrication, deep-drawn components, tanks, food equipment, architectural parts and general welded assemblies.
Not recommended for High-volume screw-machine work where machining cycle time is the dominant cost driver.
Key trade-off Better corrosion and fabrication balance, but less free-machining behavior than sulfurized 303.
Alternative 304L for extensive welding, or a verified improved-machinability 304 product when both machining and 304-level performance matter.
Reconsider both

The operating environment exceeds the grade family

Review when Warm seawater, concentrated chlorides, critical SCC conditions, strong acids or severe crevice corrosion applies.
Why Neither grade contains intentional molybdenum, and 303's sulfur inclusions can further reduce localized corrosion resistance.
Alternative 316L, duplex, a higher-alloy stainless or another material family based on the actual medium.
Engineering reversal condition: do not select 303 solely because the component is machined. If welding, forming, surface quality, corrosion or structural loading is more important than cutting productivity, 304 can still be the more appropriate and lower-risk manufacturing choice.
M04 · Technical data

303 vs 304 stainless steel properties

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The major technical difference is not a dramatic change in basic physical properties. It is the sulfur-modified metallurgy of 303, which improves machining but changes corrosion, welding and fabrication behavior.

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Published chemistry reference

303 values below use a current manufacturer free-machining product reference. 304 uses a current general-purpose manufacturer reference. Use the applicable standard and MTC for final acceptance.

Element / feature 303 / S30300 304 / S30400 Selection significance Reference condition
Carbon ≤0.035%* 0.04% typical Carbon is not the primary differentiator in this comparison. 303 free-machining manufacturer reference; 304 typical flat-product reference
Chromium 17.5% nominal* 18.1% typical 304 carries the slightly higher cited chromium level. Manufacturer data
Nickel 9% nominal* 8.1% typical Both remain Cr-Ni austenitic grades. Manufacturer data
Sulfur 0.20% nominal* Standard low-sulfur grade This is the main free-machining distinction. 303 Sanmac 4305 reference
Molybdenum No intentional addition No intentional addition Neither should be treated as a 316-type chloride grade. Grade family
Microstructure Austenitic Austenitic Both retain broadly similar base physical behavior, but sulfur inclusions change manufacturing performance. Grade classification

Machining and fabrication comparison

Machining performance depends on tooling, machine rigidity, feed, speed, coolant, part geometry and material condition.

Factor 303 304 Engineering effect Buyer action
Machinability Very high Moderate / product dependent 303 is specifically designed for machining productivity. Define cycle-time and tool-life priorities.
Chip breaking Improved by sulfur additions Longer / tougher chips more common 303 can simplify automated turning operations. Confirm tooling, feed and surface requirement.
Relative cutting-speed reference Up to about 2× Sanmac 304/304L in cited manufacturer example Reference baseline This is product-specific, not a universal cutting-speed guarantee. Run production trials for critical cycle-time targets.
Formability More limited Better Sulfur inclusions make 303 less suitable for severe forming. Use 304 for demanding bends or draws.
Weldability Requires careful control Good 303 sulfur content complicates welding behavior. Avoid casual substitution in welded structures.
Heat hardening No conventional quench hardening No conventional quench hardening Both are austenitic grades. Use cold work if higher strength is needed.

Same-manufacturer hollow-bar mechanical reference

This table uses Alleima Sanmac 4305 and Sanmac 304/304L hollow-bar data to provide a more comparable product-form reference. It is not a universal minimum for every 303 or 304 product.

Property at 20°C 303 / Sanmac 4305 304/304L Sanmac reference Unit Selection meaning
Rp0.2 minimum 210 210 MPa Same cited minimum in this product reference.
Rp1.0 minimum 230 240 MPa Only a small difference in this condition.
Tensile strength ≥515 515–680 MPa Do not select 303 because of a presumed strength advantage.
Elongation A ≥35 ≥45 % 304 reference shows the higher ductility.
Hardness ≤90 HRB ≤90 HRB HRB Same maximum cited for these hollow-bar products.

Corrosion and welding behavior

The sulfur addition that improves 303 machining performance also creates important corrosion and fabrication trade-offs.

Factor 303 304 Why it matters Selection guidance
General corrosion Good in suitable mild environments Generally stronger margin Free-machining sulfur inclusions reduce 303's corrosion margin. Prefer 304 where corrosion is a primary requirement.
Pitting / crevice corrosion Can be sensitive even at relatively low chloride levels Also chloride-sensitive, but generally preferred over 303 Neither contains intentional molybdenum. Consider 316L or duplex for more demanding chlorides.
Stress-corrosion cracking Susceptible Susceptible Both are austenitic Cr-Ni stainless steels. Review tensile stress, chlorides and temperature.
Weld fabrication Special care required Broadly suitable 303's sulfur addition affects weld behavior. Use 304/304L for heavily welded components.
Surface after machining Machining-focused Broad finish capability The final surface, crevices and contamination can affect corrosion. State Ra, passivation and finish requirements.

Typical physical properties

303 and 304 are both austenitic Cr-Ni stainless steels, so their basic physical behavior is relatively close.

Property 303 reference 304 reference Condition Selection significance
Density 7.9 g/cm³ 7.9 g/cm³ Typical No meaningful difference.
Elastic modulus 200 GPa 200 GPa 20°C manufacturer reference Same cited stiffness.
Thermal conductivity 15 W/m·K 15 W/m·K 20°C manufacturer reference No useful selection difference.
Thermal expansion 16.5 ×10⁻⁶/K 16.5 ×10⁻⁶/K Approx. 30–100°C Same cited behavior.
Specific heat 475 J/kg·K 475 J/kg·K 20°C manufacturer reference Same cited value.
Magnetic behavior Austenitic; cold work can increase response Austenitic; cold work can increase response General behavior Do not use grade name alone for strict magnetic limits.
Important: the published “up to twice the cutting speed” statement is a manufacturer comparison between specific Sanmac 4305 and Sanmac 304/304L products. It should not be treated as a universal machining multiplier for every mill, machine, tool or component.
M05 · Product form matrix

Product forms and procurement routes for 303 and 304

303 is most relevant in bar, hollow bar and machining-oriented product forms. 304 has a much broader route across sheet, coil, plate, bar, wire, tube and fabricated products.

Availability note: this matrix describes typical product and purchasing routes only. It does not represent live stock. Confirm size, tolerance, finish, product standard, quantity and certification before purchase.
Product / route 303 304 Status Procurement note
Round bar Core machining route Common product form Typical product form Specify diameter, tolerance, surface and mechanical condition.
Hexagon / square bar Free-machining application route Supplier-dependent Confirm availability Useful for nuts, fittings and screw-machine components.
Hollow bar Published 303 product route Published 304/304L product route Typical machining route Confirm OD, ID, machined component size and tolerance.
Automatic-machining stock Primary 303 use case Possible with suitable 304 product Process-specific Compare tool life, cycle time and required corrosion performance.
Cold-rolled coil / sheet Supplier-dependent / uncommon route Core 304 product form 304 typical 304 is the more natural choice for general sheet-metal fabrication.
Hot-rolled coil Supplier-dependent Core 304 product form 304 typical Confirm finish, thickness and product standard.
Plate Not the typical 303 buying route Established 304 product form Product-dependent Use 304 where plate fabrication is required.
Deep-formed components Limited suitability Strong application route Application-specific 304 provides the more favorable forming balance.
Welded equipment Not preferred for heavy welding Established application route Welding-specific Also consider 304L when extensive welding and sensitization matter.
M06 · Grade selector

Choose 303 or 304 by manufacturing route

Start with how the component will actually be made. 303 earns its place when machining productivity is valuable enough to justify reduced corrosion and fabrication flexibility.

Preliminary result

Define machining intensity first

Use 303 when machining productivity is the principal reason for the material choice. Use 304 when corrosion, welding and forming carry more weight.

Machining Corrosion Fabrication
Send Conditions to an Engineer
STEP 01 Define machining

Turning, drilling, tapping, volume and cycle time.

STEP 02 Define corrosion

Water, chloride, chemicals, pH and temperature.

STEP 03 Define fabrication

Welding, forming, finishing and passivation.

STEP 04 Define product form

Bar, hollow bar, sheet, coil, plate or component.

M08 · Applications & limitations

Where 303 works, where 304 is preferred, and where neither grade is enough

Discuss Your Application

303 should earn its place through a real machining advantage. If the component does not need the sulfur-driven chip-breaking benefit, 304 usually provides the broader service and manufacturing envelope.

303

Typical reasons to choose 303

  • CNC turned components
  • Automatic screw-machine parts
  • Machined shafts
  • Bushings and spacers
  • Threaded components
  • Machined fittings
  • Complex high-volume machined parts
304

Typical reasons to choose 304

  • Kitchen equipment
  • Food-processing equipment
  • Storage tanks
  • Architectural components
  • General sheet-metal fabrication
  • Formed components
  • Welded assemblies
Review

Conditions that may require another grade

  • Warm seawater
  • High chloride concentration
  • Severe crevice corrosion
  • Critical chloride SCC service
  • Strong reducing acids
  • High-pressure code requirements
  • High-strength structural service

Failure modes that can reverse the recommendation

303 pitting around sulfur-rich inclusions
The sulfur addition that improves free-machining behavior can reduce localized corrosion resistance. Alleima notes that its 1.4305 free-machining grade can be sensitive to pitting and crevice corrosion even in relatively low-chloride solutions.
Welding a sulfurized 303 component
303 is not the preferred choice for heavily welded fabrication. Sulfur modifies solidification behavior, so filler selection, heat input and welding procedure require more control than with 304.
Using 303 for severe forming
303 is optimized for machining, not maximum formability. For deep drawing, severe bending or multi-stage forming, 304 is generally the more appropriate starting material.
Assuming 303 is stronger because it machines better
Machinability and structural strength are different requirements. In the cited same-manufacturer hollow-bar data, both grades have a 210 MPa minimum Rp0.2, while the 304/304L reference has higher cited elongation.
304 machining work hardening
304 is machinable, but poor cutting strategy can promote work hardening, heat generation, built-up edge and reduced tool life. Tooling and feeds should keep the cutting edge below the work-hardened layer.
Chloride stress-corrosion cracking
Both remain austenitic stainless steels and can suffer chloride SCC when tensile stress, temperature and chloride exposure reach a critical combination. Choosing 304 instead of 303 does not eliminate this mechanism.
Crevice corrosion in machined assemblies
Thread roots, tight joints, deposits and stagnant gaps can create oxygen-depleted crevices. This can be especially important when a machined 303 part operates in chloride-containing fluids.
Choosing 303 only to reduce machining cost
A lower machining cycle time can be offset by shorter service life, extra welding difficulty, restricted product availability or corrosion-related maintenance. Compare total component cost, not machining time alone.
Inspection guidance: for 303 machined components, check machined surface quality, thread roots, crevices, passivation condition and early localized corrosion. For welded or formed assemblies, verify that 304 or 304L has not become the more appropriate choice.

Material image comparison

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303 stainless steel

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304 stainless steel

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M10 · Standards & equivalence

303 and 304 stainless steel grade designations

303 and 304 are related Cr-Ni austenitic stainless steels, but 303 is a distinct free-machining sulfurized grade. They should not be treated as interchangeable specifications.

Designation / standard 303 304 Procurement caveat
ASTM / AISI type 303 304 303 is specifically optimized for free machining.
UNS S30300 S30400 Use the exact UNS designation.
EN material number 1.4305 1.4301 Distinct European material numbers.
EN name X10CrNiS18-9 X5CrNi18-10 The “S” in the 303 EN name reflects the sulfurized chemistry.
JIS SUS303 SUS304 Verify the applicable JIS product standard.
GB reference Verify against project standard Verify against project standard Do not use an informal cross-reference as contractual acceptance.
ASTM A582/A582M Free-machining stainless bars Not the normal 304 bar route A582 is specifically for free-machining stainless steel bars.
ASTM A276 Verify grade / product applicability Common 304 bar route Use the appropriate bar specification.
ASTM A240/A240M Verify product applicability Common plate / sheet / strip route 304 has a much broader flat-product procurement route.
EN 10088-3 Long-product route subject to grade/product program Long-product reference Confirm dimensions, condition and certification.
Machining condition Critical selection variable Application dependent State bar finish, tolerance and final machining requirement.
303 · S30300 Free-machining austenitic stainless steel.
303 · 1.4305 EN X10CrNiS18-9 sulfurized Cr-Ni grade.
304 · S30400 General-purpose austenitic Cr-Ni stainless steel.
304 · 1.4301 EN X5CrNi18-10 general-purpose designation.

Approximate grade equivalence does not replace a purchasing specification. Verify chemistry, product standard, mechanical condition, dimensional tolerance, corrosion requirement and material certificate.

M11 · Direct answers

303 vs 304 stainless steel FAQ

Direct answers to common engineering and purchasing questions about 303 versus 304 stainless steel.

What is the main difference between 303 and 304 stainless steel?

303 is a free-machining austenitic stainless steel with increased sulfur to improve chip breaking and machining productivity. 304 is a general-purpose austenitic grade with better overall corrosion, formability and weldability.

Which is easier to machine, 303 or 304?

303 is the clear free-machining choice. In Alleima's Sanmac product comparison, cutting speeds of up to twice those used for Sanmac 304/304L are cited. Actual machining performance still depends on tooling, feed, machine rigidity and component geometry.

Which has better corrosion resistance, 303 or 304?

304 is generally preferred where corrosion resistance matters. The sulfur-rich inclusions that improve 303 machinability can reduce localized-corrosion resistance, particularly in chloride environments.

Can 303 stainless steel be welded?

303 can be welded, but it is not the preferred grade for heavy welded fabrication. The sulfur addition requires careful welding control and appropriate filler material. 304 or 304L is generally the more natural welded-fabrication choice.

Is 303 stronger than 304?

Not as a general rule. 303 is chosen primarily for machinability, not for a structural-strength advantage. Mechanical properties must be compared in the same product form and condition.

Why is sulfur added to 303 stainless steel?

Sulfur creates inclusions that improve chip formation and reduce machining difficulty. The trade-off is reduced corrosion, forming and welding performance compared with conventional 304.

Is 303 good for seawater?

303 should not be the default choice for seawater. Its free-machining sulfur addition reduces localized-corrosion margin, and neither 303 nor 304 contains intentional molybdenum. 316L, duplex or another higher-alloy grade may be required.

Can I replace 303 with 304?

Possibly, but machining cost can increase. If corrosion, welding or forming performance matters more than machining productivity, 304 may be the better choice. The drawing and purchase specification must still permit the grade change.

M13 · Sources & technical review

Technical basis for this 303 vs 304 comparison

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This comparison separates free-machining behavior from corrosion, mechanical, welding and product-form requirements so 303 is not treated simply as an easier-to-machine version of 304 without trade-offs.

Primary technical references

  • Alleima Sanmac 4305 303 / S30300 / 1.4305, free-machining positioning, nominal sulfur content, hollow-bar mechanical properties, corrosion behavior, physical properties and welding guidance.
  • Alleima Sanmac 304/304L Same-product-family reference for hollow-bar mechanical and physical properties, corrosion behavior, welding and machining.
  • Outokumpu Core 304/4301 304 / S30400 / 1.4301 typical chemistry, product forms, formability, weldability and general application guidance.
  • ASTM A582/A582M Free-machining stainless steel bar specification for applicable products.
  • ASTM A276 / A479 Applicable conventional stainless bar product-standard framework.
  • ASTM A240/A240M Plate, sheet and strip specification framework for applicable 304 products.
  • ISO 15510 designation reference 303 / S30300 / 1.4305 / X10CrNiS18-9 / SUS303 and 304 grade mappings.
  • Product-specific MTC / MTR Required to verify actual heat chemistry, mechanical properties and product-standard compliance.

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M14 · Ask an engineer

Still deciding between 303 and 304?

Send the machining, corrosion, fabrication and product requirements so the grade can be reviewed against the finished component rather than machining speed alone.

  • 303, 304, 304L or alternative grade under consideration
  • UNS S30300 or S30400 requirement
  • EN 1.4305 or 1.4301 requirement
  • Round, hex, square, hollow bar, sheet, plate or component
  • Raw-material dimensions and tolerances
  • Finished component dimensions
  • CNC turning, drilling, tapping or milling operations
  • Annual machining volume
  • Cycle-time or tool-life requirement
  • Surface roughness requirement
  • Welding requirement
  • Forming or bending requirement
  • Process medium and chloride concentration
  • Normal and peak operating temperature
  • Passivation or cleaning requirement
  • Surface finish
  • Quantity, destination and due date
  • MTC and inspection requirements
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