There are three major classes of stainless steel are:
Austenitic: Chromium-nickel-iron alloys with 16-26% chromium, 6-22% nickel (Ni), and low carbon content, with non-magnetic properties (if annealed - working it at low temperature, then heated and cooled). Nickel increases corrosion resistance. Hardenable by cold-working (worked at low temperatures) as well as tempering (heated then cooled). Type 304 (S30400) or "18/8" (18% chromium 8% nickel), is the most commonly used grade or composition.
Martensitic: Chromium-iron alloys with 10.5-17% chromium and carefully controlled carbon content, hardenable by quenching (quickly cooled in water or oil) and tempering (heated then cooled). It has magnetic properties. Commonly used in knives. Martensitic grades are strong and hard, but are brittle and difficult to form and weld. Type 420 (S42000) is a typical example.
Ferritic: Chromium-iron alloys with 17-27% chromium and low carbon content, with magnetic properties. Cooking utensils made of this type contain the higher chromium levels. Type 430 is the most commonly used ferritic.
Two additional classes worth mentioning include Duplex (with austenitic and ferritic structures), and Precipitation Hardening stainless steel, used in certain extreme conditions.The austenitic microstructure is most commonly used for knives and cooking utensils. It is very tough, hardened through a process that consists of heating, cooling and heating. It resists scaling and retains strength at high temperatures.
Both ferritic and austenitic are used in kitchenware and household appliances. Austenitics are preferred in the food industry and beverage equipment due to the superior corrosion resistance and ease of cleaning. Type 301, for example, is an austenitic stainless steel, with 17% chromium, 7% nickel, and .05% carbon, and is widely used for institutional food preparation utensils.
You can easily make do with the lesser quality cookware for most oven use. For stovetop cooking, however, don’t skimp on quality; buy only the better ones. Most manufacturers of high quality cookware use stainless steel similar to the Type 304 grade, with thick heat diffusing bottoms. Metals that provide better diffusion of heat, such as copper and aluminum, are attached to the bottom for heat diffusion, to prevent hot spots and uneven cooking.
Low quality cutlery is generally made out of grades like 409 and 430 (ferritic), while the finest Sheffield cutlery uses specially produced 410 and 420 (martensitic) for the knives, and 304 (austenitic) for the spoons and forks. Grades like the 410/420 can be hardened and tempered so that the knife blades will take a sharp edge, whereas the more ductile 304 stainless is easier to work and therefore more suitable for objects that have to undergo numerous shaping, buffing and grinding processes.
