Strengthening of steels by

8.3 Strengthening of steels by

heat-treatment

the sinusoidal composition modulation, and K depends

on the difference in bond energies between like and

8.3.1 Time –temperature –transformation

unlike atom pairs. The coherency strain energy term

diagrams

is related to the misfit ε between regions A and B, where ε D ⊲1/a⊳da/dc, the fractional change in lattice

Eutectoid decomposition occurs in both ferrous parameter a per unit composition change, and is given

(e.g. iron–carbon) and non-ferrous (e.g. cop- for an elastically isotropic solid, by

per–aluminium, copper–tin) alloy systems, but it is of particular importance industrially in governing the

G

strain Dε c 2 EV/⊲

hardening of steels. In the iron–carbon system (see Figure 3.18) the -phase, austenite, which is a solid solution of carbon in fcc iron, decomposes on cool-

molar volume. The total free energy change arising ing to give a structure known as pearlite, composed from a composition fluctuation is therefore

of alternate lamellae of cementite ⊲Fe 3 C⊳ and ferrite. However, when the cooling conditions are such that

d 2 G 2K G D

the alloy structure is far removed from equilibrium, an

C C⊲ 2ε 2

dc 2 EV/⊲

c 2

2 / 2 alternative transformation may occur. Thus, on very rapid cooling, a metastable phase called martensite,

which is a supersaturated solid solution of carbon in and a homogeneous solid solution will decompose

ferrite, is produced. The microstructure of such a trans- spinodally provided

formed steel is not homogeneous but consists of plate-

2 ⊳C⊲ like needles of martensite embedded in a matrix of 2ε 2 EV/ (8.20) the parent austenite. Apart from martensite, another

d 2 G/ dc 2 ⊳>⊲

2 G/ dc 2 ⊳C⊲ structure known as bainite may also be formed if 2ε 2 EV/ the formation of pearlite is avoided by cooling the ] D 0 is known as the coherent spinodal, as shown

austenite rapidly through the temperature range above 550 °

C, and then holding the steel at some temperature has to satisfy the condition

between 250 °

C and 550 °

C. A bainitic structure con-

sists of platelike grains of ferrite, somewhat like the

> 2K/[d 2 G/ dc 2 C⊲ 2ε 2 EV/

plates of martensite, inside which carbide particles can

be seen.

and decreases with increasing degree of undercool- The structure produced when austenite is allowed to transform isothermally at a given temperature can

be conveniently represented by a diagram of the type shown in Figure 8.18, which plots the time necessary

a diffusion distance. For large misfit values, a large at a given temperature to transform austenite of eutec- undercooling is required to overcome the strain energy

toid composition to one of the three structures: pearlite, effect. In cubic crystals, E is usually smaller along

bainite or martensite. Such a diagram, made up from

h 1 0 0i directions and the high strain energy is accom- the results of a series of isothermal-decomposition modated more easily in the elastically soft directions,

experiments, is called a TTT curve, since it relates the with composition modulations localized along this

transformation product to the time at a given tempera- direction.

ture. It will be evident from such a diagram that a wide Spinodal decompositions have now been studied in

variety of structures can be obtained from the austenite

a number of systems such as Cu–Ni –Fe, Cu–Ni –Si, decomposition of a particular steel; the structure may Ni –12Ti, Cu–5Ti exhibiting ‘side-bands’ in X-ray

range from 100% coarse pearlite, when the steel will small-angle scattering, satellite spots in electron

be soft and ductile, to fully martensitic, when the steel diffraction patterns and characteristic modulation of

will be hard and brittle. It is because this wide range structure along h1 0 0i in electron micrographs. Many

of properties can be produced by the transformation of of the alloys produced by splat cooling might be

a steel that it remains a major constructional material expected to exhibit spinodal decomposition, and it has

for engineering purposes.

Strengthening and toughening 275

Figure 8.18 TTT curves for (a) eutectoid, (b) hypo-eutectoid and (c) low alloy (e.g. Ni/Cr/Mo) steels (after ASM Metals Handbook) .

From the TTT curve it can be seen that just below the bainite transformation depends on diffusion. The

lower part of the TTT curve below about 250–300 ° C tion is slow even though the atomic mobility must

the critical temperature, A 1 , the rate of transforma-

indicates, however, that the transformation speeds up

be high in this temperature range. This is because any again and takes place exceedingly fast, even though phase change involving nucleation and growth (e.g.

atomic mobility in this temperature range must be very the pearlite transformation) is faced with nucleation

low. For this reason, it is concluded that the marten- difficulties, which arise from the necessary surface and

site transformation does not depend on the speed of strain energy contributions to the nucleus. Of course, as

migration of carbon atoms and, consequently, it is the transformation temperature approaches the temper-

often referred to as a diffusionless transformation. The ature corresponding to the knee of the curve, the trans-

austenite only starts transforming to martensite when formation rate increases. The slowness of the transfor-

the temperature falls below a critical temperature, usu- mation below the knee of the TTT curve, when bainite

ally denoted by M s . Below M s the percentage of is formed, is also readily understood, since atomic

austenite transformed to martensite is indicated on the migration is slow at these lower temperatures and

diagram by a series of horizontal lines.

276 Modern Physical Metallurgy and Materials Engineering The M s temperature may be predicted for steels

containing various alloying elements in weight per cent by the formula, due to Steven and Haynes, given by M s ⊲ ° C⊳ D 561–474C–33Mn–17Ni–17Cr–21Mo.