Time–temperature–transformation diagrams

7.3.1 Time–temperature–transformation diagrams

Eutectoid decomposition occurs in both ferrous (e.g. iron–carbon) and non-ferrous (e.g. copper– aluminum, copper–tin) alloy systems, but it is of particular importance industrially in governing the hardening of steels. In the iron–carbon system (see Figure 2.20) the γ-phase, austenite, which is a solid solution of carbon in fcc iron, decomposes on cooling to give a structure known as pearlite, composed

of alternate lamellae of cementite (Fe 3 C) and ferrite. However, when the cooling conditions are such that the alloy structure is far removed from equilibrium, an alternative transformation may occur. Thus, on very rapid cooling, a metastable phase called martensite, which is a supersaturated solid solution of carbon in ferrite, is produced. The microstructure of such a transformed steel is not homogeneous but consists of plate-like needles of martensite embedded in a matrix of the parent austenite. Apart from martensite, another structure known as bainite may also be formed if the formation of pearlite is avoided by cooling the austenite rapidly through the temperature range above 550 ◦

C, and then holding the steel at some temperature between 250 and 550 ◦

C. A bainitic structure consists of plate-like grains

of ferrite, somewhat like the plates of martensite, inside which carbide particles can be seen.

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 7.18, which plots the time necessary at a given temperature to transform austenite of eutectoid composition to one of the three structures: pearlite, bainite or martensite. Such a diagram, made up from the results of a series of isothermal decomposition experiments, is called a TTT curve, since it relates the transformation product to the time at a given temperature. It will be evident from such a diagram that a wide variety of structures can be obtained from the austenite decomposition of a particular steel; the structure may range from 100% coarse pearlite, when the steel will be soft and ductile, to fully martensitic, when the steel will be hard and brittle. It is because this wide range of properties can be produced by the transformation of a steel that it remains a major constructional material for engineering purposes.

From the TTT curve it can be seen that, just below the critical temperature, A 1 , the rate of trans- formation is slow even though the atomic mobility must be high in this temperature range. This is because any phase change involving nucleation and growth (e.g. the pearlite transformation) is faced with nucleation difficulties, which arise from the necessary surface and strain energy contributions to the nucleus. Of course, as the transformation temperature approaches the temperature corresponding to the knee of the curve, the transformation rate increases. The slowness of the transformation below the knee of the TTT curve, when bainite is formed, is also readily understood, since atomic migration is slow at these lower temperatures and the bainite transformation depends on diffusion. The lower part of the TTT curve below about 250–300 ◦

C indicates, however, that the transformation speeds up

410 Physical Metallurgy and Advanced Materials

800 A 1 Austenite

600 A⫹F⫹C Ferrite⫹Cementite

400 Temperature ( C)

M s 200

50% martensite on quenching to this temperature 90% martensite at this temperature

10 10 2 10 3 10 4 10 5 Time (s)

(a)

A 3 Austenite

transformed transformed

Austenite 500

A⫹F⫹C

Ferrite⫹Cementite

Temperature ( C) 50% martensite

Temperature ( C)

Time (s)

Time (s)

(c) Figure 7.18 TTT curves for: (a) eutectoid, (b) hypo-eutectoid and (c) low alloy (e.g. Ni–Cr–Mo)

(b)

steels (after ASM Metals Handbook).

again and takes place exceedingly fast, even though atomic mobility in this temperature range must

be very low. For this reason, it is concluded that the martensite transformation does not depend on the speed of migration of carbon atoms and, consequently, it is often referred to as a diffusionless transformation. The austenite only starts transforming to martensite when the temperature falls below

a critical temperature, usually denoted by M s . Below M s the percentage of austenite transformed to martensite is indicated on the diagram by a series of horizontal lines.

Mechanical properties II – Strengthening and toughening 411

Rate of growth

Rate of growth

Rate of

600⬚C 670⬚C 690⬚C

NG

B Temperature

A Transformation

Rate of nucleation

(nuclei mm ⫺ 3 s ⫺ 1 )

(a)

(b)

Figure 7.19 Effect of temperature on: (a) amount of pearlite formed with time and (b) rate of nucleation and rate of growth of pearlite (after Mehl and Hagel, 1956).

The M s temperature may be predicted for steels containing various alloying elements in weight percent by the formula, due to Steven and Haynes, given by M s ( ◦ C) = 561–474C–33Mn–17Ni– 17Cr–21Mo.