Jeklo: Razlika med redakcijama

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Najpogostejša so ogljikova jekla. To so jekla, ki poleg železa vsebujejo le ogljik ter manjše količine [[mangan]]a, [[silicij]]a in [[aluminij]]a. Slednje tri elemente dodamo z namenom, da bi zmanjšali ali povsem izničili negativen vpliv nečistoč, kot so [[žveplo]], [[fosfor]], [[kisik]] in [[dušik]]. Druga skupina jekel so legirana jekla. Ta – za razliko od ogljikovih jekel - vsebujejo še znatne količine [[krom]]a, [[nikelj|niklja]], [[molibden]]a ali katerega drugega elementa. Posebna legirana jekla, ki so znana kot nerjavna, vsebujejo najmanj 11,5 % kroma. Orodna jekla so posebna vrsta jekel. Namenjena so odrezovanju in oblikovanju [[kovina|kovinskih]] in nekovinskih [[material]]ov v želeno obliko. Nekatera jekla dobijo svojo končno obliko z [[litje]]m (jeklena litina), medtem ko večino jekel oblikujemo v končno obliko z gnetenjem (preoblikovanjem) in jih lahko prištevamo h gnetnim zlitinam.
 
== Lastnosti jekeljekla ==
[[Slika:Steel wire rope.png|thumb|250px|Jeklena [[vrv]]]]
Jeklo je elastično, [[prožnostni modul|modul elastičnosti]] ''E'' znaša od 2,0 do 2,2×10<sup>5</sup> N/mm².
Čisto železo ima [[trdota|trdoto]] le 60 [[trdota po Vickersu|HV]]. S postopki [[toplota|toplotne]] obdelave dosegamo v jeklu trdote tudi do 800 HV, s postopki toplotno kemične obdelave pa tudi vrednosti 2000 HV. [[Natezna trdnost]] čistega železa znaša približno 200 N/mm², jekla pa tudi do 4000 N/mm².
[[File:Steel pd.svg|thumb|420px|[[Fazni diagram]] železo-ogljik,z različnimi fazami, odvisno od temperature in vsebnosti ogljika]]
 
Železo je pogosto najti v zemeljski [[skorja (geologija) | skorji]] v obliki [[ruda|rude]], navadno železovega oksida, kot sta na primer [[magnetit]]in [[hematit]]. Železo se pridobiva iz rude tako, da se s pomočjo ogljika iz rude odstranjuje kisik, ki se sprošča v atmosfero kot ogljikov dioksid. Ta proces, znan kot [[taljenje]], je bil prvič uporabljen za kovine z nižjimi [[tališče|tališči]], kot sta na primer [[kositer]], ki se topi pri približno {{convert | 250 | C | F}} in [[baker]], ki se topi pri približno {{convert | 1100 | C | F |}}. Za primerjavo, lito železo se topi pri približno {{convert | 1375 | C | F}}. <Ref name = "Smelting">{{cite book|title=Smelting|publisher=Encyclopædia Britannica|year=2007|accessdate=2007-02-28}}</ref> V antičnih časih so topili majhne količine železa s segrevanjem rude v trdnem stanju, zakopane v goreče [[oglje]]; pridobljene grude železa so s kladivom obdelovali, da iz njih iztisnejo nečistoče. Če se pri tem previdno premika rudo po ognjišču, je mogoče obvladati vsebnost ogljika.
 
All of these temperatures could be reached with ancient methods that have been used since the [[Bronze Age]]. Since the oxidation rate of iron increases rapidly beyond {{convert|800|C|F}}, it is important that smelting take place in a low-oxygen environment. Unlike copper and tin, liquid or solid iron dissolves carbon quite readily. Smelting, using carbon to reduce iron oxides, results in an alloy ([[pig iron]]) that retains too much carbon to be called steel.<ref name="Smelting"/> The excess carbon and other impurities are removed in a subsequent step.
 
Other materials are often added to the iron/carbon mixture to produce steel with desired properties. [[Nickel]] and [[manganese]] in steel add to its tensile strength and make the [[austenite]] form of the iron-carbon solution more stable, [[chromium]] increases hardness and melting temperature, and [[vanadium]] also increases hardness while making it less prone to [[metal fatigue]].<ref name=materialsengineer>{{cite web|title=Alloying of Steels|publisher=Metallurgical Consultants|date=2006-06-28|url=http://materialsengineer.com/E-Alloying-Steels.htm|accessdate=2007-02-28}}</ref>
 
To inhibit corrosion, at least 11% chromium is added to steel so that a hard [[Passivation (chemistry)|oxide]] forms on the metal surface; this is known as [[stainless steel]]. Tungsten interferes with the formation of [[cementite]], allowing [[martensite]] to preferentially form at slower quench rates, resulting in [[high speed steel]]. On the other hand, sulfur, [[nitrogen]], and [[phosphorus]] make steel more brittle, so these commonly found elements must be removed from the steel melt during processing.<ref name="materialsengineer"/>
 
The [[density]] of steel varies based on the alloying constituents but usually ranges between {{convert|7750|and|8050|kg/m3|lb/ft3|abbr=on}}, or {{convert|7.75|and|8.05|g/cm3|oz/cuin|abbr=on}}.<ref>{{cite web|last = Elert|first = Glenn|title = Density of Steel|url = http://hypertextbook.com/facts/2004/KarenSutherland.shtml|accessdate = 2009-04-23}}</ref>
 
Even in a narrow range of concentrations of mixtures of carbon and iron that make a steel, a number of different metallurgical structures, with very different properties can form. Understanding such properties is essential to making quality steel. At [[room temperature]], the most stable form of pure iron is the [[body-centered cubic]] (BCC) structure called [[ferrite (iron)|ferrite]] or α-iron. It is a fairly soft metal that can dissolve only a small concentration of carbon, no more than 0.005% at {{Convert|0|C|F|abbr=on}} and 0.021 wt% at {{convert|723|C|F|abbr=on}}. At 910&nbsp;°C pure iron transforms into a [[face-centered cubic]] (FCC) structure, called [[austenite]] or γ-iron. The FCC structure of austenite can dissolve considerably more carbon, as much as 2.1%<ref>Sources differ on this value so it has been rounded to 2.1%, however the exact value is rather academic because plain-carbon steel is very rarely made with this level of carbon. See:
*{{harvnb|Smith|Hashemi|2006|p=363}}—2.08%.
*{{harvnb|Degarmo|Black|Kohser|2003|p=75}}—2.11%.
*{{harvnb|Ashby|Jones|1992}}—2.14%.</ref> (38 times that of ferrite) carbon at {{convert|1148|C|F|abbr=on}}, which reflects the upper carbon content of steel, beyond which is cast iron.<ref>{{harvnb|Smith|Hashemi|2006|p=363}}.</ref>
 
When steels with less than 0.8% carbon (known as a hypoeutectoid steel), are cooled, the [[austenitic]] phase (FCC) of the mixture attempts to revert to the ferrite phase (BCC). The carbon no longer fits within the FCC structure, resulting in an excess of carbon. One way for carbon to leave the [[austenite]] is for it to [[precipitate]] out of solution as [[cementite]], leaving behind a surrounding phase of BCC iron that is low enough in carbon to take the form of ferrite, resulting in a ferrite matrix with cementite inclusions. Cementite is a hard and brittle [[intermetallics|intermetallic compound]] with the [[chemical formula]] of Fe<sub>3</sub>C. At the [[eutectoid]], 0.8% carbon, the cooled structure takes the form of [[pearlite]], named for its resemblance to [[mother of pearl]]. On a larger scale, it appears as a lamellar structure of ferrite and cementite. For steels that have more than 0.8% carbon, the cooled structure takes the form of pearlite and cementite.<ref>{{harvnb|Smith|Hashemi|2006|pp=365–372}}.</ref>
 
Perhaps the most important [[polymorphism (materials science)|polymorphic form]] of steel is [[martensite]], a metastable phase that is significantly stronger than other steel phases. When the steel is in an austenitic phase and then [[quenching|quenched]] rapidly, it forms into martensite, as the atoms "freeze" in place when the cell structure changes from FCC to a distorted form of BCC as the atoms do not have time enough to migrate and form the cementite compound. Depending on the carbon content, the martensitic phase takes different forms. Below approximately 0.2% carbon, it takes on a ferrite BCC crystal form, but at higher carbon content it takes a [[body-centered tetragonal]] (BCT) structure. There is no thermal [[activation energy]] for the transformation from austenite to martensite. Moreover, there is no compositional change so the atoms generally retain their same neighbors.<ref name="smith&hashemi">{{Harvnb|Smith|Hashemi|2006|pp=373–378}}.</ref>
 
Martensite has a lower density than does austenite, so that the transformation between them results in a change of volume. In this case, expansion occurs. Internal stresses from this expansion generally take the form of [[physical compression|compression]] on the crystals of martensite and [[tension (mechanics)|tension]] on the remaining ferrite, with a fair amount of [[shear stress|shear]] on both constituents. If quenching is done improperly, the internal stresses can cause a part to shatter as it cools. At the very least, they cause internal [[work hardening]] and other microscopic imperfections. It is common for quench cracks to form when steel is water quenched, although they may not always be visible.<ref>{{cite web|title=Quench hardening of steel|url=http://steel.keytometals.com/default.aspx?ID=CheckArticle&NM=12|accessdate=2009-07-19}}</ref>
 
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