View source for Open-hearth furnace

{{Short description|A type of industrial furnace for steelmaking}}
{{Redirect|Open hearth|an open hearth in the home|hearth}}
[[File:Práce na martinské peci.jpg|thumb|300px|Open hearth furnace workers at the [[Zaporizhstal]] steel mill in Ukraine taking a steel sample, c. 2012]]
[[File:Fotothek df n-34 0000203 Metallurge für Hüttentechnik.jpg|thumb|250px|Tapping open-hearth furnace, VEB Rohrkombinat Riesa, East Germany, 1982]]
An '''open-hearth furnace''' or '''open hearth furnace''' is any of several kinds of industrial [[Industrial furnace|furnace]] in which excess [[carbon]] and other impurities are burnt out of [[pig iron]] to [[Steelmaking|produce steel]].<ref>K. Barraclough, Steelmaking 1850-1900 (Institute of Metals, London 1990), 137-203.</ref> Because [[steel]] is difficult to manufacture owing to its high [[melting point]], normal fuels and furnaces were insufficient for [[mass production]] of steel, and the open-hearth type of furnace was one of several technologies developed in the nineteenth century to overcome this difficulty. Compared with the [[Bessemer process]], which it displaced, its main advantages were that it did not [[Embrittlement|embrittle]] the steel from excessive [[nitrogen]] exposure{{Clarify|date=March 2024|reason=it sounds from the rest of the article that the steel is exposed to air (80% nitrogen) so why is the nitrogen content of the product lower than that from Bessemer? }}, was easier to control, and permitted the melting and refining of large amounts of [[scrap|scrap iron and steel]].<ref name=ONE/>

The open-hearth furnace was first developed by German/British engineer [[Carl Wilhelm Siemens]]. In 1865, the French engineer [[Pierre-Émile Martin]] took out a licence from Siemens and first applied his regenerative furnace for making [[steel]]. Their process was known as the '''Siemens–Martin process''' or '''Martin–Siemens process''', and the furnace as an "open-hearth" furnace. Most open hearth furnaces were closed by the early 1990s, not least because of their slow operation, being replaced by the [[basic oxygen furnace]] or [[electric arc furnace]].<ref name=ONE>Philippe Mioche, « Et l'acier créa l'Europe », Matériaux pour l'histoire de notre temps, vol. 47, 1997, p. 29-36</ref>

Whereas the earliest example of open-hearth steelmaking is found about 2000 years ago in the culture of the [[Haya people]], in present day [[Tanzania]],<ref>{{cite journal|last1=Avery|first1=Donald|last2=Schmidt|first2=Peter|title=Complex Iron Smelting and Prehistoric Culture in Tanzania|journal=Science|date=1978|volume=201|issue=4361|pages=1085–1089|doi=10.1126/science.201.4361.1085 |issn=0036-8075 |jstor=1746308|pmid=17830304 |bibcode=1978Sci...201.1085S |s2cid=37926350 }}</ref> and in Europe in the [[Bloomery|Catalan forge]], invented in Spain in the 8th century, it is usual to confine the term to certain 19th-century and later steelmaking processes, thus excluding [[bloomery|bloomeries]] (including the Catalan forge), [[finery forge]]s, and [[puddling furnace]]s from its application.

==Open-hearth process==

The open-hearth process is a  [[batch production|batch process]] and a batch is called a "heat". The furnace is first inspected for possible damage. Once it is ready or repaired, it is charged with light scrap, such as sheet metal, shredded vehicles or waste metal. The furnace is heated using burning gas. Once the charge has melted, heavy scrap, such as building, construction or steel milling scrap is added, together with [[pig iron]] from [[blast furnace]]s. Once all the steel has melted, slag-forming agents such as limestone are added. Atmospheric [[oxygen]] in contact with molten pig iron directly oxidizes the [[carbon]] in excess it contains to form [[carbon monoxide]] (CO). Additionally, Fe(II) present in [[iron(II) oxide]] (FeO) and other impurities also contribute to decarburize the pig iron by oxidizing carbon into CO and simultaneously reducing Fe(II) into metallic Fe. The formed carbon monoxide (CO) is flushed  away in the fumes, while steel is formed. To increase the oxidizing power of the "heat", more iron oxide ore can be added.<ref name=TWO>A Study of the Open Hearth: A Treatise on the Open Hearth Furnace and the Manufacture of Open Hearth Steel. Harbison-Walker Refractories Company. (2015), 102 pag, {{ISBN|1341212122}}, {{ISBN|978-1341212123}}</ref>
<!-- Deleted image removed: [[File:Open Hearth Furnace.jpg|thumb|right|Open Hearth Furnace charging area]] -->

The process is far slower than that of the [[Bessemer converter]] and thus easier to control and sample for quality assessment. Preparing a heat usually takes eight to eight and a half hours, and longer to finish the conversion into steel. As the process is slow, it is not necessary to burn all the carbon away as in the Bessemer process, but the process can be terminated at any given point when the desired carbon content has been achieved.<ref name=TWO/>

The furnace is tapped in the same way a [[blast furnace]] is tapped; a hole is drilled in the side of the hearth and the raw steel flows out. Once all the steel has been tapped, the slag is skimmed away. The raw steel may be cast into ingots, a process called ''teeming'', or it may be used in continuous casting in the rolling mill.<ref name=TWO/>

The regenerators are the distinctive feature of the furnace and consist of fire-brick flues filled with bricks set on edge and arranged in such a way as to have a great number of small passages between them.<ref name=TWO/>  The bricks absorb most of the heat from the outgoing waste gases and return it later to the incoming cold gases for combustion.

==History==

[[File:Tappning av martinugn.jpg|thumb|upright|Tapping open hearth furnace, Fagersta steelmill, Sweden, 1967.]]
[[Carl Wilhelm Siemens]] developed the '''Siemens regenerative furnace''' in the 1850s, and claimed in 1857 to be recovering enough heat to save 70–80% of the fuel.  This furnace operates at a high temperature by using [[Air preheater|regenerative preheating]] of fuel and air for [[combustion]]. In regenerative preheating, the exhaust gases from the furnace are pumped into a chamber containing bricks, where heat is transferred from the gases to the bricks. The flow of the furnace is then reversed so that fuel and air pass through the chamber and are heated by the bricks. Through this method, an open-hearth furnace can reach temperatures high enough to melt steel, but Siemens did not initially use it for that.<ref name=TEN>Basic Open Hearth Steelmaking, with Supplement on Oxygen in Steelmaking, third edition (The Seely W. Mudd Series) The American Institute of Mining, Metallurgical, and Petroleum Engineers (1964).  Gerhard, Derge.  ASIN B00IJLRL40.</ref>

In 1865, the French engineer [[Pierre-Émile Martin]] took out a license from Siemens and first applied his regenerative furnace for making steel. The most appealing characteristic of the Siemens regenerative furnace is the rapid production of large quantities of basic steel, used for example to construct high-rise buildings.<ref name=TEN/>  The usual size of furnaces is 50 to 100 tons, but for some special processes they may have a capacity of 250 or even 500 tons.

<gallery class="center">
Siemensmartin12nb.jpg|Siemens furnace from 1895.
Forno Martin Siemens, 1917 - san dl SAN IMG-00001419.jpg|30-ton Siemens–Martin furnace, section, 1917.
Open hearth furnace size evolution sketch.svg|lang=en|Evolution of the size of open-hearth furnaces.
</gallery>

The Siemens–Martin process complemented rather than replaced the [[Bessemer process]]. It is slower and thus easier to control, allowing production of better product. It also permits the melting and refining of large amounts of scrap steel, further lowering steel production costs and recycling an otherwise troublesome waste material. One of its important drawbacks is that melting and refining a charge takes several hours. This was an advantage in the early 20th century, as it gave plant chemists time to analyze the steel and decide how much longer to refine it. But by about 1975 electronic instruments such as atomic absorption spectrophotometers had made analysis of the steel much easier and faster. The work environment around an open-hearth furnace is said to be extremely dangerous, although that may be even more true of the environment around a basic oxygen or electric arc furnace.<ref name=TEN/>

On the one hand, the process achieves lesser [[economies of scale]] than the Bessemer, so its steel was costlier in former's heyday, but on the other, it was more suitable for countries which couldn't produce lots of steel anyway due to limitations of natural resources.<ref>{{cite journal | url=https://www.tandfonline.com/doi/abs/10.1080/00076791.2016.1172570 | doi=10.1080/00076791.2016.1172570 | title=Business and State in the development of the steel industry in Spain and Italy (C.1880–1929) | year=2017 | last1=Sáez-García | first1=Miguel A. | journal=Business History | volume=59 | issue=2 | pages=159–178 | hdl=10045/66416 | s2cid=156562137 | hdl-access=free | url-access=subscription }}</ref> 

Russian engineers invented the twin-hearth furnace in the mid-20th century. It has two melting pools separated by a brick wall, and no regenerator chambers. Instead the furnace has direct burners and oxygen lances at the ceiling of the furnace. The idea is to process two heats simultaneously, but in different phases, e.g. when one is being charged, the other is being decarburized. The idea is to burn away excess carbon and impurities with oxygen blast instead of free flame formation. All reactions which occur are exothermic, so the burners have only an auxiliary role. <ref>https://www.slideshare.net/slideshow/twin-hearth-furnace-78491031/78491031</ref> This is similar as the [[AJAX furnace]], which also uses oxygen blow instead of free flame formation and regenerator chambers.

[[Basic oxygen steelmaking]] eventually replaced the open-hearth furnace. It rapidly superseded both the Bessemer and Siemens–Martin processes in western Europe by the 1950s and in eastern Europe by the 1980s. Open-hearth steelmaking had superseded the Bessemer process in UK by 1900, but elsewhere in Europe, especially in Germany, the Bessemer and Thomas processes were used until the late 1960s when they were superseded by basic oxygen steelmaking. The last open-hearth furnace in former [[East Germany]] was stopped in 1993. In the US, steel production using the Bessemer process ended in 1968 and the open-hearth furnaces had stopped by 1992. In [[Hunedoara steel works]], [[Romania]] the last 420-tonne capacity open-hearth furnace was shut down on 12 June 1999 and demolished and scrapped between 2001 and 2003, but the eight smokestacks of the furnaces remained until February 2011. The last open-hearth shop in China was shut down in 2001. The process in the form of Twin Hearth Furnace was in use in India's Steel Authority of India Bhilai Steel Plant and some parts of Ukraine. Russia retired its last hearth furnace in March 2018, and was considering preserving it as a museum artifact. India's SAIL shut it down in April 2020 with the advent of COVID19 because of nonavailability of manpower to run the labor intensive process.<ref>{{Cite web | url=https://sdelanounas.ru/blogs/104567/ | title=В России закрывается последняя крупная мартеновская печь| date=6 March 2018}}</ref>

As of 2024, the largest steel mill in the world that still produces steel using open-hearth furnaces is the [[Zaporizhstal| Zaporizhstal steel mill]] in central [[Ukraine]], which has seven 500-ton capacity OHFs and one twin-hearth furnace as well as four [[blast furnace]]s. The availability of cheap fuel oil in large quantities, as well as the ongoing invasion, largely contribute to their profitability despite the slow process. The high cost of upgrading to new furnace technologies is prohibitive.<ref>{{Cite web |url=https://gmk.center/en/manufacturer/zaporizhstal-iron-and-steel-works/ |title=Zaporizhstal Iron and Steel works |access-date=2024-10-17 |archive-url=https://web.archive.org/web/20240104060124/https://gmk.center/en/manufacturer/zaporizhstal-iron-and-steel-works/ |archive-date=2024-01-04 |url-status=live}}</ref><ref>{{Cite web |url=http://www.energystar.gov/ia/business/industry/41724.pdf |title=Archived copy |access-date=2006-12-09 |archive-url=https://web.archive.org/web/20170809032312/https://www.energystar.gov/ia/business/industry/41724.pdf |archive-date=2017-08-09 |url-status=dead }}</ref>

==See also==
{{Portal| Engineering }}
* [[Bessemer process]]
* [[Cementation (metallurgy)]] 
* {{section link|Crucible steel#Methods of crucible steel production}}
*[[AJAX furnace]], oxygen based open hearth process

== References ==
<references/>

== Further reading ==
* {{citation| first = K.|last = Barraclough| title = Steelmaking 1850–1900| publisher = Institute of Metals, London| year =  1990| pages =137–203}}
* {{citation| first = W. K. V.|last= Gale| title = Iron and Steel| publisher = Longmans, London| year =  1969| pages= 74–77}}
* {{Cite journal | last1 = Siemens | first1 = C. W. |author-link=Carl Wilhelm Siemens| title = On a regenerative gas furnace, as applied to glasshouses, puddling, heating, etc | doi = 10.1243/PIME_PROC_1862_013_007_02 | journal =  Proceedings of the Institution of Mechanical Engineers | volume = 13 | pages = 21–26 | date = June 1862 | publisher  =Institution of Mechanical Engineers}}

==External links==
{{Commonscat}}
* [http://www.davistownmuseum.org/toolPreBlastFurnace.html Precursors to the Blast Furnace]
* [https://books.google.com/books?id=7igDAAAAMBAJ&pg=PA64 "Administering Doses of Liquid Iron to Steel Furnaces"], ''[[Popular Science]]'', February 1919, page 64, scanned by [[Google Books]].

{{Iron and steel production}}

{{Authority control}}

[[Category:Steelmaking]]
[[Category:Industrial furnaces]]