Free-Machining Stainless Steel
Free-machining stainless steels are standard grades with about 0.15–0.35 % sulfur (or selenium) added to form manganese-sulfide stringers that break the chip and lubricate the tool: 303 is 304 made to cut, 416 is 410, 430F is 430, 420F is 420 and 440F is 440C. The gain is real — 303 machines about 30 % faster than 304 with a better finish, and 416 is the most machinable stainless of all — and the price is a step down in corrosion resistance, poor weldability and lower transverse toughness.
Popular free-machining stainless grades
Compare in the explorer| Grade | Yield, MPa | Tensile, MPa | Density, g/cm³ | Machinability | Weldability | Best for |
|---|---|---|---|---|---|---|
| 303 | 450 | 655 | 7.9 | Very good | Poor | Turned fittings and shafts; 304 made to cut |
| 416 | 275 | 515 | 7.7 | Very good | Poor | Hardenable to 40 HRC; most machinable stainless |
| 430F | 380 | 550 | 7.7 | Very good | Poor | Magnetic solenoid parts and fittings |
| 420F | 380 | 690 | 7.75 | Good | Not recommended | Cutlery, valve trim and gauges at 50 HRC |
| 303Se | 415 | 690 | 7.9 | Excellent | Poor | Finer finish; parts also staked or cold-formed |
| 316F | 450 | 620 | 8 | Very good | Poor | Free-machining with molybdenum for chlorides |
| 440F | 415 | 760 | 7.7 | Good | Not recommended | Bearings and seats at 58 HRC |
Choose free-machining stainless for
Screw-machine and Swiss-turned parts in volume — fittings, adapters, shafts, spacers, standoffs, nuts, valve stems, gears, plungers — where cycle time and tool life set the cost and the part is not welded and does not see chlorides or marine spray.
Look elsewhere when
The part is welded (the sulfur cracks the weld — 304L or 316L), sits outdoors near salt or in food, pharmaceutical or medical contact (304, 316L, 316LVM), needs full corrosion resistance after hardening (410 or 17-4 PH rather than 416), takes impact or bending across the bar axis, or runs in low volume where the 30 % time saving does not pay for the lower corrosion resistance.
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Free-machining stainless by application
- Screw-machine fittings
- 303 for hydraulic adapters, pipe fittings, standoffs and spacers; 1.4570 or 303Se when the part is knurled, thread-rolled or swaged after turning.
- Hardened shafts and stems
- 416 quenched and tempered for pump shafts, valve stems and studs to about 40 HRC; 420F for gauges, nozzles and cutlery at 50 HRC; 440F for bearing races and valve seats at 58 HRC.
Working with free-machining stainless
- Machining
- Run 303 about 30 % faster than 304 with lower tool pressure and short, curling chips; 416 goes faster still. Sharp tools and moderate feeds give a mirror-like turned finish. The sulfide stringers run along the bar, so cross-drilled holes and transverse features finish slightly rougher, and 303Se is chosen where finish matters most.
- Heat treatment
- 303, 303Se, 316F and 430F cannot be hardened. 416 austenitizes at 950–1,010 °C, oil-quenches and tempers to 30–40 HRC; 420F at 980–1,030 °C to about 50 HRC; 440F at 1,010–1,065 °C to 58 HRC. Temper outside 400–550 °C, where toughness and corrosion resistance both fall.
Other stainless steel series
Free-Machining Stainless Steel — frequently asked
What is free-machining stainless?
Free-machining stainless steels are standard grades with about 0.15–0.35 % sulfur (or selenium) added to form manganese-sulfide stringers that break the chip and lubricate the tool: 303 is 304 made to cut, 416 is 410, 430F is 430, 420F is 420 and 440F is 440C. The gain is real — 303 machines about 30 % faster than 304 with a better finish, and 416 is the most machinable stainless of all — and the price is a step down in corrosion resistance, poor weldability and lower transverse toughness.
Which grade of free-machining stainless should I use?
303 for turned fittings, shafts, nuts, bushings and hydraulic adapters that will not be welded and live indoors or in fresh water; 303Se when the part is also cold-formed, staked or knurled, or needs a finer surface finish. 316F is the molybdenum-bearing version for occasional chlorides. 416 is the hardenable one: quench and temper to about 40 HRC for pump shafts, valve stems, gears and studs, still cutting far faster than 410; 416Se is its selenium twin. 420F hardens to about 50 HRC for cutlery, valve trim and gauges; 440F reaches 58 HRC for bearings and seats. 430F is the ferritic, magnetic grade for solenoid plungers, armatures and fittings that are never hardened. 1.4305 and 1.4005 are the EN designations of 303 and 416, 1.4104 is the EN grade closest to 430F, and 1.4570 is a copper-bearing 303 with lower work hardening for high-volume turned parts.
Which grade of free-machining stainless is the strongest?
303 has the highest yield strength in the series at 450 MPa; 316F follows at 450 MPa. Values are for each grade's default condition.
Which grade of free-machining stainless is easiest to machine?
1.4005 (machinability excellent, 85/100), then 303Se (85/100).
Is free-machining stainless heat-treatable?
Some grades are. Check the conditions listed on each grade page.
303 or 304 for a turned part?
303 when the part is made in quantity on a lathe, will not be welded and lives indoors or in fresh water: it cuts about 30 % faster with longer tool life and a better finish, and the strength is the same. 304 when the part is welded, formed, sees chlorides or outdoor exposure, or touches food or body fluids — the sulfides in 303 pit first and are not accepted by most food and medical specifications. For a handful of parts the time saving rarely matters; for ten thousand it decides the quote.
Why does 303 rust where 304 does not?
The manganese-sulfide stringers that make it machine are also gaps in the passive film: chloride attack starts at each one, so 303 pits and stains roughly one grade sooner than 304 and should not be used at the coast, in pools or in bleach. Passivation matters more than for 304 and has to be the citric or nitric-dichromate method, because plain nitric acid etches the sulfides out. Indoors and in fresh water, properly passivated 303 stays clean for years.