GlossaryKind of term: Manufacturing termStainless steel
What is stainless steel?
Stainless steel is steel with at least 10.5 % chromium that protects itself against corrosion with a passive layer; 1.4301 and 1.4404 are common grades.
Stainless steel is steel with at least 10.5 % chromium and at most 1.2 % carbon. The chromium forms a thin passive layer that protects the steel against corrosion, as long as the environment does not break that layer down. By structure there are five families: austenitic (1.4301, 1.4404, 1.4305), ferritic (1.4016), martensitic and hardenable (1.4057, 1.4418), duplex (1.4462) and precipitation hardening (1.4542). The family sets hardenability, magnetism, weldability, corrosion behaviour and machinability.
What does the standard say about stainless steel?
EN 10088-1 gives the definition and lists the grades with their composition; EN 10088-2 and EN 10088-3 give the technical delivery conditions for sheet, plate and strip and for bars, sections and other long products. The grade number alone is not enough: the delivery condition, such as solution annealed or cold drawn, and for sheet the surface finish belong with it. US drawings use AISI or UNS numbers and ASTM standards; their composition limits overlap with the European grades but are not identical.
How does stainless steel behave in the shop?
Austenitic stainless is tough to machine: it work-hardens under the cutting edge, chips break poorly and heat stays in the tool. An edge that rubs instead of cutting hardens the layer for the next pass; sharp edges, adequate feed and coolant help. Sulphur-bearing grades such as 1.4305 machine better, at the expense of corrosion resistance and weldability. Cutting data and tool life usually sit below those for carbon steel, and the price per kilo above, partly because of the alloy surcharge. Whether material or machining dominates the cost depends on shape, feature count and quantity. Where the drawing or the application requires it, pickling or passivation follows: pickling removes heat tint and the chromium-depleted layer beneath it, passivation removes free iron from the surface, so that the passive layer can form intact.
What is stainless steel often confused with?
When the shop says alloy steel, it usually means low-alloy steel such as 42CrMo4, chosen for strength and hardenability; stainless is high-alloy and chosen for corrosion resistance. Stainless is also not rustproof in every environment: chlorides, crevices and iron particles from tools or workholding can cause pitting or rust staining. And being non-magnetic is not a property of stainless as a whole: ferritic and martensitic grades are magnetic, and austenitic stainless can become slightly magnetic after cold working.
Frequently asked questions
Why is a stainless part often dearer than the same part in aluminium?
Usually for two reasons: a higher price per kilo, and longer machining time because of lower cutting data and more wear. Which of the two dominates varies by part and batch size.
Does it matter whether the drawing says 1.4301 or 1.4404?
Yes. They differ in corrosion resistance, price per kilo and availability, and 1.4404 is usually chosen where chlorides are present. Price the grade the drawing states; any substitute goes to the customer first.
Can stainless steel still rust?
Yes. Chlorides, crevices, low oxygen and embedded iron particles can break the passive layer locally. A suitable grade, clean tooling and, where needed, pickling or passivation limit that.