Stainless steelsUNS S30300

303 Stainless Steel

Also: AISI 303, UNS S30300, SS 303, SAE 30303, 1.4305, X8CrNiS18-9, 303 Se (S30323), free-machining 304

303 is 304 with sulfur added for machinability: the stainless to run on a screw machine. It welds poorly and needs careful passivation. Values for annealed bar.

ASTM A582 (free-machining bar)ASTM A895 (free-machining plate, sheet, strip)AMS 5640 (bar, wire, forgings)ASME SA-582

Sourcing 303 parts or stock?

Attach a drawing (PDF or STEP) or describe the part; a U.S. shop that fits quotes it.

At a glance

303 Stainless Steel typical properties
Density	8 g/cm³ (0.289 lb/in³)
Tensile strength	620 MPa (90 ksi) typical
Yield strength	240 MPa (35 ksi) typical
Hardness	160 HB (about 83 HRB) typical; cold-drawn bar runs higher
Elongation	50% typical
Modulus of elasticity	193 GPa (28 Msi)
Thermal	Melts at 1400-1420 °C (2550-2590 °F); thermal conductivity ~16 W/m·K; expansion ~17 um/m·K; scales in continuous service above about 870 °C (1600 °F)
Composition	Fe, 17-19% Cr, 8-10% Ni, 0.15% min S, up to 0.15% C, up to 2% Mn, up to 1% Si, up to 0.2% P, up to 0.6% Mo

Typical values, Annealed (Condition A) bar. Confirm against the mill certificate and the purchase spec.

MachinabilityExcellent: About 78% of B1112 and the best of the austenitic grades: chips break, tools last, and speeds run 30-50% faster than 304. It still work-hardens, so keep the feed up and never let a tool rub.
WeldabilityPoor: Sulfur causes hot cracking and porosity. If a weld is unavoidable, keep it short, use 308L or 312 filler at low heat input, and expect the joint to corrode first. Design the assembly so 303 is machined and 304L is welded.
FormabilityPoor: Sulfide stringers tear on bending, cold heading and drawing; use 304 or 303 Se for formed parts. Light staking, crimping and knurling are fine.
Corrosion resistanceFair: A step below 304: pitting starts at the sulfides, so it stains in chlorides and coastal air where 304 would not. Fine indoors, in fresh water and with mild chemicals. Passivate after machining.

303 is an 18-8 austenitic stainless with at least 0.15% sulfur. The sulfur forms manganese sulfide inclusions that break the chip and lubricate the tool, which lifts machinability to about 78% of the B1112 free-cutting steel benchmark, against roughly 45% for 304. It is the stainless that U.S. screw-machine and Swiss shops reach for when a drawing says "stainless" and the part is turned in quantity: fittings, shafts, spacers, nuts and studs.

Everything else is a step behind 304. The sulfides are where pitting starts, so 303 stains in chlorides and marine air that 304 shrugs off. The same sulfides cause hot cracking in welds, tear during cold forming, and pull out during electropolishing. It is stocked almost entirely as bar; sheet and plate exist on paper (ASTM A895) but are rarely in a warehouse.

Choose 303 when the part is machined from bar and used indoors, in fresh water or in mild chemicals. Move to 304 or 316 when it is welded, formed or sees chlorides, and to 416 when it must be magnetic or hardened. The selenium variant, 303 Se, gives a smoother machined surface and forms better in cold-headed parts, at a corrosion cost.

Heat treatment

Not hardenable by heat treatment; strength comes from cold work only. Anneal at 1010-1120 °C (1850-2050 °F) and water quench. Stress relieve below 400 °C (750 °F) when a machined part must hold size. Avoid holding at 425-870 °C (800-1600 °F), where chromium carbides sensitize the grain boundaries. If the part must be hard, use 416.

Tempers and conditions

Annealed (Condition A)
Cold drawn (Condition B, hard temper)
Precision ground and polished shaft

Stock forms

Round, hex and square bar (the main form)
Cold-drawn precision ground shafting
Wire
Plate and sheet per ASTM A895 (rarely stocked)

Relative cost

Tier 3 of 5. About 1.1-1.2x 304 bar per pound, but shorter cycle times make a 303 part cheaper than the same part in 304 at volume. Roughly 3x the price of 12L14 bar.

Typical applications

Screw-machine fittings, nuts, bolts and studs
Shafts, pins and spacers
Bushings and collars
Valve bodies, stems and trim
Electrical connector bodies and contacts
Gears and worms in light service
Aircraft and instrument fittings

Process compatibility

How 303 takes each process, and what to specify. Each process links to its own guide and to the shops that do it.

303 Stainless Steel process compatibility
Process	Rating	Notes
CNC Machining	ExcellentExcellent	The default turned stainless; 30-50% faster than 304 with broken chips and long tool life.
Screw Machining	ExcellentExcellent	Made for multi-spindle and cam machines; high-volume fittings and fasteners come from 303 bar.
Swiss Machining	ExcellentExcellent	Small precision parts from ground bar; the sulfur keeps chips short in the guide bushing.
CNC Turning	ExcellentExcellent	Shafts and stems from bar; order precision ground stock for long slender parts.
Passivation	FairFair	Passivates, but nitric baths can etch it grey ("flash attack"). Specify ASTM A967 citric, or nitric with sodium dichromate, and tell the shop it is 303.
Electropolishing	PoorPoor	Sulfide stringers pull out and leave streaks and pits; the finish never matches 304 or 316. Use those when a bright EP surface is the point.
Welding	PoorPoor	Hot-cracks from the sulfur; if forced, 308L or 312 filler, low heat input, short welds, and accept a weaker, less corrosion-resistant joint. Can 303 stainless steel be welded?
Black Oxide	FairFair	MIL-DTL-13924 Class 4 blackens it for glare control; thin and cosmetic, no corrosion benefit.
Bead Blasting	GoodGood	Uniform matte finish on turned parts; use glass or stainless media so no iron embeds.
Press Brake Forming	PoorPoor	Sheet is rare and the sulfides crack on the outside of a bend; form 304 instead.
Laser Cutting	FairFair	Cuts like 304 when sheet can be found, but 303 is a bar alloy; laser-cut parts are 304 or 316.
Heat Treating	Not recommendedNot recommended	Cannot be hardened; annealing only. Specify 416 for a hard free-machining stainless or 17-4 PH for strength.
Hot-Dip Galvanizing	Not recommendedNot recommended	Zinc does not wet the chromium oxide and adds nothing; molten zinc on hot stainless also risks liquid-metal embrittlement.

Compare 303

303 vs 304 Stainless

Design and sourcing tips

Call out "ASTM A582 Type 303, Condition A" on the drawing; add AMS 5640 for aerospace work.
Do not weld it. If the assembly must be welded, make the welded member 304L and the turned member 303, and join them mechanically.
Specify passivation per ASTM A967 by the citric method (or nitric with dichromate) and name the alloy so the shop does not flash-attack the surface.
Keep it out of chloride, pool and marine exposure; use 316 there and accept the machining cost.
Stainless-on-stainless threads gall: specify anti-seize, a plated mating fastener or a dissimilar pair (303 body, 316 screw).
For long slender turned parts, order cold-drawn precision ground bar so straightness and diameter come from the mill, not the lathe.

Frequently asked questions

Is 303 stainless steel magnetic?: No, not in the annealed condition; it is austenitic like 304. Heavy cold work (cold-drawn bar, cold-headed parts) makes it faintly magnetic. A magnet cannot tell 303 from 304; an XRF or PMI check for sulfur can.
Can 303 stainless be welded?: Only badly. The sulfur that makes it machine well segregates in the weld and causes hot cracking and porosity. Shops manage short tack welds with 308L or 312 filler at low heat input, but a structural or leak-tight weld should be made in 304L or 316L. Design so 303 parts are machined and joined mechanically.
What is the difference between 303 and 304 stainless?: Composition differs by one element: 303 has at least 0.15% sulfur, 304 has almost none. That sulfur nearly doubles machinability but lowers corrosion resistance, ruins weldability and reduces ductility. Use 303 for turned parts in mild environments; use 304 for anything welded, formed or exposed to chlorides.
Does 303 stainless rust?: It can stain and pit where 304 would not, because the sulfide inclusions break the passive film. Salt spray, pool chemicals and chloride cleaners are the usual causes, along with free iron left by steel tooling. Passivation per ASTM A967 after machining removes the free iron and buys back most of the difference for indoor and fresh-water service.

Processes that commonly use it

CNC Machining CNC Turning Swiss Machining Screw Machining Centerless Grinding Gear Cutting Plating Electroless Nickel Plating Nickel Plating Passivation

Related materials

304 Stainless Steel 316 Stainless Steel 416 Stainless Steel 12L14 Steel C360 Free-Cutting Brass

Where to verify these values

ASTM A582 and ASTM A895
AMS 5640
ASM Handbook, Volume 1: Properties and Selection: Irons, Steels, and High-Performance Alloys
Producer data sheets for 303 bar (Carpenter, Ugitech, Valbruna)

303 Stainless Steel

Also: AISI 303, UNS S30300, SS 303, SAE 30303, 1.4305, X8CrNiS18-9, 303 Se (S30323), free-machining 304

303 is 304 with sulfur added for machinability: the stainless to run on a screw machine. It welds poorly and needs careful passivation. Values for annealed bar.

ASTM A582 (free-machining bar)ASTM A895 (free-machining plate, sheet, strip)AMS 5640 (bar, wire, forgings)ASME SA-582

At a glance

303 Stainless Steel typical properties
Density	8 g/cm³ (0.289 lb/in³)
Tensile strength	620 MPa (90 ksi) typical
Yield strength	240 MPa (35 ksi) typical
Hardness	160 HB (about 83 HRB) typical; cold-drawn bar runs higher
Elongation	50% typical
Modulus of elasticity	193 GPa (28 Msi)
Thermal	Melts at 1400-1420 °C (2550-2590 °F); thermal conductivity ~16 W/m·K; expansion ~17 um/m·K; scales in continuous service above about 870 °C (1600 °F)
Composition	Fe, 17-19% Cr, 8-10% Ni, 0.15% min S, up to 0.15% C, up to 2% Mn, up to 1% Si, up to 0.2% P, up to 0.6% Mo

Typical values, Annealed (Condition A) bar. Confirm against the mill certificate and the purchase spec.

MachinabilityExcellent

About 78% of B1112 and the best of the austenitic grades: chips break, tools last, and speeds run 30-50% faster than 304. It still work-hardens, so keep the feed up and never let a tool rub.

WeldabilityPoor

Sulfur causes hot cracking and porosity. If a weld is unavoidable, keep it short, use 308L or 312 filler at low heat input, and expect the joint to corrode first. Design the assembly so 303 is machined and 304L is welded.

FormabilityPoor

Sulfide stringers tear on bending, cold heading and drawing; use 304 or 303 Se for formed parts. Light staking, crimping and knurling are fine.

Corrosion resistanceFair

A step below 304: pitting starts at the sulfides, so it stains in chlorides and coastal air where 304 would not. Fine indoors, in fresh water and with mild chemicals. Passivate after machining.

Heat treatment

Process compatibility

How 303 takes each process, and what to specify. Each process links to its own guide and to the shops that do it.

303 Stainless Steel process compatibility
Process	Rating	Notes
CNC Machining	ExcellentExcellent	The default turned stainless; 30-50% faster than 304 with broken chips and long tool life.
Screw Machining	ExcellentExcellent	Made for multi-spindle and cam machines; high-volume fittings and fasteners come from 303 bar.
Swiss Machining	ExcellentExcellent	Small precision parts from ground bar; the sulfur keeps chips short in the guide bushing.
CNC Turning	ExcellentExcellent	Shafts and stems from bar; order precision ground stock for long slender parts.
Passivation	FairFair	Passivates, but nitric baths can etch it grey ("flash attack"). Specify ASTM A967 citric, or nitric with sodium dichromate, and tell the shop it is 303.
Electropolishing	PoorPoor	Sulfide stringers pull out and leave streaks and pits; the finish never matches 304 or 316. Use those when a bright EP surface is the point.
Welding	PoorPoor	Hot-cracks from the sulfur; if forced, 308L or 312 filler, low heat input, short welds, and accept a weaker, less corrosion-resistant joint. Can 303 stainless steel be welded?
Black Oxide	FairFair	MIL-DTL-13924 Class 4 blackens it for glare control; thin and cosmetic, no corrosion benefit.
Bead Blasting	GoodGood	Uniform matte finish on turned parts; use glass or stainless media so no iron embeds.
Press Brake Forming	PoorPoor	Sheet is rare and the sulfides crack on the outside of a bend; form 304 instead.
Laser Cutting	FairFair	Cuts like 304 when sheet can be found, but 303 is a bar alloy; laser-cut parts are 304 or 316.
Heat Treating	Not recommendedNot recommended	Cannot be hardened; annealing only. Specify 416 for a hard free-machining stainless or 17-4 PH for strength.
Hot-Dip Galvanizing	Not recommendedNot recommended	Zinc does not wet the chromium oxide and adds nothing; molten zinc on hot stainless also risks liquid-metal embrittlement.

Design and sourcing tips

Call out "ASTM A582 Type 303, Condition A" on the drawing; add AMS 5640 for aerospace work.

Do not weld it. If the assembly must be welded, make the welded member 304L and the turned member 303, and join them mechanically.

Specify passivation per ASTM A967 by the citric method (or nitric with dichromate) and name the alloy so the shop does not flash-attack the surface.

Keep it out of chloride, pool and marine exposure; use 316 there and accept the machining cost.

Stainless-on-stainless threads gall: specify anti-seize, a plated mating fastener or a dissimilar pair (303 body, 316 screw).

For long slender turned parts, order cold-drawn precision ground bar so straightness and diameter come from the mill, not the lathe.

Frequently asked questions

Is 303 stainless steel magnetic?

No, not in the annealed condition; it is austenitic like 304. Heavy cold work (cold-drawn bar, cold-headed parts) makes it faintly magnetic. A magnet cannot tell 303 from 304; an XRF or PMI check for sulfur can.

Can 303 stainless be welded?

Only badly. The sulfur that makes it machine well segregates in the weld and causes hot cracking and porosity. Shops manage short tack welds with 308L or 312 filler at low heat input, but a structural or leak-tight weld should be made in 304L or 316L. Design so 303 parts are machined and joined mechanically.

What is the difference between 303 and 304 stainless?

Composition differs by one element: 303 has at least 0.15% sulfur, 304 has almost none. That sulfur nearly doubles machinability but lowers corrosion resistance, ruins weldability and reduces ductility. Use 303 for turned parts in mild environments; use 304 for anything welded, formed or exposed to chlorides.

Does 303 stainless rust?

It can stain and pit where 304 would not, because the sulfide inclusions break the passive film. Salt spray, pool chemicals and chloride cleaners are the usual causes, along with free iron left by steel tooling. Passivation per ASTM A967 after machining removes the free iron and buys back most of the difference for indoor and fresh-water service.