Editing Stainless steel (section)

== Families ==
{{anchor|Types of stainless steel}}<!-- target from [[drug-eluting stent#design]] -->
Stainless steel is classified into five different "families" of alloys, each having a distinct set of attributes. Four of the families are defined by their predominant [[crystalline structure]] - the austenitic, ferritic, martensitic, and duplex alloys. The fifth family, precipitation hardening, is defined by the type of heat treatment used to develop its properties. 

=== Austenitic ===
{{main|Austenitic stainless steel}}

Austenitic stainless steel<ref>{{Cite book|title=Stainless steels for design engineers (#05231G)|publisher=ASM International|year=2008|isbn=978-0-87170-717-8|url=https://www.asminternational.org/search/-/journal_content/56/10192/05231G/PUBLICATION|pages=69–78 (Chapter 6)|access-date=1 October 2021|archive-date=14 April 2021|archive-url=https://web.archive.org/web/20210414010819/https://www.asminternational.org/search/-/journal_content/56/10192/05231G/PUBLICATION|url-status=live}}</ref><ref>{{Cite book|url=https://www.nickelinstitute.org/library/?opt_perpage=20&opt_layout=grid&searchTerm=austenitic%20stainless%20steel&contentTypes=22f540f09ae14cab816ae22ddd6e0bf5,b412a2a14cea4a9cb03684666654b559&page=2|title=Practical Guidelines for the Fabrication of High Performance Austenitic Stainless Steels|isbn=978-0-87170-717-8|last1=McGuire|first1=Michael F.|year=2008|publisher=ASM International |access-date=1 October 2021|archive-date=14 April 2021|archive-url=https://web.archive.org/web/20210414011308/https://nickelinstitute.org/library/?opt_perpage=20&opt_layout=grid&searchTerm=austenitic%20stainless%20steel&contentTypes=22f540f09ae14cab816ae22ddd6e0bf5,b412a2a14cea4a9cb03684666654b559&page=2|url-status=live}}</ref> is the largest family of stainless steels, making up about two-thirds of all stainless steel production.<ref name="ISSF-2021" /> They have a [[face-centered cubic]] crystal structure.<ref name="totalmateria">{{Cite web|title=Microstructures in Austenitic Stainless Steels :: Total Materia Article|url=https://www.totalmateria.com/page.aspx?ID=CheckArticle&site=kts&NM=268|access-date=2020-06-23|website=www.totalmateria.com|archive-date=14 April 2021|archive-url=https://web.archive.org/web/20210414010819/https://www.totalmateria.com/page.aspx?ID=CheckArticle&site=kts&NM=268|url-status=live}}</ref> This microstructure is achieved by alloying steel with sufficient nickel, manganese, or nitrogen to maintain an austenitic microstructure at all temperatures, ranging from the [[cryogenic]] region to the melting point.<ref name="totalmateria" /> Thus, austenitic stainless steels are not hardenable by heat treatment since they possess the same microstructure at all temperatures.<ref name="totalmateria" />

Austenitic stainless steels consist of two subfamilies: 

* 200 series<ref>{{Cite web|url=https://www.bssa.org.uk/publications.php?id=96|title=200 Series Stainless Steels. An overview|last=Bristish Stainless Steel Association|date=August 2006|publisher=Stainless Steel Industry|access-date=1 October 2021|archive-date=7 August 2020|archive-url=https://web.archive.org/web/20200807114612/https://www.bssa.org.uk/publications.php?id=96|url-status=live}}</ref> 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. Representative alloys include Type 201 and Type 202. 
* 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. Representative alloys include Type [[SAE 304 stainless steel|304]] and Type [[Marine grade stainless|316]].

=== Ferritic ===
{{main|Ferritic stainless steel}}

Ferritic stainless steels have a [[body-centered cubic]] crystal structure, are magnetic, and are hardenable by cold working, but not by heat treating. They contain between 10.5% and 27% chromium with very little or no nickel. Due to the near-absence of nickel, they are less expensive than austenitic stainless steels. Representative alloys include Type 409, Type 429, Type 430, and Type 446. Ferritic stainless steels are present in many products, which include:

* Automobile exhaust pipes<ref>{{Cite book|last1=Santacreu|first1=P-O|title=K4X: A new ferritic stainless steel grade with improved durability for high temperature exhaust manifolds|last2=Faivre|first2=L.|last3=Acher|first3=A.|last4=Leseux|first4=J.|publisher=Proceedings of 7th European Stainless Steel Science & Market (Como, Italy) Paper 25|year=2011}}</ref>
*Architectural and structural applications<ref>{{Cite journal|last1=Cashell|first1=K. A.|last2=Baddoo|first2=N.R.|date=2014|title=Ferritic stainless steels in structural applications|journal=Thin-walled Structures|publisher=Elsevier B.V.|volume=83|pages=169–181|doi=10.1016/j.tws.2014.03.014|url=https://bura.brunel.ac.uk/handle/2438/9834|access-date=1 October 2021|archive-date=24 November 2020|archive-url=https://web.archive.org/web/20201124152713/https://bura.brunel.ac.uk/handle/2438/9834|url-status=live}}</ref>
*Building components, such as slate hooks, roofing, and chimney ducts{{Citation needed|date=January 2025}}
*Power plates in [[solid oxide fuel cell]]s operating at temperatures around {{cvt|700|C|F|sigfig=2}}<ref>{{Cite journal|last1=Shaigan|first1=Nima|last2=Qu|first2=Wei|last3=Ivey|first3=Douglas|last4=Chen|first4=Weixing|date=2010|title=A review of recent progress in coatings, surface modifications and alloy developments for solid oxide fuel cell ferritic stainless steel interconnects|journal=Journal of Power Sources|publisher=Elsevier B.V.|volume=195|issue=6|pages=1529–1542|doi=10.1016/j.jpowsour.2009.09.069|bibcode=2010JPS...195.1529S}}</ref>

=== Martensitic ===
{{main|Martensitic stainless steel}}

Martensitic stainless steels have a body-centered tetragonal crystal structure, are magnetic, and are hardenable by heat treating and by cold working. They offer a wide range of properties and are used as stainless engineering steels, stainless tool steels, and [[Creep (deformation)|creep]]-resistant steels. They are not as corrosion-resistant as ferritic and austenitic stainless steels due to their low chromium content. They fall into four categories (with some overlap):<ref>{{Cite web|url=http://worldstainless.org/news/show/2125|title=Martensitic Stainless Steels|date=21 November 2017|website=worldstainless.org/|access-date=28 January 2019|archive-date=9 July 2018|archive-url=https://web.archive.org/web/20180709154627/http://worldstainless.org/news/show/2125|url-status=live}}</ref>

* Fe-Cr-C grades. These were the first grades used and are still widely used in engineering and wear-resistant applications. Representative grades include Type 410, Type 420, and Type 440C. 
* Fe-Cr-Ni-C grades. Some carbon is replaced by nickel. They offer higher toughness and higher corrosion resistance. Representative grades include Type 431. 
* Martensitic precipitation hardening grades. 17-4 PH (UNS S17400), the best-known grade, combines martensitic hardening and [[precipitation hardening]] to increase strength and toughness. 
* Creep-resisting grades. Small additions of niobium, [[vanadium]], [[boron]], and [[cobalt]] increase the strength and creep resistance up to about {{cvt|650|C|F|sigfig=2}}.

Martensitic stainless steels can be heat treated to provide better mechanical properties. The heat treatment typically involves three steps:<ref>{{Cite book|url=https://www.asminternational.org/web/hts/home/-/journal_content/56/10192/ZASMHBA0005985/BOOK-ARTICLE|title=ASM Handbook Vol 4D Heat treating of irons and steels|editor1=Dossett, J |editor2=Totten, GE |publisher=ASM International|year=2014|pages=382–396|doi=10.31399/asm.hb.v04d.a0005985|access-date=1 October 2021|archive-date=23 June 2018|archive-url=https://web.archive.org/web/20180623221947/https://www.asminternational.org/web/hts/home/-/journal_content/56/10192/ZASMHBA0005985/BOOK-ARTICLE|url-status=live}}</ref>

# Austenitizing, in which the steel is heated to a temperature in the range {{Convert|980-1050|C|F|abbr=}}, depending on grade. The resulting austenite has a face-centered cubic crystal structure.
# [[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.
# Tempering. Martensite is heated to around {{cvt|500|C|F|sigfig=2}}, held at temperature, then air-cooled. Higher tempering temperatures decrease [[yield strength]] and [[ultimate tensile strength]] but increase the elongation and impact resistance.

=== Duplex ===
{{main|Duplex stainless steel}}

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{{Citation needed|date=January 2025}}. Their mixed microstructure provides improved resistance to chloride stress corrosion cracking in comparison to austenitic stainless steel types 304 and 316{{Citation needed|date=January 2025}}. 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]].{{Citation needed|date=January 2025}}

=== Precipitation hardening ===
[[Precipitation hardening]] stainless steels are characterized by the ability to be precipitation hardened to higher strength. There are three types of precipitation hardening stainless steels which are classified according to their crystalline structure:<ref>{{Cite book|last=De Cooman|first=Bruno Charles|date=April 2016|others=Pohang University of Science and Technology Korea Graduate Institute of Ferrous Technology|title=Lecture on stainless steel_9|url=https://www.researchgate.net/publication/301692427|doi=10.13140/RG.2.1.1950.2488}}</ref>

* Martensitic precipitation hardenable stainless steels are martensitic at room temperature in both the solution annealed and precipitation hardened conditions. Representative alloys include 17-4 PH (UNS S17400), 15-5 PH (UNS S15500), Custom 450 (UNS S45000) and Custom 465 (UNS S46500). 

* Semi-austenitic precipitation hardenable stainless steels are initially austenitic in the solution annealed condition for ease of fabrication, but are subsequently transformed to martensite to provide higher strength and to be precipitation hardened. Representative alloys include 17-7 PH (UNS S17700), 15-7 PH (UNS S15700), AM-350 (UNS S35000), and AM-355 (UNS S35500). 

* Austenitic precipitation hardenable stainless steels are austenitic at room temperature in both the solution annealed and precipitation hardened conditions. Representative alloys include A-286 (UNS S66286) and Discalloy (UNS S66220).<ref>Precipitation-Hardening Stainless Steels, Stainless Steels for Design Engineers, By Michael F. McGuire, ASM International, 2008, p 137–146, https://doi.org/10.31399/asm.tb.ssde.t52310137</ref>