Stainless Steel CNC Machining
303, 304, 316L, 17-4PH, 420 and 440C with the work-hardening discipline stainless demands: sharp tooling, constant engagement, and passivation or passivate-ready finishes.
Grades We Machine
| Grade | Type | Tensile (typ.) | Machinability | Wins on |
|---|---|---|---|---|
| 303 | Austenitic + S | approx. 620 MPa | 78% (best) | Complex parts without welding |
| 304 | Austenitic | 515 MPa min | 45% | General corrosion resistance, weldable |
| 316L | Austenitic + Mo | 485 MPa min | 36% | Chloride and salt exposure, medical |
| 17-4PH | Precipitation hardening | 1310 MPa (H900) | Condition dependent | High strength + corrosion, HRC 40-44 |
| 420 | Martensitic | hardened HRC 48-52 | Moderate | Cutting edges, instrument blades |
| 440C | Martensitic | hardened HRC 58-60 | Lower | Bearing seats, valve needles |
Full comparison: Stainless Grade Comparison.
Stainless typical parts Grade comparison
Chemical Composition (typical limits, wt %)
From the KGL engineering standard (2022-01-08); 303, 304 and 316L limits follow GB/T 20878 / JIS G4303, cross-checked against the Mechanical Design Handbook 6th ed. Vol. 1.
| Grade | C | Si | Mn | P | S | Cr | Ni | Mo | Others |
|---|---|---|---|---|---|---|---|---|---|
| 303 | ≤ 0.15 | ≤ 1.00 | ≤ 2.00 | ≤ 0.20 | ≥ 0.15 | 17.0-19.0 | 8.0-10.0 | ≤ 0.60 | - |
| 304 | ≤ 0.08 | ≤ 1.00 | ≤ 2.00 | ≤ 0.045 | ≤ 0.030 | 18.0-20.0 | 8.0-10.5 | - | - |
| 316L | ≤ 0.08 | ≤ 1.00 | ≤ 2.00 | ≤ 0.045 | ≤ 0.030 | 16.0-18.0 | 10.0-14.0 | 2.00-3.00 | - |
| 17-4PH | ≤ 0.07 | ≤ 1.00 | ≤ 1.00 | ≤ 0.040 | ≤ 0.030 | 15.5-17.5 | 3.00-5.00 | - | Cu 3.00-5.00, Nb+Ta 0.15-0.45 |
| 304L | ≤ 0.030 | ≤ 1.00 | ≤ 2.00 | ≤ 0.045 | ≤ 0.030 | 18.0-20.0 | 8.0-12.0 | - | - |
| 316 | ≤ 0.08 | ≤ 1.00 | ≤ 2.00 | ≤ 0.045 | ≤ 0.030 | 16.0-18.0 | 10.0-14.0 | 2.00-3.00 | - |
| 316Ti | ≤ 0.08 | ≤ 1.00 | ≤ 2.00 | ≤ 0.045 | ≤ 0.030 | 16.0-18.0 | 10.0-14.0 | 2.00-3.00 | Ti 5×C-0.70 |
| 321 | ≤ 0.08 | ≤ 1.00 | ≤ 2.00 | ≤ 0.045 | ≤ 0.030 | 17.0-19.0 | 9.0-12.0 | - | Ti 5×C-0.70 |
| 347 | ≤ 0.08 | ≤ 1.00 | ≤ 2.00 | ≤ 0.045 | ≤ 0.030 | 17.0-19.0 | 9.0-12.0 | - | Nb+Ta 10×C-1.10 |
| 310S | ≤ 0.08 | ≤ 1.50 | ≤ 2.00 | ≤ 0.045 | ≤ 0.030 | 24.0-26.0 | 19.0-22.0 | - | High-temperature service |
| 904L | ≤ 0.020 | ≤ 1.00 | ≤ 2.00 | ≤ 0.045 | ≤ 0.030 | 19.0-23.0 | 23.0-28.0 | 4.00-5.00 | Cu 1.00-2.00 |
| 410 | ≤ 0.15 | ≤ 1.00 | ≤ 1.00 | ≤ 0.040 | ≤ 0.030 | 11.5-13.5 | - | - | Martensitic baseline |
| 416 | ≤ 0.15 | ≤ 1.00 | ≤ 1.25 | ≤ 0.060 | ≥ 0.15 | 12.0-14.0 | - | - | Free-machining martensitic |
| 431 | ≤ 0.20 | ≤ 1.00 | ≤ 1.00 | ≤ 0.040 | ≤ 0.030 | 15.0-17.0 | 1.50-2.50 | - | High-strength martensitic |
| 440A | 0.60-0.75 | ≤ 1.00 | ≤ 1.00 | ≤ 0.040 | ≤ 0.030 | 16.0-18.0 | - | - | Cutlery class, HRC 55-58 |
| 17-7PH | ≤ 0.09 | ≤ 1.00 | ≤ 1.00 | ≤ 0.040 | ≤ 0.030 | 16.0-18.0 | 6.50-7.75 | - | Al 0.75-1.50, spring temper |
| 2205 | ≤ 0.030 | ≤ 1.00 | ≤ 2.00 | ≤ 0.030 | ≤ 0.020 | 22.0-23.0 | 4.50-6.50 | 3.00-3.50 | N 0.14-0.20, duplex |
| 2507 | ≤ 0.030 | ≤ 0.80 | ≤ 1.20 | ≤ 0.035 | ≤ 0.020 | 24.0-26.0 | 6.00-8.00 | 3.00-5.00 | N 0.24-0.32, super duplex |
Grade Cross-Reference
| ASTM/UNS | GB | JIS | DIN/EN | Note |
|---|---|---|---|---|
| 303 / S30300 | Y12Cr18Ni9 | SUS303 | 1.4305 | Free-machining austenitic |
| 304 / S30400 | 06Cr19Ni10 | SUS304 | 1.4301 | General purpose austenitic |
| 316L / S31603 | 022Cr17Ni12Mo2 | SUS316L | 1.4404 | Mo-bearing, chloride resistant |
| 17-4PH / S17400 | 05Cr17Ni4Cu4Nb | SUS630 | 1.4542 | Precipitation hardening |
| 15-5PH / S15500 | 05Cr15Ni5Cu4Nb | - | 1.4545 | PH, aerospace structural forgings |
| 420 / S42000 | 30Cr13 | SUS420J2 | 1.4028 | Martensitic, hardened edges |
| 440C / S44004 | 11Cr17 | SUS440C | 1.4125 | Highest hardness martensitic |
| 304L / S30403 | 022Cr19Ni10 | SUS304L | 1.4307 | Low-carbon, post-weld corrosion |
| 316 / S31600 | 06Cr17Ni12Mo2 | SUS316 | 1.4401 | Mo-bearing general purpose |
| 316Ti / S31635 | 06Cr17Ni12Mo2Ti | SUS316Ti | 1.4571 | Ti-stabilised, high temperature |
| 321 / S32100 | 06Cr18Ni11Ti | SUS321 | 1.4541 | Ti-stabilised, aerospace exhaust and fasteners |
| 347 / S34700 | 06Cr18Ni11Nb | SUS347 | 1.4550 | Nb-stabilised, welding-intensive assemblies |
| 310S / S31008 | 06Cr25Ni20 | SUS310S | 1.4845 | High-temperature furnace and manifold parts |
| 904L / N08904 | 015Cr21Ni26Mo5Cu2 | - | 1.4539 | Sulfuric acid and severe chloride service |
| 410 / S41000 | 12Cr13 | SUS410 | 1.4006 | Basic martensitic, pump and valve parts |
| 416 / S41600 | Y12Cr13 | SUS416 | 1.4005 | Free-machining martensitic |
| 431 / S43100 | 14Cr17Ni2 | SUS431 | 1.4057 | High-strength martensitic, aircraft fittings |
| 440A / S44002 | 68Cr17 | SUS440A | - | Cutlery and instrument blades |
| 17-7PH / S17700 | 07Cr17Ni7Al | SUS631 | 1.4568 | PH spring and diaphragm stock |
| 2205 / S32205 | 022Cr22Ni5Mo3N | - | 1.4462 | Duplex, high strength + chloride resistance |
| 2507 / S32750 | 022Cr25Ni7Mo4N | - | 1.4410 | Super duplex, offshore and seawater |
Designations per GB/T 20878 with ASTM/UNS, JIS and DIN/EN equivalents from the KGL engineering standard, cross-checked with the Mechanical Design Handbook 6th ed. Vol. 1 (stainless steels, tables 3.1-128 to 3.1-136). Aerospace callouts center on 321, 347, 431, 15-5PH, 17-4PH and 17-7PH families.
Work-Hardening: The Rules We Machine By
- Sharp, honed edges and constant feed: tool dwell is what hardens 304 and 316 and kills inserts
- Heat at the shear zone, not in the part: high-pressure coolant on deep bores
- Thread milling or form rolling where thread galling has bitten previous suppliers
- Leaving 0.2 to 0.5 mm of finish stock after roughing so the finish pass cuts under the hardened skin
Finishes and Verification
Passivation (ASTM A967), electropolish for medical, Ra 0.4 achievable on sealing faces. CMM reports on critical dimensions come with every order; RoHS and REACH declarations on request.
Finish detail pages: passivation, polishing and electropolishing, electroplating.
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Stainless Machining FAQ
Why does my 304 part cost more to machine than 303?
303 has added sulfur that breaks chips cleanly, so it cuts faster and tools last longer. 304 work-hardens and galls, which costs cycle time. If your part is not welded and corrosion demands are moderate, 303 can cut the unit price.
Can you machine 17-4PH in the hardened condition?
We normally machine 17-4PH in the solution-treated condition and age it afterward (H900 to H1150), then finish machine or grind critical seats. This avoids tool wear on the hardened condition while keeping final dimensions in spec.
How do you prevent passivation from changing dimensions?
Passivation removes surface iron and grows no measurable layer, so dimensions hold. Dimensions are verified after passivation on sealing and bearing faces.