Common Grades of Stainless Steels

Huyett Marketing Department
10/21/2020

Stainless steel, also referred to as corrosion‑resistant steel (CRES), is defined as a steel alloy that has a minimum of 11.5% chromium by mass. Carbon steel rusts when exposed to air and moisture. Iron oxide (the product of rusting) is unsightly and can weaken the structural strength of steel over time. The presence of chromium stimulates the formation of chromium oxide, which prevents surface corrosion.

There are numerous grades of stainless steel which divide into four major groups: austenitic general purpose stainless steel (100 series), austenitic chromium/nickel/manganese alloys (200 series), austenitic chromium/nickel alloys (300 series), and ferritic and martensitic chromium alloys (400 series). (The SAE steel grades system includes the 500 & 600 series also, but these are infrequently used.

Austenitic, or 300 series, account for roughly three‑fourths of stainless steel production. Austenitic stainless steels contain a maximum of 0.15% carbon and a minimum of 0.16% chromium, with added nickel or manganese. The nickel and manganese retain what is known as an austenitic structure which stabilizes iron, and, therefore, inhibits the formation of iron oxide.

304 stainless steel, also known as 18/8, a common austenitic stainless steel term, refers to steel containing 18.00% chromium and 8.00% nickel. 304 stainless steel and 316 stainless steel, which has added molybdenum, is used to avoid corrosion problems stimulated by welding. Nickel is an expensive compound, so other major grades of stainless have been created.

Ferritic stainless steels have better engineering properties, but reduced corrosion resistance due to lower chromium and nickel content. They are also less expensive. Martensitic stainless steels are even less corrosion resistant, containing chromium (12.00‑14.00%), and higher carbon and manganese. Martensitic stainless steels are harder and can be heat treated, but are also magnetic.

Precipitation‑hardened martensitic steels have less corrosion resistance than austenitic steels, but by using precipitation hardening can be made even harder and stronger than standard martensitic varieties.

Internationally, stainless steel has two commonly used looser designations that are used in DIN specifications, not only for shaft keys, but also for fastener categories. A2 refers to 300 series stainless such as AISI 303. A4 is a high nickel content, referencing AISI 316. For example, “DIN 6885A A4” refers to a full radiused machine key (both ends round), made from 316 (A4) stainless steel.

Common Grades of Stainless Steels

Stainless steel is the term used for grades of steel that contain more than 11.00% chromium, with or without other alloying elements. Stainless steel resists corrosion, maintains its strength at high tolerances, and is easily maintained. It is used predominately in the automotive, aerospace, and construction industries.

302

A common heat‑resisting stainless steel, 302 is austenitic, non‑magnetic, extremely tough, and ductile. Cold‑working will dramatically increase its hardness. It is amenable to stamping, spinning, and wire forming. 302 is commonly used for washers, springs, screens, and cables. It can be found in bridge pins, spring pins, and spiral rings.

303

303 is a popular free machining stainless steel providing excellent speed and feed capabilities. 303 is used extensively in the screw and machining industries to make hardware, fasteners such as lock nuts, valve parts, nozzles, and trim. It can also be used for keyed shafts, pins, and grease fittings.

304

Originally known as 18‑8, 304 is the most commonly specified austenitic (chromium‑nickel stainless class) stainless steel, accounting for more than half of the stainless steel produced in the world. This grade withstands ordinary corrosion in architecture, is durable in typical food processing environments, and resists most chemicals. 304 is available in virtually all product forms and finishes and is used for cookware, appliances, sinks, and tabletops. It can be found in bridge pins, thread inserts, hose clamps, and spring pins.

316

Austenitic (chromium‑nickel stainless class) stainless steel containing 2.00%‑3.00% molybdenum (whereas 304 has none). The inclusion of molybdenum gives 316 greater resistance to various forms of deterioration. It is well‑suited for extreme situations such as laboratory equipment, chloride environments, and sub‑zero temperatures. 316 is frequently used for chemical, food, paper, mining, pharmaceutical, and petroleum equipment. It is one of the most common stainless steels and can be found in most types of pins, hose clamps, keyed shafts, threaded rod, machine keys, and keystock.

410

The most widely used martensitic (plain chromium stainless class with exceptional strength) stainless steel, featuring the high‑level of strength conferred by martensitic steels. It is a low‑cost, heat treatable grade suitable for non‑severe corrosion applications. It is used for cutlery, turbine blades, bushings, valve components, fasteners, screens, kitchen utensils, and spring pins.

416

This was one of the first free machining stainless steels. It has excellent machinability and non‑galling characteristics and is magnetic. It is used for nuts, bolts, screws, gears, and pinions, valve trim, shafts, axles, dowel pins, and keystock.

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