What is stainless steels?

Stainless steel is a group of iron-based alloys that contain a minimum of approximately 11% chromium, a composition that prevents the iron from rusting and also provides heat-resistant properties. Different types of stainless steel include the elements carbon (from 0.03% to greater than 1.00%), nitrogen, aluminium, silicon, sulfur, titanium, nickel, copper, selenium, niobium, and molybdenum. Specific types of stainless steel are often designated by a three-digit number, e.g., 304 stainless.

Stainless steel's resistance to rusting results from the presence of chromium in the alloy, which forms a passive film that protects the underlying material from corrosion attack, and can self-heal in the presence of oxygen. Corrosion resistance can be increased further, by:

increasing the chromium content to levels above 11%;

addition of 8% or higher amounts of nickel;

addition of molybdenum (which also improves resistance to "pitting corrosion").

The addition of nitrogen also improves resistance to pitting corrosion and increases mechanical strength. Thus, there are numerous grades of stainless steel with varying chromium and molybdenum contents to suit the environment the alloy must endure. Resistance to corrosion and staining, low maintenance, and familiar luster make stainless steel an ideal material for many applications where both the strength of steel and corrosion resistance are required. Moreover, stainless steel can be rolled into sheets, plates, bars, wire, and tubing.

Stainless steel series

There are many kinds of stainless steel, it can be divided into several categories according to the organizational structure:

100 Series

Type 102—austenitic general purpose stainless steel

200 Series—austenitic chromium-nickel-manganese alloys

Type 201—austenitic that is hardenable through cold working

Type 202—austenitic general purpose stainless steel

300 Series—austenitic chromium-nickel alloys

Type 301—highly ductile, for formed products. Also hardens rapidly during mechanical working. Good weldability. Better wear resistance and fatigue strength than 304.

Type 302—same corrosion resistance as 304, with slightly higher strength due to additional carbon.

Type 303—free machining version of 304 via addition of sulfur and phosphorus. Also referred to as "A1" in accordance with ISO 3506.

Type 304—the most common grade; the classic 18/8 (18% chromium, 8% nickel) stainless steel. Outside of the US it is commonly known as "A2 stainless steel", in accordance with ISO 3506 (not to be confused with A2 tool steel). The Japanese equivalent grade of this material is SUS304.

Type 304L—same as the 304 grade but lower carbon content to increase weldability. Is slightly weaker than 304.

Type 304LN—same as 304L, but also nitrogen is added to obtain a much higher yield and tensile strength than 304L.

Type 305—same as 304, but with more nickel to decrease work hardening.

Type 308—used as the filler metal when welding 304.

Type 309—better temperature resistance than 304, also sometimes used as filler metal when welding dissimilar steels, along with inconel.

Type 310 310S— is a highly alloyed austenitic stainless steel used for high temperature application. The high chromium and nickel content give the steel excellent oxidation resistance as well as high strength at high temperature. This grade is also very ductile, and has good weldability enabling its widespread usage in many applications. 

Type 316—the second most common grade (after 304); for food and surgical stainless steel uses; alloy addition of molybdenum prevents specific forms of corrosion. It is also known as marine grade stainless steel due to its increased resistance to chloride corrosion compared to type 304. 316 is often used for building nuclear reprocessing plants.

Type 316L—is an extra low carbon grade of 316, generally used in stainless steel watches and marine applications, as well exclusively in the fabrication of reactor pressure vessels for boiling water reactors, due to its high resistance to corrosion. Also referred to as "A4" in accordance with ISO 3506.

Type 316Ti—variant of type 316 that includes titanium for heat resistance. It is used in flexible chimney liners.

Type 321—similar to 304 but lower risk of weld decay due to addition of titanium. See also 347 with addition of niobium for desensitization during welding.

400 Series—ferritic and martensitic chromium alloys

Type 405—ferritic for welding applications

Type 408—heat-resistant; poor corrosion resistance; 11% chromium, 8% nickel.

Type 409—cheapest type; used for automobile exhausts; ferritic (iron/chromium only).

Type 410—martensitic (high-strength iron/chromium). Wear-resistant, but less corrosion-resistant.

Type 416—easy to machine due to additional sulfur

Type 420—Cutlery Grade martensitic; similar to the Brearley's original rustless steel. Excellent polishability.

Type 430—decorative, e.g., for automotive trim; ferritic. Good formability, but with reduced temperature and corrosion resistance.

Type 439—ferritic grade, a higher grade version of 409 used for catalytic converter exhaust sections. Increased chromium for improved high temperature corrosion/oxidation resistance.

Type 440—a higher grade of cutlery steel, with more carbon, allowing for much better edge retention when properly heat-treated. It can be hardened to approximately Rockwell 58 hardness, making it one of the hardest stainless steels. Due to its toughness and relatively low cost, most display-only and replica swords or knives are made of 440 stainless. Available in four grades:

Type 440A—has the least amount of carbon making this the most stain-resistant.

Type 440B—slightly more carbon than 440A.

Type 440C—has the greatest amount of carbon of the Type 440 variants. Strongest and considered more desirable in knifemaking than the Type 440A variant[citation needed], except for diving or other salt-water applications. This variant is also more readily available than other variants of Type 440.

Type 440F—a free-machining variant. Contains the same high carbon content as Type 440C.

Type 446—For elevated temperature service

500 Series—heat-resisting chromium alloys

600 Series—originally created for proprietary alloys (which are no longer given SAE grade numbers)[9]

601 through 604: Martensitic low-alloy steels.

610 through 613: Martensitic secondary hardening steels.

614 through 619: Martensitic chromium steels.

630 through 635: Semiaustenitic and martensitic precipitation hardening stainless steels.

Type 630 is most common PH stainless, better known as 17-4; 17% chromium, 4% nickel.

650 through 653: Austenitic steels strengthened by hot/cold work.

660 through 665: Austenitic superalloys; all grades except alloy 661 are strengthened by second-phase precipitation.

900 series—austentic chromium-molybdenum alloys

Type 904—similar to 316 but with higher chromium and molybdenum content for more corrosion resistance