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304 vs 316 Stainless Steel: Machinability, Corrosion Resistance & Cost
304 and 316 are the two most commonly specified austenitic stainless steels in CNC machined parts. This guide compares their composition, mechanical properties, machinability, corrosion resistance, and cost — written from the perspective of a U.S.-based, ITAR-registered precision CNC machining shop in Hanover, Pennsylvania.
Quick Reference
- 304 stainless (UNS S30400): 18% Cr, 8% Ni austenitic. ~75 ksi tensile. The general-purpose stainless — food, architectural, indoor industrial. ASTM A276 / A479. Often called "18/8".
- 316 stainless (UNS S31600): 16% Cr, 10% Ni, 2–3% Mo austenitic. ~75 ksi tensile. Marine, medical, chemical-processing grade. ASTM A276 / A479. Molybdenum content drives the corrosion advantage.
Property Comparison Table
| Property |
304 (UNS S30400) |
316 (UNS S31600) |
| Chromium |
18.0–20.0% |
16.0–18.0% |
| Nickel |
8.0–10.5% |
10.0–14.0% |
| Molybdenum |
None |
2.0–3.0% |
| Ultimate tensile strength |
75 ksi (515 MPa) min |
75 ksi (515 MPa) min |
| Yield strength (0.2%) |
30 ksi (205 MPa) min |
30 ksi (205 MPa) min |
| Elongation at break |
40% min |
40% min |
| Brinell hardness |
≤201 HB |
≤217 HB |
| Density |
8.00 g/cm³ |
8.00 g/cm³ |
| Machinability rating |
~45 (vs 12L14 = 100) |
~36 (vs 12L14 = 100) |
| Thermal conductivity |
16.2 W/m·K |
16.3 W/m·K |
| Magnetic response (annealed) |
Non-magnetic |
Non-magnetic |
| Pitting resistance (PREN) |
~19 |
~26 |
| Chloride / saltwater service |
Limited |
Excellent |
| Max continuous service temp |
~870 °C (1,600 °F) |
~870 °C (1,600 °F) |
| Relative cost (per lb) |
1.0× (baseline) |
1.3–1.6× |
Values are typical for annealed bar stock per ASTM A276 / A479. Actual values vary by mill, form, condition, and certification. PREN = %Cr + 3.3×%Mo + 16×%N.
Corrosion Resistance
Corrosion behavior is the single most important reason to choose between these grades. Both rely on a passive chromium-oxide film, but 316's molybdenum addition raises the pitting-resistance equivalent number (PREN) from roughly 19 to 26 — a substantial advantage anywhere chloride ions are present.
- 304: Excellent in clean water, food acids, atmospheric exposure, and most non-chloride industrial environments. Vulnerable to pitting and crevice corrosion in saltwater, de-icing-salt spray, and chlorinated processing streams. Not recommended for coastal or marine service.
- 316: The default for saltwater, marine atmospheres, brine, swimming-pool hardware, pharmaceutical and food-processing equipment exposed to sanitizing chlorides, and surgical implants. The 2–3% molybdenum stabilizes the passive film against chloride attack and dramatically reduces pitting initiation.
- Sensitization and the L grades: When standard 304 or 316 is welded or held at 425–870 °C (800–1,600 °F), chromium carbides can precipitate at grain boundaries, depleting nearby chromium and causing intergranular corrosion. 304L and 316L (low-carbon, ≤0.030% C) resist this and are the standard specification for welded assemblies.
Machinability Notes
Both grades are austenitic and work-hardening. Cutting practice that works for carbon steel will destroy tools on stainless. We treat 304 and 316 as separate setups with distinct speeds, feeds, and tooling.
- 304: Tough and stringy. Long, ribbon-like chips require chip-breaker geometries and high-pressure coolant. We run roughly 250–350 SFM with sharp positive-rake carbide (AlTiN or TiAlN coated) and maintain a continuous feed to prevent dwell — dwelling on stainless work-hardens the cut zone and dulls the next pass instantly. Heavier depth of cut with moderate feed outperforms light shaving passes.
- 316: Gummier and more abrasive than 304 because of the molybdenum content. Speeds drop roughly 20% — we typically run 200–280 SFM with the same tooling family. Built-up edge is a constant risk; flood coolant or through-spindle coolant is required for any drilling or boring operation. Tool life on 316 is meaningfully shorter than on 304, which is reflected in our per-part pricing on bar-stock production.
- Free-machining substitutes: If corrosion requirements allow, 303 (a sulfur-bearing variant of 304) machines roughly twice as fast as 304 and is the default for high-volume turned fittings that do not see welding or saltwater. Improved-machinability 316 grades (Ugima 316, 316LSM) approach 303's machinability while retaining most of 316's corrosion advantage and are worth specifying for production runs above a few hundred pieces.
- Tapping and threading: Both grades require spiral-flute taps with TiCN or TiN coating, generous tapping fluid, and conservative speeds. Form taps work well in 304 up to about ½″; bottoming form taps are preferred in 316 to avoid galling. For ⅜-16 and larger we routinely thread-mill instead of tap on long runs to extend tool life.
Design-for-Manufacturability (DFM) Guidance
- Default to 304 unless chlorides are involved. 304 is cheaper, easier to source in any bar diameter or plate thickness, and easier to machine. Switching to 316 should be a deliberate corrosion-driven choice — not a default "stainless is stainless" specification.
- Specify the L grade for any welded assembly. 304L and 316L cost the same as standard 304 and 316 and prevent sensitization in the heat-affected zone. For machined-only parts that will not see welding, standard 304/316 are acceptable.
- Avoid sharp internal corners. Both grades concentrate stress and accumulate work-hardening at sharp internal corners. Specify a generous radius (≥ ⅓ of cutter diameter) on milled pockets to extend tool life and reduce chatter. This single change typically improves cycle time by 10–20%.
- Tight tolerance bores require honing or finish-pass strategy. Boring stainless to ±0.0005″ requires a dedicated finish pass at light depth and steady feed. We size finish boring at 0.005–0.010″ depth on the final pass to avoid dwell-induced work hardening.
- Surface finish: passivate after machining. Both grades benefit from citric or nitric acid passivation per ASTM A967 after machining to dissolve embedded iron from tooling and restore the passive layer. This is non-optional for medical, food-contact, and marine parts.
- Plan for material premium on 316. 316 bar stock typically runs 30–60% higher per pound than 304, and machining time is roughly 20–25% longer. Budget both effects when costing a 316 part against a 304 baseline.
Common Applications We Machine
304 / 304L
- • Food-processing fittings and brackets
- • Architectural and decorative hardware
- • General industrial fasteners and standoffs
- • Indoor pressure-vessel components
- • Sanitary tubing flanges (non-marine)
- • Equipment frames and machine guards
316 / 316L
- • Marine fittings and deck hardware
- • Surgical instruments and implant components
- • Pharmaceutical and bioprocess fittings
- • Chemical-processing valve bodies
- • Brine and saltwater pump hardware
- • Defense coastal and shipboard fixtures
How Olympus Machines 304 and 316 Stainless
304 and 316 are routine production materials at our Hanover, PA shop. We run them on live-tool CNC lathes and 3-axis milling centers with coated carbide tooling (AlTiN / TiAlN), high-pressure flood coolant, and chip-breaker insert geometries selected for austenitic stainless. Setups are dedicated — we do not run stainless on a fixture that has just run aluminum without cleaning, to avoid galvanic contamination on parts destined for passivation.
Material certifications per ASTM A276 or A479 are retained for every lot, with mill heat numbers traced to the finished part on Certificates of Conformance. CMM inspection on our Haas HMM 430 supports tight-tolerance bores, GD&T true-position callouts, and AS9102 Rev C First Article Inspection. For medical and marine parts, we coordinate post-machining passivation per ASTM A967 (citric or nitric) and can supply passivation certifications with the shipment.
Frequently Asked Questions
Is 316 always better than 304?
No. 316 is better for chloride and marine environments, but it costs more, machines slower, and offers no advantage in non-chloride service. For indoor food-processing or architectural parts, 304 is the correct engineering choice. Over-specifying 316 adds cost without performance benefit.
Are 304 and 316 magnetic?
In the annealed condition, both are essentially non-magnetic. Cold work from machining, bending, or drawing can induce a small ferromagnetic response in 304 (more so than 316). For applications requiring guaranteed non-magnetic behavior, 316 in the solution-annealed condition is the more reliable choice.
Can 304 and 316 be welded to each other?
Yes. The dissimilar weld is common in industry and is typically filled with 316L or 309L filler. The joint inherits the lower corrosion resistance of the two parents, so place the joint away from the most aggressive exposure zone.
What is the difference between 304 and 303?
303 is a sulfur-bearing free-machining variant of 304. It machines roughly twice as fast but loses some corrosion resistance and is not recommended for welded or marine applications. For high-volume turned fittings that will not be welded and will not see chlorides, 303 is the preferred stock.
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Last reviewed: June 9, 2026