Stainless Steel

Stainless steel is specified for corrosion resistance — but "stainless" doesn't mean "corrosion-proof." 304 rusts in marine environments. 303 machines beautifully but has half the corrosion resistance. 316 costs 30% more than 304 but is often unnecessary. This page helps you pick the right grade and avoid overpaying.

Which Stainless Steel Do You Need?

Your Situation	Use This	Why
General indoor use, food equipment, aesthetic parts	304	Best all-around. Good corrosion resistance, formable, weldable. Covers 80% of stainless needs.
Marine, chemical, chloride exposure	316/316L	Molybdenum gives excellent pitting resistance. The "marine grade."
High-volume screw machine parts	303	Free-machining grade with sulfur. Cuts like brass. But reduced corrosion resistance.
Need hardness + corrosion resistance	17-4PH	Precipitation hardening. Can reach HRC 40. Aerospace, medical, valve stems.
High strength + extreme corrosion	2205 duplex	Twice the strength of 304, excellent SCC resistance. Offshore, chemical plants.
Non-magnetic requirement	304L or 316L	L-grades have lower carbon, better weldability, slightly less magnetic after cold work.
Budget is tight, mild corrosion resistance OK	303 or 430	303 machines cheapest (fastest). 430 is cheapest ferritic grade.

Default choice If you don't know which stainless to use, pick 304. It handles most environments except saltwater and strong acids. Only upgrade to 316 when you have a specific chloride or acid exposure requirement.

Stainless Steel Data at a Glance

Property	304	316	303	17-4PH	2205
Type	Austenitic	Austenitic	Austenitic (free-mach)	Precip. hardening	Duplex
Tensile (MPa)	515	515	620	1000 (H900)	620
Yield (MPa)	205	205	240	880 (H900)	450
Max hardness	88 HB (annealed)	95 HB	26 HRC	40 HRC (H900)	32 HRC
Machinability	Poor (gummy)	Poor (gummy)	Excellent	Fair	Poor
Weldable	Yes	Yes	Not recommended	Difficult	Yes (preheat)
Pitting resistance	Good	Excellent	Fair	Good	Excellent
SCC resistance	Poor (>60°C)	Fair	Poor	Good	Excellent
Relative cost	1.0x	1.3–1.5x	1.2–1.4x	2–3x	1.8–2.5x

304 vs 316 — The Most Common Question

316 has 2–3% molybdenum. That's it. That small addition gives dramatically better resistance to pitting corrosion in chloride environments (saltwater, bleach, certain acids). In most indoor environments, there's zero difference.

Environment	304	316
Indoor, dry, atmospheric	Fine	Overkill
Outdoor, urban atmosphere	Fine	Overkill
Food processing, mild acids	Fine	Better
Marine / saltwater splash	Surface staining, pitting possible	Fine
Continuous saltwater immersion	Pitting within weeks	May still need 317 or super duplex
Chloride solutions (>100ppm Cl−)	Pitting	Good resistance
High-temp (>60°C) chloride	SCC risk	Reduced but not immune

Stop over-specifying 316 316 costs 30–50% more than 304 and is harder to source in some forms. If your part is for indoor use, 304 is the right choice. Only specify 316 when you have a documented chloride or acid exposure requirement.

303 — The Machining Cheat Code

303 is 304 with sulfur added (0.15% min). The sulfur creates chip-breaking inclusions that make it machine like brass. Tool life doubles, cycle times drop 30–40%. The trade-off: reduced corrosion resistance and it cannot be welded reliably.

When to use 303 High-volume screw machine parts where machinability is the priority and corrosion resistance is secondary (mild indoor environments). Common for fittings, connectors, valve bodies, and any part with lots of threaded features.

17-4PH — Hardness + Corrosion

17-4PH is a precipitation-hardening stainless steel. You machine it in the solution-treated condition (soft, ~35 HRC), then age it to H900 for 40 HRC. It's the answer when you need both hardness and corrosion resistance — something 304 and 316 can't provide.

Condition	Temp/Time	HRC	Tensile (MPa)	Notes
A (solution treated)	1040°C / air cool	33–38	1035	Machine in this condition
H900	480°C / 1 hr	40–44	1310	Most common aged condition
H1150	620°C / 4 hrs	28–36	1000	Overaged for toughness

Dimensional change on aging 17-4PH grows slightly (0.0005–0.001 in/in) during aging. For tight-tolerance parts, machine slightly undersize before aging, or account for the growth in your tolerance stack.

Machining Stainless — Real Talk

Austenitic stainless (304, 316) is notoriously difficult to machine. It work-hardens, galls, and generates high heat. Here's what actually works:

Rule	Detail
Low feed, high speed	Keep the feed high enough to get under the work-hardened surface. Never dwell — the tool rubs and work-hardens the spot.
Sharp tools only	Dull tools create more work-hardening. Replace end mills before they look worn. Use TiAlN or diamond-coated tools.
Coolant flood	Stainless generates a lot of heat. Flood coolant is mandatory for any extended cut. Air blast alone won't work.
Rigid setup	Stainless pushes back hard. Use the shortest tool possible, minimize overhang, rigid workholding.
Chip evacuation	Stainless chips are stringy and can recut. Use tools with chipbreaker geometry and high-pressure coolant if available.
Expect 2x the cost vs carbon steel	Slower feeds, faster tool wear, more setups. Stainless machining costs roughly double vs 4140 for the same geometry.

Common Mistakes

Mistake	What happens	Correct approach
Specifying 316 when 304 suffices	Paying 30–50% more for zero benefit	Use 316 only for documented chloride/acid exposure
Using 303 for welded assemblies	Sulfur causes weld porosity and cracking	Use 304 for any welded parts
Dwelling during 304 machining	Work-hardens the surface, tool chatters, part is scrap	Keep the tool moving. Never pause mid-cut.
Not accounting for 17-4PH growth	Aged part goes oversize	Machine undersize by 0.01–0.03mm before aging
Using stainless for high-strength application	304 yield is only 205 MPa — weaker than aluminum 6061-T6	Need strength? Use 17-4PH or switch to alloy steel
Calling it "stainless" without a grade	Supplier ships whatever's cheapest — could be 201 or 430	Always specify: "304 stainless" or "316L stainless"

← Steel

Titanium →