Editing Stainless steel (section)

=== 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.