CNC machining of 304 / 304L stainless steel.

Why 304 is the default stainless specification

Ask any procurement engineer to "make it stainless" with no further context and they will get 304 quoted. This is rational: 304 covers the 70% of applications where corrosion resistance matters but the environment isn't chloride-rich. It is more weldable, more formable, and 15–20% cheaper than 316, yet visually indistinguishable after passivation. The job of this page is to help you decide whether that default is correct for your part, or whether you should escalate to 316, 304L, or drop down to 303.

The "L" in 304L designates low carbon (max 0.03% vs 0.08% in standard 304). This is a welding specification — when 304 is welded, the heat-affected zone can precipitate chromium carbides at grain boundaries, depleting local chromium and creating intergranular corrosion sites. 304L eliminates this risk. If your part will be welded and used in corrosive service, specify 304L. If your part is machined-only with no welding, 304 is fine and slightly stronger.

§01 — Chemical composition (ASTM A276)

Element	304 min %	304 max %	304L max %	Role
Carbon (C)	—	0.08	0.030	Affects weldability, sensitization
Chromium (Cr)	18.0	20.0	20.0	Passive layer, corrosion resistance
Nickel (Ni)	8.0	10.5	12.0	Austenite stabilizer, ductility
Manganese (Mn)	—	2.00	2.00	Austenite stabilizer, deoxidizer
Silicon (Si)	—	0.75	0.75	Deoxidizer
Phosphorus (P)	—	0.045	0.045	Impurity limit
Sulfur (S)	—	0.030	0.030	Impurity limit (303 adds S for machining)
Nitrogen (N)	—	0.10	0.10	Strengthener

§02 — Mechanical properties (annealed bar/plate)

Property	304 (metric)	304 (imperial)	304L	Test method
Tensile strength	515 MPa	75 ksi	485 MPa	ASTM E8
Yield strength (0.2%)	205 MPa	30 ksi	170 MPa	ASTM E8
Elongation (50mm)	40%	40%	40%	ASTM E8
Hardness (max)	201 HB	92 HRB	201 HB	ASTM E10
Impact (Charpy, 20°C)	120 J	88 ft-lbf	130 J	ASTM E23
Modulus of elasticity	193 GPa	28 × 10⁶ psi	193 GPa	ASTM E111
Thermal expansion (20–100°C)	17.2 µm/m·K	9.6 µin/in·°F	17.2 µm/m·K	ASTM E228
Thermal conductivity	16.2 W/m·K	9.4 BTU/hr·ft·°F	16.2 W/m·K	ASTM E1225

§03 — Cutting parameters

304 is considered moderately difficult to machine. It work-hardens rapidly, has low thermal conductivity (heat stays at the cutting edge), and produces stringy chips. The cardinal rules: maintain consistent feed (never let the tool dwell), use generous coolant, and prefer coated carbide inserts with positive-rake geometry. Dwelling is the main failure mode — a stopped tool creates a work-hardened layer that then destroys the edge when cutting resumes.

Operation	Tool	Surface speed (SFM)	Feed per tooth (mm)	DOC (mm)
Face milling	TiAlN coated carbide	250–400	0.10–0.20	1.0–3.0
End milling (roughing)	4-flute TiAlN carbide	200–300	0.05–0.12	0.5×D axial
End milling (finishing)	4-flute AlTiN carbide	300–400	0.03–0.08	0.2 mm radial
Drilling	Cobalt HSS or carbide	60–100	0.08–0.15/rev	—
Tapping	Cobalt spiral flute tap	15–30	—	—
Turning (roughing)	CVD-coated carbide, −5° rake	200–300	0.25–0.40/rev	2.0–5.0
Turning (finishing)	PVD-coated carbide	250–350	0.10–0.20/rev	0.25–0.5
Reaming	Cobalt HSS, TiN coated	30–50	0.05–0.15/rev	—

§04 — Work-hardening control

304's work-hardening rate is about 3× that of carbon steel. A dwelling tool raises the local hardness from ~200 HB to ~450 HB in seconds, which then chips or dulls the cutter on the next pass. Three rules eliminate this:

1. Never let the tool rub without cutting. Feed rate must always produce a chip thicker than the tool-edge radius (~0.02 mm for sharp carbide). Light "finish passes" at 0.02 mm depth of cut cause work-hardening — go deeper or skip it.
2. Climb-mill, don't conventional-mill. Climb milling engages with maximum chip thickness and exits at zero, minimizing rubbing. Conventional milling does the opposite and is a major cause of premature tool failure in 304.
3. Maintain continuous cut on interrupted surfaces. For slots or pockets with varying stock, use trochoidal (adaptive) toolpaths that maintain consistent chip load. Standard pocket-out strategies cause repeated engagement/disengagement and work-harden the slot floor.

§05 — Achievable tolerances

§06 — Surface finishes

§07 — Applications

§08 — 304 vs 316 vs 303 decision matrix

Property	304 (default)	316 (marine/chemical)	303 (free-machining)
Yield strength	205 MPa	205 MPa	240 MPa
Chloride resistance	Moderate — pits in seawater	Excellent	Moderate
Machinability	Fair (45%)	Fair (40%)	Excellent (80%)
Weldability	Excellent	Excellent	Poor (sulfur cracks)
Food & medical	Yes (FDA)	Yes (FDA, implantable)	No (sulfur)
Cost (relative)	1.00×	1.25–1.40×	0.95×
When to specify	Default stainless	Seawater, chlorides, implants	High-volume turned parts

Quick decision: 304 unless you're in seawater (→ 316), or making high-volume screw-machine parts with no welding or food contact (→ 303).