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Classes of Stainless Steels

What is Stainless Steel?

Stainless steel is a low carbon steel containing chromium; it is this addition of chromium that gives ordinary steel its unique anti-fouling and corrosion resistance properties. The chromium in steel permits the formation of a rough, invisible, corrosion-resistant chromium oxide film on the surface of the steel. If the material is mechanically or chemically damaged, the film will heal itself (provided oxygen is present). With the addition of chromium and other elements (such as molybdenum, nickel and nitrogen), the steel has higher corrosion resistance and other properties.

Martensitic Stainless Steels

Stainless steels are usually divided into five different categories. Each is identified by the alloying element that affects its microstructure and each is named after an alloying element.

  • Martensitic stainless steels
  • Ferritic stainless steels
  • Austenitic stainless steels
  • Duplex (ferritic-austenitic) stainless steels
  • Precipitation hardening (PH) stainless steels
Stainless Steels

Martensitic Stainless Steels

Martensitic stainless grades are a group of corrosion resistant and harden able (using heat treatment) stainless steel alloys. All martensitic grades are simple chromium steels that do not contain nickel. All these grades are magnetic. Martensitic grades are mainly used where hardness, strength and wear resistance are required.

Grade Type

  1. Type 410: A basic martensitic grade with a low alloy content. It has a relatively low cost and is a general purpose, heat treatable stainless steel. It is typically used in applications where corrosion is less severe (e.g. air, water, certain chemicals and food acids). Applications for this product may include components where a combination of strength and corrosion resistance is required, such as fasteners.
  2. Type 410S: Lower carbon content compared to Type 410, but provides improved weldability but lower hardenability. This is a general purpose corrosion and heat resistant chromium steel.
  3. Type 414: This type has an increased nickel content (2%) to improve corrosion resistance. Typical applications include springs and cutlery.
  4. Type 416: This type has phosphorus and sulphur to improve machinability. Typical applications include screw machine parts.
  5. Type 420: The addition of carbon to this type helps to improve mechanical properties. Typical applications include surgical instruments.
  6. Type 431: With a higher chromium content, it has higher corrosion resistance and good mechanical properties. Typical applications include high strength components such as valves and pumps.
  7. Type 440: Further increases in chromium and carbon content help to improve the toughness and corrosion resistance of this type. Typical applications include surgical instruments

Ferritic Stainless Steels

Ferritic stainless steel grades resist corrosion and oxidation while maintaining resistance to stress and cracking. Although these steels are magnetic, they cannot be hardened by heat treatment. Once annealed, these grades can be cold worked. They have higher corrosion resistance than martensitic grades but are mostly inferior to austenitic grades. These grades are nickel-free straight chromium steels and are commonly used for decorative trim, sinks and certain automotive applications, such as exhaust systems.

  1. Type 430: A basic grade less resistant to corrosion than Type 304. This type is resistant to corrosion such as nitric acid, sulphury gases and many organic and food acids.
  2. Type 405: This type has a lower chromium content and added aluminums. This chemical composition helps to prevent hardening on cooling at high temperatures. Typical applications include heat exchangers.
  3. Type 409: One of the cheapest grades of stainless steel due to its reduced chromium content. This type should only be used for internal and external components in non-critically corrosive environments. Typical applications include mufflers.
  4. Type 434: This type has an increased molybdenum content, giving it a higher corrosion resistance. Typical applications include automotive trim and fasteners.
  5. Type 436: This grade has added niobium for corrosion and heat resistance. Most typical applications include deep-drawn parts.
  6. Type 442: Higher resistance to fouling due to increased chromium content. Applications can include furnace and heater components.
  7. Type 446: Higher chromium content added to further improve corrosion and scale resistance at high temperatures. This grade has good resistance to oxidation in a sulphury environment.
Stainless Steels

Austenitic Stainless Steels

Austenitic stainless steels are the most commonly used group of stainless steels. The high chromium and nickel content of the grades in this group provides excellent corrosion resistance and very good mechanical properties. They cannot be hardened by heat treatment but can be hardened considerably by cold working. None of the grades in this category are magnetic.

Standard Grade

Standard grades of austenitic stainless steel contain a maximum of 0.08% carbon; there is no minimum carbon requirement.

Low Carbon Grades (L Grade)

The “L” grade is used to provide additional corrosion resistance after welding. The letter “L” after the stainless-steel grade indicates low carbon. The carbon content is kept at or below 0.03% to avoid carbide precipitation, which can lead to corrosion. Due to the temperatures generated during the welding process (which can lead to carbon precipitation) – the “L” grade is usually used. Typically, stainless steel manufacturers offer these stainless-steel grades as dual certification, for example 304 / 304L or 316 / 316L.

High Carbon Grade (H grade)

Stainless steel grade “H” has a carbon content of at least 0.04% and a maximum carbon content of 0.10%. The higher carbon content helps to maintain strength at extreme temperatures. These grades are indicated by the letter “H” at the end of the stainless-steel grade. This designation will be used when the end use involves extreme temperature environments.

  1. Type 304: one of the most commonly used grades of (austenitic) stainless steel. Its high content of chromium and nickel makes it the preferred choice for the manufacture of processing equipment for the chemical (mild chemicals), food/dairy and beverage industries. This grade offers an excellent combination of strength, corrosion resistance and fabric properties.
  2. Type 316: This stainless-steel grade contains 18% chromium, 14% nickel and added molybdenum; these combine to increase its corrosion resistance. In particular, the molybdenum used helps to control the pitted erosion of corrosion. type 316 is used in chemical processing, the pulp and paper industry, food and beverage processing and distribution and in more corrosive environments. Due to its corrosion resistance, it is also used in the marine industry.
  3. Type 317: It contains a higher percentage of molybdenum than 316 and is used in highly corrosive environments. The molybdenum content of this grade must be greater than 3%. It is commonly used in scrubber systems for air pollution control units to remove particulate matter and/or gases from industrial exhaust streams.
  4. Type 321: Contains a titanium addition with at least five times the carbon content. This addition is intended to reduce or eliminate chromium carbide precipitation due to welding or exposure to high temperatures. Used in the aerospace industry.
  5. Type 347: Offers slightly improved corrosion resistance in strong oxidizing environments compared to Type 321 stainless steel. Type 347 should be considered for applications requiring intermittent heating between 800ºF (427ºC) and 1650ºF (899ºC) or for welding under conditions that prevent post-weld annealing.

Duplex (ferritic-austenitic) Stainless Steels

Duplex stainless steels grades are a combination of austenitic and ferritic materials. These grades are approximately twice as strong as austenitic and ferritic grades. They do not reach the level of austenitic grades, although they do have better toughness and ductility than ferritic grades. The corrosion resistance of duplex grades is very close to that of austenitic grades, such as 304 and 316. 2205 grade is the most widely used of the duplex classes.

  1. Type 2205: Duplex stainless steel 2205 is ideally suited to high pressure and highly corrosive environments. It also has high corrosion and erosion fatigue properties, as well as lower thermal expansion and higher thermal conductivity than austenitic. Use of this grade should be restricted to temperatures below 315°C as prolonged exposure to high temperatures can cause the material to become brittle.
  2. Type 2304: Duplex 2304 is commonly used for the same applications using alloys 304 and 316L. Its corrosion resistance is very close to or slightly better than that of austenitic grades 304 and 316, but it has almost double the yield strength. It is suitable for temperatures between -50° and 300° C. This grade has high mechanical strength and high resistance to stress corrosion cracking. It has good weldability, machinability and is easy to manufacture.
  3. Type 2507: Duplex stainless steel 2507 is a super duplex stainless steel. It is suitable for applications requiring exceptional strength and corrosion resistance, such as chemical processes, petrochemicals and seawater equipment. This grade offers excellent resistance to chloride stress, corrosion cracking, high thermal conductivity and a low coefficient of thermal expansion. The high chromium, molybdenum and nitrogen content provides excellent resistance to pitting, crevice and general corrosion.

Precipitation Hardening (PH) Stainless Steels

Precipitation hardening stainless steels can be strengthened and hardened by heat treatment. This provides designers with a unique combination of fabric properties, strength, ease of heat treatment and corrosion resistance not found in any other type of material. These grades include 17Cr-4Ni (17-4PH) and 15Cr-5Ni (15-5PH).

  1. Type 17-4: Alloy 17-4 is a chromium-copper precipitation hardening stainless steel for applications requiring high strength and medium corrosion resistance. It has high strength and good corrosion resistance at all heat treatment conditions. The grade can be heat treated at a wide range of temperatures, producing a wide range of finished properties. This grade should not be used at temperatures above 300°C or at very low temperatures.
  2. Type 15-5: This is a variant of the older 17-4 chromium-nickel-copper precipitation-hardening martensitic stainless steel. 15-5 alloy is designed to have greater toughness than 17-4. It is used for applications requiring better corrosion resistance and lateral properties than other similar martensitic grades.