146 8·Iron and Steel (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 Mr 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 most146 8 • Iron and Steel (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