Stainless steels
Chosen for the environment, not for strength: where the part sits in moisture, in a chemical agent or in food contact. It costs more to machine than it does per kilogram, and the reason lies in how it behaves under the tool.
Usual grades
How it behaves at the cutting edge
Austenitic grades (1.4301, 1.4404) are non-magnetic, tough and poor conductors of heat - the heat stays at the cutting edge instead of leaving with the chip. They want a firm feed and generous coolant; too light a pass work-hardens the surface and makes it harder to cut than it was. 1.4404 contains molybdenum and so resists chlorides better. 1.4542 (17-4PH) is a precipitation-hardening martensitic steel: machined in the solution-annealed condition, then hardened at low temperature with little distortion - which is why it is chosen where both strength and close tolerances are required.
What to state on the drawing
The surfaces that stay in contact with the aggressive medium are marked on the drawing: there, surface finish matters more than tolerance, because a rough surface holds contamination and starts corrosion where it would not have started on its own. For 1.4542 / 17-4PH the required heat-treatment condition is stated - the material is supplied in several, and hardness differs noticeably between them.
What it means for the routing
Austenitic stainless work-hardens: the layer already cut becomes harder, and the next pass cuts a material harder than the one on the certificate. That is why passes are taken deep and without hesitation, not light and frequent. An edge that rubs the surface without cutting it raises the very hardness it then has to overcome. In practice: fewer passes, firm feed, tools changed on time.
Where it is used
- Food-industry parts
- Components exposed to moisture and chemicals
- Fittings and valve bodies
- Laboratory equipment parts
- Parts that cannot be painted or coated
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