EFFECTS OF ALLOYING ELEMENTS ON STEEL PROPERTIES

EFFECTS OF ALLOYING ELEMENTS ON STEEL PROPERTIES PRIMARY ALLOYING ELEMENTS: In the production of Carbon Steels, elements that are added to refine molten steel in the ladle (or other refining vessel) in order to achieve desired metallurgical properties. THE PRIMARY ALLOYING ELEMENTS ARE: C, Mn and Si.

SECONDARY ALLOYING ELEMENTS: Added to refine molten steel in the ladle ( or other refining vessel ) along with Primary Alloying Elements to enhance properties & performance.SECONDARY ALLOYING ELEMENTS INCLUDE:

Cu, Ni, Cr, Mo, Al, V, Nb, B, Co, and WRESIDUAL ELEMENTS: A Residual Element is one that reports to the molten bath, and for which no minimum content is specified for the grade to be produced.

A maximum content may or may not be required by the grade/ specification. Whether covered by grade /specifications or not, residuals may affect steel properties and performance for better or worse.In Carbon Steel making, the term "Residual Elements" usually refers to Cu, Ni, Cr, and Mo ( and may include Co and W, if certain highly alloyed specialty steels are included in a furnace charge )

TRAMP ELEMENTS:

There is no universally accepted distinction between "Residual Elements" vs. "Tramp Elements". The two phrases are frequently used interchangeably.

Tramp Elements will be applied to certain elements that serve no useful purpose in the furnace charge. Tramp Elements may report to the molten bath, and / or to the slag, and / or to the furnace atmosphere.

Tramp Elements that report to the bath, if present in sufficient amounts, almost always have adverse effects on steel properties, ease and cost of steel production, and / or have undesirable environmental characteristics.

The "Tramp Elements" are principally: S, P, Pb, Sn, Sb, Zn, Cd, and Hg.

Steel making raw materials, including items of Ferrous Scrap, may contribute "Residual Elements and Tramp Elements to a furnace charge in two ways:RESIDUAL & TRAMP ELEMENTS ENDOGENOUSLY: Already present as alloy constituents in scrapped steel items ( or other steel-making raw materials) -their presences were intentional or unintentional. Such elements or the substances that contain them are almost never removable from the raw material by mechanical means.

Present as items or substances, or contained in items or substances, that coat, or are attached to, or otherwise accompany materials that are placed in a furnace charge. Usually removable from the raw material by mechanical means( but not necessarily economically ).

The control of Residual Element content in commercial scrap is essential to high quality steel making in an electric furnace. This claim is based upon metallurgical considerations that originate either from difficulties experienced in fabrication, both by steel makers and their customers, or the negative effect of high residuals on properties or performance of finished products.

ALLOYING

Primary Alloying Elements- C(Carbon) The principal element responsible for hardness in steel, due to formation of Fe3C upon cooling through the Transformation Temperature, when Gamma Iron (Austenite) decomposes into Alpha Iron (Ferrite) + Fe3C (Iron Carbide).EFFECT OF CARBON

Increases Hardness, tensile strength in steels. .Ductility generally decreases as C increases, but in most cases this effect can be offset with proper heat treatment..Affects melting pointWeldability decreases as C increases. Excess oxygen usage may be required to remove excess C ( takes furnace time ). When C added to an oxidizing slag, causes slag to foam, like froth on beer. The insulating effect of a "foamy slag" reduces heat loss in the furnace, prolongs electrode life, and reduces refractory wear.Certain applications specify a required Carbon Equivalent content in order to achieve desired properties. Example of a Carbon Equivalent calculation Carbon Equivalent = % C + 0.25 * % Mn+ % V + % NbMn( MANGANESE )

The formation in steel microstructures of Carbide compounds, especially Iron Carbide ( Fe3C ), is a key to steels superior metallurgical performance. Mn acts as a Carbide Stabilizer. Steels with Mn contents as low as 0.3%may insure that Fe3C won't degrade to Fe + Graphite under many heat treat conditions; as little as 0.3% Mn will offset the tendency of Si to promote graphitization, even when Si contents are as high as 2 %.SULPHUR

When S is present in steel, Mn may help prevent Hot Shortness( tearing during hot rolling or forging). In the absence of Mn, S may combine with Fe to form FeS, which appears in the steel microstructure as low melting point, brittle inclusions that weaken the steel. When Mn is present in steels that contain S, MnS is formed instead FeS. EFFECT OF SULPHUR

MnS appears in solidified steels as randomly distributed globules which are soft enough to deform during hot working, and may form elongated stringers which are frequently not harmful. If the Mn:S ratio is greater than 8:1, hot shortness due to FeS formation will generally not occur.EFFECT OF Mn

In addition, Mn may be used to improve hardenablilty ("through hardening"), toughness & tensile strength, but may decrease ductility & weldability.Si( SILICON )

During the O2 Blow phase of steel melting production, Si is readily burned to form Si02. The oxidation of Si is exothermic, and while the burning of Si represents a yield loss, there may benefit in that the heat produced may help "melt in" faster. SiO2 promotes fast slag formation by lowering melting point of CaO ( Lime ). However, an appropriate ratio of CaO to SiO2 (about 2.5 to 1 ) must be maintained in order to assure that the refining capability of the slag( to remove S and P from molten steel bath) is not compromised, and to not shorten furnace refractory life.

Si is an important, but not exceptionally strong deoxidizer. After the refining step in the furnace is complete, but before continuous casting may begin, excess O2 must be removed, which is accomplished by adding deoxidizers to the molten steel.

Elements used for deoxidation purposes include primarily Si and Al, but may also include Ca and Ti. Si is the primary deoxidizer used in the production of continuously cast billets for bar and structural applications. Si is not a grain refiner.Si can improve steel tensile strength, but may adversely affect machinability ( due to formation of silicate inclusions ). Si is the principal alloying element in lectrical steels used in transformers, and electric motor laminations, generators & relays ( Si increases permeability, raises electrical resistivity and reduces hysteresis loss ).Si promotes graphitization, i.e., decomposition of Iron Carbide ( Fe3C ) into metallic Fe and graphite. SECONDARY ALLOYING ELEMENTSCu( COPPER )

Cu is completely soluble in molten steel, and won't form oxides, carbides or sulfides in steel. Thus the Cu content of molten steel can only be lowered economically by dilution.

Can cause hot hortness(tearing during hot working, including forging, at seemingly low concentrations, i.e. over 0.2 % Cu ). Even if hot shortness does not occur, Cu can result in poor surface quality.The risk of hot shortness increases with increasing Sn and / or C contents, hot work furnace preheat time and temperature, and when the heating is done in an oxidizing atmosphere. The tendency to cause hot shortness may be somewhat offset by Ni. When present in solid solution, Cu "stiffens" Ferrite and decreases ductility. Can be used to increases atmospheric corrosion resistance. Can inhibit acid solution pickling by increasing scale adhesion.Ni( NICKEL )

Improves hardenability. When present in solid solution, Ni "stiffens" Ferrite and decreases ductility. Reduces distortion in heat treating. Permits use of milder quenching media. Permits ability to achieve strength & toughness levels at lower carbon contents.

Improves weldability, plasticity & fatique properties Improves toughness, especially at low temperatures. Improves ability to case harden. Improves corrosion resistance.Like Cu, Ni cannot be removed from molten steel. Ni content can only be lowered by dilution. Ni is an Austenite stabilizer in Stainless Steels.Cr( CHROMIUM )

-A strong carbide former.-improves wear resistance.-increase resistance to softening during tempering..When present in solid solution, Cr "stiffens" Ferrite and decreases ductility. -Improves hardenability depth. - Promotes the response of steel to carburzing heat treatment. In combination with even very low P, Sn, As or Sb contents,

Chromium and Ni-Cr alloy steels are particularly susceptible to "temper embrittlement" (loss of ductility when tempering or slow cooling in the range 375 -600C). When Cr > 4%, corrosion resistance greatly improves ( Responsible for corrosion resistance in Stainless Steels ). Not readily oxidized from bath; requires high temperatures, increased heat time and slag volume.

Mo(MOLYBDENUM)

A strong Carbide former. Mo has a high effect on hardenability. When present in solid solution, Mo "stiffens" Ferrite and decreases ductility. Improves control of heat treatment by inhibiting formation of certain microstructures e.g., Pearlite).

Can improves high temperature corrosion resistance. Can improve toughness & fatique properties. Not readily oxidizable ( removable ) from liquid steel. Can inhibit acid solution pickling. Expensive.V(VANADIUM)

An effective grain refiner ( i.e., restricts Austenitic grain growth ). Strong carbide and nitride former( improves abrasion resistance).Improves yield strength, toughness and hot hardness. Strongly increases resistance to softening during tempering. Ties up Nitrogen to inhibit strain ageing. Expensive.

Nb(NIOBIUM)

. Also called Columbium An effective grain refiner (i.e., restricts Austenitic grain growth). Strong carbide former.

Improves yield strength, tensile strength and hot hardness. Strongly increases resistance to softening during tempering.Ties up Nitrogen to inhibit strain ageing. .Expensive.Al(ALUMINUM)

A powerful deoxidizer and nitride former. In small amounts, serves as a powerful, inexpensive grain refiner (i.e., restricts Austenitic grain growth).Can improve toughness, especially at low temperatures.Added to Nitriding steels to promote high surface hardness & wear resistance.Difficult to use in continuous billet casting (due to precipitation of Al2O3 particles that can clog tundish nozzles).

B(Boron)

Strongly increases hardenability, by

suppressing Ferrite precipitation during

transformation from Austenite during

heat treatment.

Effective in very small amounts (less

than 0.003% B).

Highly reactive with Oxygen and

Nitrogen; difficult to use in EAF / CC

practice vs. BOF billet steel production

due to generally higher Oxygen and

Nitrogen levels in EAF liquid steels.

Co(Cobalt)

Like Cu, a Ferrite "stiffener" (by means of

solid solution strengthening), an effect that

persists at high tempertures (i.e.,

increases hot hardness).

Like Ni, cannot remove Co from molten steel.

Can find radioactive Co isotopes in certain

scrap.

If Radioactive Co Melted Into Bath, Steel maker

has BIG PROBLEMS.

Very Expensive

W(TUNGSTEN)

Forms extremely hard, stable carbides. Used almost exclusively in High Speed and other tool steels (requiring wear resistance and high hot hardness). Very Expensive. Used in the manufacture of High Speed Tool Steel, but otherwise almost never used due to extremely high cost.

M.PRABHU

QUALITYCONTROL

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