Steels also contain carbon in amounts ranging from very small, of the order of 0.005 wt% in ultralow-carbon, vacuumdegassed

sheet steels, to a maximum of 2.00 wt% in the highest-carbon tool steels. Carbon profoundly changes the phase

relationships, microstructure, and properties in steels. Generally, carbon content is kept low in steels that require high

ductility , high toughness, and good weldability, but is maintained at higher levels in steels that require high strength, high

hardness, fatigue resistance, and wear resistance.

Figure 1 shows the iron-carbon phase diagram and the changes that carbon induces in the phase equilibria of pure iron.

Carbon is an austenite stabilizer and expands the temperature range of stability of austenite. Its solubility is much higher

in austenite (a maximum of 2.11 wt% in equilibrium with cementite at 1148 C, or 3000 F) than in ferrite (a maximum of

0.0218 wt% in equilibrium with cementite at 727 C, or 1340 F). The solubility of carbon in ferrite and austenite is a

function of temperature; when the carbon atoms can no longer be accommodated in the octahedral interstitial sites

between the iron atoms, a new phase that can accommodate more carbon atoms in its crystal structure will form (Ref 2).

This phase is designated as cementite or iron carbide (Fe3C) and has an orthorhombic crystal structure. Cementite

formation and the temperature-dependent solubility of carbon in austenite and ferrite, as controlled by alloying and

processing, account for the great variety of microstructures and properties produced in steels.

Alloys of iron and carbon that contain up to 2.00 wt% C are classified as steels, while those containing over 2.00 wt% C

are classified as cast irons.

The austenite phase field shown in Fig. 1 is the basis for the hot workability and heat treatability of carbon steels. Singlephase

austenite is readily hot worked; therefore, massive sections of steel can be hot reduced to smaller sections and

structural shapes

In addition to iron and carbon, steels contain many other elements that shift the boundaries of the iron-carbon phase

diagram. Elements such as manganese and nickel are austenite stabilizers, which lower critical temperatures. Elements

such as silicon, chromium, and molybdenum are ferrite stabilizers and carbide formers, which raise critical temperatures

and shrink the austenite phase field (Ref 3). Other elements, such as titanium, niobium, and vanadium, may form

temperature-dependent dispersions of nitrides, carbides, or carbonitrides in the austenite. These effects must be taken into

account when setting processing temperature ranges for commercial alloys.

Fig. 2 Hardness as a function of carbon content for various microstructures in steels. Crosshatched area shows

effect of retained austenite.

All types of microstructures increase in strength with increasing carbon content, but martensitic microstructures show the

most dramatic increases. Because of the low solubility of carbon in ferrite (except for as-quenched martensite), the carbon

is primarily concentrated in carbide phases. Therefore, much of the higher strength of medium- and high-carbon steels is

due to higher volume fractions and finer dispersions of carbides in ferrite. Ferritic matrix grain sizes and morphology also

significantly affect mechanical behavior at any given carbon level.

Figure 2 shows that all types of microstructures could be produced in a steel of a given carbon content. There are,

however, practical limits to this observation. Low-carbon steels do not have sufficient hardenability to form martensite

except in the thinnest sections and are therefore produced primarily with ferritic microstructures, which have excellent

ductility for cold-working and forming operations. At the other extreme, medium- and high-carbon steels alloyed with

chromium, nickel, and/or molybdenum may have such high hardenability for the formation of martensite or bainite that

other microstructures are formed only by special annealing treatments.

The preceding comments should help explain why various types of steels have evolved based on the most readily

attainable microstructures and the property requirements that they satisfy for certain types of applications. Alloy design

and processing approaches have also evolved and are still evolving to exploit the best features of each type of steel. The

following sections in this article describe in detail several important ferrous microstructures and the steels and processing

methods used to produce them.

Isothermal transformation diagram for 1080 steel containing 0.79 wt% C and 0.76 wt% Mn. Specimens

were austenitized at 900 C (1650 F) and had an austenitic grain size of ASTM No. 6. The Ms, M50, and M90

temperatures are estimated. Source: Ref 1

STEELS constitute the most widely used category of metallic material, primarily because they can be manufactured

relatively inexpensively in large quantities to very precise specifications. They also provide a wide range of mechanical

properties, from moderate yield strength levels (200 to 300 MPa, or 30 to 40 ksi) with excellent ductility to yield strengths

exceeding 1400 MPa (200 ksi) with fracture toughness levels as high as 110 MPa m (100 ksi in ).

The term low-alloy steel rather than the more general term alloy steel is being used to differentiate the steels

covered in this article from high-alloy steels. High-alloy steels include steels with a high degree of fracture

toughness (Fe-9Ni-4Co), which are described in the article "Ultrahigh-Strength Steels" in this Section of the

Handbook. They also include maraging steels (Fe-18Ni-4Mo-8Co), austenitic manganese steels (Fe-1C-

12Mn), tool steels, and stainless steels, which are described in separate articles in the Section "Specialty

Steels and Heat-Resistant Alloys" in this Volume.

Classification of Steels

Steels can be classified by a variety of different systems depending on:

The composition, such as carbon, low-alloy, or stainless steels

The manufacturing methods, such as open hearth, basic oxygen process, or electric furnace methods

The finishing method, such as hot rolling or cold rolling

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

The deoxidation practice, such as killed, semikilled, capped, or rimmed steel

The microstructure, such as ferritic, pearlitic, and martensitic (Fig. 1)

The required strength level, as specified in ASTM standards

The heat treatment, such as annealing, quenching and tempering, and thermomechanical processing

Quality descriptors, such as forging quality and commercial quality

Fig. 1 Classification of steels. Source: D.M. Stefanescu, University of Alabama, Tuscaloosa

systems, chemical composition is the most widely used internationally and will be

emphasized in this article.

Effects of Alloying Elements (Ref 6)

Steels form one of the most complex group of alloys in common use. The synergistic effect of alloying elements and heat

treatment produce a tremendous variety of microstructures and properties (characteristics). Given the limited scope of this

article, it would be impossible to include a detailed survey of the effects of alloying elements on the iron-carbon

equilibrium diagram. This complicated subject, which is briefly reviewed in the article "Microstructures, Processing, and

Properties of Steels" in this Volume, lies in the domain of ferrous physical metallurgy and has also been reviewed

extensively in the literature (Ref 7, 8, 9, 10, 11). 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 elements are modified by the influence of other elements. These interrelations must be considered when

evaluating a change in the composition of a steel. For the sake of simplicity, however, the various alloying elements listed

below are discussed separately.

Carbon. The amount of carbon required in the finished steel limits the type of steel that can be made. As the carbon

content of rimmed steels increases, surface quality becomes impaired. Killed steels in approximately the 0.15 to 0.30% C

content level may have poorer surface quality and require special processing to attain surface quality comparable to steels

with higher or lower carbon contents. Carbon has a moderate tendency to segregate, and carbon segregation is often more

significant than the segregation of other elements. Carbon, which has a major effect on steel properties, is the principal

hardening element in all steel. Tensile strength in the as-rolled condition increases as carbon content increases (up to

about 0.85% C). Ductility and weldability decrease with increasing carbon.

Manganese has less of a tendency toward 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 is particularly beneficial in resulfurized steels. It contributes to strength and

hardness, but to a lesser degree than does carbon; the amount of increase is dependent upon the carbon content. Increasing

the manganese content decreases ductility and weldability, but to a lesser extent than does carbon. Manganese has a

strong effect on increasing the hardenability of a steel.

Phosphorus segregates, but to a lesser degree than carbon and sulfur. Increasing phosphorus increases strength and

hardness and decreases ductility and notch impact toughness in the as-rolled condition. The decreases in ductility and

toughness are greater in quenched and tempered higher-carbon steels. Higher phosphorus is often specified in low-carbon

free-machining steels to improve machinability (see the article "Machinability of Steels" in this Volume).

Sulfur. Increased sulfur content lowers transverse ductility and notch impact toughness but has only a slight effect on

longitudinal mechanical properties. Weldability decreases with increasing sulfur content. This element is very detrimental

to surface quality, particularly in the lower-carbon and lower-manganese steels. For these reasons, only a maximum limit

is specified for most steels. The only exception is the group of free-machining steels, where sulfur is added to improve

machinability; in this case a range is specified (see the article "Machinability of Steels" in this Volume). Sulfur has a

greater segregation tendency than any of the other common elements. Sulfur occurs in steel principally in the form of

sulfide inclusions. Obviously, a greater frequency of such inclusions can be expected in the resulfurized grades.

Silicon is one of the principal deoxidizers used in steelmaking; therefore, the amount of silicon present is related to the

type of steel. Rimmed and capped steels contain no significant amounts of silicon. Semikilled steels may contain

moderate amounts of silicon, although there is a definite maximum amount that can be tolerated in such steels. Killed

carbon steels may contain any amount of silicon up to 0.60% maximum.

Silicon is somewhat less effective than manganese in increasing as-rolled strength and hardness. Silicon has only a slight

tendency to segregate. In low-carbon steels, silicon is usually detrimental to surface quality, and this condition is more

pronounced in low-carbon resulfurized grades.

Copper has a moderate tendency to segregate. Copper in appreciable amounts is detrimental to hot-working operations.

Copper adversely affects forge welding, but it does not seriously affect arc or oxyacetylene welding. Copper is

detrimental to surface quality and exaggerates the surface defects inherent in resulfurized steels. Copper is, however,

beneficial to atmospheric corrosion resistance when present in amounts exceeding 0.20%. Steels containing these levels

of copper are referred to as weathering steels and are described in the article "High-Strength Structural and High-Strength

Low-Alloy Steels" in this Volume; they are also included in the descriptions of high-strength low-alloy steels given later

in this article.

Lead is sometimes added to carbon and alloy steels through mechanical dispersion during teeming for the purpose of

improving the machining characteristics of the steels. These additions are generally in the range of 0.15 to 0.35% (see the

article "Machinability of Steels" in this Volume for details).

Boron is added to fully killed steel to improve hardenability. Boron-treated steels are produced to a range of 0.0005 to

0.003%. Whenever boron is substituted in part for other alloys, it should be done only with hardenability in mind because

the lowered alloy content may be harmful for some applications. Boron is most effective in lower carbon steels. Boron

steels are discussed in the Section "Hardenability of Carbon and Low-Alloy Steels" in this Volume.

Chromium is generally added to steel to increase resistance to corrosion and oxidation, to increase hardenability, to

improve high-temperature strength, or to improve abrasion resistance in high-carbon compositions. Chromium is a strong

carbide former. Complex chromium-iron carbides go into solution in austenite slowly; therefore, a sufficient heating time

before quenching is necessary.

Chromium can be used as a hardening element, and is frequently used with a toughening element such as nickel to

produce superior mechanical properties. At higher temperatures, chromium contributes increased strength; it is ordinarily

used for applications of this nature in conjunction with molybdenum.

Nickel, when used as an alloying element in constructional steels, is a ferrite strengthener. Because nickel does not form

any carbide compounds in steel, it remains in solution in the ferrite, thus strengthening and toughening the ferrite phase.

Nickel steels are easily heat treated because nickel lowers the critical cooling rate. In combination with chromium, nickel

produces alloy steels with greater hardenability, higher impact strength, and greater fatigue resistance than can be

achieved in carbon steels.

Molybdenum is added to constructional steels in the normal amounts of 0.10 to 1.00%. When molybdenum is in solid

solution in austenite prior to quenching, the reaction rates for transformation become considerably slower as compared

with carbon steel. Molybdenum can induce secondary hardening during the tempering of quenched steels and enhances

the creep strength of low-alloy steels at elevated temperatures. Alloy steels that contain 0.15 to 0.30% Mo display a

minimized susceptibility to temper embrittlement (see the article "Embrittlement of Steels" in this Volume for a

discussion of temper embrittlement and other forms of thermal embrittlement).

Niobium. Small additions of niobium increase the yield strength and, to a lesser degree, the tensile strength of carbon

steel. The addition of 0.02% Nb can increase the yield strength of medium-carbon steel by 70 to 100 MPa (10 to 15 ksi).

This increased strength may be accompanied by considerably impaired notch toughness unless special measures are used

to refine grain size during hot rolling. Grain refinement during hot rolling involves special thermomechanical processing

techniques such as controlled rolling practices, low finishing temperatures for final reduction passes, and accelerated

cooling after rolling is completed (further discussion of controlled rolling can be found in the article "High-Strength

Structural and High-Strength Low-Alloy Steels" in this Volume).

Aluminum is widely used as a deoxidizer and for control of grain size. When added to steel in specified amounts, it

controls austenite grain growth in reheated steels. Of all the alloying elements, aluminum is the most effective in

controlling grain growth prior to quenching. Titanium, zirconium, and vanadium are also effective grain growth

inhibitors; however, for structural grades that are heat treated (quenched and tempered), these three elements may have

adverse effects on hardenability because their carbides are quite stable and difficult to dissolve in austenite prior to

quenching.

Titanium and Zirconium. The effects of titanium are similar to those of vanadium and niobium, but it is only useful in

fully killed (aluminum-deoxidized) steels because of its strong deoxidizing effects.

Zirconium can also be added to killed high-strength low-alloy steels to obtain improvements in inclusion characteristics,

particularly sulfide inclusions where changes in inclusion shape improve ductility in transverse bending.

Carbon Steels

The American Iron and Steel Institute defines carbon steel as follows (Ref 2, 3):

Steel is considered to be carbon steel when no minimum content is specified or required for

chromium, cobalt, columbium [niobium], molybdenum, nickel, titanium, tungsten, vanadium or

zirconium, or any other element to be added to obtain a desired alloying effect; when the

specified minimum for copper does not exceed 0.40 per cent; or when the maximum content

specified for any of the following elements does not exceed the percentages noted: manganese

1.65, silicon 0.60, copper 0.60

Low-carbon steels contain up to 0.30% C. The largest category of this class of steel is flat-rolled products (sheet or

strip) usually in the cold-rolled and annealed condition. The carbon content for these high-formability steels is very low,

less than 0.10% C, with up to 0.4% Mn. Typical uses are in automobile body panels, tin plate, and wire products.

For rolled steel structural plates and sections, the carbon content may be increased to approximately 0.30%, with higher

manganese up to 1.5%. These latter materials may be used for stampings, forgings, seamless tubes, and boiler plate.

Medium-carbon steels are similar to low-carbon steels except that the carbon ranges from 0.30 to 0.60% and the

manganese from 0.60 to 1.65%. Increasing the carbon content to approximately 0.5% with an accompanying increase in

manganese allows medium-carbon steels to be used in the quenched and tempered condition. The uses of medium carbonmanganese

steels include shafts, couplings, crankshafts, axles, gears, and forgings. Steels in the 0.40 to 0.60% C range are

also used for rails, railway wheels, and rail axles.

High-carbon steels contain from 0.60 to 1.00% C with manganese contents ranging from 0.30 to 0.90%. High-carbon

steels are used for spring materials and high-strength wires.

Ultrahigh-carbon steels are experimental alloys containing approximately 1.25 to 2.0% C. These steels are

thermomechanically processed to produce microstructures that consist of ultrafine, equiaxed grains of ferrite and a

uniform distribution of fine, spherical, discontinuous proeutectoid carbide particles (Ref 13). Such microstructures in

these steels have led to superplastic behavior (Ref 14). Properties of these experimental steels are described in Forming

and Forging, Volume 14 of ASM Handbook, formerly 9th Edition Metals Handbook (see the Appendix to the article

"Superplastic Sheet Forming," entitled "Superplasticity in Iron-Base Alloys").

High-Strength Low-Alloy Steels

High-strength low-alloy (HSLA) steels, or microalloyed steels, are designed to provide better mechanical properties

and/or greater resistance to atmospheric corrosion than conventional carbon steels. They are not considered to be alloy

steels in the normal sense because they are designed to meet specific mechanical properties rather than a chemical

composition (HSLA steels have yield strengths of more than 275 MPa, or 40 ksi). The chemical composition of a specific

HSLA steel may vary for different product thickness to meet mechanical property requirements. The HSLA steels have

low carbon contents (0.50 to ~0.25% C) in order to produce adequate formability and weldability, and they have

manganese contents up to 2.0%. Small quantities of chromium, nickel, molybdenum, copper, nitrogen, vanadium,

niobium, titanium, and zirconium are used in various combinations

HSLA Classification. The types of HSLA steels commonly used include (Ref 15):

Weathering steels, designed to exhibit superior atmospheric corrosion resistance

Control-rolled steels, hot rolled according to a predetermined rolling schedule designed to develop a

highly deformed austenite structure that will transform to a very fine equiaxed ferrite structure on

cooling

Pearlite-reduced steels, strengthened by very fine-grain ferrite and precipitation hardening but with low

carbon content and therefore little or no pearlite in the microstructure

Microalloyed steels, with very small additions (generally