A stainless steel is a steel containing at least 10.5% chromium (and at most 1.2% carbon, according to EN 10088). On contact with oxygen, the chromium forms a very thin, invisible oxide layer on the surface, able to re-form if scratched: this passive layer protects the steel from corrosion. Hence the slightly misleading name: stainless steel is not unalterable, it can pit in chloride environments or degrade if its surface is contaminated with free iron.
There are four main families. Austenitic steels (304L, 316L) are the most widespread: highly corrosion-resistant, easy to form and weld, not hardenable and generally non-magnetic in the annealed state. Ferritic steels (such as 1.4016) are magnetic, nickel-free and cheaper. Martensitic steels (such as 1.4021) can be hardened and are used for hard parts, cutting tools or shafts. Duplex steels, which combine two structures, offer high mechanical strength together with good corrosion resistance.
304L (1.4307) is enough for most indoor and food-processing uses; 316L (1.4404), which contains molybdenum, resists chlorides better, in marine, chemical or medical settings, and costs more. The “L” stands for low carbon, which limits sensitisation to intergranular corrosion after welding. In the workshop, austenitic stainless steel work-hardens when machined, wears tools and is worked more slowly than aluminium or steel. After machining or welding, passivation restores its resistance. TIG welding suits thin sheet and plate fabrication well.
In a quote request
Give the precise grade (316L, 1.4404, X2CrNiMo17-12-2) and not just “stainless”, the form and surface finish (raw, brushed, polished, or 2B for cold-rolled sheet), the material certificate wanted (type 3.1, for example) and the treatments after manufacture. Specify the use (food contact, marine environment) and whether an equivalent grade is accepted. The guide stainless steel 304 or 316L helps you choose.