Stainless Steel Grades

Stainless steel is steel with at least 10.5 % chromium, enough to grow a self-healing oxide skin that stops rust. That single addition splits into five very different families: the soft, non-magnetic austenitic 300 series that most people mean by "stainless", the magnetic hardenable 400-series martensitics used for knives and shafts, the cheap ferritics for trim and exhaust, the precipitation-hardening grades that reach alloy-steel strength, and the duplex grades that beat 316 in chlorides at half the nickel. Pick the structure first; the grade number follows.

CAD render of a laser-fused 316L stainless steel aviation bracket — two clevis lugs on a saddle with four bolt bosses — sand-blasted as printed.

Stainless steel by structure

Austenitic (300 & 200 series)16–26 % Cr, 6–22 % Ni · 304, 316L, 321, 310363210 – 1,450The general-purpose stainless: tanks, enclosures, fittings, food and medical hardware. Non-magnetic, very weldable and formable; strengthens only by cold work.

Ferritic (400 series, low C)11–18 % Cr, no Ni · 430, 409, 439, 44485220 – 620Appliance trim, exhaust systems, heat shields, brackets — magnetic, cheaper than 304, moderate corrosion resistance, limited weld thickness.

Martensitic (400 series, higher C)12–18 % Cr, 0.1–1.2 % C · 410, 420, 440C, 431109225 – 1,850Shafts, valve parts, knife blades, surgical instruments, bearings — quench-hardenable to 40–60 HRC, magnetic, the least corrosion-resistant stainless family.

Precipitation-hardening (PH)Cr-Ni + Cu, Al or Mo · 17-4 PH, 15-5 PH, 13-8 Mo, 17-7 PH22275 – 2,000Aerospace fittings, pump shafts, high-strength fasteners, mold components — machine in the soft condition, then age to 1,000–1,400 MPa with little distortion.

Duplex & super-duplex22–25 % Cr, 4–7 % Ni, Mo, N · 2205, 2507, 230456380 – 800Seawater, brine, oil and gas, chemical plant — roughly twice the yield strength of 316 with much better chloride pitting and stress-corrosion resistance.

Super-austenitic20–25 % Cr, 18–25 % Ni, 6 % Mo, N · 904L, 254 SMO, AL-6XN, Alloy 2035220 – 490Sulfuric and phosphoric acid, hot seawater, flue-gas scrubbers — the step between 316L and a nickel alloy.

Free-machining stainlessS or Se added · 303, 416, 430F, 420F12260 – 450Screw-machine fittings, shafts and fasteners in volume — 303 cuts about 30 % faster than 304; 416 is the hardenable equivalent of 410.

Choose stainless steel for

Food, medical, marine and outdoor parts that must stay bare and clean; fasteners, fittings, manifolds and enclosures in wet or chemical service; anything welded that cannot be painted afterwards; and hardenable parts — shafts, valve stems, knife blades, surgical tools — that also need corrosion resistance. 304 handles most of it, 316L when there is salt or chloride, 17-4 PH when strength matters.

Look elsewhere when

Cost or machining time is the driver and the part will be coated anyway (carbon steel machines two to three times faster), weight matters (aluminum or titanium), the part sees strong reducing acids, hot chlorides or seawater at temperature (nickel alloys, titanium or super-duplex), or very high hardness with dimensional stability is required (tool steel). Austenitic grades are also non-magnetic and work-harden aggressively — thin walls and deep pockets cost more than in aluminum.

Yield strength against max service temp, every stainless steel grade

No materials in view have both of these properties.

Loading the library…

Stainless Steel by application

General machined and welded parts: 304 unless there is a reason not to; 304L for anything welded thicker than ~5 mm; 201 as a cheaper trim-grade substitute.
Marine, chloride and chemical exposure: 316L for salt spray and mild chemicals; 2205 for immersed seawater and brine at higher strength; 904L / 254 SMO / 2507 for hot chlorides and acids.
High strength with corrosion resistance: 17-4 PH H900 (1,170 MPa yield) is the workhorse; 15-5 PH for better toughness; 13-8 Mo for aerospace; 440C or 420 when hardness matters more than strength.
Fast turned parts in volume: 303 for non-hardened fittings and shafts; 416 where the part will be hardened; avoid both in welded or marine service.
Heat and oxidation to 1,100 °C: 321 to ~800 °C where 304 would sensitize; 309 / 310 for furnace parts and burners; 446 ferritic for sulfur-bearing atmospheres.
Wear, knives and bearings: 440C at 58–60 HRC for blades, bearings and valve seats; Nitronic 60 for galling resistance without hardening.
Medical and implant hardware: 316LVM (ASTM F138) for implants; 17-4 PH and 420 for instruments; passivate everything per ASTM A967.
Corrosion-resistant mold tooling: For PVC, optical and medical molds where a P20 cavity would pit; hardens to 48–52 HRC and polishes to mirror.

FabAgentAI

AI suggestions — verify with an engineer before ordering. Estimates are indicative; the quote page prices your exact geometry.

Manufacturing notes

CNC machining

Feed per tooth mm/tooth

Ø6 mm · 3-flute carbide end mill

Distance the cutter advances between successive teeth (chip load). Published per material; independent of tool diameter.

6061-T6 has the highest feed per tooth (0.0508 mm/tooth); 304 the lowest (0.0127 mm/tooth).

n = 1,000 v_c / (πD)v_f = f_z × z × nD = 6 mm · z = 3 flutes · diagram not to scale

Austenitic grades work-harden as they cut — rigid setups, positive rake, and no dwelling. 303 and 416 cut the fastest; 304 / 316 run at roughly half the speed of 1018 steel; 17-4 PH machines best in condition A or H1150 and is aged afterwards; 440C and 420 are machined annealed and finished by grinding after hardening.

Sheet metal

Minimum inside radius mm

Thickness

Smallest published inside radius for a 90° bend at the selected sheet thickness.

6061-T651 has the highest minimum inside radius at 1.6 mm (5.6 mm); 304 annealed the lowest (1.6 mm).

Rᵢ = (Rᵢ/t) × tt = sheet thicknessOutside radius = Rᵢ + t90° bends · grain direction and temper matter

304 and 316 sheet bends and forms readily but springs back more than mild steel — allow for it in the tooling. Ferritic 430 forms well for trim; 301 for spring clips. Watch for chromium-carbide sensitization on thick welded 304: use the L grade.

Welding: Austenitic and duplex grades weld well by TIG / MIG with matching filler (308L, 316L, 2209). Martensitic and PH grades need preheat and post-weld heat treatment; free-machining 303 and 416 crack. Always remove heat tint or passivate after welding — the discolored zone is where it will pit.
Heat treatment: Solution-anneal austenitics only for stress relief; harden martensitics by quench and temper (410 to ~40 HRC, 440C to ~58 HRC); age PH grades in one low-temperature step (H900–H1150) after machining. Duplex is never heat-treated above 300 °C in service.
Finishing: Passivation (citric or nitric, ASTM A967) is standard after machining. Bead blast, brushed #4, mirror #8, electropolish for cleanability and fatigue; black oxide, PVD or electroless nickel for color or wear. Stainless is not anodized.

Most-quoted stainless steel grades

Grade	Structure	Yield, MPa	Tensile, MPa	Density, g/cm³	Machinability	Weldability	Best for
304	Austenitic (300 & 200 series)	250	585	8	Fair	Excellent	The default stainless part
316L	Austenitic (300 & 200 series)	290	580	7.98	Fair	Excellent	Marine, chemical and welded parts
303	Free-machining stainless	450	655	7.9	Very good	Poor	Fast turned fittings and shafts
17-4 PH	Precipitation-hardening (PH)	760	1,030	7.78	Fair	Very good	High-strength shafts and fittings
410	Martensitic (400 series, higher C)	310	517	7.7	Good	Fair	Hardenable shafts and valve parts
440C	Martensitic (400 series, higher C)	450	760	7.68	Fair	Not recommended	Blades, bearings, wear parts
2205	Duplex & super-duplex	540	790	7.8	Poor	Very good	Seawater and brine at high strength
430	Ferritic (400 series, low C)	310	500	7.7	Fair	Good	Low-cost trim and brackets
13-8 PH	Precipitation-hardening (PH)	827	1,000	7.76	Poor	Good	Ultra-high strength stainless
15-5 PH	Precipitation-hardening (PH)	795	1,030	7.8	Fair	Very good	Versatile high-strength PH

Finishes for stainless steel

All 111 finishes

83 of our 111 surface finishes take stainless steel: 42 recommended outright, 29 with process care, 12 in limited cases. Each finish opens to its datasheet — thickness, hardness, salt-spray hours, spec codes and the stainless steel-specific notes — and the configurator prices it on the grade you pick.

Finish	Group	On stainless steel	Thickness, µm	Hardness, HV	Salt spray, h	Cost
Electropolishing	Anodizing, conversion & passivation	Yes	removes 5–25	—	—	$$$
Stainless Steel Passivation	Anodizing, conversion & passivation	Yes	0	—	—	$
Stainless Steel Pickling	Anodizing, conversion & passivation	Yes	0	—	—	$
Electroless Nickel Plating	Plating & metallizing	Yes	5–75	450–1,000	100–1,000	$$$
Nickel-PTFE Composite Plating	Plating & metallizing	Yes	10–25	250–500	—	$$$$
Copper Plating	Plating & metallizing	With care	5–30	50–250	—	$$
Anti-Fingerprint Oleophobic Coating	Paint, powder & polymer coatings	Yes	0.01–2	—	—	$$$
Cerakote	Paint, powder & polymer coatings	Yes	20–30	—	500–1,000	$$$
PTFE Dry-Film Coating	Paint, powder & polymer coatings	Yes	15–50	—	250–1,000	$$$
Chromium Nitride PVD	PVD, CVD & thermal spray	Yes	1–5	1,600–2,000	—	$$$$
Diamond-Like Carbon Coating	PVD, CVD & thermal spray	Yes	1–3	2,000–5,000	—	$$$$
HVOF Carbide Coating	PVD, CVD & thermal spray	Yes	50–500	1,000–1,400	—	$$$$
Bead Blasting	Mechanical finishing & post-processing	Yes	0	—	—	$
Brushed Finish	Mechanical finishing & post-processing	Yes	0	—	—	$$
Deburring	Mechanical finishing & post-processing	Yes	0	—	—	$
Chemical Etching	Marking, decoration & colour	Yes	5–100	—	—	$$$
Dot Peen Marking	Marking, decoration & colour	Yes	50–500	—	—	$
Electrochemical Etching	Marking, decoration & colour	Yes	2–25	—	—	$$

65 more finishes accept stainless steel — see the full material × finish matrix →

Stainless Steel — frequently asked

304 or 316 — when is the upgrade worth it?

Upgrade when there are chlorides: seawater, road salt, de-icing spray, brine, bleach, pool chemistry. The 2–3 % molybdenum in 316 roughly doubles pitting resistance. For indoor, food-contact, fresh-water and architectural parts 304 is as good and about 20 % cheaper. Choose the L (low-carbon) version of either if the part is welded.

Is stainless steel magnetic?

Depends on the structure. Austenitic 300-series (304, 316) is non-magnetic when annealed and only faintly magnetic after heavy cold work. Ferritic (430, 409), martensitic (410, 420, 440C), PH (17-4) and duplex grades are all magnetic. If a magnet sticks to a "304" part, it is probably 430 or a heavily worked area.

Which stainless can be hardened?

Martensitic 400-series (410 to ~40 HRC, 420 to ~50 HRC, 440C to ~58–60 HRC) by quench and temper, and the precipitation-hardening grades (17-4 PH, 15-5 PH, 13-8 Mo) by a single low-temperature age to 1,000–1,400 MPa yield. Austenitic, ferritic and duplex grades cannot be hardened by heat treatment — only by cold work.

Why does stainless cost more to machine than steel?

Austenitic grades work-harden at the cutting edge, conduct heat poorly (so the tool takes it) and produce a tough, stringy chip. Typical cutting speeds are 40–60 % of mild steel and tool life is shorter. 303 recovers most of that gap for turned parts; 17-4 PH machines well in the soft condition and is hardened afterwards.

Does stainless steel rust?

It can. Chlorides pit it, embedded carbon-steel particles (from a shared grinding wheel or shop dust) rust on the surface, weld heat tint corrodes, and the free-machining grades 303 / 416 tarnish outdoors. Passivation after machining, dedicated tooling and picking the right grade for chloride exposure keep it stain-free in practice.