Fahlman B.D. Materials Chemistry

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The phases of austenite, pearlite, and ferrite are relatively soft; hence, the

observed high hardness of steels is obtained through processing of these materials.

For instance, hypoeutectoid steel may be heated to form austenite and then slowly

cooled so the cementite/ferrite phases may be worked into desired shapes. If the

material is re-austenized and quickly quenched to room temperature, a very hard

phase known as martensite is formed. Some of the remaining pearlite and ferrite

phases (if present) would still remain in the matrix. Hence, only when the steel has

been heated to temperatures sufﬁciently high to convert all of the ferrite into

austenite, that quenching will result in pure martensitic steel. It should be noted

that the martensite phase does not appear in the above Fe–C phase diagram since it is

a non-equilibrium phase.

The temperature range is not the only crucial variable affecting the properties of

the material, but rather the rate of heating and cooling. For example, when austenitic

Figure 3.21. Top: The Al-rich side of the Al-Cu phase diagram. Bottom: schematic of the formation of

the equilibrium precipitate s phase: (a) a supersaturated a solid solution, (b) a transition, y

, precipitate

phase, and (c) the precipitate y phase within the a-matrix phase. Reproduced with permission from

Callister, W. D. Materials Science and Engineering: An Introduction, 7th ed., Wiley: New York, 2007.

188 3 Metals

steel is very slowly cooled to room temperature, the resulting solid will be soft and

malleable. However, when the same steel is rapidly quenched in cold water to

temperatures less than 250



C, the normal phase transformations to pearlite/ferrite

or pearlite/cementite (depending on %C present) are suppressed. Rather, the g-Fe

phase is conver ted to martensite, the hardest and strongest of all possible Fe–C

microstructures. Intere stingly, the formation of martensite may also occur through

fast quenching of other austenitic mixtures, such as ferrite- or cementite-rich

austenite (Eq. 16):

g  Fe þa  Fe

!

rapid cooling

martensite þ a  Fe

g  Fe þ Fe

!

rapid cooling

martensite þ Fe

ð16Þ

To summarize, the relative hardness of the various phases discussed thus far

(Brinell hardness values in parentheses): martensi te (300–700) > tempered mar-

tensite (300–450) > bainite (ca. 400) > ﬁne pearlite (100–300) > coarse pearlite

(100–220) > spheroidite (90–180). The hardness and brittleness of cementite is

much greater than ferrite, whereas the latter has signiﬁcantly greater ductility.

In order to predict the resultant phase arising from varying the cooling rate of

austenitic steel, one would use a time–temperature-transformation (TTT) curve

(e.g., Figure 3.22). To generate a TTT diagram, thin metal specimens are suitably

heated to form austenitic steel. This temperature is held at varying temperatures to

ensure full conversion of the microstructure to austenite. This is to allow the metal

carbides to fully dissolve in austenite; incomplete conversion of carbides will result

in ferrite grains that will ultimately weaken the material. The austenized specimens

are then removed at speciﬁc times and quenched in cold water. Using optical and

electron microscopies, the microstructure of the products is determined (i.e., pure

martensite, ferrite, pearlite, etc.). As one would expect, a large number of samples

are required to determine the time intervals required for the initial and full transfor-

mation of austenite to other phases, making this process an extremely labor-inten-

sive exercise.

As shown in Figure 3.22a, cooling austenitic steel at a rate fast enough to avoid

the nose of the transformation curve (i), will result in 100% martensite. Cooling

curve (ii) is tangent to the nose of the TTT plot. This will also result in 100%

martensite, but with signiﬁcantly lower internal stresses and distortions than (i).

Such a tangent line represents the slowest cooling rate that prevents formation of the

non-martensitic decomposition products of pearlite or bainite, referred to as the

critical cooling rate (CCR). A tangent to the 50% transformation curve (cooling

curve (iii)) will result in a mixture of 50% pearlite and 50% martensite. Cooling curve

(iv) represents relatively slow cooling to ca. 600



C, where 50% of the austenitic steel

is converted to pearlite (i.e., still above the pearlite/bainite threshold temperature

of 538



C). Subsequent fast cooling to ca. 450



C, and isothermal equilibration for

a few hundred seconds converts the remaining austenite into bainite. Hence, the

ﬁnal product from (iv) will be 50% pearlite and 50% bainite. The cool ing curve

(v) will result in 100% bainite, since fast cooling to ca. 500



C avoids the nose of

the TTT diagram and is held isothermally below the pearlite/bainite boundary.

3.2. Metallic Structures and Properties 189

Figure 3.22. Time-temperature-transformation (TTT) diagrams for (a) austenitic steel, and (b) an alloy

steel (type 4340); A ¼ austenite, B ¼ bainite, P ¼ pearlite, M ¼ martensite, F ¼ proeutectoid ferrite.

Reproduced with permission from Callister, W. D. Materials Science and Engineering: An Introduction,

190 3 Metals

Finally, cooling curve (vi) illustrates very slow cooling (e.g., furnace cooling), which

will result in 100% pearlite.

Solute hardening

The introduction of foreign species into the metallic lattice through alloy formation

introduces alien crystallites that will also impede slip in steel crystals by increasing

the lattice energy in the vicinity of the dopant. Strengthening occurs since more

work is required to propagate a dislocation through these areas. In particular, if the

alloying agent is carbon, hard crystallites of iron carbide may form that changes the

microstructure. By comparison, austenite usually does not contain iron carbide, and

is quite susceptible to slip.

From an analysis of various types of steels, only the following carbides will be

present: Fe

C, Mn

C, Cr

,Cr

,Fe

C, Fe

C, Mo

C, W

C, WC, VC,

TiC, NbC, TaC, Ta

C, and ZrC. The occurrence of these species will depend on the

type and concentration of the transition metal dopants within the iron lattice. For

interstitial carbides, the size of the metal atoms will govern the type of carbide

formed. In gener al, the metal radius must be >1.35 A

(e.g., Ti, Zr, Hf, V, Nb, Ta,

Mo, and W) to generate an interstitial vacancy large enough to accommodate C

atoms. Metals with smaller radii (e.g., Cr, Mn, Fe, Co, Ni) do not form MC species,

and form carbides with relatively com plex crystal structures (Figure 3.23 ). It should

be noted that metal carbides do not generally exist as isolated pure species. That is,

carbides of all alloying elements will exist as clusters that also contain iron. Further,

when several carbide-forming dopants are present that share the same crystal

structure, the resultant carbide will be present as a combination of those elements.

As an example, steel containing Cr and Mn dopants will contain particulates of the

complex carbide (Cr, Mn, Fe)

, rather than isolated Cr

and Mn

C species.

We have seen that only certain transition metals will form stable carbides; as a

relevant digression, let us consider the chemical rationale behind such reactivity.

The general trend for increasing carbide-forming ability of transition metals is:

Fe < Mn < Cr < Mo < W < V < Nb < Ta < Ti < Zr < Hf

If one follows this sequence using the Periodic Table, this grouping consists of

early transition metals that are relatively electron deﬁcient. As you may recall, the

valence shell of zero-valent transition metals in a crystal lattice is [(ns

)((n  1)d

)].

In the bulk solid state, the outer s electrons are completely delocalized, whereas the

wave functions of the d electrons remain localized on the respective metal atoms.

When a carbon atom enters the crystal ﬁeld it behaves as a ligand toward the metal,

with the ligand and metal electrons electrostatically interacting causing the d orbitals

to lose their original degeneracy.

[10]

Since this is an electrostatic effect, stronger

metal–carbon bonds will result from more diffuse metal d orbitals (5d vs.4dvs. 3d),

and metals with fewer d electrons (i.e., both corresponding to less electron–electron

repulsions between ligands and the metal ).

The transference of electron density from the metal to carbon will result in the

formation of a strongly polar covalent bond, between carbide ions (C

n

) and

3.2. Metallic Structures and Properties 191

transition metal ions. It should be noted that only the portions of the alloying

elements and carbon that cannot be dissolved in austenite at a given temperature

may be used in carbide formation. Two types of carbides are possible. If the atomic

radius ratio of carbon/metal is <0.59, an interstitial phase will result; otherwise,

carbides of complex compositions will form, having a different crystal lattice from

Figure 3.23. Crystal structures of typical carbides in steels. Shown are (a) orthorhombic Cr

(space

group: Pnma) (b) cubic Cr

(space group: Fm3m), (c) cubic Fe

C (space group: Fd3m), (d)

orthorhombic Fe

C (space group: Pnma), and (e) cubic TiC (space group: NaCl) and hexagonal W

(space group: CdI

). Reproduced with permission from Handbook of Ceramic Hard Materials, Riedel, R.

192 3 Metals

the host metal. The latter carbides are characteristic of Cr, Mn, and Fe, which have

ﬁve or more d electrons and a correspondingly weaker interaction with ligands such

as carbon. Hence, such complex carbides (e.g., Figure 3.23a–d) will have lower

melting points and hardness than analogous interstitial carbides of the 5d and/or

early transition metals (e.g.,W

C used as cutting blades).

It is interesting to note that alloying metals that are present in steel will cause stark

changes in TTT diagrams (Figure 3.22b). In particular, the nose of the austenite/

pearlite transformation will be compres sed, allowing for pearlite (and martensite)

formation at slower cooling rates. In addition, a second bainitic nose will appear in

the TTT diagram. For a given steel with constant %C, as the concentration of

metallic dopant s increase, there will be a decrease in the temperature required for

the onset of martensite formation, M

(Eq. 17):



CÞ¼539  423ð%CÞ30:4ð%MnÞ17:7ð%NiÞ

 12:1ð%CrÞ7:5ð%MoÞ

ð17Þ

The greatest effects are seen for the austenite-forming elements of C, Mn, and Ni

where even small concentrations result in a sharp decrease in M

. Whereas pure g-iron

may be converted to martensite at temperatures in excess of 500



C, hypereutectoid

steel is not transformed to martensite until a temperature of ca.160



C is reached during

quenching. For steels with carbon concentrations above 0.7% and/or high dopant

Figure 3.23. Continued

3.2. Metallic Structures and Properties 193

concentrations, martensite may be formed at temperatures well below 0



C. Hence,

high-C steels must be quenched in low-temperature media (e.g., dry ice/acetone, liquid

nitrogen) to ensure full conversion of austenite to martensite.

The effect of alloying elements may be understood by examining the austenite/

ferrite regions of the Fe–C phase diagram with respect to the alloy concentrations

(Figure 3.24). Ferrite-forming elements such as Al, Si, W, Cr, and Mo result in a

contraction of the austenite region, forming a gamma loop. By contrast, austenite-

stabilizing elements such as C, N, Mn, Ni, and Cu cause an expansion of the

austenitic phase boundary. Austenite stabilizers inhibit the nucleation and growth

of ferrite and pearlite/bainite phases, assisting in the formation of pure martensite

upon quenching. In general, since bcc ferrite contains more voidspace than fcc

austenite, larger interstitial dopant s may be incorpor ated into these lattices. Hence,

ferrite stabilizers tend to be larger in contrast to the smaller size of austenite

stabilizers. Most importantly, in accord with the “like dissolves like” principle,

bcc and fcc dopants will tend to stabilize ferrite and austenite, respectively.

A slow cooling rate will give the greatest opportunity for controlled atomic

migration within the lattice, and growth of large ordered crystallites. However, in

the rapid non-equilibrium conditions used to form martensite, there is no time for

carbon diffusion to occur; this yields a supersaturated solution, with >2 wt.% C

present as an interstitial impurity. During this phase transition, the fcc lattice of

austenite is transformed to a distorted bcc lattice, commonly referred to as body-

centered tetragonal bct (Figure 3.25). The degree of distortion from a perfect bcc

lattice (cubic lattice axes ratio, c/a ¼ 1) is ampliﬁed with increasing carbon con-

centration (Eq. 18):

¼ 1 þ 0.045(wt% C)ð18Þ

The fcc–bct conversion, known as the Bain transformation, is a diffusionless

process. That is, unlike the previous high-temperature conversions we saw earlier

(e.g., austenite to ferrite), martensite can form at temperatures signiﬁcantly below

room temperature, within 1  10

7

s. Such a fast growth rate precludes the decipher-

ing of the exact mechanism for the nucleation and growth of martensite. However,

leading theories suggest that the growth initiates from dislocations in the solid.

[11]

Tempering

Even though martensite exhibits a high hardness, the as-quenched material is much too

brittle and highly stressed for structural applications. The ductility and toughness of

martensite is greatly improved through post-annealing, a process known as tempering.

This process relieves stresses in the solid through conversion of bct martensite into bcc

ferrite, with precipitation of iron carbide particulates. It is important to note that the

annealed structure is not simply pearlite/ferrite, but is best referred to as tempered

martensite. During the annealing process for martensite, a number of key transforma-

tions occur:

(i) 50–250



C: Interstitial carbon atoms in martensite begin to diffuse within the

bct lattice. This results in precipitation hardening from the formation of

194 3 Metals

hexagonal e-iron carbide crystallites, Fe

C(2< x < 3). The martensite bct

crystal lattice begins to lose its tetragonality.

(ii) 250–350



C: Decomposition of retained austenite to ﬁne aggregates of ferrite

plates (acicular ferrite) and cementite crystallites. This intermediate micro-

structure is referred to as bainite, nucleating from the surface of ferrite crys tal-

lites. As the temperature approaches 300



C, cementite begins to dominate the

Figure 3.24. The phase diagram for the Fe–Cr system, illustrating the expanded ferrite region, relative to

the normal Fe–C diagram. Below is shown the “gamma loop” illustrating the size of the austenite region

from the presence of various dopants. Reproduced with permission from (top) Steels: Microstructure and

(bottom) Basic Metallurgy: Volume I, Principles, Grosvenor, A. W.; 3rd ed.; American Society for

3.2. Metallic Structures and Properties 195

microstructure at the expense of the e-Fe

C particles. At temperatures

approaching 350



C, the bct lattice is transformed to ferritic bcc.

(iii) 350–700



C: The cementite particles undergo a coarsening process (between

300



C and 400



C), and spheroidization (near 700



C). These processes drasti-

cally reduce the hardness of the material, but improve overall ductility and

brittleness characteristics (more desirable for particular appl ications).

Hence, to obtain high-strength tempered steels, it is essential to anneal at low

temperatures (<350



C). As one might expect, the presence of alloying elements

Figure 3.25. Crystallographic representation of the phase transformation from austenite to martensite.

Two neighboring fcc unit cells of austenite associate, resulting in a body-centered tetragonal (bct) unit

cell. A postanneal known as tempering converts the bct structure to a-Fe. Also shown is the placement of

an interstitial carbon atom, remaining in an octahedral site among lattice iron atoms.

196 3 Metals

will have a dramatic effect on the microstructure during annealing. For instance, if

1–2 wt.% Si is present, e-iron carbide particles are stabilized up to temperatures of

400



C, yielding a much harder material at elevated temperatures. Further, transition

metals such as Cr, Mo, V, W, and Ti will form stable carbides with higher enthalpies

of formation than Fe

C, typically at temperatures between 500



C and 600



C. A high

temperature is required due to the relatively low diffusivity of the alloying elements

that must substitutionally diffuse through the iron lattice. By contrast, int erstitial

dopants such as C, N, and B move between the iron lattice sites with a much greater

diffusivity. It is important to note that the alloy carbides remain as ﬁne suspensions

even after prolonged tempering. This results in substantial strengthening referred to

as secondary hardening.

One serious drawback of the above austenization/rapid quench method for mar-

tensite formation is the possibility of distor ting and crackin g the metal due to the

rapid cooling event. During the quenching process, thermal stresses arise from the

varying cooling rates experienced by outer and interior areas of the steel. In addition,

there is a volume change when austenite is transformed to martensite. Two methods

that have been used to reduce quenching stresses are martempering and austemper-

ing (Figure 3.26). Martempering allow s the transformation of austenite to martensite

to take place at the same time throughout the structure of the metal part. By using

interrupted quench, the cooling is stopped at a point above the martensite transf or-

mation region to allow sufﬁcient time for the center to cool to the same temperature

as the surface. Then cooling is continued through the martensite region, followed by

the usual tempering process. By comparison, in austempering, the austenized steel is

quenched at a rate faster than that required for pearlite formation, but above the

temperature required for martensite growth. Hence, rather than transforming to

martensite, the center and surface are converted to bainite – a strong material that

shares the hardness of martensite with the toughness of pearlite.

Surface hardening

The above changes in the microstructure upon annealing do not only apply to the

bulk material, but also for the surface. If an iron material is placed at high tempera-

ture in the presence of carbon vapor, a procedure known as carburization occurs,

where carbon atoms diffuse into the surface of the steel, increasing the surface

hardness. There must be careful control of the annealing atmosphere; if the steel is

brought into cont act with an oxidizing atmosphere, decarburization of the surface

will occur through preferential formation of CO

Other surface hardening techniques introduce nitrogen to steels containing metals

such as Al, Cr, and V that form stable nitrides (Eq. 19). Surface hardening techni-

ques add a variety of attractive properties to steel components such as increas ing

wear and stress resistances, decreasing the odds of fracturing, and increasing

corrosion resistance.

4Cr þ 3NH

3ðgÞ

!

600



þ9=2H

2ðgÞ

ð19Þ

Strengthening of the exterior of a material may be achieved through either

diffusional incorporation of dopants (e.g., B, C, N), or annealing selective portions

3.2. Metallic Structures and Properties 197