to nucleate is called the pearlite start time, or abbreviated Ps. Upon holding the work piece further at the same temperature, the nuclei grow in size until all austenite has been eventually transformed into ferrite and cementite platelets, that is, into pearlite. This has oc-curred at the pearlite finish time, Pf. Since the transformation tem-perature is quite high, the diffusion is fast and the diffusion dis-tances may be long. For this reason, and because the density of the nuclei was small, the pearlite is coarse and the hardness of the work piece is relatively low; see Figure 8.4. In summary: A small tem-perature difference during quenching causes little undercooling which yields only a small number of nuclei. As a consequence the pearlite is coarse and the hardness is relatively small.
(b) The situation is somewhat different if austenitic steel is quenched to a lower temperature, as indicated by “b” in Figure 8.4. The undercooling is now larger, which causes a shorter nu-cleation time. Moreover, one encounters shorter diffusion dis-tances and a larger number of nuclei due to the lower tempera-ture. As a consequence, the pearlite is finer and thus harder. The time until the entire transformation is completed is relatively short, as can be deduced from Figure 8.4.
(c) If the temperature to which austenitic, eutectoid steel is quenched is reduced even further, the interplay between an en-hanced tendency toward nucleation and a reduced drive for dif-fusion causes the cementite to precipitate in microscopically small, elongated particles (needles) that are imbedded in a ferrite matrix.
This new microconstituent has been named bainite, and the re-spective times for the start and finish of the transformation have been designated as Bsand Bf. Heat treatments just below the “nose”
of the TTT curves (e.g., “c” in Figure 8.4) produce upper or coarse bainite. Bainite is harder than pearlite, and the presence of a fer-rite matrix causes the steel to be ductile and tough.
T
FIGURE8.4.Schematic repre-sentation of a TTT diagram for eutectoid steel. The an-nealing temperatures (a) through (e) refer to specific cases as described in the text. Note that the hardness scale on the right points downward.
(d) Lower or fine bainite is even harder (but less ductile) than coarse bainite due to its larger number of small cementite par-ticles. This microconstituent forms by reducing the quenching temperature even further, as indicated by “d” in Figure 8.4. The long times for heat treatment to complete the transformation are, however, often prohibitive for proceeding on this avenue, par-ticularly since other treatments can be applied to achieve simi-lar results; see below.
(e) If austenitic, eutectoid steel is very rapidly quenched to room temperature (to prevent the formation of pearlite or bai-nite), a very hard and brittle, body-centered tetragonal (BCT) structure, called martensite, is instantly formed. No diffusion of atoms is involved. Instead, a slight shift of the location of atoms takes place. This allows the transformation from FCC to BCT to occur with nearly the velocity of sound. Indeed, needle-shaped microconstituents can be observed in the electron microscope to shoot out from the matrix. The reason for the increased hard-ness and the greatly reduced ductility is that BCT has no close-packed planes on which dislocations can easily move. Another cause is the large c/a ratio, which distorts the lattice and leads to substantial twinning. The hardness of steel martensite in-creases with rising carbon content, leveling off near 0.6% C.
When austenitic steel is quenched to temperatures between the Ms and Mf temperatures (see Figure 8.4) only a portion of the austenite is transformed into martensite. Specifically, the amount of martensite, and thus the hardness, increase with decreasing temperature. Prolonging the annealing time at a given tempera-ture does not change the amount of martensite, as can be de-duced from Figure 8.4.
The quenching medium has an influence on the martensitic transformation. It affects the rate at which a work piece is cooled from austenite to below the Mf temperature without allowing pearlite or bainite microconstituents to form. As an example, the cooling rate in brine is five times faster than in oil and two times faster than in plain water. The quench rate can be even doubled by stirring the medium. (The severity of a quench is determined by the H-coefficient of the medium.)
Further, the shape and size of a piece to be heat-treated influ-ences the rate of transformation and thus its hardness. For ex-ample, if a thick part is quenched from austenite, the surface is affected more severely than the interior. This may cause a more complete martensitic transformation on the outside compared to the interior, and may thus result in quench cracks due to resid-ual stresses. Moreover, a large mass as a whole may not be ef-fectively quenched because of a lack of efficient heat removal.
Martensitic steel is essentially too brittle to be used for most
engineering applications. Thus, a subsequent heat treatment, called tempering, needs to be applied. This causes the precipita-tion of equilibrium ferrite in which very fine cementite particles are dispersed. The result is an increase in ductility at the expense of hardness. Tempering between 450 and 600°C is typical. Con-siderable skill and experience are involved when performing quenching and tempering. Because of the importance of these heat treatments, many metal shops have wall charts that provide guidance for the proper procedures which allow one to obtain specific mechanical properties.
It should be noted in passing that diffusionless phase transfor-mations (i.e., martensitic transfortransfor-mations) are also observed in other alloys or substances. Among them are martensitic transfor-mations in certain copper–zinc alloys, in cobalt, or many poly-morphic ceramic materials. Some alloys (such as NiTi, Cu-Al-Ni, Au-Cd, Fe-Mn-Si, Mn-Cu, Ag-Cd, or Cu-Zn-Al) which have under-gone a thermo-mechanical treatment that yields a martensitic structure possess a shape memory effect. After deformation of these alloys, the original shape can be restored by a proper heat treat-ment which returns the stress-induced martensite into the origi-nal austenite. Some materials also change their shape upon re-cooling. They are called two-way shape memory alloys in contrast to one-way alloys which change only when heated. Only those ma-terials that exert a significant force upon shape change are of commercial interest, such as Ni-Ti and the copper-based alloys.
(An Italian entrepreneur exploited this effect to create a smart shirt that automatically rolls up its sleeves at elevated temperatures and that can be smoothed out by activating a hair dryer.)
The TTT diagrams for noneutectoid steels need to be modi-fied somewhat to allow for the austenite-containing two-phase regions (i.e., or Fe3C); see Figure 8.1. Let us consider, for example, a hypoeutectoid steel. To accommodate for the transformation from to ( ) and from there to pearlite, etc., an additional line has to be inserted beginning at the nose of the TTT diagram and reaching to higher temperatures. It rep-resents the ferrite start temperature Fs; see Figure 8.5. Let us con-sider again a few specific cases.
(a) Quenching a hypoeutectoid steel from above Af (i.e., the highest temperature at which ferrite can form) to a temperature between Afand the eutectoid temperature results in a mixture of and primary ; see Figures 8.1 and 8.5. Once formed, the amount of ferrite does not change any further when extending the annealing time; see “a” in Figure 8.5.
(b) Austenitizing and quenching a hypoeutectoid steel to a tem-perature slightly above the nose in a TTT diagram yields rela-tively quickly a mixture of and primary . The remaining
austenite eventually transforms into pearlite upon some further isothermal annealing. The transformation into pearlite is com-pleted at the pearlite finish temperature, Pf; see “b” in Figure 8.5.
(c) Finally, quenching and holding the same steel to a tem-perature just below the nose yields, after crossing the Bfline, only bainite, which has, in contrast to pearlite, no fixed composition.
Similar TTT diagrams as in Figure 8.5 are found for hyper-eutectoid plain carbon steels. The differences are an Fe3C field (instead of the field) and a cementite start curve, Cs
(instead of the ferrite start line, Fs).
The martensitic transformations for hypo- and hypereutectoid steels behave quite similar as outlined above. However, the Ms
and Mftemperatures depend on the carbon content, as shown in Figure 8.6. Unfortunately, the Mftemperature cannot be clearly determined by visual inspection only. Other techniques, such as resistivity or X-ray diffraction measurements, need to be applied to obtain a reliable value. Further, the martensitic
transforma-T FIGURE8.5.Schematic
repre-sentation of a TTT diagram for a hypoeutectoid plain carbon steel. Afis the highest temperature at which ferrite can form; see Figure 8.1. Fs
is the ferrite start tempera-ture.
Retained austenite (Vol. %)
FIGURE8.6. Schematic representa-tion of the influence of carbon con-centration on the Msand Mf tem-peratures in steel and on the amount of retained austenite (given in volume percent).
tion is seldom entirely completed even at very low temperatures.
This results in some retained austenite, as indicated in Figure 8.6. Retained austenite can be of concern after tempering in that it may lead to some brittleness due to its transformation to martensite upon tempering and cooling to room temperature.
Alloying elements, such as Mn, Si, Ni, Cu, Mo, and V, are often added to steel in various quantities (often well under 1%) in or-der to favorably alter its properties. These additional constituents generally shift the nose in a TTT diagram to longer times. As a consequence, no pearlite or bainite is inadvertently formed upon quenching, and the martensitic transformation can be brought to completion even in large work pieces despite the fact that the cooling rate might have been relatively slow. This feature is re-ferred to as hardenability and expresses the ease with which martensite is formed upon quenching.
A second effect that alloying elements provide is a shift of the eutectoid composition to lower carbon concentrations. One mass
% of molybdenum, for example, reduces the eutectoid composi-tion of plain carbon steel from 0.77 to 0.4% C. This has some in-fluence on the primary microconstituent which is formed upon cooling. Specifically, a reduction of the eutectoid composition might lead to primary cementite instead of primary ferrite in a given plain-carbon–steel.
Third, some constituents (such as Mn and Ni) considerably de-crease the eutectoid temperature, TE, and the temperatures at which ferrite and cementite are first formed (Figure 8.1). For ex-ample, 5% Ni decreases TEof plain carbon steel by about 70°C.
Other elements, such as Cr, W, and Mo, increase the eutectoid temperature instead. These changes have to be considered when austenitizing treatments are conducted. Fourth, the martensitic start and finish temperatures are reduced by alloying elements.
Moreover, the entire TTT diagram might undergo some varia-tions. Fifth, the time needed for tempering is generally dimin-ished by alloying.
Sixth, and nearly most importantly, appreciable additions of chromium to iron (at least 12%) yield corrosion-resistant steels called stainless steels. They derive this property from a protective layer of chromium oxide which forms on the free surface. How-ever, with rising Cr concentrations the amount of austenite de-creases in either binary Fe–Cr or some chromium-containing iron–carbon steels which cause the ferrite to be the dominant