Stainless Steel CNC Machining: Grades, Work-Hardening and Passivation
News
Stainless steel is the material most often specified when a part has to survive its environment — food equipment, medical fixtures, marine hardware, chemical handling, outdoor enclosures. It is also the material that punishes a careless process hardest: it work-hardens under a dull cutter, it holds heat at the cutting edge, and it will gall onto a tool if the setup is wrong. We machine 300- and 400-series stainless to drawing, with the cutting strategy chosen for the grade rather than for the geometry alone.
Typical parts
- Shafts, bushings, spacers and threaded fittings
- Weld-neck and sanitary flanges for food and beverage equipment
- Instrument housings, brackets and panel hardware
- Marine and outdoor fastening components
- Valve bodies, pump components and manifolds
Matching the grade to the job
Buyers usually arrive with a grade already chosen for corrosion resistance, and it is worth confirming why before machining:
- 303 machines most freely and is the usual choice for high-volume turned parts where corrosion demand is moderate. It is not the right grade for welded assemblies.
- 304 / 304L is the general-purpose austenitic grade — good corrosion resistance, weldable, widely available. It work-hardens noticeably, so light passes and a rigid setup matter.
- 316 / 316L adds molybdenum and is specified where chlorides are present — marine, chemical, and most food-processing environments.
- 17-4PH is used where the part needs to be hardened after machining; it is machined in the solution-treated condition and heat-treated afterwards.
- 400-series (410, 420) is chosen for wear resistance and hardness rather than for corrosion resistance.
If the grade on your drawing was chosen for a service environment rather than for manufacturability, tell us what the part is exposed to. In several cases a different grade machines better and performs the same.
Standard capability
| Item | What we work to |
|---|---|
| Tolerance | ±0.05 mm on machined features |
| Materials | 303, 304/304L, 316/316L, 17-4PH, 410, 420 |
| Surface finish | Passivation, sandblasting, electropolishing on request, as-machined |
| Order size | 1 pc prototype through to volume production |
| Quality system | ISO 9001 / ISO-TS 16949, in-house dimensional inspection |
What we do differently on stainless
- Sharp, dedicated tooling. Stainless work-hardens the moment a cutter starts rubbing instead of cutting. Tooling is changed on a schedule rather than when a part looks wrong.
- Coolant at the cutting edge. Stainless carries heat into the tool. Flood coolant and controlled chip evacuation keep the insert alive and the surface finish predictable.
- Passivation after machining. Machining leaves free iron on the surface, which is exactly where corrosion starts. Passivation removes it — on parts that will see moisture, it is not optional.
- Galling control on threads. Stainless-on-stainless threads gall. Threading strategy and, where needed, a lubricant or a dissimilar material choice for the mating part are decided at the drawing review stage.
How an order runs
- Send the drawing with the grade, the finish and what the part is exposed to in service.
- We review it and quote within one business day.
- Sample or first batch is machined and inspected in-house before shipment.
- Mass production follows the approved sample.
Questions buyers ask
Can you supply the material certificate?
Yes — mill certificates for the heat used on your order are available with the shipment. If your quality system requires them on file before production starts, ask at the quote stage so the material is booked against your order.
Do you passivate in-house?
Passivation is part of our standard finishing route for stainless parts that will see moisture in service. Tell us at the drawing stage if the part will be used in a food or medical environment, as the finish requirement is stricter.
What is the hardest part of quoting stainless?
Grade and condition. The same geometry in 303 and in 316 can differ substantially in machining time, and 17-4PH adds a heat-treatment step that changes the schedule. That is why the grade on the drawing matters more than the shape.
Will the part be magnetic?
Austenitic grades (303, 304, 316) are essentially non-magnetic in the annealed condition, but cold working during machining can make them slightly magnetic at the surface. If non-magnetic behaviour is a functional requirement, say so — it affects both grade choice and how much material is removed.