1.2 EFFECTS OF ALLOYING ELEMENTSSteels contain alloying elements and impurities that must be associated with austenite, ferrite, and cementite. The combined effects of 

alloying elements and heat treatment produce an enormous variety of microstructures and properties. Given the limited scope of this

chapter, it would be difficult to include a detailed survey of the effects of alloying elements on the iron – carbon equilibrium diagram,

allotropic transformations, and forming of new phases. This complicated subject, which lies in the domain of ferrous physical

metallurgy, has been revie wed extens ively in Chapt er 2 of this handb ook an d elsewh ere in the literat ure [4,5 ,8 – 12].

In this section, the effects of various elements on steelmaking (deoxidation) practices and steel characteristics will be briefly outlined.

It should be noted that the effects of a single alloying element on either practice or characteristics is modified by the influence of other 

elements. The interaction of alloying elements must be considered [5].

According to the effect on matrix, alloying elements can be divided into two categories:

1. By expending the g-field, and encouraging the formation of austenite, such as Ni, Co, Mn, Cu, C, and N (these elements are called

austenite stabilizers)

2. By contracting the g-field, and encouraging the formation of ferrite, such as Si, Cr, W, Mo, P, Al, Sn, Sb, As, Zr, Nb, B, S, and Ce

(these elements are called ferrite stabilizers)

Alloying elements can be divided into two categories according to the interaction with carbon in steel:

1. Carbide-forming elements, such as Mn, Cr, Mo, W, V, Nb, Ti, and Zr. They go into solid solution in cementite at low

concentrations. At higher concentrations, they formmore stable alloy carbides, though Mn only dissolves in cementite.

2. Noncarbide-forming elements, such as Ni, Co, Cu, Si, P, and Al. They are free from carbide in steels, and normally found in the

matrix [5,11,12].

To simplify the discussion, the effects of various alloying elements listed below are summarized separately.

1.2.1 C ARBON

The amount of carbon (C) required in the finished steel limits the type of steel that can be made.

As the C content of rimmed steels increases, surface quality deteriorates. Killed steels in the approximate range of 0.15 – 0.30% C may

have poorer surface quality and require special processing to attain surface quality comparable to steels with higher or lower C

contents.

Carbon has a moderate tendency for macrosegregation during solidification, and it is often more significant than that of any other 

alloying elements. Carbon has a strong tendency to segregate at the defects in steels (such as grain boundaries and dislocations).

Carbide forming elements may interact with carbon and form alloy carbides. Carbon is the main hardening element in all steels except

the austenitic precipitation hardening (PH) stainless steels, managing steels, and interstitial-free (IF) steels. The strengthening effect of 

C in steels consists of solid solution strengthening and carbide dispersion strengthening. As the C content in steel increases, strength

increases, but ductility and weldability decrease [4,5].

1.2.2 M ANGANESE

Manganese (Mn) is present in virtually all steels in amounts of 0.30% or more [13]. Manganese is essentially a deoxidizer and a

desulfurizer [14]. It has a lesser tendency for macrosegregation than any of the common elements. Steels above 0.60% Mn cannot be

readily rimmed. Manganese is beneficial to surface quality in all carbon ranges (with the exception of extremely low-carbon rimmed

steels) and reduction in the risk of red-shortness. Manganese favorably affects forgeability and weldability.

Manganese is a weak carbide former, only dissolving in cementite, and forms alloying cementite in steels [5]. Manganese is an

austenite former as a result of the open g-phase field.

Large quantities (>2% Mn) result in an increased tendency toward cracking and distortion during quenching [4,5,15]. The presence of 

alloying element Mn in steels enhances the impurities such as P, Sn, Sb, and As segregating to grain boundaries and induces temper 

embrittlement [5].

1.2.3 SILICON

Silicon (Si) is one of the principal deoxidizers used in steelmaking; therefore, silicon content also determines the type of steel

 produced. Killed carbon steels may contain Si up to a maximum of 0.60%. Semikilled steels may contain moderate amounts of Si. For 

example, in rimmed steel, the Si content is generally less than 0.10%.

Silicon dissolves completely in ferrite, when silicon content is below 0.30%, increasing its strength without greatly decreasing

ductility. Beyond 0.40% Si, a marked decrease in ductility is noticed in plain carbon steels [4].

If combined with Mn or Mo, silicon may produce greater hardenability of steels [5]. Due to the addition of Si, stress corrosion can be

eliminated in Cr  –  Ni austenitic steels. In heat treated steels, Si is an important alloy element, and increases hardenability, wear 

resistance, elastic limit and yield strength, and scale resistance in heat-resistant steels [5,15]. Si is a noncarbide former, and free from

cementite or carbides; it dissolves in martensite and retards the decomposition of alloying martensite up to 3008C.

1.2.4 PHOSPHORUS

Phosphorus (P) segregates during solidification, but to a lesser extent than C and S. Phosphorus dissolves in ferrite and increases the

strength of steels. As the amount of P increases, the ductility and impact toughness of steels decrease, and raises the cold-shortness

[4,5].

Phosphorus has a very strong tendency to segregate at the grain boundaries, and causes the temper embrittlement of alloying steels,

especially in Mn, Cr, Mn – Si, Cr  –  Ni, and Cr  – Mn steels. Phosphorus also increases the hardenability and retards the decomposition of 

martensite-like Si in steels [5]. High P content is often specified in low-carbon free-machining steels to improve machinability. In

low-alloy structural steels containing ~0.1% C, P increases strength and atmospheric corrosion resistance. In austenitic Cr  –  Ni steels,

the addition of P can cause precipitation effects and an increase in yield points [15]. In strong oxidizing agent, P causes grain

 boundary corrosion in austenitic stainless steels after solid solution treatment as a result of the segregation of P at grain boundaries [5].

 

1.2.5 SULFUR

Increased amounts of sulfur (S) can cause red- or hot-shortness due to the low-melting sulfide eutectics surrounding the grain in

reticular fashion [15,16]. Sulfur has a detrimental effect on transverse ductility, notch impact toughness, weldability, and surface

quality (particularly in the lower carbon and lower manganese steels), but has a slight effect on longitudinal mechanical properties.

Sulfur has a very strong tendency to segregate at grain boundaries and causes reduction of hot ductility in alloy steels. However, sulfur 

in the range of 0.08 – 0.33% is intentionally added to free-machining steels for increased machinability [5,17] .

Sulfur improves the fatigue life of bearing steels [18], because (1) the thermal coefficient on MnS inclusion is higher than that of 

matrix, but the thermal coefficient of oxide inclusions is lower than that of matrix, (2) MnS inclusions coat or cover oxides (such as

alumina, silicate, and spinel), thereby reducing the tensile stresses in the surrounding matrix [5,10,19].

1.2.6 ALUMINUM

Aluminum (Al) is widely used as a deoxidizer and a grain refiner [9]. As Al forms very hard nitrides with nitrogen, it is usually an

alloying element in nitriding steels. It increases scaling resistance and is therefore often added to heat-resistant steels and alloys. In

 precipitation hardening stainless steels, Al can be used as an alloying element, causing precipitation hardening reaction. Aluminum is

also used in maraging steels. Aluminum increases the corrosion resistance in low-carbon corrosion-resisting steels. Of all the alloying

elements, Al is one of the most effective elements in controlling grain growth prior to quenching.

Aluminum has the drawback of a tendency to promote graphitization.

1.2.7 NITROGEN

 Nitrogen (N) is one of the important elements in expanded g-field group. It can expand and stabilize the austenitic structure, and partly

substitute Ni in austenitic steels. If the nitride forming elements V, Nb, and Ti are added to high-strength low-alloy (HSLA) steels,

fine nitrides and carbonitrides will form during controlled rolling and controlled cooling. Nitrogen can be used as an alloying element

in microalloying steels or austenitic stainless steels, causing precipitation or solid solution strengthening [5]. Nitrogen induces strain

aging, quench aging, and blue brittleness in low-carbon steels.

1.2.8 CHROMIUM

Chromium (Cr) is a medium carbide former. In the low Cr/C ratio range, only alloyed cementite (Fe,Cr)3C forms. If the Cr/C ratio

rises, chromium carbides (Cr,Fe)7C3 or (Cr,Fe)23C6 or both, would appear. Chromium increases hardenability, corrosion and oxidation

resistance of steels, improves high-temperature strength and high-pressure hydrogenation properties, and enhances abrasion resistance

in high-carbon grades. Chromium carbides are hard and wear-resistant and increase the edge-holding quality. Complex chromium – 

iron carbides slowly go into solution in austenite; therefore, a longer time at temperature is necessary to allow solution to take place

 before quenching is accomplished [5,6,14]. Chromium is the most important alloying element in steels. The addition of Cr in steels

enhances the impurities, such as P, Sn, Sb, and As, segregating to grain boundaries and induces temper embrittlement.

1.2.9 NICKEL

 Nickel (Ni) is a noncarbide-forming element in steels. As a result of the open g-phase field, Ni is an austenite-forming element

[5,11,15]. Nickel raises hardenability. In combination with Ni, Cr and Mo, it produce greater hardenability, impact toughness, and

fatigue resistance in steels [5,10,11,18]. Nickel dissolving in ferrite improves toughness, decreases FATT 50% (8C), even at the subzero

temperatures [20]. Nickel raises the corrosion resistance of Cr  –  Ni austenitic stainless steels in nonoxidizing acid medium.

1.2.10 MOLYBDENUM

Molybdenum (Mo) is a pronounced carbide former. It dissolves slightly in cementite, while molybdenum carbides will form when the

Mo content in steel is high enough. Molybdenum can induce secondary hardening during the tempering of quenched steels and

improves the creep strength of low-alloy steels at elevated temperatures.

The addition of Mo produces fine-grained steels, increases hardenability, and improves fatigue strength. Alloy steels containing 0.20 – 

0.40% Mo or V display a delayed temper embrittlement, but cannot eliminate it. Molybdenum increases corrosion resistance and is

used to a great extent in high-alloy Cr ferritic stainless steels and with Cr  –  Ni austenitic stainless steels. High Mo contents reduce the

stainless steel’s susceptibility to pitting [5,15]. 

Molybdenum has a very strong solid solution strengthening in austenitic alloys at elevated temperatures. Molybdenum is a very

important alloying element for alloy steels.

1.2.11 TUNGSTEN

Tungsten (W) is a strong carbide former. The behavior of W is very similar to Mo in steels.

Tungsten slightly dissolves in cementite. As the content of W increases in alloy steels, W forms very hard, abrasion-resistant carbides,

and can induce secondary hardening during the tempering of quenched steels. It promotes hot strength and red-hardness and thus

cutting ability. It prevents grain growth at high temperature. W and Mo are the main alloying elements in high-speed steels [5,13].

However, W and Mo impair scaling resistance.

1.2.12 V ANADIUM

Vanadium (V) is a very strong carbide former. Very small amounts of V dissolve in cementite. It dissolves in austenite, strongly

increasing hardenability, but the undissolved vanadium carbides decrease hardenability [5].

Vanadium is a grain refiner, and imparts strength and toughness. Fine vanadium carbides and nitrides give a strong dispersion

hardening effect in microalloyed steels after controlled rolling and controlled cooling. Vanadium provides a very strong secondary

hardening effect on tempering, therefore it raises hot-hardness and thus cutting ability in high-speed steels. Vanadium increases

fatigue strength and improves notch sensitivity.

Vanadium increases wear resistance, edge-holding quality, and high-temperature strength.

It is therefore used mainly as an additional alloying element in high-speed, hot-forging, and creep-resistant steels. It promotes the

weldability of heat-treatable steels. The presence of V retards the rate of tempering embrittlement in Mo-bearing steels.

 

1.2.13 NIOBIUM AND T ANTALUM

 Niobium (Nb) and tantalum (Ta) are very strong carbide and nitride formers. Small amounts of Nb can form fine nitrides or 

carbonitrides and refine the grains, therefore increasing the yield strength of steels. Niobium is widely used in microalloying steels to

obtain high strength and good toughness through controlled rolling and controlled cooling practices. A 0.03% Nb in austenite can

increase the yield strength of medium-carbon steel by 150 MPa. Niobium containing non quenched and tempered steels, including

microalloyed medium-carbon steels and low-carbon bainite (martensite) steels, offer a greatly improved combination of strength and

toughness. Niobium is a stabilizer in Cr  –  Ni austenitic steels to eliminate intergranular corrosion.

1.2.14 TITANIUM

Titanium (Ti) is a very strong carbide and nitride former. The effects of Ti are similar to those of Nb and V, but titanium carbides and

nitrides are more stable than those of Nb and V. It is widely used in austenitic stainless steels as a carbide former for stabilization to

eliminate intergranular corrosion. By the addition of Ti, intermetallic compounds are formed in maraging steels, causing age

hardening. Titanium increases creep rupture strength through formation of special nitrides and tends significantly to segregation and

 banding [15].

Ti, Nb, and V are effective grain inhibitors because their nitrides and carbides are quite stable and difficult to dissolve in austenite. If 

Ti, Nb, and V dissolve in austenite, the hardenability of alloy steels may increase strongly due to the presence of Mn and Cr in steels.

Mn and Cr decrease the stability of Ti-, Nb-, and V-carbides in steels [5].

1.2.15 R ARE E ARTH METALS

Rare earth metals (REMs) constitute the IIIB group of 17 elements in the periodic table. They are scandium(Sc) of the fourth period,

yttrium(Y) of the fifth period, and the lanthanides of the sixth period, which include the elements, lanthanum (La), cerium (Ce),

 praseodymium (Pr), neodymium (Nd), promethium (Pm), samarium (Sm), europium (Eu), gadolinium (Gd), terbium (Tb), dysprosium

(Dy), holmium (Ho), erbium (Er), thulium (Tm; Tu), ytterbium (Yb), and lutecium (or lutecium, Lu). Their chemical and physical

 properties are similar. They generally coexist and are difficult to separate in ore beneficiation and metal extraction so they are usually

supplied as a mixture and used in various mixture states in metallurgical industries. REMs are strong deoxidizers and desulfurizers,

and they also react with the low-melting elements, such as antimony (Sb), tin (Sn), arsenic (As), and phosphorus (P), forming high-

melting compounds and preventing them from causing the red-shortness and temper embrittlement [21,22]. The effects of REM on

shape control and modification of inclusions would improve transversal plasticity and toughness, hot ductility, fatigue strength, and

machinability. REMs tend strongly to segregate at the grain boundaries and increase the hardenability of steels [21,23].

1.2.16 COBALT

Cobalt (Co) is a noncarbide former in steels. It decreases hardenability of carbon steels, but by addition of Cr, it increases

hardenability of Cr  – Mo alloy steels. Cobalt raises the martensitic transformation temperature ofMs (8C) and decreases the amount of 

retained austenite in alloy steels. Cobalt promotes the precipitation hardening [5]. It inhibits grain growth at high temperature and

significantly improves the retention of temper and high-temperature strength, resulting in an increase in tool life. The use of Co is

generally restricted to high-speed steels, hot-forming tool steels, maraging steels, and creep-resistant and high-temperature materials

[13,15].

1.2.17 COPPER

Copper (Cu) addition has a moderate tendency to segregate. Above 0.30% Cu can cause precipitation hardening. It increases

hardenability. If Cu is present in appreciable amounts, it is detrimental to hot-working operations. It is detrimental to surface quality

and exaggerates the surface defects inherent in resulfurized steels. However, Cu improves the atmospheric corrosion resistance (when

in excess of 0.20%) and the tensile properties in alloy and low-alloy steels, and reportedly helps the adhesion of paint [6,14]. In

austenitic stainless steels, a Cu content above 1% results in improved resistance to H2SO4 and HCl and stress corrosion [5,15].

1.2.18 BORON

Boron (B), in very small amounts (0.0005 – 0.0035%), has a starting effect on the hardenability of steels due to the strong tendency to

segregate at grain boundaries. The segregation of B in steels is a non equilibrium segregation. It also improves the hardenability of 

other alloying elements. It is used as a very economical substitute for some of the more expensive elements.

The beneficial effects of B are only apparent with lower- and medium-carbon steels, with no real increase in hardenability above 0.6%

C [14]. The weldability of boron-alloyed steels is another reason for their use. However, large amounts of B result in brittle,

unworkable steels.

1.2.19 ZIRCONIUM

Zircon ium (Zr) is added to killed HSLA steel s to obtain improvement in inclusion characteristics, particularly sulfide inclusions,

where modifications of inclusion shape improve ductility in transverse bending. It increases the life of heat-conducting materials. It is

also a strong carbide former and produces a contracted austenite phase field [5,15].

1.2.20 LEAD

Lead (Pb) is sometime s added (in the range of 0.2 – 0.5 %) to carbon and alloy steels through mechanical dispersion during teeming to

improve machinability.

1.2.21 TIN

Tin (Sn ) in relatively small amounts is harmful to steels. It ha s a very strong tendency to segregate at grain boundaries and induces

temper embrittlement in alloy steels. It has a detrimental effect on the surface quality of continuous cast billets containing small

amounts of Cu [24]. Small amounts of Sn and Cu also de crease the hot ductility of steels in the austenite þ ferrite region [25].

1.2.22 ANTIMONY

 

Antimony (Sb) has a strong tendency to segregate during the freezing process, and has a detrimental effect on the surface quality of 

continuous cast billets. It also has a very strong tendency to segregate at grain boundaries and cause temper embrittlement in alloy

steels.

1.2.23 C ALCIUM

Calcium (Ca) is a strong deoxidizer; silicocalcium is used usually in steelmaking. The combination of Ca, Al, and Si forms low-

melting oxides in steelmaking, and improves machinability.