clasificación de los aceros al carbono y los aceros de baja aleación.doc

8/12/2019 Clasificacin de los aceros al carbono y los aceros de baja aleacin.doc

1/6

Classification of Carbon and Low-Alloy Steels

Abstract:The American Iron and Steel Institute (AISI) defines carbon steel as follows: Steel is consideredto be carbon steel when no minimum content is specified or required for chromium, cobalt,columbium [niobium], molybdenum, nicel, titanium, tun!sten, "anadium or #irconium, or anyother element to be added to obtain a desired alloyin! effect$ when the specified minimum forcopper does not e%ceed &'& per cent$ or when the ma%imum content specified for any of thefollowin! elements does not e%ceed the percenta!es noted: man!anese '*+, silicon &'*&,copper &'*&'

Steels can be classified by a "ariety of different systems dependin! on:

The composition, such as carbon, lowalloy or stainless steel'

The manufacturin! methods, such as open hearth, basic o%y!en process, or electric furnacemethods'

The finishin! method, such as hot rollin! or cold rollin!

The product form, such as bar plate, sheet, strip, tubin! or structural shape

The deo%idation practice, such as illed, semiilled, capped or rimmed steel

The microstructure, such as ferritic, pearlitic and martensitic The required stren!th le"el, as specified in AST- standards

The heat treatment, such as annealin!, quenchin! and temperin!, and thermomechanicalprocessin!

.uality descriptors, such as for!in! quality and commercial quality'


2/6

Carbon SteelsThe American Iron and Steel Institute (AISI) defines carbon steel as follows:

Steel is considered to be carbon steel when no minimum content is specified or required forchromium, cobalt, columbium [niobium], molybdenum, nicel, titanium, tun!sten, "anadium or#irconium, or any other element to be added to obtain a desired alloyin! effect$ when the

specified minimum for copper does not e%ceed &'& per cent$ or when the ma%imum contentspecified for any of the followin! elements does not e%ceed the percenta!es noted:man!anese '*+, silicon &'*&, copper &'*&'

/arbon steel can be classified, accordin! to "arious deo%idation practices, as rimmed, capped,semiilled, or illed steel' 0eo%idation practice and the steelmain! process will ha"e an effecton the properties of the steel' 1owe"er, "ariations in carbon ha"e the !reatest effect onmechanical properties, with increasin! carbon content leadin! to increased hardness andstren!th' As such, carbon steels are !enerally cate!ori#ed accordin! to their carbon content'2enerally speain!, carbon steels contain up to 34 total alloyin! elements and can besubdi"ided into lowcarbon steels, mediumcarbon steels, hi!hcarbon steels, and ultrahi!hcarbon steels$ each of these desi!nations is discussed below'

As a !roup, carbon steels are by far the most frequently used steels' -ore than 5+4 of thesteel produced and shipped in the 6nited States is carbon steel'

Low-carbonsteels contain up to &'7&4 /' The lar!est cate!ory of this class of steel is flatrolled products (sheet or strip), usually in the coldrolled and annealed condition' The carboncontent for these hi!hformability steels is "ery low, less than &'&4 /, with up to &'4 -n'Typical uses are in automobile body panels, tin plate, and wire products'

8or rolled steel structural plates and sections, the carbon content may be increased toappro%imately &'7&4, with hi!her man!anese content up to '+4' These materials may beused for stampin!s, for!in!s, seamless tubes, and boiler plate'

Medium-carbonsteels are similar to lowcarbon steels e%cept that the carbon ran!es from&'7& to &'*&4 and the man!anese from &'*& to '*+4' Increasin! the carbon content toappro%imately &'+4 with an accompanyin! increase in man!anese allows medium carbonsteels to be used in the quenched and tempered condition' The uses of medium carbonman!anese steels include shafts, a%les, !ears, cranshafts, couplin!s and for!in!s' Steels inthe &'& to &'*&4 / ran!e are also used for rails, railway wheels and rail a%les'

1i!hcarbon steels contain from &'*& to '&&4 / with man!anese contents ran!in! from &'7&to &'9&4' 1i!hcarbon steels are used for sprin! materials and hi!hstren!th wires'

Ultrahigh-carbonsteels are e%perimental alloys containin! '3+ to 3'&4 /' These steels arethermomechanically processed to produce microstructures that consist of ultrafine, equia%ed

!rains of spherical, discontinuous proeutectoid carbide particles'

High-Strength Low-Alloy Steels1i!hstren!th lowalloy (1SA) steels, or microalloyed steels, are desi!ned to pro"ide bettermechanical properties and;or !reater resistance to atmospheric corrosion than con"entionalcarbon steels in the normal sense because they are desi!ned to meet specific mechanicalproperties rather than a chemical composition'

The 1SA steels ha"e low carbon contents (&'&+&'3+4 /) in order to produce adequateformability and weldability, and they ha"e man!anese contents up to 3'&4' Small quantities ofchromium, nicel, molybdenum, copper, nitro!en, "anadium, niobium, titanium and #irconiumare used in "arious combinations'

1SA /lassification:


3/6

Weathering steels, desi!nated to e%hibit superior atmospheric corrosion resistance

Control-rolled steels, hot rolled accordin! to a predetermined rollin! schedule,desi!ned to de"elop a hi!hly deformed austenite structure that will transform to a "ery fineequia%ed ferrite structure on coolin!

Pearlite-reduced steels, stren!thened by "ery fine!rain ferrite and precipitationhardenin! but with low carbon content and therefore little or no pearlite in the

microstructure Microalloyed steels, with "ery small additions of such elements as niobium, "anadium,

and;or titanium for refinement of !rain si#e and;or precipitation hardenin!

Acicular ferrite steel, "ery low carbon steels with sufficient hardenability to transformon coolin! to a "ery fine hi!hstren!th acicular ferrite structure rather than the usualpoly!onal ferrite structure

Dual-phase steels, processed to a microstructure of ferrite containin! small uniformly distributedre!ions of hi!hcarbon martensite, resultin! in a product with low yield stren!th and a hi!h rate of worhardenin!, thus pro"idin! a hi!hstren!th steel of superior formability'

The "arious types of 1SA steels may also ha"e small additions of calcium, rare earth elements, or #irconiumfor sulfide inclusion shape control'

Low-alloy Steelsowalloy steels constitute a cate!ory of ferrous materials that e%hibit mechanical properties superior to plaincarbon steels as the result of additions of alloyin! elements such as nicel, chromium, and molybdenum' Totalalloy content can ran!e from 3'&ecause of the wide "ariety of chemical compositions possible and the fact that some steels are used in morethan one heattreated, condition, some o"erlap e%ists amon! the alloy steel classifications' In this article, fourma=or !roups of alloy steels are addressed: () lowcarbon quenched and tempered (.T) steels, (3) mediumcarbon ultrahi!hstren!th steels, (7) bearin! steels, and () heatresistant chromiummolybdenum steels'

Low-carbon quenched and tempered steelscombine hi!h yield stren!th (from 7+& to &7+ -?a) and hi!htensile stren!th with !ood notch tou!hness, ductility, corrosion resistance, or weldability' The "arious steelsha"e different combinations of these characteristics based on their intended applications' 1owe"er, a fewsteels, such as 1@5& and 1@&&, are co"ered by military specifications' The steels listed are used primarily asplate' Some of these steels, as well as other, similar steels, are produced as for!in!s or castin!s'

Medium-carbon ultrahigh-strength steelsare structural steels with yield stren!ths that can e%ceed 75&-?a' -any of these steels are co"ered by SA;AISI desi!nations or are proprietary compositions' ?roductforms include billet, bar, rod, for!in!s, sheet, tubin!, and weldin! wire'

earing steels used for ball and roller bearin! applications are comprised of low carbon (&'& to &'3&4 C)casehardened steels and hi!h carbon ('&4 /) throu!hhardened steels' -any of these steels are co"ered bySA;AISI desi!nations'

Chromium-molybdenum heat-resistant steelscontain &'+ to 94 Crand &'+ to '&4 Mo' The carbon contentis usually below &'34' The chromium pro"ides impro"ed o%idation and corrosion resistance, and themolybdenum increases stren!th at ele"ated temperatures' They are !enerally supplied in the normali#ed andtempered, quenched and tempered or annealed condition' /hromiummolybdenum steels are widely used in theoil and !as industries and in fossil fuel and nuclear power plants'

Classification of Stainless Steels


4/6

Abstract:

Stainless steels are commonly di"ided into fi"e !roups: martensitic stainless steels, ferritic stainless steels,austenitic stainless steels, duple% (ferriticaustenitic) stainless steels, and precipitationhardenin! stainlesssteels' Stainless steels are a"ailable in the form of plate, sheet, strip, foil, bar, wire, semifinished products,pipes, tubes, and tubin!'

Stainless steels are ironbased alloys containin! at least &'+4 Cr' 8ew stainless steels contain more than7&4 Cror less than +&4 !e' They achie"e their stainless characteristics throu!h the formation of an in"isibleand adherent chromiumrich o%ide surface film' This o%ide forms itself in the presence of o%y!en'

Bther elements added to impro"e characteristics include nicel, molybdenum, copper, titanium, aluminum,silicon, niobium, nitro!en, sulfur, and selenium' /arbon is normally present in amounts ran!in! from less than&'&74 to o"er '&4 in certain martensitic !rades'

The selection of stainless steels may be based on corrosion resistance, fabrication characteristics, a"ailability,mechanical properties in specific temperature ran!es and product cost' 1owe"er, corrosion resistance andmechanical properties are usually the most important factors in selectin! a !rade for a !i"en application'

Stainless steels are commonly di"ided into fi"e !roups: martensitic stainless steels, ferritic stainless steels,austenitic stainless steels, duple% (ferriticaustenitic) stainless steels, and precipitationhardenin! stainlesssteels'

The de"elopment of precipitation-hardenable stainless steelswas spearheaded by the successfulproduction of Stainless C by 6'S' Steel in 9+' The problem of obtainin! raw materials has been a real one,particularly in re!ard to nicel durin! 9+&s when ci"il wars ra!ed in Africa and Asia, prime sources of nicel,and /old Car politics played a role because asternbloc nations were also prime sources of the element'This led to the de"elopment of a series of alloys (AISI 3&& type) in which man!anese and nitro!en are partiallysubstituted for nicel' These stainless steels are still produced today'

B"er the years, stainless steels ha"e become firmly established as materials for cooin! utensils, fasteners,cutlery, flatware, decorati"e architectural hardware, and equipment for use in chemical plants, dairy and foodprocessin! plants, health and sanitation applications, petroleum and petrochemical plants, te%tile plants, and

the pharmaceutical and transportation industries' Some of these applications in"ol"e e%posure to eitherele"ated or cryo!enic temperatures$ austenitic stainless steels are well suited to either type of ser"ice'

-odifications in composition are sometimes made to facilitate production' 8or instance, basic compositionsare altered to mae it easier to produce stainless steel tubin! and castin!' Similar modifications are made forthe manufacture of stainless steel weldin! electrodes$ here combinations of electrode coatin! and wirecomposition are used to produce desired compositions deposited weld metal'

Martensitic stainless steelsare essentially alloys of chromium and carbon that possess a distorted bodycentered cubic (bcc) crystal structure (martensitic) in the hardened condition' They are ferroma!netic,hardenable by heat treatments, and are !enerally resistant to corrosion only to relati"ely mild en"ironments'/hromium content is !enerally in the ran!e of &'+ to 54, and carbon content may e%ceed '34' Thechromium and carbon contents are balanced to ensure a martensitic structure after hardenin!'

2eneral corrosion is often much less serious than locali#ed forms such as stress corrosion cracin!, cre"icecorrosion in ti!ht spaces or under deposits, pittin! attac, and inter!ranular attac in sensiti#ed material suchas weld heataffected #ones (1AD)' Such locali#ed corrosion can cause une%pected and sometimescatastrophic failure while most of the structure remains unaffected, and therefore must be considered carefullyin the desi!n and selection of the proper !rade of stainless steel'

/orrosi"e attac can also be increased dramatically by seemin!ly minor impurities in the medium that may bedifficult to anticipate but that can ha"e ma=or effects, e"en when present in only partpermillionconcentrations$ by heat transfer throu!h the steel to or from the corrosi"e medium$ by contact trimmed only onthe ends'

Stainless steels are a"ailable in the form of plate, sheet, strip, foil, bar, wire, semifinished products, pipes,tubes, and tubin!'

SheetSheet is a flatrolled product in coils or cut len!ths at least *& mm wide and less than '


5/6

Sheet from the con"entional !rades is almost e%clusi"ely produced on continuous mills' 1and mill productionis usually confined to alloys that cannot be produced economically on continuous mills, such as certain hi!htemperature alloys'

The steel is cast in in!ots, and the in!ots are rolled on a slabbin! mill or a bloomin! mill into slabs or sheetbars' The slabs or sheet bars are then conditioned prior to bein! hot rolled on a finishin! mill' Alternati"ely, thesteel may be continuous cast directly into slabs that are ready for hot rollin! on a finishin! mill' The current

trend worldwide is toward !reater production from continuous cast slabs'

Sheet produced from slabs on continuous rollin! mills is coiled directly off the mill' After they are descaled,these hot bands are cold rolled to the required thicness and coils off the cold mill are either annealed anddescaled or bri!ht annealed' >elt !rindin! to remo"e surface defects is frequently required at hot bands or atan intermediate sta!e of processin!' 8ull coils or len!ths cut from coils may then be li!htly cold rolled on eitherdull or bri!ht rolls to produce the required finish' Sheet may be shipped in coils, or cut sheets may beproduced by shearin! len!ths from a coil and flattenin! them by roller le"elin! or stretcher le"elin!'

StripStrip is a flatrolled product, in coils or cut len!ths, less than *& mm wide and &'7 to ', and &/ can be produced in the hardened and tempered

condition'

%perience in the use of stainless steels indicates that many factors can affect their corrosion resistance'Some of the more prominent factors are:

/hemical composition of the corrosi"e medium includin! impurities

?hysical state of the mediumliquid, !aseous, solid, or combinations thereof

Temperature

Temperature "ariations

Aeration of the medium

B%y!en content of the medium

>acteria content of the medium

Ioni#ation of the medium

Fepeated formation and collapse of bubbles in the medium

Felati"e motion of the medium with respect to the steel

/hemical composition of the metal

Eature and distribution of microstructural constituents etc'

Surface !inish# Bther characteristics in the stainless steel selection checlist are "ital for some speciali#edapplications but of little concern for many applications' Amon! these characteristics, surface finish is importantmore often than any other e%cept corrosion resistance' Stainless steels are sometimes selected because theyare a"ailable in a "ariety of attracti"e finishes' Surface finish selection may be made on the basis ofappearance, frictional characteristics, or sanitation'

$late

?late is a flatrolled or for!ed product more than 3+& mm (& in') in width and at least '


6/6

directly cast or rolled from in!ots and that usually ha"e been conditioned to impro"e plat surface' Some platemay be produced by direct rollin! from in!ot'

8or strip, ed!e condition is often more important than it usually is for sheet' Strip can be furnished with "arioused!e specifications:

-ill ed!e (as produced, condition unspecified)

Eo' ed!e (ed!e rolled, rounded, or square)

Eo'7 ed!e (as slit)

Eo'+ ed!e (square ed!e produced by rollin! or filin! after slittin!)

!oil8oil is a flatrolled product, in coil form, up to &'7 mm thic and less than *& mm wide' 8oil is produced in slitwidths with ed!e conditions correspondin! to Eo'7 and Eo'+ ed!e conditions for strip' 8oil is made from types3&, 3&3, 7&, 7&3, 7&, 7&, 7&+, 7*, 7*, 73, 7ar is a product supplied in strai!ht len!ths$ it is either hot or cold finished and is a"ailable in "arious shapes,si#es, and surface finishes' This cate!ory includes small shapes whose dimensions do not e%ceed

clasificación de los aceros al carbono y los aceros de baja aleación.doc

Documents