Frequently Asked Questions about Stainless Steel

Have you ever wondered, what is stainless steel? Below we have tried to answer a few of the commonly asked questions about stainless steel.

Q. What is stainless steel?

Stainless steel is the generic name for a number of different steels used primarily because of their corrosion resistance. All stainless steels share a minimum percentage of 10.5% chromium. It is this element that reacts with the oxygen in the air to form a complex chrome-oxide surface layer that is invisible but strong enough to prevent further oxygen from "staining" (rusting) the surface.

The film itself is extremely thin, about 130 Angstroms and one Angstrom is one millionth of a centimeter. This layer is described as passive (does not react or influence other materials), tenacious (clings to the layer of steel and is not transferred elsewhere) and self-renewing (if damaged, more chromium from the steel will be exposed to the air and form more chromium oxide).

This means that over a period of years a stainless steel knife can literally be worn away by daily use and will still remain stainless. Higher levels of chromium and the addition of other alloying elements such as nickel and molybdenum enhance this surface layer and improve the corrosion resistance of the stainless material. Chromium is always the deciding factor, although other elements, particularly nickel and molybdenum, are added to improve corrosion resistance.

Furthermore, stainless steel is fully recyclable.

The 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 temperatures, then heated and cooled). Nickel increases corrosion resistance. Harden able 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, harden able 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.

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Q. What does chrome-nickel content mean?

Chrome-Nickel content refers to the percentage of chrome and nickel in the stainless steel. Chrome gives the stainless steel its luster & durability, while the nickel is for the hardness & strength. 18/10 Stainless Steel means it contains 18% Chromium or Chrome and 10% Nickel. A sink with an 18/10 chrome-nickel content is considered to be an excellent grade, due to its corrosive resistance and greater durability.

Q. What is the difference between 18/8 and 18/10 stainless steel?

The first number is the amount of chromium that is contained in the stainless, i.e., 18 is 18% chromium. The second number is the amount of nickel, i.e., 8 stands for 8% nickel. So 18/8 means that this stainless steel contains 18% chromium and 8% nickel. 18/10 is 18% chromium and 10% nickel.

The higher the numbers the more corrosion resistant the material. Both 18/8 and 18/10 contain nickel and are part of the grade family "300 series" stainless. 18/0 means that there is 18% chromium but zero nickel. When there is no nickel the stainless grade family is the "400 series".

400 series are not as corrosion resistant as the 300 series and are magnetic, where the 300 series are non-magnetic and is the grade mainly used for sinks, Food processing equipment, Restaurant food preparation areas etc.

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Q. What is the difference between 304 and 316 stainless steel?

304 contains 18% chromium and 8% nickel. 316 contains 16% chromium, 10% nickel and 2% molybdenum. The "moly" is added to help resist corrosion to chlorides (like sea water and de-icing salts.

Q. Can stainless steel be "welded"?

YES. Stainless steel is easily welded, but the welding procedure is different than that used with carbon steel. The "filler" rod or electrode must be stainless steel.

Q. What temperatures can stainless steel withstand?

Stainless steel has excellent properties at both extremes of the temperature scale. Some stainless steel can be used down to liquid nitrogen temperatures and some up to about 1800° F.

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Q. What is the recycle content of stainless steel?

Stainless steel can be recycled 100%. That is all stainless steel can be re-melted to make new stainless steel. The typical amount of recycled stainless steel "scrap" that is used to make new stainless steel is between 65 & 80%.

Q. Can stainless steel rust? Why? (I thought stainless did not rust!)

Stainless does not "rust" as you think of regular steel rusting with a red oxide on the surface that flakes off. Corrosion is generally caused by contaminants settling on the surface of the stainless steel.

Q. Is stainless steel magnetic?

There are several "types" of stainless steel. The 300 series (which contains nickel) is NOT magnetic. The 400 series (which just contains chromium and no nickel) ARE magnetic.

Q. How are different classes of stainless steels used?

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 ferritics and austenitics 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.

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