Materials Science and Engineering Handbook Part 8 pdf

Modern rail steels consist of a fully pearlitic microstructure with a fine pearlite interlamellar spacing, as shown in Fig.. Bramfitt, Homer Research Laboratories, Bethlehem Steel Corpor

Ferritic stainless steels

Annealed at 815 °C (1500 °F) and cold drawn 607 88 462 67 26 64 96 HRB

Martensitic stainless steels(b)

Annealed bar 860 125 655 95 20 55 260

Oil quenched from 980 °C (1800 °F); tempered at

Hardened and tempered at 315 °C (600 °F) 1970 285 1900 275 2 10 580

Austenitic stainless steels(b)

In the selection process, what is required for one application may be totally inappropriate for another application For example, steel beams for a railway bridge require a totally different set of properties than the steel rails that are attached

to the wooden ties on the bridge deck In designing the bridge, the steel must have sufficient strength to withstand substantial applied loads In fact, the designer will generally select a steel with higher strength than actually required Also, the designer knows that the steel must have fracture toughness to resist the growth and propagation of cracks and must be capable of being welded so that structural members can be joined without sacrificing strength and toughness The steel bridge must also be corrosion resistant This can be provided by a protective layer of paint If painting is not allowed, small amounts of certain alloying elements such as copper and chromium can be added to the steel to inhibit or reduce corrosion rates Thus, the steel selected for the bridge would be a high-strength low-alloy (HSLA) structural steel such as ASTM A572, grade 50 or possibly a weathering steel such as ASTM A588 A typical HSLA steel has a ferrite-pearlite microstructure as seen in Fig 2 and is microalloyed with vanadium and/or niobium for strengthening

(Microalloying is a term used to describe the process of using small additions of carbonitride forming elements titanium,

vanadium, and niobium to strengthen steels by grain refinement and precipitation hardening.)

Fig 2 Microstructure of a typical HSLA structural steel (ASTM A572, grade 50) 2% nital + 4% picral etch

200×

On the other hand, the steel rails must have high strength coupled with excellent wear resistance Modern rail steels consist of a fully pearlitic microstructure with a fine pearlite interlamellar spacing, as shown in Fig 3 Pearlite is unique because it is a lamellar composite consisting of 88% soft, ductile ferrite and 12% hard, brittle cementite (Fe3C) The hard cementite plates provide excellent wear resistance, especially when embedded in soft ferrite Pearlitic steels have high strength and are fully adequate to support heavy axle loads of modern locomotives and freight cars Most of the load is applied in compression Pearlitic steels also have relatively poor toughness and cannot generally withstand impact loads without failure The rail steel could not meet the requirements of the bridge builder, and the HSLA structural steel could not meet the requirements of the civil engineer who designed the bridge or the rail system

Fig 3 Microstructure of a typical fully pearlitic rail steel showing the characteristic fine pearlite interlamellar

spacing 2% nital + 4% picral etch 500×

A similar case can be made for the selection of cast irons A cast machine housing on a large lathe requires a material with adequate strength, rigidity, and durability to support the applied load and a certain degree of damping capacity in order to rapidly attenuate (dampen) vibrations from the rotating parts of the lathe The cast iron jaws of a crusher require a material with substantial wear resistance For this application, a casting is required because wear-resistant steels are very difficult to machine For the machine housing, gray cast iron is selected because it is relatively inexpensive, can be easily cast, and has the ability to dampen vibrations as a result of the graphite flakes present in its microstructure These flakes are dispersed throughout the ferrite and pearlite matrix (Fig 4) The graphite, being a major nonmetallic constituent in the gray iron, provides a tortuous path for sound to travel through the material With so many flakes, sound waves are easily reflected and the sound dampened over a relatively short distance However, for the jaw crusher, damping capacity is not

a requirement In this case, an alloy white cast iron is selected because of its high hardness and wear resistance The white cast iron microstructure shown in Fig 5 is graphite free and consists of martensite in a matrix of cementite Both of these constituents are very hard and thus provide the required wear resistance Thus, in this example the gray cast iron would not meet the requirements for the jaws of a crusher and the white cast iron would not meet the requirements for the lathe housing

Fig 4 Microstructure of a gray cast iron with a ferrite-pearlite matrix 4% picral etch 320× Courtesy of A.O

Benscoter, Lehigh University

Fig 5 Microstructure of an alloy white cast iron White constituent is cementite and the darker constituent is

martensite with some retained austenite 4% picral etch 250× Courtesy of A.O Benscoter, Lehigh University

References cited in this section

1 Engineering Properties of Steel, P.D Harvey, Ed., American Society for Metals, 1982

2 G Krauss, Principles of the Heat Treatment of Steel, American Society for Metals, 1980

Effects of Composition, Processing, and Structure on Properties of Irons and Steels

Bruce L Bramfitt, Homer Research Laboratories, Bethlehem Steel Corporation

Role of Microstructure

In steels and cast irons, the microstructural constituents have the names ferrite, pearlite, bainite, martensite, cementite, and austenite In most all other metallic systems, the constituents are not named, but are simply referred to by a Greek letter ( , , , etc.) derived from the location of the constituent on a phase diagram Ferrous alloy constituents, on the other hand, have been widely studied for more than 100 years In the early days, many of the investigators were petrographers, mining engineers, and geologists Because minerals have long been named after their discoverer or place

of origin, it was natural to similarly name the constituents in steels and cast irons

It can be seen that the four examples described above have very different microstructures: the structural steel has a ferrite + pearlite microstructure; the rail steel has a fully pearlitic microstructure; the machine housing (lathe) has a ferrite + pearlite matrix with graphite flakes; and the jaw crusher microstructure contains martensite and cementite In each case, the microstructure plays the primary role in providing the properties desired for each application From these examples, one can see how material properties can be tailored by microstructural manipulation or alteration Knowledge about microstructure is thus paramount in component design and alloy development In this section, each microstructural constituent will be described with particular reference to the properties that can be developed by appropriate manipulation

of the microstructure through deformation (e.g., hot and cold rolling) and heat treatment Further details about these microstructural constituents can be found in Ref 2, 3, 4, 5, and 6

Ferrite

A wide variety of steels and cast irons fully exploit the properties of ferrite However, only a few commercial steels are completely ferritic An example of the microstructure of a fully ferritic, ultralow carbon steel is shown in Fig 6

Fig 6 Microstructure of a fully ferritic, ultralow carbon steel Marshalls etch + HF, 300× Courtesy of A.O

Benscoter, Lehigh University

Ferrite is essentially a solid solution of iron containing carbon or one or more alloying elements such as silicon, chromium, manganese, and nickel There are two types of solid solutions: interstitial and substitutional In an interstitial solid solution, elements with small atomic diameter, for example, carbon and nitrogen, occupy specific interstitial sites in the body-centered cubic (bcc) iron crystalline lattice These sites are essentially the open spaces between the larger iron atoms In a substitutional solid solution, elements of similar atomic diameter replace or substitute for iron atoms The two types of solid solutions impart different characteristics to ferrite For example, interstitial elements like carbon and nitrogen can easily diffuse through the open bcc lattice, whereas substitutional elements like manganese and nickel diffuse with great difficulty Therefore, an interstitial solid solution of iron and carbon responds quickly during heat treatment, whereas substitutional solid solutions behave sluggishly during heat treatment, such as in homogenization

According to the iron-carbon phase diagram (Fig 7(a)), very little carbon (0.022% C) can dissolve in ferrite ( Fe), even

at the eutectoid temperature of 727 °C (1330 °F) (The iron-carbon phase diagram indicates the phase regions that exist over a wide carbon and temperature range The diagram represents equilibrium conditions Figure 7(b) shows an expanded iron-carbon diagram with both the eutectoid and eutectic regions.) At room temperature, the solubility is an order of magnitude less (below 0.005% C) However, even at these small amounts, the addition of carbon to pure iron increases the room-temperature yield strength of iron by more than five times, as seen in Fig 8 If the carbon content exceeds the solubility limit of 0.022%, the carbon forms another phase called cementite (Fig 9) Cementite is also a constituent of pearlite, as seen in Fig 10 The role of cementite and pearlite on the mechanical properties of steel is discussed below

Fig 7(a) Iron-carbon phase diagram showing the austenite ( Fe) and ferrite ( Fe) phase regions and

eutectoid composition and temperature Dotted lines represent iron-graphite equilibrium conditions and solid lines represent iron-cementite equilibrium conditions Only the solid lines are important with respect to steels Source: Ref 2

Fig 7(b) Expanded iron-carbon phase diagram showing both the eutectoid (shown in Fig 7(a)) and eutectic

regions Dotted lines represent iron-graphite equilibrium conditions and solid lines represent iron-cementite equilibrium conditions The solid lines at the eutectic are important to white cast irons and the dotted lines are important to gray cast irons Source: Ref 2

Fig 8 Increase in room-temperature yield strength of iron with small additions of carbon Source: Ref 7

Fig 9 Photomicrograph of an annealed low-carbon sheet steel with grain-boundary cementite 2% nital + 4%

picral etch 1000×

Fig 10 Photomicrograph of pearlite (dark constituent) in a low-carbon steel sheet 2% nital + 4% picral etch

1000×

The influence of solid-solution elements on the yield strength of ferrite is shown in Fig 11 Here one can clearly see the strong effect of carbon on increasing the strength of ferrite Nitrogen, also an interstitial element, has a similar effect Phosphorus is also a ferrite strengthener In fact, there are commercially available steels containing phosphorus for strengthening These steels are the rephosphorized steels (type 1211 to 1215 series) Compositions and mechanical property data for these steels can be found in Tables 1 and 5

Fig 11 Influence of solid-solution elements on the changes in yield stress of low-carbon ferritic steels Source:

Ref 5

In Fig 11, the substitutional solid solution elements of silicon, copper, manganese, molybdenum, nickel, aluminum, and chromium are shown to have far less effect as ferrite strengtheners than the interstitial elements In fact, chromium, nickel, and aluminum in solid solution have very little influence on the strength of ferrite

In addition to carbon (and other solid-solution elements), the strength of a ferritic steel is also determined by its grain size according to the Hall-Petch relationship:

where y is the yield strength (in MPa), o is a constant, ky is a constant, and d is the grain diameter (in mm)

The grain diameter is a measurement of size of the ferrite grains in the microstructure, for example, note the grains in the ultralow carbon steel in Fig 6 Figure 12 shows the Hall-Petch relationship for a low-carbon fully ferritic steel This relationship is extremely important for understanding structure-property relationships in steels Control of grain size through thermomechanical treatment, heat treatment, and/or microalloying is vital to the control of strength and toughness

of most steels The role of grain size is discussed in more detail later in this article

Fig 12 Hall-Petch relationship in low-carbon ferritic steels Source: Ref 8

There is a simple way to stabilize ferrite, thereby expanding the region of ferrite in the iron-carbon phase diagram, namely

by the addition of alloying elements such as silicon, chromium, and molybdenum These elements are called ferrite stabilizers because they stabilize ferrite at room temperature through reducing the amount of solid solution (austenite) with the formation of what is called a -loop as seen at the far left in Fig 13 This iron-chromium phase diagram shows that ferrite exists up above 12% Cr and is stable up to the melting point (liquidus temperature) An important fully ferritic family of steels is the iron-chromium ferritic stainless steels These steels are resistant to corrosion, and are classified as type 405, 409, 429, 430, 434, 436, 439, 442, 444, and 446 stainless steels These steels range in chromium content from

11 to 30% Additions of molybdenum, silicon, niobium, aluminum, and titanium provide specific properties Ferritic stainless steels have good ductility (up to 30% total elongation and 60% reduction in area) and formability, but lack strength at elevated temperatures compared with austenitic stainless steels Room-temperature yield strengths range from

170 to about 440 MPa (25 to 64 ksi), and room-temperature tensile strengths range from 380 to about 550 MPa (55 to 80 ksi) Table 5 lists the mechanical properties of some of the ferritic stainless steels Type 409 stainless steel is widely used for automotive exhaust systems Type 430 free-machining stainless steel has the best machinability of all stainless steels other than that of a low-carbon, free-machining martensitic stainless steel (type 416)

Fig 13 Iron-chromium phase diagram Source: Ref 9

Another family of steels utilizing a ferrite stabilizer ( -loop) are the iron-silicon ferritic alloys containing up to about 6.5% Si (carbon-free) These steels are of commercial importance because they have excellent magnetic permeability and low core loss High-efficiency motors and transformers are produced from these iron-silicon electrical steels (aluminum can also substitute for silicon in them)

Over the past 20 years or so, a new breed of very-low-carbon fully ferritic sheet steels has emerged for applications requiring exceptional formability (see Fig 6) These are the interstitial-free (IF) steels for which carbon and nitrogen are reduced in the steelmaking process to very low levels, and any remaining interstitial carbon or nitrogen is tied up with small amounts of alloying elements (e.g., titanium or niobium) that form preferentially carbides and nitrides These steels have very low strength, but are used to produce components that are difficult or impossible to form from other steels Very-low-carbon, fully ferritic steels (0.001% C) are now being manufactured for automotive components that harden during the paint-curing cycle These steels are called bake-hardening steels, and have controlled amounts of carbon and nitrogen that combine with other elements, such as titanium and niobium, during the baking cycle (175 °C, or 350 °F, for

30 min) The process is called aging, and the strength derives from the precipitation of titanium/niobium carbonitrides at the elevated temperature

Another form of very-low-carbon, fully ferritic steel is motor lamination steel The carbon is removed from these steels

by a process known as decarburization The decarburized (carbon-free) ferritic steel has good permeability and sufficiently low core loss (not as low as the iron-silicon alloys) to be used for electric motor laminations, that is, the stacked steel layers in the rotor and stator of the motor

As noted previously, a number of properties are exploited in fully ferritic steels:

• Iron-silicon steels: Exceptional electrical properties

• Iron-chromium steels: Good corrosion resistance

• Interstitial-free steels: Exceptional formability

• Bake-hardening steels: Strengthens during paint cure cycle

• Lamination steels: Good electrical properties

Pearlite

As the carbon content of steel is increased beyond the solubility limit (0.02% C) on the iron-carbon binary phase diagram,

a constituent called pearlite forms Pearlite is formed by cooling the steel through the eutectoid temperature (the temperature of 727 °C in Fig 7(a) and 7(b)) by the following reaction:

Austenite cementite + ferrite (Eq 2)

The cementite and ferrite form as parallel plates called lamellae (Fig 14) This is essentially a composite microstructure consisting of a very hard carbide phase, cementite, and a very soft and ductile ferrite phase A fully pearlitic microstructure is formed at the eutectoid composition of 0.78% C As can be seen in Fig 3 and 14, pearlite forms as colonies where the lamellae are aligned in the same orientation The properties of fully pearlitic steels are determined by the spacing between the ferrite-cementite lamellae, a dimension called the interlamellar spacing, , and the colony size

A simple relationship for yield strength has been developed by Heller (Ref 10) as follows:

Fig 15 Relationship between pearlite interlamellar spacing and yield strength for eutectoid steels Source: Ref

10

It has also been shown by Hyzak and Bernstein (Ref 11) that strength is related to interlamellar spacing, pearlite colony size, and prior-austenite grain size, according to the following relationship:

YS = 52.3 + 2.18( -1/2) - 0.4( ) - 2.88(d-1/2) (Eq 4)

where YS is the yield strength (in MPa), dc is the pearlite colony size (in mm), and d is the prior-austenite grain size (in

mm) From Eq 3 and 4, it can be seen that the steel composition does not have a major influence on the yield strength of a fully pearlitic eutectoid steel There is some solid-solution strengthening of the ferrite in the lamellar structure (see Fig 11)

The thickness of the cementite lamellae can also influence the properties of pearlite Fine cementite lamellae can be deformed, compared with coarse lamellae, which tend to crack during deformation

Although fully pearlitic steels have high strength, high hardness, and good wear resistance, they also have poor ductility and toughness For example, a low-carbon, fully ferritic steel will typically have a total elongation of more than 50%, whereas a fully pearlitic steel (e.g., type 1080) will typically have a total elongation of about 10% (see Table 5) A low-carbon fully ferritic steel will have a room-temperature Charpy V-notch impact energy of about 200 J (150 ft · lbf), whereas a fully pearlitic steel will have room-temperature impact energy of under 10 J (7 ft · lbf) The transition temperature (i.e., the temperature at which a material changes from ductile fracture to brittle fracture) for a fully pearlitic steel can be approximated from the following relationship (Ref 11):

TT = 217.84 - 0.83( ) - 2.98(d-1/2) (Eq 5)

where TT is the transition temperature (in °C)

From Eq 5, one can see that both the prior-austenite grain size and pearlite colony size control the transition temperature

of a pearlitic steel Unfortunately, the transition temperature of a fully pearlitic steel is always well above room temperature This means that at room temperature the general fracture mode is cleavage, which is associated with brittle fracture Therefore, fully pearlitic steels should not be used in applications where toughness is important Also, pearlitic steels with carbon contents slightly or moderately higher than the eutectoid composition (called hypereutectoid steels) have even poorer toughness

From Eq 4 and 5, one can see that for pearlite, strength is controlled by interlamellar spacing, colony size, and austenite grain size, and toughness is controlled by colony size and prior-austenite grain size

prior-Unfortunately, these three factors are rather difficult to measure To determine interlamellar spacing, a scanning electron microscope (SEM), or a transmission electron microscope (TEM) is needed in order to resolve the spacing Generally, a magnification of 10,000× is adequate, as seen in Fig 14 Special statistical procedures have been developed to determine

an accurate measurement of the spacing (Ref 12) The colony size and especially the prior austenite grain size are very difficult to measure and require a skilled metallographer using the light microscope or SEM and special etching procedures

Because of poor ductility/toughness, there are only a few applications for fully pearlitic steels, including railroad rails and wheels and high-strength wire By far, the largest tonnage application is for rails A fully pearlitic rail steel provides excellent wear resistance for railroad wheel/rail contact Rail life is measured in millions of gross tons (MGT) of travel and current rail life easily exceeds 250 MGT The wear resistance of pearlite arises from the unique morphology of the ferrite-cementite lamellar composite where a hard constituent is embedded into a soft-ductile constituent This means that the hard cementite plates do not abrade away as easily as the rounded cementite particles found in other steel microstructures, that is, tempered martensite and bainite, which will be discussed later Wear resistance of a rail steel is directly proportional to hardness This is shown in Fig 16, which indicates less weight loss as hardness increases Also, wear resistance (less weight loss) increases as interlamellar spacing decreases, as shown in Fig 17 Thus, the most important microstructural parameter for controlling hardness and wear resistance is the pearlite interlamellar spacing Fortunately, interlamellar spacing is easy to control and is dependent solely on transformation temperature

Fig 16 Relationship between hardness and wear resistance (weight loss) for rail steels Source: Ref 13

Fig 17 Relationship between pearlite interlamellar spacing and wear resistance (weight loss) for rail steels

Source: Ref 13

Figure 18 shows a continuous cooling transformation (CCT) diagram for a typical rail steel A CCT diagram is a time versus temperature plot showing the regions at which various constituents ferrite, pearlite, bainite, and martensite form during the continuous cooling of a steel component Usually several cooling curves are shown with the associated start and finish transformation temperatures of each constituent These diagrams should not be confused with isothermal transformation (IT or TTT) diagrams, which are derived by rapidly quenching very thin specimens to various temperatures, and maintaining that temperature (isothermal) until the specimens begin to transform, partially transform, and fully transform, at which time they are quenched to room temperature An IT diagram does not represent the transformation behavior in most processes where steel parts are continuously cooled, that is, air cooled, and so forth

Fig 18 A CCT diagram of a typical rail steel (composition: 0.77% C, 0.95% Mn, 0.22% Si, 0.014% P, 0.017%

S, 0.010% Cr) Source: Ref 14

As shown in Fig 18, the pearlite transformation temperature (indicated by the pearlite-start curve, Ps) decreases with increasing cooling rate The hardness of pearlite increases with decreasing transformation temperature Thus, in order to provide a rail steel with the highest hardness and wear resistance, one must cool the rail from the austenite at the fastest rate possible to obtain the lowest transformation temperature This is done in practice by a process known as head hardening, which is simply an accelerated cooling process using forced air or water sprays to achieve the desired cooling rate (Ref 15) Because only the head of the rail contacts the wheel of the railway car and locomotive, only the head requires the higher hardness and wear resistance

Another application for a fully pearlitic steel is high-strength wire (e.g., piano wire) Again, the composite morphology of lamellar ferrite and cementite is exploited, this time during wire drawing A fully pearlitic steel rod is heat treated by a process known as patenting During patenting, the rod is transformed at a temperature of about 540 °C (1000 °F) by passing it through a lead or salt bath at this temperature This develops a microstructure with a very fine pearlite interlamellar spacing because the transformation takes place at the nose of the CCT diagram, that is, at the lowest possible pearlite transformation temperature (see Fig 18) The rod is then cold drawn to wire Because of the very fine interlamellar spacing, the ferrite and cementite lamellae become aligned along the wire axis during the deformation process Also, the fine cementite lamella tend to bend and deform as the wire is elongated during drawing The resulting wire is one of the strongest commercial products available; for example, a commercial 0.1 mm (0.004 in.) diam wire can have a tensile strength in the range of 3.0 to 3.3 GPa (439 to 485 ksi), and in special cases a tensile strength as high as 4.8 GPa can be obtained These wires are used in musical instruments because of the sound quality developed from the high tensile stresses applied in stringing a piano and violin and are also used in wire rope cables for suspension bridges

Ferrite-Pearlite. The most common structural steels produced have a mixed ferrite-pearlite microstructure Their applications include beams for bridges and high-rise buildings, plates for ships, and reinforcing bars for roadways These steels are relatively inexpensive and are produced in large tonnages They also have the advantage of being able to be produced with a wide range of properties The microstructure of typical ferrite-pearlite steels is shown in Fig 19

Fig 19 Microstructure of typical ferrite-pearlite structural steels at two different carbon contents (a) 0.10% C

(b) 0.25% C 2% nital + 4% picral etch 200×

In most ferrite-pearlite steels, the carbon content and the grain size determine the microstructure and resulting properties For example, Fig 20 shows the effect of carbon on tensile and impact properties The ultimate tensile strength steadily increases with increasing carbon content This is caused by the increase in the volume fraction of pearlite in the microstructure, which has a strength much higher than that of ferrite Thus, increasing the volume fraction of pearlite has

a profound effect on increasing tensile strength

Fig 20 Mechanical properties of ferrite-pearlite steels as a function of carbon content Source: Ref 2

However, as seen in Fig 20, the yield strength is relatively unaffected by carbon content, rising from about 275 MPa (40 ksi) to about 415 MPa (60 ksi) over the range of carbon content shown This is because yielding in a ferrite-pearlite steel

is controlled by the ferrite matrix, which is generally considered to be the continuous phase (matrix) in the microstructure Therefore, pearlite plays only a minor role in yielding behavior

From Fig 20, one can also see that ductility, as represented by reduction in area, steadily decreases with increasing carbon content A steel with 0.10% C has a reduction in area of about 75%, whereas a steel with 0.70% C has a reduction

in area of only 25% Percent total elongation would show a similar trend, however, with values much less than percent reduction in area

Much work has been done to develop empirical equations for ferrite-pearlite steels that relate strength and toughness to microstructural features, for example, grain size and percent of pearlite as well as composition One such equation for ferrite-pearlite steels under 0.25% C is as follows (Ref 16):

YS = 53.9 + 32.34 (Mn) + 83.2(Si) + 354.2(Nf) + 17.4(d-1/2) (Eq 6)

where Mn is the manganese content (%), Si is the silicon content (%), Nf is the free nitrogen content (%), and d is the

ferrite grain size (in mm) Equation 6 shows that carbon content (percent pearlite) has no effect on yield strength, whereas the yield strength in Fig 20 increases somewhat with carbon content According to Eq 6, manganese, silicon, and nitrogen have a pronounced effect on yield strength, as does grain size However, in most ferrite-pearlite steels nitrogen is quite low (under 0.010%) and thus has minimal effect on yield strength In addition, as discussed below, nitrogen has a detrimental effect on impact properties

The regression equation for tensile strength for the same steels is as follows (Ref 16):

TS = 294.1 + 27.7(Mn) + 83.2(Si)

where TS is the tensile strength (in MPa) and P is pearlite content (%) Thus, in distinction to yield strength, the

percentage of pearlite in the microstructure plays an important role on tensile strength

Toughness of ferrite-pearlite steels is also an important consideration in their use It has long been known that the absorbed energy in a Charpy V-notch test is decreased by increasing carbon content, as seen in Fig 21 In this graph of impact energy versus test temperature, the shelf energy decreases from about 200 J (150 ft · lbf) for a 0.11% C steel to about 35 J (25 ft · lbf) for a 0.80% C steel Also, the transition temperature increases from about -50 to 150 °C (-60 to 300

°F) over this same range of carbon content The effect of carbon is due mainly to its effect on the percentage of pearlite in the microstructure This is reflected in the regression equation for transition temperature below (Ref 16):

TT = -19 + 44(Si) + 700( )

+ 2.2(P) - 11.5(d-1/2)

(Eq 8)

Fig 21 Effect of carbon content in ferrite-pearlite steels on Charpy V-notch transition temperature and shelf

energy Source: Ref 17

It can be seen in all these relationships that ferrite grain size is an important parameter in improving both strength and toughness It can also be seen that while pearlite is beneficial for increasing tensile strength and nitrogen is beneficial for increasing yield strength, both are harmful to toughness Therefore, methods to control the grain size of ferrite-pearlite steels have rapidly evolved over the past 25 years The two most important methods to control grain size are controlled rolling and microalloying In fact, these methods are used in conjunction to produce strong, tough ferrite-pearlite steels

Controlled rolling is a thermomechanical treatment in which steel plates are rolled below the recrystallization temperature of austenite This process results in elongation of the austenite grains Upon further rolling and subsequent cooling to room temperature, the austenite-to-ferrite transformation takes place The ferrite grains are restricted in their growth because of the "pancake" austenite grain morphology This produces the fine ferrite grain size required for higher strength and toughness

Microalloying is the term applied to the addition of small amounts of special alloying elements (vanadium, niobium, or titanium) that aid in retarding austenite recrystallization, thus allowing a wide window of rolling temperatures for controlled rolling Without retarding recrystallization, as in normal hot rolling, the pancake-type grains do not form and a fine grain size cannot be developed Microalloyed steels are used in a wide variety of high tonnage applications including structural steels for the construction industry (bridges, multistory buildings, etc.), reinforcing bar, pipe for gas transmission, and numerous forging applications

Bainite

Like pearlite, bainite is a composite of ferrite and cementite Unlike pearlite, the ferrite has an acicular morphology and the carbides are discrete particles Because of these morphological differences, bainite has much different property characteristics than pearlite In general, bainitic steels have high strength coupled with good toughness, whereas pearlitic steels have high strength with poor toughness

Another difference between bainite and pearlite is the complexity of the bainite morphologies compared with the simple lamellar morphology of pearlite The morphologies of bainite are still being debated in the literature For years, since the classic work of Bain and Davenport in the 1930s (Ref 18), there were two classifications of bainite: upper and lower bainite This nomenclature was derived from the temperature regions at which bainite formed during isothermal (constant temperature) transformation Upper bainite formed isothermally in the temperature range of 400 to 550 °C (750 to 1020

°F), and lower bainite formed isothermally in the temperature range of 250 to 400 °C (480 to 750 °F) Examples of the microstructure of upper and lower bainite are shown in Fig 22 One can see that both types of bainite have an acicular morphology, with upper bainite being coarser than lower bainite The true morphological differences between the microstructures can only be determined by electron microscopy Transmission electron micrographs of upper and lower bainite are shown in Fig 23 In upper bainite, the iron carbide phase forms at the lath boundaries, whereas in lower bainite, the carbide phase forms on particular crystallographic habit planes within the laths Because of these differences

in morphology, upper and lower bainite have different mechanical properties Lower bainite, with a fine acicular structure and carbides within the laths, has higher strength and higher toughness than upper bainite with its coarser structure

Fig 22 Microstructure of (a) upper bainite and (b) lower bainite in a Cr-Mo-V rotor steel 2% nital + 4% picral

etch 500×

Fig 23 TEM micrographs of (a) upper bainite and (b) lower bainite in a Cr-Mo-V rotor steel

Because during manufacture most steels undergo continuous cooling rather than isothermal holding, the terms upper and lower bainite can become confusing because "upper" and "lower" are no longer an adequate description of morphology Bainite has recently been reclassified by its morphology, not by the temperature range in which it forms (Ref 19) For example, a recent classification of bainite yields three distinct types of morphology

• Class 1 (B1): Acicular ferrite associated with intralath (plate) iron carbide, that is, cementite (replaces

the term "lower bainite")

• Class 2 (B2): Acicular ferrite associated with interlath (plate) particles or films of cementite and/or austenite (replaces the term "upper bainite")

• Class 3 (B3): Acicular ferrite associated with a constituent consisting of discrete islands of austenite and/or martensite

The bainitic steels have a wide range of mechanical properties depending on the microstructural morphology and composition; for example, yield strength can range from 450 to 950 MPa (65 to 140 ksi), and tensile strength from 530 to

1200 MPa (75 to 175 ksi) Another aspect of a bainitic steel is that a single composition, Mo-B steel for example, can yield a bainitic microstructure over a wide range of transformation temperatures The CCT diagram for this steel is shown

in Fig 24 Note that for this steel the bainite start (Bs) temperature is almost constant at 600 °C (1110 °F) This flat transformation region is important because transformation temperature plays an important role in the development of microstructure A constant transformation temperature permits the development of a similar microstructure and properties over a wide range of cooling rates This has many advantages in the manufacturing of bainitic steels and is particularly advantageous in thick sections where a wide range in cooling rates is found from the surface to the center of the part

Fig 24 A CCT diagram of a Mo-B steel Composition: 0.093% C, 0.70% Mn, 0.36% Si, 0.51% Mo, 0.0054%

B Austenitized at Ac3 + 30 °C for 12 min Bs, bainite start; Bf, bainite finish; Fs, ferrite start; Ff, ferrite finish Numbers in circles indicate hardness (HV) after cooling to room temperature Source: Ref 20

In designing a bainitic steel with a wide transformation region, it becomes critical that the pearlite and ferrite regions are pushed as far to the right as possible on the CCT diagram; that is, pearlite and ferrite form only at slow cooling rates Alloying elements such as nickel, chromium, and molybdenum (and manganese) are selected for this purpose

For low-carbon bainitic steels, the relationship between transformation temperature and tensile strength is shown in Fig

25 (martensite will be discussed in next section of this article) Note the rapid increase in tensile strength as the transformation temperature decreases For these steels, a regression equation for tensile strength has been developed as follows (Ref 21):

TS = 246.4 + 1925(C) + 231(Mn + Cr) + 185(Mo)

+ 92(W) + 123(Ni) + 62(Cu) + 385(V + Ti) (Eq 9)

Fig 25 Relationship between transformation temperature and tensile strength of ferrite-pearlite, bainitic, and

martensitic steels Source: Ref 5

In addition to the elements carbon, nickel, chromium, molybdenum, vanadium, and so forth, it is well known that boron in very small quantities (for example, 0.003%) has a pronounced effect on retarding the ferrite transformation Thus, in a boron-containing steel (e.g., Mo + B), the ferrite nose in the CCT diagram is pushed to slower cooling rates Boron retards the nucleation of ferrite on the austenite grain boundaries and, in doing so, permits bainite to be formed (Fig 24) Whenever boron is added to steel, it must be prevented from combining with other elements such as oxygen and nitrogen Generally, aluminum and titanium are added first in order to lower the oxygen and nitrogen levels of the steel Even when adequately protected, the effectiveness of boron decreases with increasing carbon content and austenite grain size

Attempts have been made to quantitatively relate the microstructural features of bainite to mechanical properties One such relationship is (Ref 22):

YS = -194 + 17.4(d-1/2) + 15(n1/4) (Eq 10)

where YS is the 0.2% offset yield strength (in MPa), d is the bainite lath size (mean linear intercept) (in mm), and n is the

number of carbides per mm2 in the plane of section

With bainitic steels, the lath width of the bainite obeys a Hall-Petch relationship as shown in Fig 26 The lath size is directly related to the austenite grain size and decreases with decreasing bainite transformation temperature Because of the fine microstructure of bainite, the measurement of lath size and carbide density can only be done by scanning or transmission electron microscopy (SEM or TEM)

Fig 26 Relationship between bainite lath width (grain size) and yield strength Source: Ref 5

In low-carbon bainitic steels, type B2 (upper) bainite has inferior toughness to type B1 (lower) bainite In both cases, strength increases as the transition temperature decreases In type B2 (upper) bainite, the carbides are much coarser than in type B1 (lower) bainite and have a tendency to crack and initiate cleavage (brittle) fracture In type B1 bainite, the small carbides have less tendency to fracture One can lower the transition temperature in type B1 bainitic steels by providing a finer austenite grain size through lower-temperature thermomechanical treatment and grain refinement

Bainitic steels are used in many applications including pressure vessels, backup rolls, turbine rotors, die blocks, casting molds, nuclear reactor components, and earthmoving equipment One major advantage of a bainitic steel is that an optimal strength/toughness combination can be produced without expensive heat treatment, for example, quenching and tempering as in martensitic steels

die-Martensite

Martensite is essentially a supersaturated solid solution of carbon in iron The amount of carbon in martensite far exceeds that found in solid solution in ferrite Because of this, the normal body-centered cubic (bcc) lattice is distorted in order to accommodate the carbon atoms The distorted lattice becomes body-centered tetragonal (bct) In plain-carbon and low-alloy steels, this supersaturation is generally produced through very rapid cooling from the austenite phase region (quenching in water, iced-water, brine, iced-brine, oil or aqueous polymer solutions) to avoid forming ferrite, pearlite, and bainite Some highly alloyed steels can form martensite upon air cooling (see the discussion of maraging steels later in this section) Depending on carbon content, martensite in its quenched state can be very hard and brittle, and, because of this brittleness, martensitic steels are usually tempered to restore some ductility and increase toughness

Reference to a CCT diagram shows that martensite only forms at high cooling rates in plain-carbon and low-alloy steels

A CCT diagram for type 4340 is shown in Fig 27, which indicates that martensite forms at cooling rates exceeding about

1000 °C/min Most commercial martensitic steels contain deliberate alloying additions intended to suppress the formation

of other constituents that is ferrite, pearlite, and bainite during continuous cooling This means that these constituents form at slower cooling rates, allowing martensite to form at the faster cooling rates, for example, during oil and water quenching This concept is called hardenability and is essentially the capacity of a steel to harden by rapid quenching Most all the conventional alloying elements in steel promote hardenability For example, type 4340 steel shown in Fig 27 has significant levels of carbon, manganese, nickel, copper, and molybdenum to promote hardenability More details about hardenability can be found in Ref 2

Fig 27 The CCT diagram for type 4340 steel austenitized at 845 °C (1550 °F) Source: Ref 22

The martensite start temperature (Ms) for type 4340 is 300 °C(570 °F) Carbon lowers the Ms temperature, as shown in Fig 28, and alloying elements such as carbon, manganese, chromium, nickel, and molybdenum also lower Ms

temperature Many empirical equations have been developed over the past 50 years relating Ms temperature to composition One recent equation by Andrews (Ref 24) is:

Ms (°C) = 539 - 423(C) - 30.4(Mn) - 12.1(Cr) - 17.7(Ni) - 7.5(Mo) (Eq 11)

Fig 28 Effect of carbon content on Ms temperature in steels Source: Ref 6

With sufficient alloy content, the Ms temperature can be below room temperature, which means that the transformation is incomplete and retained austenite can be present in the steel

The microstructure of martensitic steels can be generally classed as either lath martensite, plate martensite, or mixed lath and plate martensite In plain carbon steels, this classification is related to carbon content, as shown in Fig 28 Lath martensite forms at carbon contents up to about 0.6%, plate martensite is found at carbon contents greater than 1.0%, and

a mixed martensite microstructure forms for carbon contents between 0.6 and 1.0% An example of lath martensite is shown in Fig 29 and plate martensite in Fig 30 Generally, plate martensite can be distinguished from lath martensite by its plate morphology with a central mid-rib Also, plate martensite may contain numerous microcracks, as shown in Fig

31 These form during transformation when a growing plate impinges on an existing plate Because of these microcracks, plate martensite is generally avoided in most applications The important microstructural units measured in lath martensite are lath width and packet size A packet is a grouping of laths having a common orientation

Fig 29 Microstructure of a typical lath martensite 4% picral + HCl 200×

Fig 30 Microstructure of a typical plate martensite 4% picral + HCl 1000×

Fig 31 Microcracks formed in plate martensite 4% picral + HCl/sodium metabisulfite etch 1000×

Plain-carbon and low-alloy martensitic steels are rarely used in the as-quenched state because of poor ductility To increase ductility, these martensitic steels are tempered (reheated) to a temperature below 650 °C (1200 °F) During tempering, the carbon that is in supersaturated solid solution precipitates on preferred crystallographic planes (usually the octahedral {111} planes) of the martensitic lattice Because of the preferred orientation, the carbides in a tempered martensite have a characteristic arrangement as seen in Fig 32

Fig 32 TEM micrograph showing carbide morphology in tempered martensite

Tempered martensite has similar morphological features to type B1 (lower) bainite However, a distinction can be made in terms of the orientation differences of the carbide precipitates This can be seen by comparing type B1 bainite in Fig 23 with tempered martensite in Fig 32 However, unless the carbide morphology is observed it is very difficult to distinguish between B1 bainite and tempered martensite

The hardness of martensite is determined by its carbon content, as shown in Fig 33 Martensite attains a maximum hardness of 66 HRC at carbon contents of 0.8 to 1.0% The reason that the hardness does not monotonically increase with carbon is that retained austenite is found when the carbon content is above about 0.4% (austenite is much softer than martensite) Figure 34 shows the increase in volume percent retained austenite with increasing carbon content Yield strength also increases with increasing carbon content as seen in Fig 35 This empirical relationship between the yield strength and carbon content for untempered low-carbon martensite is (Ref 25):

YS (MPa) = 413 + 17.2 × 105(C1/2) (Eq 12)

Lath martensite packet size also has an influence on the yield strength, as shown in Fig 36 The linear behavior follows a

Hall-Petch type relationship of (d-1/2)

Fig 33 Effect of carbon content on the hardness of martensite Source: Ref 4

Fig 34 Effect of carbon content on the volume percent of retained austenite ( ) in as-quenched martensite

Source: Ref 4

Fig 35 Relationship between carbon content and the yield strength of martensite Source: Ref 4

Fig 36 Relationship between lath martensite packet size (d) and yield strength of Fe-0.2%C (upper line) and

Fe-Mn (lower line) martensites Source: Ref 2

Most martensitic steels are used in the tempered condition where the steel is reheated after quenching to a temperature less than the lower critical temperature (Ac1) Figure 37 shows the decrease in hardness with tempering temperature for a number of carbon levels Plain-carbon or low-alloy martensitic steels can be tempered in lower or higher temperature ranges, depending upon the balance of properties required Tempering between 150 and 200 °C (300 and 390 °F) will maintain much of the hardness and strength of the quenched martensite and provide a small improvement in ductility and toughness (Ref 26) This treatment can be used for bearings and gears that are subjected to compression loading Tempering above 425 °C (796 °F) significantly improves ductility and toughness but at the expense of hardness and strength The effect of tempering temperature on the tensile properties of a typical oil-quenched low-alloy steel (type 4340) is shown in Fig 38 These data are for a 13.5 mm (0.53 in.) diam rod quenched in oil The as-quenched rod has a

hardness of 601 HB Note that by tempering at 650 °C (1200 °F), the hardness (see x-axis) decreased to 293 HB; or to less

than half the as-quenched hardness The tensile strength has decreased from 1960 MPa (285 ksi) at a 200 °C (400 °F) tempering temperature to 965 MPa (141 ksi) at a 650 °C (1200 °F) tempering temperature However, the ductility, represented by total elongation and reduction in area, increases dramatically The tempering process can be retarded by the addition of certain alloying elements such as vanadium, molybdenum, manganese, chromium, and silicon Also, for tempering, temperature is much more important than time at temperature

Fig 37 Decrease in the hardness of martensite with tempering temperature for various carbon contents

Source: Ref 2

Fig 38 Effect of tempering temperature on the mechanical properties of type 4340 steel Source: Ref 2

Temper embrittlement is possible during the tempering of alloy and low-alloy steels This embrittlement occurs when quenched-and-tempered steels are heated in, or slow cooled through the 340 to 565 °C (650 to 1050 °F) temperature range Embrittlement occurs when the embrittling elements, antimony, tin, and phosphorus, concentrate at the austenite grain boundaries and create intergranular segregation that leads to intergranular fracture The element molybdenum has been shown to be beneficial in preventing temper embrittlement

The large variation in mechanical properties of quenched-and-tempered martensitic steels provides the structural designer

with a large number of property combinations Data, like that shown in Fig 38, are available in Volume 1 of ASM Handbook as well as from other sources Hardnesses of quenched-and-tempered steels can be estimated by a method

established by Grange, et al (Ref 27) The general equation for hardness is:

HV = HVC + HVMn + HVP + HVSi + HVNi + HVCr + HVMo + HVV (Eq 13)

where HV is the estimated hardness value (Vickers)

In order to use this relationship, one must determine the hardness value of carbon (HVC) from Fig 39 For example, if one assumes that a tempering temperature of 540 °C (1000 °F) is used and the carbon content of the steel is 0.2% C, the HVC

value after tempering will be 180 HV Second, the effect of each alloying element must be determined from a figure such

as Fig 40 This graph represents a tempering temperature of 540 °C (1000 °F) Graphs representing other tempering temperatures can be found in Ref 27

Fig 39 Relationship between hardness of tempered martensite with carbon content at various tempering

temperatures Source: Ref 2

Fig 40 Effect of alloying elements on the retardation or softening during tempering at 540 °C (1000 °F)

relative to iron-carbon alloys Source: Ref 2

To illustrate the use of the Grange, et al method, the same type 4340 steel shown in Fig 38 is used The composition of the steel is 0.41% C, 0.67% Mn, 0.023% P, 0.018% S, 0.26% Si, 1.77% Ni, 0.78% Cr, and 0.26% Mo Assuming a 540

°C (1000 °F) tempering temperature, the estimated hardness value for carbon is 210 HV From Fig 39, the hardness values for each of the other alloying elements are:

Element Content, % Hardness, HV

According to Fig 38, the hardness value after tempering at 540 °C (1000 °F) was 363 HB (see Brinell hardness values

along x-axis) From the ASTM E 48 conversion table (included in Mechanical Testing, Volume 8 of ASM Handbook), a

Brinell hardness of 363 HB equates to a Vickers hardness of 383 HV The calculated value of 380 HV (in the table above)

is very close to the actual measured value of 383 HV Thus, this method can be used to estimate a specific hardness value after a quenching-and-tempering heat treatment for a low-alloy steel Also, as a rough approximation, the derived Brinell hardness value can be used to estimate tensile strength by the following equation (calculated from ASTM E 48 conversion table):

TS (MPa) = -42.3 + 3.6 HB (Eq 14)

For the above example, a type 4340 quenched-and-tempered (540 °C, or 1000 °F) steel with a calculated hardness of 363

HB would have an estimated tensile strength from Eq 14 of 1265 MPa (183 ksi) From Table 5, this measured tensile strength of a type 4340 quenched-and-tempered (540 °C, or 1000 °F) steel is 1255 MPa (182 ksi)

It is seen that quenched-and-tempered martensitic steels provide a wide range of properties The design engineer can choose from a large number of plain-carbon and low-alloy steels (Tables 1 and 2) In addition to this large list of steels, there are two other commercially important categories of fully martensitic steels, namely, martensitic stainless steels (Table 3) and maraging steels (Table 4)