Stainless steel families

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There are five main families, which are primarily classified by their crystalline structure: austenitic, ferritic, martensitic, duplex, and precipitation hardening.

Austenitic stainless steeledit

Austenitic stainless steel is the largest family of stainless steels, making up about two-thirds of all stainless steel production (see production figures below). They possess an austenitic microstructure, which is a face-centered cubic crystal structure. This microstructure is achieved by alloying steel with sufficient nickel and/or manganese and nitrogen to maintain an austenitic microstructure at all temperatures, ranging from the cryogenic region to the melting point. Thus, austenitic stainless steels are not hardenable by heat treatment since they possess the same microstructure at all temperatures.

Austenitic stainless steels can be further subdivided into two sub-groups, 200 series and 300 series:

• 200 series are chromium-manganese-nickel alloys that maximize the use of manganese and nitrogen to minimize the use of nickel. Due to their nitrogen addition, they possess approximately 50% higher yield strength than 300 series stainless sheets of steel.
  • Type 201 is hardenable through cold working.citation needed
  • Type 202 is a general-purpose stainless steel. Decreasing nickel content and increasing manganese results in weak corrosion resistance.
• 300 series are chromium-nickel alloys that achieve their austenitic microstructure almost exclusively by nickel alloying; some very highly-alloyed grades include some nitrogen to reduce nickel requirements. 300 series is the largest group and the most widely used.
  • Type 304: The best-known grade is Type 304, also known as 18/8 and 18/10 for its composition of 18% chromium and 8%/10% nickel, respectively.citation needed
  • Type 316: The second most common austenitic stainless steel is Type 316. The addition of 2% molybdenum provides greater resistance to acids and localized corrosion caused by chloride ions. Low-carbon versions, such as 316L or 304L, have carbon contents below 0.03% and are used to avoid corrosion problems caused by welding.

Ferritic stainless steelsedit

Ferritic stainless steels possess a ferrite microstructure like carbon steel, which is a body-centered cubic crystal structure, and contain between 10.5% and 27% chromium with very little or no nickel. This microstructure is present at all temperatures due to the chromium addition, so they are not hardenable by heat treatment. They cannot be strengthened by cold work to the same degree as austenitic stainless steels. They are magnetic.

Additions of niobium (Nb), titanium (Ti), and zirconium (Zr) to Type 430 allow good weldability (see welding section below).

Due to the near-absence of nickel, they are cheaper than austenitic steels and are present in many products, which include:

• Automobile exhaust pipes (Type 409 and 409 Cb are used in North America; stabilized grades Type 439 and 441 are used in Europe)
• Architectural and structural applications (Type 430, which contains 17% Cr)
• Building components, such as slate hooks, roofing, and chimney ducts
• Power plates in solid oxide fuel cells operating at temperatures around 700 °C (1,292 °F) (high-chromium ferritics containing 22% Cr)

Martensitic stainless steelsedit

Martensitic stainless steels offer a wide range of properties and are used as stainless engineering steels, stainless tool steels, and creep-resistant steels. They are magnetic, and not as corrosion-resistant as ferritic and austenitic stainless steels due to their low chromium content. They fall into four categories (with some overlap):

1. Fe-Cr-C grades. These were the first grades used and are still widely used in engineering and wear-resistant applications.
2. Fe-Cr-Ni-C grades. Some carbon is replaced by nickel. They offer higher toughness and higher corrosion resistance. Grade EN 1.4303 (Casting grade CA6NM) with 13% Cr and 4% Ni is used for most Pelton, Kaplan, and Francis turbines in hydroelectric power plants because it has good casting properties, good weldability and good resistance to cavitation erosion.
3. Precipitation hardening grades. Grade EN 1.4542 (also known as 17/4PH), the best-known grade, combines martensitic hardening and precipitation hardening. It achieves high strength and good toughness and is used in aerospace among other applications.
4. Creep-resisting grades. Small additions of niobium, vanadium, boron, and cobalt increase the strength and creep resistance up to about 650 °C (1,202 °F).

Heat treatment of martensitic stainless steelsedit

Martensitic stainless steels can be heat treated to provide better mechanical properties.

The heat treatment typically involves three steps:

1. Austenitizing, in which the steel is heated to a temperature in the range 980–1,050 °C (1,800–1,920 °F), depending on grade. The resulting austenite has a face-centered cubic crystal structure.
2. Quenching. The austenite is transformed into martensite, a hard body-centered tetragonal crystal structure. The quenched martensite is very hard and too brittle for most applications. Some residual austenite may remain.
3. Tempering. Martensite is heated to around 500 °C (932 °F), held at temperature, then air-cooled. Higher tempering temperatures decrease yield strength and ultimate tensile strength but increase the elongation and impact resistance.

Nitrogen-alloyed martensitic stainless steelsedit

Replacing some carbon in martensitic stainless steels by nitrogen is a recent development.when? The limited solubility of nitrogen is increased by the pressure electroslag refining (PESR) process, in which melting is carried out under high nitrogen pressure. Steel containing up to 0.4% nitrogen has been achieved, leading to higher hardness and strength and higher corrosion resistance. As PESR is expensive, lower but significant nitrogen contents have been achieved using the standard argon oxygen decarburization (AOD) process.

Duplex stainless steeledit

Duplex stainless steels have a mixed microstructure of austenite and ferrite, the ideal ratio being a 50:50 mix, though commercial alloys may have ratios of 40:60. They are characterized by higher chromium (19–32%) and molybdenum (up to 5%) and lower nickel contents than austenitic stainless steels. Duplex stainless steels have roughly twice the yield strength of austenitic stainless steel. Their mixed microstructure provides improved resistance to chloride stress corrosion cracking in comparison to austenitic stainless steel Types 304 and 316.

Duplex grades are usually divided into three sub-groups based on their corrosion resistance: lean duplex, standard duplex, and super duplex.

The properties of duplex stainless steels are achieved with an overall lower alloy content than similar-performing super-austenitic grades, making their use cost-effective for many applications. The pulp and paper industry was one of the first to extensively use duplex stainless steel. Today, the oil and gas industry is the largest user and has pushed for more corrosion resistant grades, leading to the development of super duplex and hyper duplex grades. More recently, the less expensive (and slightly less corrosion-resistant) lean duplex has been developed, chiefly for structural applications in building and construction (concrete reinforcing bars, plates for bridges, coastal works) and in the water industry.

Precipitation hardening stainless steelsedit

Precipitation hardening stainless steels have corrosion resistance comparable to austenitic varieties, but can be precipitation hardened to even higher strengths than other martensitic grades. There are three types of precipitation hardening stainless steels:

• Martensitic 17-4 PH (AISI 630 EN 1.4542) contains about 17% Cr, 4% Ni, 4% Cu, and 0.3% Nb.

Solution treatment at about 1,040 °C (1,900 °F)followed by quenching results in a relatively ductile martensitic structure. Subsequent aging treatment at 475 °C (887 °F) precipitates Nb and Cu-rich phases that increase the strength up to above 1000 MPa yield strength. This outstanding strength level is used in high-tech applications such as aerospace (usually after remelting to eliminate non-metallic inclusions, which increases fatigue life). Another major advantage of this steel is that aging, unlike tempering treatments, is carried out at a temperature that can be applied to (nearly) finished parts without distortion and discoloration.

• Semi-austenitic 17-7PH (AISI 631 EN 1.4568) contains about 17% Cr, 7.2% Ni, and 1.2% Al.

Typical heat treatment involves solution treatment and quenching. At this point, the structure remains austenitic. Martensitic transformation is then obtained either by a cryogenic treatment at −75 °C (−103 °F) or by severe cold work (over 70% deformation, usually by cold rolling or wire drawing). Aging at 510 °C (950 °F)—which precipitates the Ni₃Al intermetallic phase—is carried out as above on nearly finished parts. Yield stress levels above 1400 MPa are then reached.

• Austenitic A286(ASTM 660 EN 1.4980) contains about Cr 15%, Ni 25%, Ti 2.1%, Mo 1.2%, V 1.3%, and B 0.005%.

The structure remains austenitic at all temperatures.

Typical heat treatment involves solution treatment and quenching, followed by aging at 715 °C (1,319 °F). Aging forms Ni₃Ti precipitates and increases the yield strength to about 650 MPa at room temperature. Unlike the above grades, the mechanical properties and creep resistance of this steel remain very good at temperatures up to 700 °C (1,292 °F). As a result, A286 is classified as an Fe-based superalloy, used in jet engines, gas turbines, and turbo parts.

Gradesedit

There are over 150 grades of stainless steel, of which 15 are most commonly used. There are several systems for grading stainless and other steels, including US SAE steel grades.